Evaluation of the Cranial Nerves
George Sachs
Summary
Electrophysiological testing of the cranial nerves remains an important and perhaps under utilized area of neurophysiological evaluation. Motor responses from individual branches of the facial nerve can be obtained to assist in the diagnosis and prognosis of facial neuropathies. In blink reflex testing, the first division of the trigeminal nerve is stimulated and responses from the orbicularis oculi are obtained. This form of testing can be used for a variety of purposes including helping to localize dis- orders of the Vth and VIIth cranial nerves, assisting in the evaluation of Guillain–Barré syndrome, and even in the assessment of brainstem disorders. It is also useful in the assessment of hemifacial spasm.
The masseter reflex and master silent period can also be assessed when clinically indicated. Although less commonly pursued, other cranial nerves can also be evaluated electrophysiologically, including the XIth and XIIth cranial nerves. Assessment of these cranial nerves can be important in assessing other diseases, including amyotrophic lateral sclerosis.
Key Words: Bell’s palsy; blink reflex; facial neuropathy; hemifacial spasm; trigeminal neuropathy.
1. INTRODUCTION
Although clinical neurophysiological techniques can be applied to nearly all cranial nerves, this chapter will focus on those readily studied in standard EMG laboratories. Evoked poten- tial studies of the optic and auditory nerves are discussed in Chapters 25 and 26, and only brief mention will be made of specialized techniques applied to extraocular and laryngeal muscles.
Standard nerve conduction studies and EMG can assess the facial, trigeminal, spinal accessory, and hypoglossal nerves and the muscles they innervate. However, a few general caveats apply to the study of these cranial nerves. First, normal values for conduction studies are not as well- established as those for limb nerves and, therefore, side-to-side comparisons become particu- larly important. Second, the short or inaccessible extracranial course of some nerves prohibits direct assessment of conduction velocity. In such cases, reflex studies and latency measure- ments provide the next best option. Finally, EMG of cranial muscles requires a delicate approach and familiarity with specific motor unit potential (MUP) characteristics.
2. CONDUCTION STUDIES 2.1. Facial Nerve
2.1.1. Relevant Anatomy
Of all cranial nerves, the facial nerve (Fig. 1) is the one most frequently studied in EMG labs. It emerges from the pons, coursing across the cerebello-pontine angle to the internal
From: The Clinical Neurophysiology Primer
Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ