Corticospinal and peripheral conductions to proximal lower limb muscles: a combined neurophysiological study

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KAUNAS UNIVERSITY OF MEDICINE

Miglė Ališauskienė

Corticospinal and peripheral conductions to

proximal lower limb muscles: a combined

neurophysiological study

Doctoral Dissertation

Biomedical Sciences, Medicine (07B)

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Dissertation has been prepared at Kaunas University of Medicine during the period 2002 – 2006.

Scientific supervisor:

Assoc. Prof. Dr. Nerija Vaičienė

(Kaunas University of Medicine, Biomedical Sciences, Medicine – 07B)

Research advisor:

Prof. Michel R. Magistris (Medical Faculty of Geneva University, Switzerland, Biomedical Sciences, Medicine – 07B)

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ACKNOWLEDGEMENTS

This work has been possible thanks to the Convention of collaboration between Medical Faculty of Geneva University and Kaunas University of Medicine.

I express my sincere gratitude to Professor Michel R. Magistris from Geneva University for teaching me electroneuromyography and for giving me the opportunity to work in one of the best electroneuromyography laboratories. He suggested that I should specialize in transcranial stimulation studies and has guided my scientific work. His skill, experience, enthusiasm and patience have been the greatest school for me.

I am very grateful to Assoc. Prof. Dr. Nerija Vaičienė from Neurology Department, Kaunas University of Medicine, for the possibility to study in doctorantura program, for supervising my research, for her support in every matter.

I express my gratitude to Dr. André Truffert for his help in understanding of electroneuromyography, for suggesting of the topic of my scientific work, for enthusiastic collaboration and patience.

I thank to Dr. Jovita Švilpauskė for teaching me electroneuromyography, for her support in every matter and for her friendship.

My cordial thanks are addressed to Assoc. Prof. Habil. Dr. Daiva Rastenytė, for her invaluable advices.

I thank my many colleagues and friends of the Department of Neurology in Kaunas and Geneva for their help in my endeavors to achieve experience in neurology and for having played the role of healthy controls.

I am very grateful to my family for their patience, everlasting support and understanding during the years of my studies.

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ABBREVIATIONS

CMAP – compound motor action potential CMCT – central motor conduction time CNS – central nervous system

ENG – electroneurography ENMG – electroneuromyography MEP – motor evoked potential

PMCT – peripheral motor conduction time T – patellar T reflex

QCT – quadriceps combined technique SD – standard deviation

TMS – transcranial magnetic stimulation TST – triple stimulation technique

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1. INTRODUCTION

Clinical neurophysiology uses many tools to study the central and peripheral motor pathways. The corticospinal tract can be investigated through the motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and the peripheral motor conduction through the direct (M) or indirect (F, H or T) motor responses elicited by electrical stimuli applied over peripheral nerves and by mechanical stimuli of tendons.

In clinical routine, the neurophysiological evaluation usually concerns the distal limb muscles. Proximal muscles are rarely investigated, despite their major functional role, possibly because percutaneous stimulation of proximal nerves is difficult technically.

We were prompted to undertake this study because of the large number of patients in whom study of proximal muscles would be of interest. These patients are referred for gait disorders, limping, thigh weakness, myelopathies and radiculopathies. Initially, recordings from the quadriceps were done to study the distal femoral nerve (Gassel, 1963; Johnson et al., 1968; Schubert and Keil, 1968; Borenstein and Desmedt, 1969; Echternach and Hayden, 1969). Later, peripheral conduction of the proximal segments used the H response (Mongia, 1972; Guihéneuc and Ginet, 1974; Aiello et al., 1982; Kameyama et al., 1989) or the T reflex response (Stam et al., 1987; Kuruoglu and Oh, 1993; Frijns et al., 1997; Struys et al., 1997; Péréon et al., 2004). In the 1990s, Ravnborg et al. (1991) and Furby et al. (1992) recorded MEPs to the quadriceps.

We report the development of a new method combining the neurography of the femoral nerve with recording from the vastus medialis of the quadriceps muscle, MEPs, and recording of the patellar T reflex (T). This method allows a comprehensive assessment of the conduction to and from the quadriceps muscle. It is simple, quickly performed and well tolerated by the subjects tested. In this study we investigated 100 normal controls (2 subgroups of 50 subjects each) and 180 patients, to evaluate the clinical utility of this new method.

This method demonstrastes useful in routine electrophysiological examination when the question concerns the thigh muscles, lower limb girdle, and in case of gait disorders.

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2. AIM AND OBJECTIVES OF THE STUDY

2.1 Aim of the study

The aim of the study was to evaluate central and peripheral conductions to proximal lower limb muscles using a method combining the neurography of the femoral nerve, transcranial magnetic stimulation and recording of the patellar T reflex – quadriceps combined technique (QCT).

2.2 Objectives of the study

1. To determine reference values of central and peripheral conductions, corrected by height and/or age and limits of normal in control subjects.

2. To estimate interside asymmetry for corrected reference values and limits of normal for asymmetry in control subjects.

3. To evaluate changes of parameters in patients with impairment of central motor pathways, to compare these results with those of control subjects and to distinguish most discriminative parameters for disclosing of central motor disorders.

4. To estimate changes of parameters in patients with impairment of both central and peripheral motor pathways, to compare these results with those of control subjects and to determine most sensitive and specific parameters for disclosing a central and peripheral components.

5. To evaluate changes of parameters in patients with impairment of peripheral motor pathways, to compare these results with those of control subjects and to distinguish most discriminative parameters for disclosing of peripheral motor disorders.

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6. To determine the most sensitive and specific parameters for disclosing a peripheral proximal and peripheral distal/diffuse disorders in patients with impairment of peripheral motor pathways and to compare these results with those of control subjects.

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3. NOVELTY, SCIENTIFIC AND PRACTICAL SIGNIFICANCE

OF THE STUDY

3.1. Novelty and scientific significance of the study

Despite their important functional and clinical role, motor pathways to proximal muscles of lower limbs have been rarely investigated in clinical neurophysiology. Only few works reported MEPs registration from quadriceps muscles. Calculation of PMCT and CMCT was technically difficult, not standardized and could not be exploited in practice. TMS has not been studied in Lithuania previously. During this study for the first time a new quadriceps combined technique (QCT) was developed which:

- combines the neurography of the femoral nerve, MEPs, and recording of the patellar T reflex;

- allows a comprehensive assessment of the conduction to and from the quadriceps muscle.

Reference values and normal limits for 11 parameters according to height and age for each parameter were established (100 control subjects).

Large sample of patients with proximal limb weakness were studied and the best parameters to detect or discard central and peripheral disorders were established.

3.2. Practical significance of the study

We introduced a new combined method that gives a precise evaluation of the motor pathways to vastus medialis muscles:

- provides information on central and peripheral motor pathways conduction properties in a simple and rapidly performed procedure;

- has a diagnostic value and can be used to monitor the deficits in time;

- helps to distinguish pathological hyperreflexia from physiological brisk reflexes; - allows disclosing a central component even though it may be clinically hidden by peripheral involvement.

We implemented QCT to the routine tests of Kaunas and Geneva ENMG laboratories in proximal lower limbs or pelvic girdle symptoms of unclear origin.

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4. REVIEW OF THE LITERATURE

4.1. Transcranial magnetic stimulation 4.1.1. History

In the 1950s and 1960s, several studies used single pulse electric stimulation of the exposed motor cortex of monkeys to examine its action on pyramidal tract (Patton et al., 1954; Hern et al., 1962).

At the beginning of the 1980s, Merton and Morton showed that high-voltage electrical stimulation over the scalp was able to activate the motor cortex and to evoke twitches in the corresponding muscles (Merton and Morton, 1980). This technique has been used to investigate the central motor pathways in normal subjects and in patients with various neurological disorders. However, owing to the high resistance of the scalp and skull structures, much of the applied stimulating current flows along the skin and subcutaneous tissues inducing scalp muscle contraction and cutaneous pain (Hern et al., 1962).

Electromagnetic induction, producing a current in a conductive object by using a moving or time-varying magnetic field, was first described by Michael Faraday in 1831, and is probably the most relevant experimental observation for magnetic stimulation. Faraday put two coils on an iron ring and found that whenever the coil on one side was connected or disconnected from a battery, an electrical current passed through the coil on the other side. The iron ring acted as a channel linking the magnetic field from the first coil to the second. A change in the magnetic field, related to the changing current in the first coil, induced a current in the second coil.

In 1985, Barker and colleagues achieved magnetic stimulation of the human motor cortex. This led to a new era of research in motor control and cortical function (Barker et al., 1985).

4.1.2. Motor effects of brain stimulation

Transcranial magnetic stimulation (TMS) is a method of inducting electric current flow in the brain. It is this induced current, not the magnetic field, which activates neural tissue. To understand how TMS activates the brain, it is useful to review what is known about conventional electric stimulation.

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Electrical brain stimulation

Several studies in monkeys used single pulse electric stimulation of the exposed motor cortex to examine its action on pyramidal tract neurons (Patton et al., 1954; Hern et al., 1962). The main result was that an electrical pulse activated these neurons at proximal portions of the axon (rather than at the cell body). Activation of a number of these neurons gave rise to a descending volley in the pyramidal tract which could be recorded from the surface of the brainstem or spinal cord and which was termed the direct wave (D wave). As the intensity of stimulation was increased, the pulse began to recruit activity in other structures which could excite the pyramidal tract neurons transsynaptically and give rise to indirect waves (I waves) in the pyramidal tract. Precisely what elements were stimulated at these intensities was unclear, but they could have included cortical interneurons, or afferent fibers from subcortical or cortical structures. Each electrical pulse evoked a number of I waves at 1.5 – 2 ms intervals.

Several authors have now recorded the descending volleys in the pyramidal tract of humans, either during surgery on the exposed spinal cord, or from electrodes implanted into the epidural space at the spinal column in patients being treated with chronic stimulation for relief of pain (Boyd et al., 1986; Burke et al., 1998). The latter method has the adventage that the descending volleys can be recorded in conscious subjects (Di Lazzaro et al., 1998). Recordings in the operating theatre can be affected by the change in sensitivity of cortical structures produced by anaesthesia (Burke et al., 2000).

Transcranial magnetic stimulation

TMS of the motor cortex has been studied in detail. Two factors distinguish it from transcranial electrical stimulation. First, there is a preferred direction for recruiting descending activation (Marsden et al., 1983; Day et al., 1989). Second, TMS tends to evoke I waves at a lower intensity than D waves (Di Lazzaro et al., 1998). The latter is due to the fact that magnetic stimulation induced electric current flows parallel to the surface of the brain with no radial component of flow (Thompson et al., 1987).

The technical principle of TMS is to pass a brief surge of current through a coil, which induces a rapidly changing magnetic field. This magnetic field passes into the surrounding medium, where it again induces an electrical field. Applied over the human

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scalp it excites cortical neurons. Whereas, peripheral nerve stimuli, when maximal, excite all motor axons and evoke compound muscle action potentials (CMAPs) with latencies and sizes that do not vary if stimulation is repeated, transcranial stimuli evoke multiple descending volleys in corticospinal neurons. The initial volley – the direct (D) wave – is thought to arise from excitation of the pyramidal cell. This D-wave is followed by a number of indirect (I) waves that possibly stem from transsynaptic excitation of corticospinal cells by different sets of intracortical neurons (Berardelli et all., 1990, Burke et al., 1990, 1993, Rösler, 2001). Weak magnetic stimuli do not usually evoke a D-wave but only a succession of I-waves, as demonstrated by recordings in monkeys and in patients undergoing spinal surgery. The onset latencies of MEP are consistent with monosynaptic corticomotoneuronal excitation in most muscles. Temporal and spatial summation of the descending activity converging on spinal motor neurons is needed to drive them to discharge (Fig. 1). The complex segmental inhibitory and excitatory interconnections may significantly alter the excitability of the motor neuron pool, thereby influencing not only the size of the resulting MEP but also its latency. Depending on these influences and on the stimulation strength, some spinal motor neurons will not reach the firing threshold, whereas others may discharge several times (Day et all., 1987, Hess et al., 1987a, Rösler, 2001) (Fig. 1).

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Figure 1. An illustration of the multiple descending volleys after transcranial magnetic stimulation

A. Mechanism at the upper motoneurone level. I waves are mediated by the cortical interneurons and D wave generated directly at the axon hillock of the pyramidal neuron. In epidural recordings, the D wave (if present) is followed by multiple descending volleys of I waves.

B. Mechanism at the lower motoneurone level. Individual excitatory postsynaptic potentials (EPSPs) conveyed by multiple descending volleys summate temporally and spatially to give total EPSP amplitudes enough to reach the threshold potential for firing the lower motoneurone. (From Kaji and Kohara, 2006).

Motor evoked potentials (MEPs) vary in latency and size from one stimulus to another. If the target muscle is voluntary contracted, facilitation ensues. This facilitation, which relates to an increased excitability, diffuse magnetic excitation performed over the scalp causes a focal response from one or a group of muscle, with a latency of the response that decreases and an amplitude that increases (Hess et al., 1987a, Maertens de Noordhout et al., 1992; Kischka et al., 1993; Magistris MR et al., 1998).

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4.1.3. Assessment of cortico-spinal tract conduction

Motor evoked potential

When TMS is applied to the motor cortex at appropriate stimulation intensity, MEPs can be recorded from muscles of the contralateral extremity (Hess et al., 1987a; Rossini et al., 1999).

The size of MEPs is measured on neurographic recordings. The amplitude of the negative phase (in mV) may be expressed as a percentage of the amplitude of the maximum M wave recorded from the same muscle following supramaximal electrical stimulation of the corresponding peripheral nerve.

The amplitude of the MEP reflects not only the integrity of the corticospinal tract, but also the excitability of motor cortex, nerve roots and conduction along the peripheral motor pathway to the muscles. Patients with dysfunction of any of these parameters may show abnormal MEPs. A reduced size ratio is suggestive of either a reduced excitability of the cortico-spinal motor neurons, a conduction block on the cortico-spinal tract, a loss of cortical motor neurons or axons.

Presence of intact MEPs suggests integrity of the pyramidal tract. But in healthy people the size of MEPs shows great variability due to dispersion of the response of spinal motor neurons to the descending volley in the corticospinal tract, leading to a broad range of normal values. This problem has been solved by the “triple stimulation technique” (Magistris et al., 1998). The triple stimulation technique (TST) provides a quantitative electrophysiological measurement of central motor conduction failures. This technique involves three stimuli (cortex, distal and proximal site on the nerve) timed to produce two collisions. The initial transcranial stimulus is followed by a stimulus applied distally to the nerve, the antidromic impulses from which collide with the descending impulses evoked by TMS. A third stimulus is then applied on the proximal site on the nerve and orthodromic impulses from this stimulus cancel any uncollided impulses from the distal stimulus. The response from the third stimulus therefore reflects the number of peripheral nerve fibres activated from the initial cortical stimulus (Fig. 2). By suppressing the phase cancellation due to the dispersion of the MEP, the TST is more sensitive than conventional MEPs in detecting corticospinal conduction failures and it provides a precise measurement of the corticospinal tract conduction and of central motor conduction failures (Magistris et al., 1999).

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Figure 2. Scheme of the triple stimulation technique (TST)

The motor tract is simplified to four spinal motor neurons with their axons. Horizontal lines represent the muscle fibers of the four motor units. Solid arrows depict action potentials giving rise to a trace deflection, open arrows depict action potentials that are not recorded. A1: In the example, only three of four motor neurons are brought to discharge by the brain stimulus due to upper motor neuron lesion. A2: Following the brain stimulus, action potentials descend in axons 1-3. Desynchronization of the three action potentials has occurred. Motor neurons 1 and 2 discharge twice so that a second action potential descends (*). After a delay, a maximal second stimulus is given at the wrist (W), leading to descending (orthodromic) action potentials causing a first negative deflection of TSTtest curve, and to ascending (antidromic) action potentials in all axons. Three of the ascending action potentials collide and cancel with the action potentials descending in axons 1-3. The sites of collision are different due to the desynchronization of the descending action potentials. A3: The multiple discharges (*) on motor neurons 1 and 2 are not cancelled and continue to descend. They give rise to a small deflection in the trace (*). The action potential on axon 4 continues to ascend, since no collision occurred. A4: After a delay, a maximal third stimulus is given at Erb`s point, evoking action potentials, which descend on axons 1-3, while a collision occurs in axon 4. A5: Finally, a synchronized response from the three axons (1-3) which were initially excited by the transcranial stimulus is recorded as a second main deflection of the TSTtest curve. B1-B5: The TSTcontrol curve is recorded by replacing the first stimulus at the cortex by a supramaximal stimulus at Erb`s point (succession of stimuli: Erb-wrist-Erb) with appropriate adjustments of the delays. C: Superimposition of TSTtest and TSTcontrol curves. The TST amplitude ratio is 75%, indicating that three of four neurons were excited by the transcranial stimulus. (From Rösler and Magistris, 2004).

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Central motor conduction time

The MEP latency is measured in milliseconds from the stimulus artifact to the motor response onset. The conduction time from motor cortex to spinal cord alpha-motor neurons is referred as the central motor conduction time (CMCT). Several different methods exist for measuring the CMCT. These methods compute the difference between conduction time from cortex to the muscle and peripheral motor conduction time. The peripheral motor conduction time can be measured by different methods. The most frequently used are F wave latency (Robinson et al., 1988) and electrical (Merton et al., 1982) or magnetic (Maertens de Noordhout et al., 1999) stimulation of the spinal nerve roots. Alternatively, it could use the latency of the H reflex.

For the F wave method, PMCT is calculated by using this formula: PMCT = (F + M – 1) / 2

where F is the shortest of 16 F wave latencies, M is the M wave latency. 1 ms is the estimated delay time for antidromic activation of the spinal motor neuron. For the other methods PMCT equals the onset latency of the compound muscle action potential (CMAP) evoked by stimulation of the spinal nerve roots. With the spinal nerve root stimulation techniques, excitation takes place in the region of the intervertebral foramen where the spinal nerve exits the vertebral column (Mills and Murray, 1986). Therefore, the CMCT obtained by the spinal nerve root stimulation techniques is longer by 0.5 to 1.4 ms for the cervical roots and by 3.0 to 4.1 ms for the lumbosacral roots (Briton et al., 1990) compared to the CMCT obtained by the F-wave method. The large difference for the lumbosacral roots is explained by the longer distance from the spinal motor neuron to the exit point at the intervertebral foramen compared to the cervical roots.

It is recommended to measure CMCT while the target muscle contracts at 5% to 20% of its maximum strength (Rossini et al., 1999), because the MEP size saturates for stronger contractions (Hess et al., 1986). Facilitation is better during phasic contraction than during a steady isometric contraction. The CMCT to the active muscle is shorter by 2–3 ms than that to the resting muscle (Hess et al., 1986). This shortening is due to the fact that voluntary activation reduces the need for temporal summation to reach firing threshold, particularly of spinal motor neurons. Furthermore, the recruitment of larger, more rapidly conducting spinal motor neurons is facilitated during muscle contraction. CMCT shortens slightly with increasing intensity but this effect is usually less prominent than the effect of muscle contraction (Hess et al., 1986).

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The CMCT is also affected by the type and position of the stimulating coil. Studies may use either a circular, a figure-of-eight and a double cone coils (Mills et al., 1992; Brasil-Neto et al., 1992; Pascual-Leone et al., 1994; Terao et al., 2000; Di Lazaro et al., 2002). The shortest CMCT is obtained when the coil is placed at the optimal position for eliciting MEP in the target muscle. The representations of face, hand, arm, and leg are somatotopically aligned along an antero-lateral to postero-medial axis, with the face representation most lateral (approximately 10 cm away from the midline) and the leg representation in the midline.

Finally, the CMCT depends on the direction of TMS induced current in the motor cortex. Using a circular coil with the coil current flowing clockwise when viewed from above, the left hemisphere will be excited preferentially. Turning the coil over so that current now flows anticlockwise, the right hemisphere will be excited preferentially. With a figure-of-eight coil, the central linear segment should be over the motor area (Mills et al., 1992; Sakai et al., 1997).

The conduction time from the motor cortex to the muscle clearly depends on the length of the conduction pathway that depends on the height, and particularly so for the lower limbs, as shown by several authors (Rossini et al., 1987; Ravnborg et al., 1991). Investigators have found weak correlation between CMCT and the age, and no correlation between CMCT and the gender (Claus, 1990; Tobimatsu et al., 1998). The shorter CMCT and MEP latency to the lower limbs observed in women relate only to the parameter of height (Tobimatsu et al., 1998).

The main reasons for pathological CMCT lengthening are demyelination of the corticospinal fibers and degenerative or ischemic changes (Rossini et al., 1999). CMCT measurements are of interest in central demyelinating disorders (Hess et al., 1987b; Boniface et al., 1991; Mayr et al., 1991; Britton et al., 1995; Kandler et al., 1995; Jones et al., 1996; Ravnborg et al., 1996; Sale et al., 1997; Ingram et al., 1998, Kidd et al., 1998; Mathis et al., 1998; Fuhr et al., 2001, 2002; Kuhlmann et al., 2002; Humm et al. 2003), cerebral ischemic stroke (Berardelli et al., 1991; Heald et al., 1993, 1995; Turton et al., 1996; Traversa et al., 1997; Pennisi et al., 1999; Trompetto et al., 2000), myelopathies (Eisen et al., 1990; Herdman et al., 1992; Di Lazzaro et al., 1992; De Mattei et al., 1993; Maertens de Noordhout et al., 1993; Tavy et al., 1994; Mathis et al., 1996; Brunholzl et al., 1994; Linden et al., 1994; 1996; Kaneyama et al., 1995; Bondurant et al., 1997; Bednaric et al., 1998; Maertens de Noordhout et al., 1998; Jakolski et al., 1998; Tani et al., 1999; Kalita et al., 2000; Truffert et al., 2000; Kaneko et al., 2001; Misawa et al., 2001) and in

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neurodegenerative diseases, affecting the corticospinal tract (Cantello et all., 1991; Cruz Martinez et al., 1992; Mondelli et al., 1995; Abele et al., 1997; Cruz Martinez et al., 1997; Yokota et al., 1998; Miscio et al. 1999; Restivo et al., 1999; Triggs et al., 1999; Ugawa et al., 2000; Young et al., 2001; Wessel et al., 2001; Tremblay et al., 2002; Cantello et al., 2002). In these disorders CMCT may be useful in disclosing abnormal changes before clinical manifestation occurs.

Normal values of CMCT and MEPs in adults are available for many muscles of the upper limbs, of the distal region of the lower limbs and cranial muscles (Mills, 1999) (Table 1).

Table 1. Normal values for central conduction to lower limb muscles

N Muscle Stimulating

device

Coil MEP lat

ms (SD) CMCT ms Abbruzzese et al. (1988) Ugawa et al. (1989) Eisen et al. (1990) Booth et al. (1991) Chu et al. (1989) Ugawa et al. (1989) Kukowski et al. (1993) Tobimatsu et al. (1998) 11 40 150 30 52 40 20 86 Tibialis anterior Tibialis anterior Tibialis anterior Tibialis anterior Tibialis anterior Quadriceps Abductor hallucis Abductor hallucis Electrical Electrical Magnetic Magnetic Magnetic Electrical Magnetic Magnetic - - Circular Circular Circular - Circular Double cone 26.2 (1.5) 27.3 (2.0) 27.7 (2.4) 30.3 (2.2) 26.5 (1.7) 21.0 (1.3) - 39.3 (2.4) 12.6 (D) 13.1 (D) 13.1 (F) 13.8 (F) 14.8 (D) 13.0 (D) 13.8 (F) 17.3 (D)

F : calculated using F-wave method D : calculated using direct method N : number of subjects

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4.1.4. TMS in clinical neurology

Many abnormalities revealed by magnetic stimulation are not disease specific and, like most other neurophysiological tests, the results must be considered in the light of clinical data. TMS has been studied and applied the most in multiple sclerosis, stroke, amyotrophic lateral sclerosis, mielopathies and others.

Multiple sclerosis (MS) causes multiple central white matter lesions. These lesions, that are disseminated in time and space, frequently affect cortico-nuclear and cortico-spinal conductions. This function can be assessed using TMS, by studying MEPs to cranial and limb muscles. Various abnormalities can be observed in MS that relate to demyelination and to axonal loss (Hess et al., 1987b; Caramia et al., 1991; Ravnborg et al., 1992; Britton et al., 1995; Nielsen, 1996; Facchetti et al., 1997; Di Lazzaro et al., 1999). Demyelination of central motor pathways induces slowed conduction or conduction block. The latency of MEPs can be prolonged, the response may be dispersed, of smaller size or absent. A reduced MEPs size may indicate a central conduction deficit, but this relation is obscured by the desynchronization of the descending action potentials in response to TMS. The TST eliminates these effects, allows quantification of conducting central motor neurons. Thereby, it increases the sensitivity to detect a central motor conduction deficit (Magistris et al., 1999). Abnormalities of interhemispheric inhibition may be observed, that reflect demyelination or axonal lesions of corpus callosum fibers (Schmierer et al., 2000). The combination of CMCT and trancallosal inhibition data may be useful to estimate the disease progression and prognosis (Schmierer et al., 2002).

In stroke patients with hemiplegia, MEPs after cortical stimulation of the damaged hemisphere are often absent. Low amplitude MEPs with increased motor threshold and prolonged CMCT can be observed in patients with paresis (Berardelli et al. 1991). TMS is a good predictor of stroke outcome (Heald et al. 1993; Arac et al. 1994; Heald et al., 1995; Rapisarda et al., 1996; Turton et al., 1996; Cicinelli et al., 1997; Traversa et al., 1997; Cruz Martinez et al., 1999; Trompetto et al., 2000). During the early stage, obtainable MEPs correlates with a favorable outcome, whereas absent responses predict a poor recovery. Patients with delayed but present MEPs, recover more slowly than those with normal MEPs, but both groups are similar at 12 months (Heald et al., 1995).

In amyotrophic lateral sclerosis (ALS) patients, a common finding is that MEPs are of reduced size or absent. This relates to cortico-spinal inexcitability, or to impairement of both cortical and spinal motor neurons. CMCT can be prolonged in ALS but the degree of

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prolongation is usually modest (Ingram et al., 1987; Eisen et al., 1990, Kohara et al., 1996,1999). The correlation of central motor conduction prolongation with other MEP abnormalities and with clinical upper motor neuron signs is poor (Schriefer et al., 1989). The TST is of interest in detecting and quantifying the central conduction deficit while simultaneously yielding information concerning the peripheral motor neuron (Magistris et al., 1999, Rösler et al., 2000). Information on the responses of single spinal motor neurons to the corticospinal input in ALS reveals evidence of reduced firing frequency in corticospinal fibers with consequent impaired temporal summation at the motor neuron (Mills et al., 1995).

Cervical spondylotic myelopathy is characterized by a marked and early CMCT prolongation. Abnormalities of central conduction may precede clinical evidence of myelopathy. Slowed central conduction may be an early manifestation of cord compression, before it becomes evident on magnetic resonance imaging (Travlos et al., 1992).

Sometimes clinically and with routine ENMG examination distinction between cervical spondylotic myelopathy and ALS may be difficult. These disorders impair both upper and lower motoneurones and may share similar clinical features, including muscle wasting and fasciculations. TMS can distinguish these disorders. CMCT is usually more prolonged in cervical spondylotic myelopathy than in ALS, although this may not discriminative in an individual patient. Studies made from the muscles spared in cervical spondylotic myelopathy, but concerned in ALS, such as the masseter muscle (Trompetto et al., 1998) or the trapezius muscle (Truffert et al., 2000), are helpful for this distinction.

In patients with hereditary spastic paraplegia, lower limb responses are almost always abnormal: absent, reduced, or delayed. Upper limb responses, however, are usually normal even in the presence of clinical upper motor neuron signs (Pelosi et al., 1991; Schady et al., 1991). A similar pattern can be seen in patients with hereditary motor and sensory neuropathy with pyramidal signs (Schnider et al., 1991). The CMCT to small hand muscles in Friedreich’s ataxia is most often prolonged. Moreover, the MEP is frequently of small amplitude and dispersed. The sensitivity is even greater when recording from lower limb muscles. In other cerebellar ataxias, abnormalities are less severe and less frequent, with the highest rate of impairment being observed in spinocerebellar ataxias (Perretti et al., 1996; Schöls et al., 1997; Claus et al., 1998; Schwenkreis et al., 2002). Prolongation of central motor conduction is also common finding in patients with human T-cell lymphotrophic virus type I associated tropical spastic paraparesis. Responses in the lower

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limbs typically show marked prolongation. Upper limb responses may be normal or show slowing of central conduction, less prominent than when recording from leg muscles (Young et al., 2001).

4.2. Electroneurography of peripheral nerves

M response

The electroneurographic (ENG) examination helps in localizing dysfunction of peripheral nerves – focal or diffuse lesions, severity and type of damage (myelinic, axonal or both) (Oh, 1976; Brown et al., 1982). Supramaximal electrical stimulation is used for peripheral nerve conduction studies. Maximal current is defined as the smallest stimulating current that will produce M response of maximal amplitude and shortest latency. Supramaximal current is 25% above maximal. In motor nerve studies the following parameters are usually measured: latency from the stimulus to the M response in milliseconds (ms); amplitude from baseline to the negative peak of the M response in millivolts (mV); area, expressed as msmV and conduction velocity, obtained by dividing the distance between two stimulus points by the difference in conduction time between these two points. Conduction velocity measures the speed of the fastest conducting fibers.

Neurography of femoral nerve

The femoral nerve arises from the lumbar plexus and is formed from the posterior divisions of the ventral rami of the L2, L3 and L4 spinal nerves (Fig. 3).

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Figure 3. Femoral nerve (from Stewart, 2000)

Femoral nerves studies with recordings from the quadriceps muscle have been reported previously (Gassel, 1963; Johnson et al., 1968; Schubert and Keil, 1968; Borenstein and Desmedt, 1969; Echtarnach and Hayden, 1969). These early studies do not fully agree on the amplitude of the M responses due to different recording and stimulating methods. Results of these studies are provided in Table 2.

The femoral nerve may be damaged by trauma, surgical procedures (abdominal and pelvic, inguinal area, femoral artery angioplasty), childbirth, iliacus compartment syndromes (hematoma, abscess), tumors (muscle, bone, nerve, metastatic), diabetes mellitus, and involved in polyneuropathies, plexopathies and lumbosacral radiculopathies (Coppack et al., 1991; Sharma et al., 1991; Seid et al., 1994; Jarosz et al., 1995; Hall MC et al., 1995; Chevallier et al., 1996; Kuntzer et al., 1997; Hsieh et al., 1998; Takao et al., 1999).

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Table 2. Normal values of femoral nerve conduction in previous studies Recordings (muscle) Latency ms (SD) Amplitude mV Gassel (1963) Johnson et al. (1968) Schubert and Keil (1968) Echternach and Hayden (1969)

Borenstein and Desmedt (1969) Rectus femoris Vastus medialis Rectus femoris Vastus medialis Rectus femoris Vastus lateralis Vastus medialis 6.0 (0.6) 6.0 (0.7) 5.5 (0.47) 5.7 (2.57) 2.5 4-6 4-6 - - - 0.2 -11.0 17 15 18.4

Late F and H responses

Investigation of late responses (F wave and H reflexe) has been proven to be useful in examination of proximal parts of motor fibers. F waves and H reflexes are also useful in the diagnosis of polyneuropathy and radicular syndromes (Guihéneuc et al., 1976; Troni, 1981; Attarian et al., 2001).

The F waves are evoked by supramaximal stimulation during the motor nerve conduction study and are caused by antidromic volleys in the motor fibers. F waves can be recorded easily from the distal muscles of hands and legs. But F waves are limited ‘persistence’ in proximal muscles, their onset is difficult to identify within the repolarization phase of the M wave, and they require repeated maximal stimuli that are uncomfortable at this site.

The H reflex is a monosynaptic reflex that is elicited by electrical stimulation of the muscle spindle afferents due to their large diameter and thus hight excitability. It is obtained by weak stimulus intensity and is maximal at stimulus strength below threshold for a maximal M wave. At higher stimulus intensity the H reflex is obliterated by collision with the antidromic impulses in the motor axons. The H reflex is the electrophysiological analog of the tendon reflexes. The H reflex duration and amplitude are less useful clinically than the H reflex latency.

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H waves from the quadriceps muscle can also be obtained, they do not require strong stimuli, and their latency is easier to determinate as they are undisturbed by the M wave (Bratzlavsky, 1971; Mongia, 1972; Guihéneuc et al., 1974; Aielo et al., 1982; Spencer et al., 1984, Kameyama et al., 1989). But they have the major drawback of being less constant than T responses, without providing additional information. The nervous circuitry of the H and T responses is otherwise identical. A drawback of the T response is that the length of the pathways is unclear due to uncertain localization of the muscle spindles within the muscle.

4.3. Investigation of tendon reflexes

Several works report normal values of patellar tendon reflexes (T) (de Weerd et al., 1986; Stam et al., 1987,1989,1990; Kuruoglu et al., 1993; Ertekin et al., 1995; Frijns et al., 1997; Struys et al., 1997; Péréon et al., 2004). Normal values of previous studies are in Table 3.

Human tendon reflexes normally show a large variability within subjects, especially their amplitudes (Stam et al., 1987, 1989). This fact is observed both in clinical neurology and in physiological studies. Several factors explain the variation of T latency, such as the angulation of the knee joint, strength of percussion, degree of excitability of the neuromuscular spindles and spinal motor neurons (Péréon et al., 2004).

Various mechanical hand-held hammers and instrumental stimulators have been constructed to produce constant and reproducible tendon taps, but the reflexes evoked with these instruments still show considerable variability of amplitudes and less of latencies (Frijns et al., 1997).

In tendon reflexes studies the following parameters are usually measured: latency from the stimulus artifact of the tendon percussion and the onset of the T response (because there is an unpredictable variability between the mechanical contact of the hammer and the trigger of the sweep), and amplitude from baseline to the negative peak of the T response in mV.

Measurement of tendon reflex latency provides a simple method for assessing peripheral nerve conduction and is particularly useful in studying the proximal segment where the nerve fibers are deeply located and thus poorly available to direct stimulation.

Reflex testing is more likely to be of value in clinical practice when disease produces hyporeflexia rather hyperreflexia, and when the process is unilateral rather than bilateral

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(Uysal et al., 1999). Testing of T reflex is particularly useful for patients with lower motor neuron disorders, involving roots, plexus or peripheral nerves, and less useful for patients with upper motor neuron disorders (Stam et al., 1990; Uysal et al., 1999).

Table 3. Normal values of patellar T reflexes

Nb Latency ms (SD) Amplitude mV (SD) Interside differencies Ms (SD) Height R (p) Stam et al. (1987) Kuruoglu et al. (1993) Husemann et al. (1993) Frijns et al. (1996) Struys et al. (1997) Péréon et al. (2004) 40 24 39 102 51 268 Not known 17.2 (2.0) 19.4 (1.05) 21.0 (1.5) - 19.9 (1.7) - 1.4 (0.9) - 2.43 (1.38) 0.1-11.0 - - 0.9 (1.0) - - 1.0-1.6 - 0.44 (<0.005) 0.58 (<0.001) 0.72 (<0.01) 0.71 (<0.0001) 0.89 (<0.0001) 0.95 (<0.0001) R : correlation coefficient of latency and height (H)

To summarize, clinical neurophysiology uses several tools to study the central (by TMS) and peripheral (by ENG and mechanical stimuli of tendons) motor pathways. The combination of this measurements (MEP, M an T responses) has been used at the upper limb to investigate the conduction to biceps brachii in patients with compressive cervical myelopathy (Ofuji et al., 1998). However, these authors used the T reflex only to estimate the PMCT without taking the amplitude parameter into account. Wochnik-Dyjas et al. (1996), aimed at a comprehensive assessment of central and peripheral motor conductions to the quadriceps muscle (vastus lateralis); they provided normal values for latency, amplitude and asymmetry of parameters. Both magnetic stimulation of lumbar roots and F-waves were used by these authors to estimate the true PMCT. However, despite an attempt to standardize the results, amplitude parameters could not be fully exploited, because root and cortical motor responses were inframaximal and not expressed as ratios. This technique was not developed further and was not the subject of a published clinical study.

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5. SUBJECTS AND METHODS

5.1 Subjects

5.1.1. Control subjects

The study was performed in two subgroups of 50 subjects each.

Subgroup A was composed of healthy volunteers (29 women and 21 men); mean age 34.5 years, SD 10,4 years (range 18-64 years); mean height 171 cm, SD 9,4 cm (range 153-200 cm). A neurological examination was performed to ascertain absence of any neurological deficit. Exclusion criteria were: current or previous symptoms of radiculopathy or polyneuropathy, intoxications or disorders predisposing to polyneuropathy and signs of upper motor neuron disorder. All subjects gave informed consent and the experimental protocol was approved by the regional bioethics committee of Kaunas, where examinations of these healthy volunteers took place (BE – 2 – 43) (investigator: Miglė Ališauskienė).

Subgroup B was composed of patients (28 women and 22 men) referred for an electrophysiological examination; mean age 39.4 years, SD 13,8 years (range 16-83 years); mean height 167 cm, SD 11,4 cm (range 133-195 cm). These patients were free from motor and sensory symptoms. They were referred to rule out any sub-clinical dysfunction of central or peripheral motor pathways. Their complaints were mainly pain in one or more limbs. The exclusion criteria were otherwise the same as in subgroup A, except that patients might have had symptoms of radiculopathy without neurological deficits. Both their clinical and electrophysiological investigations were normal. All patients gave informed consent for the investigation. This subgroup was examined at the Geneva University Hospital as part of the routine clinical activity of the ENMG unit (investigator: André Truffert).

Subgroups A and B were first studied separately and compared. In a second step, the two subgroups were joined to form a group of hundred subjects (57 women and 43 men; mean age 37 years, SD 12.4 years; mean height 169.4 cm, SD 10.6cm), all studied on both sides, i.e. 200 sides were studied. The purpose of this fusion was to derive robust normative data that could be used in a collaborative clinical study, despite minor methodological differences described here after. Inside this group, man and women were also compared.

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5.1.2. Patients

We examined 180 patients referred to our laboratories for evaluation of proximal weakness of their lower limbs (105 women and 75 men; mean age 54.0 years, SD 17.2 years; mean height 168.6 cm, SD 9.8 cm). They presented with various neurological disorders and were divided into 3 groups based on clinical, laboratory and radiological findings (Table 4).

- Group 1 consisted of 71 patients with impairment of central motor pathways (mean age 49.5 years, SD 18.1 years; mean height 167.4 cm, SD 9.7 cm). In addition to proximal lower limbs weakness, these patients exhibited clinical pyramidal signs (hyperreflexia, Babinski’s sign, spasticity) and lacked clinical signs of peripheral nervous system involvement.

- Group 2 consisted of 36 patients with impairment of both central and peripheral motor pathways (mean age 58.4 years, SD 16.5 years; mean height 167.3 cm, SD 11.1 cm). All patients exhibited both pyramidal and peripheral clinical signs in lower limbs.

- Group 3 consisted of 73 patients with impairment of peripheral motor pathways (mean age 56.8 years, SD 15.7 years; mean height 170.4 cm, SD 9.2 cm). This group was further subdivided into two subgroups according to the level of impairment: Subgroup 3a, with impairment of the proximal part of the peripheral nervous system, i.e. L2-L4 roots, lumbar plexus or proximal part of the femoral nerve (segment located above inguinal ligament); Subgroup 3b, with either impairment of the distal part or the femoral nerve or diffuse polyneuropathy.

All patients gave informed consent for the investigation using the QCT which was performed at Kaunas (investigator: MA) and Geneva (investigator: AT) University Hospitals as part of the routine clinical activity of our ENMG laboratories.

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Table 4. Distribution of patients by a level of disorder

Level of disorder Diagnosis n

Group 1: central (n = 71) Multiple sclerosis Spondylotic myelopathy CNS degenerative disorders Other myelopathies Traumatic myelopathy Inflammatory myelopathy Stroke 33 22 7 3 2 2 2 Group 2:

mixed (central + peripheral)(n = 36)

Myeloradiculopathy ALS Myelopathy + polyneuropathy Stroke + polyneuropathy 14 11 9 2 Group 3: peripheral (n = 73) - Subgroup 3a: proximal peripheral (n = 40) L3 or L4 monoradiculopathy Lumbar plexopathy

Polyradiculopathy (lumbal stenosis) Proximal motor neuropathy (diabetes mellitus)

Multifocal motor neuropathy with conduction blocks 27 6 4 2 1 - Subgroup 3b: distal/diffuse peripheral (n = 33) Other polyneuropathies

Sensory axonal polyneuropathy Femoral neuropathy Guillain-Barré syndrome 13 9 6 5 Total 180 29

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5.2. Methods

5.2.1. Equipment

The investigation was performed using a Viking IV (subgroup A) or Viking Select (subgroup B) electromyograph (Nicolet, Madison, Wisconsin, USA). In subgroup A, the magnetic stimulator was a Dantec (Medtronic, Skovlunde, Denmark) and the probe used was a circular (90 mm) Medtronic simple coil. In subgroup B the magnetic stimulator was a Magstim 200 (Magstim Company, Spring Gardens, Whitland, Dyfed, UK) and the probe used a nude double cone coil. In both subgroups, tendon reflexes were elicited by the same model of electrical hand-held hammer (Medelec, Oxford Instruments, Wiesbaden, Germany) which was connected to the trigger input of the electromyograph (Fig. 4).

Figure 4. Experimental setting, showing patient position, coil position over the head, and reflex hammer

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5.2.2. Recordings

Recording was from both vastus medialis muscles with surface electrodes. The active electrode was placed on the thigh, on a line joining the superior edge of the patella to the contralateral anterior superior iliac spine at a distance from the superior edge of the patella equal to the quarter of the distance between the ipsilateral anterior superior iliac spine and the superior edge of the patella; the reference electrode was on the patella (Fig.5). The ground electrode was placed on the thigh between the site of electrical stimulation and the active electrode. An additional ground electrode was sometimes placed on the chest of patient to avoid or minimize the TMS stimulus artifact. During examination the subject laid supine, with a pillow placed under the knees. The bandpass of the amplifier was 2 Hz-5 kHz. The sampling rate during recordings was 12 kHz.

aE (-)

rE (+) D

D/4 _D/4

Figure 5. Recording electrodes position. D: distance from anterior superior iliac spine to upper border of patella; aE: active electrode (motor point), rE: reference electrode (patella)

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5.2.3. Stimuli

In subgroup A, face `A` (face visible) of the simple coil stimulated the left hemisphere and face `B` the right hemisphere. The coil was placed over the lower limbs motor area at the optimum scalp position to elicit a MEP from the contralateral vastus medialis muscle. In subgroup B, the double cone coil was applied over the vertex, with induced current flowing in a postero-anterior direction. Stimulation intensities, expressed as a percentage of the maximum output of 2.0 Tesla, were increased until the amplitude of the MEP did not increase further and maximal amplitude was reached. TMS was applied while the subject was contracting the target muscle, with a strength that was not standardized but only monitored by audio feedback, depending on the amplitudes of the MEPs recorded and until the best possible MEP amplitude was reached. The latency of MEP was defined as the shortest latency from 6 responses and was measured in milliseconds from the stimulus artifact to the first negative deflection from baseline. Amplitude (in mV) was that of the negative peak. The minimal latency and maximal amplitude were chosen for calculations.

Electrical stimulation site was below the inguinal ligament, 2 cm laterally from the femoral artery. A monopolar stimulation was used with a hand-held surface cathode probe (diameter = 0.8 cm) applied over the femoral nerve and a large plate anode (30 cm2) placed on the gluteal region (Roth and Magistris, 1987) and the stimulus duration was 1 ms. Stimulus intensity was increased until the M response reached its maximal amplitude.

The T was obtained by tapping the patellar tendon with the hammer. Sixteen or more responses were recorded on each side using either Jendrassik`s maneuver, adjusting knee joint angle or strength of percussion to reach the best possible amplitude and the shortest possible latency. The latency of the T response was measured from the onset of the stimulus artifact to the onset of the first deflection from baseline. Amplitude was measured from baseline to negative peak. The minimal latencies and maximal amplitudes were chosen for statistical calculations.

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5.2.4. Parameters

The latency and amplitude measurements are not readily usable for optimal interpretation and need a processing.

- Raw data

M response: the latency of the M response (Mlat) was measured from the electrical stimulus artifact to the onset of the negative deflection of the maximal M response. The amplitude of the M response (Mampl) was that of the maximal negative peak.

T reflex: the latency of the T (Tlat) was measured from the onset of the mechanical stimulus artifact (usually a small positive wave) to the onset of the negative deflection. Tlat was defined as the shortest T latency of a series of 16 (or more) percussions. T amplitude (Tampl) was that of the maximal negative peak.

MEPs: the latency of MEP (MEPlat) was the shortest from 8 (or more) responses and was measured from the stimulus artifact to the negative deflection from baseline. Amplitude was that of the maximal negative peak.

- Derived parameters

The peripheral motor conduction time (PMCT) was defined as (Tlat - 1)/2. For further analysis of peripheral conduction, PMCT was divided in two segments, distal (PMCTdist = Mlat) and proximal (PMCTprox = PMCT - PMCTdist).

The central motor conduction time (CMCT) was defined as MEPlat - PMCT.

MEPampl was related to Mampl by use of the MEP/M amplitude ratio in percent. Likewise Tampl was related to Mampl by use of the T/M amplitude ratio in percent. In addition we studied the T/MEP amplitude ratio which we expected to be a good marker of pyramidal tract dysfunction if increased, and the T/MEP latency ratio to reflect the respective proportion of central and peripheral conduction slowing.

- Correction of parameters

Since most parameters depend on height, and to a lesser degree on age, they need to be corrected. This correction is convenient for graphical representation and suppresses the need of height and age-matched samples for statistical comparisons in patients groups (see in statistical analysis).

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- Asymmetry of parameters

For each corrected parameter the amount of interside asymmetry was determined using the relative difference in percent, i.e. the percentage of difference between right and left sides: Δ% = (higher value - lower value)/mean value)*100 (Bouquiaux et al., 2003).

- Graphical representation

After the processing described above the most informative data was extracted and represented in two-dimensional graphics, with latency parameters in abscissa and amplitude parameters in ordinate. These diagrams summarize the conduction properties of the motor pathways at three levels, distal peripheral, proximal peripheral, and central. Normal limits could be easily represented, including limits of normal asymmetry which appear as boxes on the graph.

Detailed description of the procedure

Acquisition of data was carried out in three steps. The first step was to record M response by monopolar stimulation of femoral nerve. Maximal M responses were obtained at rest and during voluntary contraction of the quadriceps muscles, so that MEPs and M comparison could be done from acquisitions made under the same conditions. The second step was to record MEPs to TMS during voluntary contraction. Each side was recorded sequentially with the simple coil, whereas both sides were recorded simultaneously with the double cone coil. The third and last step consisted of T recordings.

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5.3. Statistical analysis

5.3.1. Normal controls

For all parameters, the Pearson’s coefficient of correlation R with height and age was calculated using the SPSS statistical software (version 12.0.1, Apache Software Foundation). The analysis was performed separately for right and left sides to verify that unpaired data yielded similar results. For the asymmetry coefficient Δ% correlations were sought not only with height and age, but with the variable itself.

Comparisons between subgroups A and B were made using a non-parametric test (Mann-Whitney). The tests were performed with raw data and after normalization of data according to height and age of the subject. To obtain normalized data a regression analysis was first performed and values were corrected using the following method:

1. Transformation of raw data according to the height of the subject. The formula is:

yH = mo + yo – (aH + b), (1)

there: yH - value of parameter after correction for height; mo - mean of observed values; yo - observed parameter; b - positive or null value of the intercept of the regression line with the ordinate axis. For all latency parameters this value was equalized to 0, because the linear relationship between height and latency is of the same nature as a conduction velocity (i.e. a distance equal to 0 corresponds to a latency equal to 0).

2. Transformation of height-corrected values according to the age. The formula is:

yHA = mH + yH - (a’A + b’), (2)

there: yHA - value of parameter after correction for height and age; mH - mean of height-corrected values; b’- positive or null value of the intercept of the regression line with the ordinate axis.

At this point we obtain the equation giving the corrected value as a function of height and age:

yHA = yo - aH - a’A + c, (3)

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there c is a constant value equal to:

mo + mH - b - b’. (4)

This corrected value yHA corresponds to the value of the parameter once the effects of height and age have been cancelled. Further analyses were made in the group resulting of the fusion of A +B, where male and female data were compared using the same method.

The aim of statistical analysis was to derive predicted normal limits for all parameters given the height and age of the subjects. Normal limits for each parameter were defined as the rounded value closest to the 2.5 th (lower limit) or 97.5 th (upper limit) percentiles. If these were different from mean ± 2 standard deviations, an intermediate rounded value between percentiles and m ± 2 SD was chosen. If the distribution of the data were not Gaussian, the skew was corrected as described by Robinson et al. (1991) for better estimation of confidence intervals, using data transformation with simple mathematical functions. We used the following equation to calculate the expected limits of normal value as a function of height, and age if necessary:

Limit of normal (LON) = aH + a’A + e, (5)

There:

e = LON – c (6)

LON for Δ% were established using the same method. For all calculations, the degree of precision was one tenth of percent which represents the order of precision of the measures, i.e. all coefficients that were less than one were rounded to the third decimal.

The normal limits can be readily calculated for each patient on an Excel sheet (Microsoft) given the age and height.

5.3.2. Patients

The mean and SD of each parameter and related coefficient of asymmetry (Δ%) were determined for each patient group relative to the reference values.

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Means of parameters were compared in the groups of patients and in the control group using a non-parametric Mann-Whitney-Wilcoxon test. The levels of statistical significance:

p > 0.05 – not significantly;

p < 0.05 (and p ≥ 0.01) – poorly sensitive parameter; p < 0.01 (bet p ≥ 0.001) – moderately sensitive parameter; p < 0.001 – highly sensitive parameter.

In patients the sensitivity and the specificity were determined for each of these parameters using Gold standard test (Table 5).

The results calculated in patients with mixed disorders (Group 2) are not reported.

Table 5. Gold standard test

Test results Disease (Number of subjects) No disease (Number of subjects) Total Abnormal Normal Total a c a + c b d b + d a + b c + d n = a + b + c + d

The sensitivity is the proportion of individuals with the disease who are correctly identified by the test: Sensitivity = a / (a + c);

The specificity, as the proportion of individuals without the disease who are correctly identified by the test: Specificity = d / (b + d).

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6. RESULTS

6.1 Reference values of central and peripheral conductions in normal controls

MEPs were easily elicited in all subjects also when the simple coil was used, although a high level of voluntary contraction or high magnetic stimulation strength (100% of the stimulator output) were sometimes required to reach maximal amplitudes of the MEPs. Figure 6 gives an example of the curves with simultaneous recording of the MEPs on both sides. Figure 7 depicts successive recordings of T responses showing variability between iterative percussions, and placement of the latency markers to get a correct measure of T latency.

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A. B.

F igure 6. Examples of curves ob ta ine d in on e complet e se t o f ex amination Top t race s : r ight s ide. B o ttom tra ces : lef t sid e. (A ): M re spon ses and MEPs af ter TM S ob tai ned d ur ing a voluntary co nt rac tio n of th e quad ri cep muscles us ing th e dou ble c one co il wi th s imultan eous reco rdi ng of bo th sid es. On e M and 3 MEP s r eco rdi ngs ar e s uper imposed. (B) : M res pons es a pate ll ar T re fl exes a ft er pe rcus sion o f the p at el lar ten don reco rde d from rela xed quadr icep s muscles. Two M and 2 T resp onse s a re s uper impose Note th at T r espo nse lat enc ies a re on ly appar ent, n ot t rue la tenc ie s (s ee exp lan at io n in f igure 7). In both A an d B, the in it ia l de fle ct ions are th ose o f th M r espon se (CMAP) obt ai ned by sur fac e monopola r supr amaximal ele ct ric al s timulat ion of femoral nerv es be low the inguina l liga me nts (sho dura tion : 1 ms). 39

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A . B . F igure 7. Examples of p at el lar T re fl ex cu rves Eight suc ces sive r eco rding s of the T, sh owing variab ili ty of bo th on se t of th e m echan ica l s ignal re la tive to the sweep l ine (f ir st markers ), an d o f respo nse s ons et (se cond markers). T he t rue T la ten cy is the time in terval betw een markers 1 and 2. (A): succ ess ive t race s show

ing marker placement. (B

): same trac es s uper imposed. Note vari abi li ty of l at enc ies and ampli tude s. 40

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6.1.1. Comparison of subgroups

Measured values and correlations obtained in both subgroups are shown in Table 6. Subjects were on average 4 cm taller and 5 years younger in subgroup A than in subgroup B. There was a uniform tendency for all latency parameters to be slightly longer in subgroup A than in subgroup B. This difference was biggest for the longest latencies (i.e. T latencies, 0.6 ms, and MEP latencies, 0.9 ms) and of a magnitude expected for a simple distance effect (a 4 cm difference in height corresponds to 0.66 ms at 60 m/s). Normalized parameters were identical in both subgroups (Mann-Whitney p > 0.05). There was an inverse correlation of height with age in subgroup A (i.e. the taller subjects were also the younger) which was not observed in subgroup B. A strong correlation with height was found in both subgroups for all parameters except MEP amplitude ratios. The subgroups differed slightly with respect to age correlations which were only weak or absent in subgroup A, whereas they were slightly stronger in subgroup B. Sex ratio was similar in both subgroups (0.72 in subgroup A, 0.79 in subgroup B). Women were on average shorter by 15 cm than men in both subgroups. The statistical analysis showed that results in subgroup A and B were not different. Therefore, the two subgroups were joined for further determination of gender differences and calculation of reference values.

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Table 6. Electrophysiol

ogical data and correlat

ions in Kaunas (su

bgroup A) and Geneva (subgroup B) and i

n the resulting control group (A + B)

Height (cm) Age (yrs) M T MEP PMCT (ms) CMCT (ms) Latency (ms) Amplitude (mV) Latency (ms) Amplitude rat io (%) Latency (ms) Amplitude rat io (%) Subgroup A (n = 10 0) Subgroup B (n = 10 0) Group A + B (n = 2 00 ) Mean (SD) 17 1. 7 ( 9. 39 ) 1 67 .2 (1 1. 35 ) 1 69 .4 (1 0. 64 ) 34. 5 (1 0. 41 ) 39. 4 (1 3. 80 ) 37. 0 (1 2. 44 ) 4 .6 (0 .6 2) 4 .5 (0 .6 4) 4 .6 (0 .6 3) 11. 5 (2 .9 0) 11. 4 (2 .6 0) 11. 5 (2 .7 4) 22. 4 (2 .1 7) 21. 8 (2 .2 3) 22. 1 (2 .2 2) 32. 7 (1 5. 75 ) 37. 4 (1 8. 98 ) 35. 1 (1 7. 55 ) 21. 1 (2 .0 9) 20. 2 (1 .7 8) 20. 6 (1 .9 9) 59. 2 (1 5. 12 ) 60. 9 (1 6. 39 ) 60. 0 (1 5. 75 ) 10. 7 (1 .0 9) 10. 4 (1 .1 1) 10. 6 (1 .1 1) 10. 4 (1 9 .7 (1 .0 10. 1 (1 Subgroup A (n = 10 0) Subgroup B (n = 10 0) Group A + B (n = 2 00 ) Height cor re la tion R (p) - 0. 29 ( <0 .01 ) 0.0 5 (n.s. ) - 0 .1 2 (n. s.) 0 .41 ( < 0. 00 1) 0 .66 ( < 0. 00 1) 0 .56 ( < 0. 00 1) 0 .35 ( < 0. 00 1) 0 .29 ( < 0. 01 ) 0 .32 ( < 0. 00 1) 0 .76 ( < 0. 00 1) 0 .74 ( < 0. 00 1) 0 .75 ( < 0. 00 1) - 0. 36 ( <0 .00 1) - 0 .0 02 (n.s .) - 0. 17 ( <0 .05 ) 0 .80 ( < 0. 00 1) 0 .79 ( < 0. 00 1) 0 .80 ( < 0. 00 1) - 0 .0 8 (n. s.) - 0 .0 4 (n. s.) - 0 .0 8 (n. s.) 0 .74 ( < 0. 00 1) 0 .73 ( < 0. 00 1) 0 .75 ( < 0. 00 1) 0 .61 ( < 0. 0 .54 ( < 0. 0 .60 ( < 0. (1) : w ithout cor re ct ion Subgroup A (n = 10 0) Subgroup B (n = 10 0) Group A + B (n = 2 00 ) (2) : w ith co rr ec tion* Subgroup A (n = 10 0) Subgroup B (n = 10 0) Group A + B (n = 2 00 ) Age cor rel at ion R (p) - 0. 29 ( <0 .01 ) 0.0 5 (n.s. ) - 0 .1 2 (n. s.) 0.1 1 (n.s. ) 0 .27 ( < 0. 01 ) 0 .17 ( < 0. 05 ) 0 .25 ( < 0. 05 ) 0 .32 ( < 0. 01 ) 0 .28 ( < 0. 01 ) - 0. 25 ( <0 .05 ) - 0. 35 ( <0 .01 ) - 0. 32 ( <0 .00 1) - 0. 20 ( <0 .05 ) - 0. 37 ( <0 .00 1) - 0. 27 ( <0 .00 1) - 0 .0 9 (n. s.) 0 .39 ( < 0. 00 1) 0 .15 ( < 0. 05 ) 0.1 1 (n.s. ) 0 .53 ( < 0. 00 1) 0 .35 ( < 0. 00 1) 0 .21 ( < 0. 05 ) 0 .24 ( < 0. 05 ) 0 .25 ( < 0. 00 1) 0 .30 ( < 0. 01 ) - 0 .23 ( < 0. 00 1) - 0 .1 7 (n. s.) 0 .38 ( < 0. 00 1) 0.0 6 (n.s. ) - 0 .0 2 (n. s.) 0 .56 ( < 0. 00 1) 0 .24 ( < 0. 01 ) 0.0 8 (n.s. ) - 0. 24 ( <0 .05 ) - 0 .0 9 (n. s.) - - - - 0 .0 9 (n. s.) 0 .39 ( < 0. 00 1) 0 .15 ( < 0. 05 ) 0.1 1 (n.s. ) 0 .53 ( < 0. 00 1) 0 .34 ( < 0. 00 1) - 0 .1 9 (n. 0 .23 ( < 0. - 0 .0 3 (n. - 0 .0 9 (n. 0 .23 ( < 0. 0.0 4 (n.s.

M: motor reponse, T: pat

ellar T re

flex, MEP: mot

or evoked potential, PM

CT: peripheral motor con

duction time, CMCT: central moto

r conduction time,

SD: standard deviation, n

: refers to the numb

er of

lower limbs examined, R

: Pearson’s cor relation co efficient. p : level of si gnific ance; n.s. : not si gnificant (i.e. p ≥ 0.05), p < 0.05 ( and ≥ 0.01) : low si gnific anc e, p <0.01 (and ≥ 0.001) : mild significan ce, p <0.0 01 : highl y signifi cant cor rela tion.

* with correction : understand once influence of

height on paramete

r has been canc

elled b

y

al

geb

raic corr

ection (see meth

ods). This was applied o

nl y when hei ght cor relation was found si gnificant (p <0.05). 42

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6.1.2. Gender differences

Gender differences are summarized in table 7 and figure 8. Mean values of all latency parameters were shorter in women than in men, and regression analysis clearly showed that these differences were explained by the shorter height of women (see Fig. 8a). For the MEP latency parameter, the slope of the regression line was identical (12%) in both genders. The mean amplitude of the M response was smaller in women than in men and regression analysis showed that this difference was not explained by height, or by age differences (Fig 8b). No gender differences were disclosed for T and MEP amplitude ratios.

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Table 7. Gender differences in Kaunas (subgroup A) and Geneva (subgroup B) and i

n the resulting control group (A+B)

M: motor response,

T:

patellar T

re

flex, MEP: motor evoked

potentia

l, PMCT: peripheral m

otor conduction time,

CMCT: central mot

conduction time, SD: standard deviation, n : re

fer

s to the number of lower

limbs examined Height, cm m (SD) Ag e, y rs m (SD) M T ME P P MCT, ms m (SD) CMCT, m m (SD) Latenc y, ms m (SD) A mplitude, mV m (SD) Latenc y, ms m (SD) A mplitude ratio, % m (SD) Latenc y, m s m (SD) A mplitude ratio, % m (SD) Men Subgro up A (n = 4 2) Subgro up B ( n = 4 4) Group A + B (n = 8 6) 179 .8 (7. 81) 175 .3 (8. 70) 177 .7 (8. 48) 33. 9 ( 9.6 9) 43. 6 ( 14. 65) 38. 9 ( 13. 32) 4.9 (0. 63) 4.8 (0. 68) 4.9 (0. 66) 13. 5 ( 2.9 9) 12. 2 ( 2.7 2) 12. 8 ( 2.9 0) 24. 1 ( 1.8 9) 23. 3 ( 1.9 0) 22. 7 ( 1.9 2) 28. 1 ( 17. 15) 39. 3 ( 20. 49) 33. 8 ( 19. 65) 22. 6 ( 2.2 2) 21. 5 ( 1.2 9) 22. 0 ( 1.8 8) 59. 7 ( 16. 59) 58. 9 ( 19. 19) 59. 2 ( 17. 86) 11. 6 ( 0.9 4) 11. 2 ( 0.9 5) 11. 4 ( 0.9 6) 11. 0 ( 1.5 10. 3 ( 1.0 10. 7 ( 1.3 Women Subgro up A (n = 5 8) Subgro up B ( n = 5 6) Group A + B (n = 1 14) 165 .8 (5. 08) 160 .4 (8. 23) 163 .2 (7. 30) 34. 9 ( 10. 97) 36. 2 ( 12. 26) 35. 5 ( 11. 59) 4.4 (0. 53) 4.2 (0. 48) 4.3 (0. 51) 10. 1 ( 1.8 3) 10. 7 ( 2.3 1) 10. 4 ( 2.1 0) 21. 2 ( 1.4 1) 20. 6 ( 1.0) 20. 9 ( 1.5 8) 36. 1 ( 13. 85) 35. 9 ( 17. 75) 36. 0 ( 15. 82) 21. 2 ( 1.4 1) 19. 14 (1. 41) 19. 6 ( 1.3 1) 58. 8 ( 14. 09) 62. 5 ( 13. 78) 60. 6 ( 14. 00) 10. 1 ( 0.7 0) 9.8 (0. 85) 10. 0 ( 0.7 9) 9.9 (1. 06) 9.3 (0. 91) 9.6 (1. 03) 44

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A . B . F igure 8. MEPs la ten cie s (A), an d M respon ses ampli tude s (B), plot ted agai nst h eight (up p

er diagrams) and age of th

e sub jec ts ( lowe r diag rams). Fill ed ci rc les repr ese nt men, a nd op en circ les women. Line ar co rre lat ion s a re rep res en ted as sol id lin es (men’s d at a) or dot ted l ine s (women’s da ta). Th e s ame stro ng corre la tion o f MEP s lat enc ies wi th he ight (s lope of r egress ion l ine s = 12%) is obs erved fo r both

men and women, whereas

no signif ic an corr el at ion wi th age i s obs erved fo r eac h gender (a w eak posit ive cor re la tion ap pear s, however, whe n bot h genders a re an alysed toget her). T her e i s a robus t inver se co rr ela ti on o f M r espon ses ampli tud es with age fo r bot h genders with a s imilar slo pe of abo ut -8% of the regres sion li nes in men an women. No correl at ion o f M amplitu des e xis t wi th he ight if each gende r i n ana lysed se par ate ly, but a we ak posi tive co rre la tio n app ear s if bo th gender are a nalysed togeth er. S ex dif fer enc es o f mean M ampli tude s values cann ot b e ex pla ined by he ight, or by age dif fe renc e. 45

Corticospinal and peripheral conductions to proximal lower limb muscles: a combined neurophysiological study

KAUNAS UNIVERSITY OF MEDICINE

Miglė Ališauskienė