Kenneth W. Wright

Complex Strabismus:

Restriction, Paresis, Dissociated Strabismus,

and Torticollis

Kenneth W. Wright

T his chapter on complex strabismus reviews the evaluation and management of incomitant strabismus associated with rectus muscle paresis and ocular restriction. Other topics include dissociated strabismus complex, torticollis, and nystag- mus. Incomitant strabismus is a deviation that changes in different fields of gaze. Incomitance can be caused by ocular restriction, extraocular muscle paresis, or oblique muscle dys- function or can be associated with a primary A- or V-pattern.

The diagnosis and treatment of oblique muscle dysfunction (palsy and overaction), Brown’s syndrome, and A- and V-patterns are covered in Chapter 9.

PARALYTIC RECTUS MUSCLES AND RESTRICTIVE STRABISMUS:

GENERAL PRINCIPLES

If an eye has limited ductions, there are only two basic causes:

extraocular muscle paresis or ocular restriction. Therefore, a strabismus associated with limited ductions is secondary to extraocular muscle paresis, ocular restriction, or both.

Paresis

Extraocular muscle paresis means weak muscle pull, whereas palsy indicates a complete lack of muscle function. Cranial

10

323

nerve paresis and primary muscle disease are obvious reasons for a weak muscle that can cause limited ocular rotations. A muscle paresis can also be caused by ineffective muscle pull on the eye, or mechanical disadvantage of muscle pull. Clinical examples of conditions that cause mechanical disadvantage of muscle pull include:

• A scarred or tethered muscle preventing transmission of muscle pull to the globe (e.g., floor fracture with entrapped inferior rectus muscle)

• A posteriorly displaced rectus muscle (e.g., slipped muscle)

• A muscle shifted out of its appropriate plane, thus dimin- ishing the vector force in the field of action of the muscle (e.g., high myopia with displaced lateral rectus muscle) Table 10-1 lists the three major causes of a mus-cle paresis:

(1) cranial nerve paresis, (2) primary muscle disease, and (3) mechanical disadvantage of muscle pull. Specific types of para- lytic strabismus, including sixth and third nerve palsies, are covered later in this chapter.

TABLE 10-1. Causes of Muscle Paresis.

Primary muscle Mechanical disadvantage Cranial nerve palsy disease of muscle pull

Third nerve palsy Botulism Stretched scar after muscle surgery

Fourth nerve palsy

Myasthenia gravis Slipped muscle or lost muscle (superior oblique

palsy)

Sixth nerve palsy CPEO Canine tooth syndrome with scarring of trochlea causing Brown’s syndrome with superior oblique palsy Trauma to muscle Miller–Fisher Floor fracture with an

syndrome entrapped inferior rectus (Guillain-Barré) muscle causing limited

depression Cranial nerve aberrant Agenesis of an High myopia with large

innervation extraocular muscle posterior staphyloma, and syndromes (e.g., often associated slippage of lateral rectus Duane’s syndrome) with a craniofacial below globe reducing lateral

disorder rectus abduction force, causing esotropia

aSee Chapter 9.

Ocular Restriction

Classically, the term ocular restriction describes a mechanical tether or leash that limits ocular rotations. Ocular restriction, however, can be caused by at least two general mechanisms: a mechanical tether on eye movements or misdirected muscle forces that work against the normal agonist muscle function.

The term restriction is often loosely used as a general term for limited eye movements; however, a clear distinction should be made between ocular restriction and rectus muscle palsy. If the cause of diminished eye movements is not known, then use the term limited rotations or limitation of eye movements until the etiology is determined. Table 10-2 lists the causes of restric- tive strabismus.

Mechanical restriction of eye movement is caused by adhe- sions to an extraocular muscle or sclera, a tight or inelastic extraocular muscle, or an orbital mass. Restrictive adhesions can occur from conjuctival scarring, scarring of Tenon’s capsule, orbital fat adherence, and, rarely, congenital fibrotic bands that attach to the eye or extraocular muscles. Inelastic muscle or muscle fibrosis occurs with thyroid myopathy, local anesthesia myotoxicity, and congenital muscle fibrosis (e.g., monocular elevation deficit and congenital fibrosis syndrome). An orbital mass, such as an orbital hemangioma, or a glaucoma implant can cause ocular restriction either by direct interference of rota- tion of the eye or by pressure on an extraocular muscle that tightens the muscle. Restriction resulting from misdirected muscle force vectors occurs in conjunction with aberrant inner- vation of an antagonist muscle and abnormal muscle–pulley location or a displaced extraocular muscle.

An example of aberrant innervation causing restriction is limited adduction, often associated with Duane’s syndrome. Restricted adduction occurs because the lateral rectus muscle is aberrantly innervated by part of the medial rectus nerve. When the eye attempts to adduct, the lateral rectus muscles contracts against the con- tracting medial rectus muscle, thus restricting adduction.

An example of displaced extraocular muscle is the V-pattern

strabismus and superior oblique muscle underaction that are fre-

quently seen in patients with craniosynostosis.

These patients

have excyclorotation of the orbits that results in superior

displacement of the medial rectus muscle and limited ocular

depression in adduction. The superiorly displaced medial rectus

muscle pulls the eye up in addition to its normal function of

T ABLE 10-2. Causes of Ocular Restriction. Mechanical restriction T ight extraocular muscle Str uctural adhesions Orbital mass Misdirected muscle forces Thyroid: Graves disease Fat adherence to muscle or sclera High myopia with large Congenital cranial ner ve aber rant (e.g., after strabismus surger y, posterior staphyloma inner vation retinal detachment surger y, (Duane’ s syndrome) or periocular trauma) Congenital fibrosis syndrome Congenital fibrotic band Orbital tumor causing Congenital ectopic extraocular muscle mass effect on globe inser tion and or pulley movement (craniosynostosis, extor ted orbit) Congenital Brown’ s syndrome: inelastic Acquired Brown’ s syndrome: Glaucoma explant with Iatrogenic displaced muscle inser tion; SO muscle tendon scar ring or inflammation large bleb causing antielevation after inferior oblique complex (see Chapter 9) around the trochlea mass effect on globe anteriorization with J-defor mity , and movement or displace limited depression after anterior SO tendon (acquired displacement of SO tendon by Brown’ s syndrome) retinal band Entrapped muscle after orbital fracture (inferior rectus most common) Fibrosis after local anesthetic injection High myopia with large posterior into a muscle (inferior most common) staphyloma and slippage of lateral rectus below globe Fat adherence to extraocular muscle (e.g., after strabismus surger y, retinal surger y, or periocular trauma) Monocular elevation deficit syndrome caused by a fibrotic inferior rectus

adduction and limits depression in the field of action of the supe- rior oblique.

A rare example of restriction caused by a displaced muscle–pulley was reported by Oh et al.

They described a patient with limitation of elevation in adduction, or a pseudo- Brown’s syndrome, caused by a congenitally inferiorly displaced lateral rectus muscle and its pulley. These authors hypothesized that the infraplaced lateral rectus muscle and pulley act to pull the eye down, limiting elevation on adduction. Iatrogenic dis- placement of extraocular muscles during strabismus surgery can also cause limited eye movements. Inferior oblique muscle anteriorization anterior to the inferior rectus insertion can also cause active restriction and limited elevation (see Chapter 2, Fig. 2–17).

In some cases, restriction and paresis coexist, such as with paretic lateral rectus muscle and secondary con- tracture of its antagonist medial rectus muscle. It is important to diagnoses the cause of limited ductions to formulate an effec- tive surgical plan. The next section describes methods for diag- nosing extraocular muscle paresis and ocular restriction.

Diagnosing Restriction Versus Paresis

The principal diagnostic tests that differentiate paresis from restriction include saccadic velocity measurements, forced duc- tions, and forced-generation test. Other signs influencing diag- nosis include intraocular pressure changes in various fields of gaze and lid fissure changes in sidegaze.

S ACCADIC V ELOCITY M EASUREMENTS

Saccadic velocity measurements can help differentiate restric- tion from paresis by observation, without touching the eye.

Therefore, this method is useful in young children as well as

adults. Saccadic movements are fast, jerk-like eye movements

that require normal rectus muscle function. The rectus muscles

are the major movers of the eye and are responsible for saccadic

eye movements. The presence of a saccadic eye movement indi-

cates normal rectus muscle function whereas the inability to

stimulate a saccade suggests a rectus muscle palsy. A paretic

rectus muscle does not have the power to generate a saccadic

eye movement, and the eye drifts slowly to the intended field

of gaze. Strabismus associated with limited ductions and

diminished saccadic velocity is caused by a rectus muscle

paresis, not an oblique muscle palsy.

In contrast to a rectus muscle paresis, ocular restriction is associated with normal, but shortened, saccadic movements as the eye stops abruptly when the restriction is met. This eye movement pattern of a fast eye movement that stops abruptly as it meets the restriction is termed the dog on a leash; it is anal- ogous to a dog lunging after a cat, then being abruptly stopped by its leash (Fig. 10-1). In patients with limited eye movements, it is important to clinically test for saccadic eye movements before surgery to assess muscle function. At the time of surgery when the patient is under anesthesia, it is impossible to test muscle function. Positive forced ductions at the time of surgery indicate only passive restriction and do not exclude the possi- bility of coexisting muscle palsy.

Horizontal and vertical eye movements can be measured by laboratory tests including electro-oculogram (EOG) recordings and infrared eye trackers. Clinical observation of eye move- ments can also be used in clinical practice for evaluating the presence of a saccadic movement; this is facilitated through the use of an optokinetic nystagmus (OKN) drum for young children who are not able to follow instructions as well as for coopera- tive patients to compare eye movements (Fig. 10-2). Rotate the OKN drum and observe the patient’s eyes for a brisk redress

FIGURE 10-1. “Dog on a Leash.” The pattern of a fast eye movement

that stops abruptly indicates a mechanical restriction. Upper: cartoon

shows a dog on a leash walking toward a cat behind a tree. Lower: The

dog sees the cat and leaps for the cat but is stopped abruptly by the leash.

movement opposite to the direction of the drum rotation.

Compare eye to eye and look for asymmetry of the OKN response. An inability to generate a saccadic movement indi- cates a paretic rectus muscle.

F ORCED D UCTIONS

Forced ductions identify the presence of a mechanical restric-

tion to ocular rotation; these are performed by grasping the eye

with a forceps and then passively moving the eye into the field

of limited ocular rotation. If the eye shows a resistance to rota-

tion with the forceps (positive forced ductions), then there is a

mechanical restriction. When performing forced ductions for

possible rectus muscle restriction, proptose the eye to stretch

the rectus muscles. This maneuver will allow identification of

restriction caused by a tight rectus muscle. If the examiner inad-

vertently retropulses the eye, the rectus muscles slacken and

produce a negative forced-duction test, even if the rectus muscle

is tight (Fig. 10-3). The opposite holds true for oblique muscle

forced ductions, because retropulsing the eye will stretch the

oblique muscles and accentuate a tight oblique muscle. If a

restriction is worse with retropulsion of the eye, then the res-

triction is not caused by a tight rectus muscle but, instead, is

FIGURE 10-2. Photograph of a child being examined with an optokinetic

nystagmus (OKN) drum. The saccadic movement will be in the direction

opposite to the drum rotation. This is a good clinical method to estimate

if a saccade is present.

secondary to either a periocular adhesion or a tight oblique muscle.

Forced-duction testing can be used as an in-office test using topical anesthesia, or at the time of strabismus surgery. In most cases, the pattern of the eye movements, including the clinical evaluation for saccades, establishes the diagnosis of restriction or paresis. Therefore, in-office forced-duction testing is usually not necessary. If surgery is indicated, forced- duction testing can be performed at the time of surgery to verify the diagnosis. It is important to remember that positive forced ductions does not exclude the presence of a coexisting palsy. In fact, most cases of long-standing rectus muscle palsy also have contracture of the antagonist muscle, so forced ductions will be positive. Preoper- ative evaluation of muscle function by saccadic eye movement

A B

FIGURE 10-3A,B. (A) The proper technique for rectus muscle forced duc-

tions includes grasping the conjunctiva with a 2  3 Lester forceps at the

limbus, just anterior to the muscle insertion. First, proptose the eye, and

then pull the eye away from the muscle being tested, thus placing the

rectus muscle on stretch. This maneuver allows identification of even

mildly tight or restricted muscles. (B) The improper technique for rectus

muscle forced ductions shows the eye being retropulsed during the

maneuver, causing iatrogenic slackening of the muscle and a false-normal

forced-ductions test. Positive forced ductions that do not improve when

the eye is intentionally retropulsed suggest the presence of a nonrectus

muscle restriction, such as periocular scarring (e.g., fat adherence).

testing or the forced-generation test (see next section) is required to diagnose a rectus muscle palsy.

F ORCED -G ENERATION T EST

The forced-generation test directly measures active muscle force and is useful for diagnosing a rectus muscle palsy. To perform this test, the eye is topically anesthetized and grasped with forceps; the patient is asked to look into the field of limited rota- tion. A sterile cotton-tipped applicator can also be used to push against the eye to feel the abduction force, as noted in Chapter 5 (Fig. 5-16A,B). The examiner feels the pull of the muscle against the forceps or cotton-tipped applicator and compares this to the fellow eye or the antagonist muscle. If there is diminished pull from the muscle into the field of limited rotation, then a paresis is present. Forced ductions can be used in conjunction with forced-generation testing. If forced ductions are positive and the force-generation test shows poor muscle function, then the diagnosis is a combination of restriction and paresis.

I NTRAOCULAR P RESSURE C HANGE ON E YE M OVEMENT Another sign of restriction is increased intraocular pressure on attempted duction into the field of limited movements and away from a restriction or tight muscle. Intraocular pressure increases as the eye forcibly attempts to move against the restriction.

Patients with thyroid myopathy and strabismus may show increased intraocular pressure when the pressure reading is made with the restricted eye in forced primary position.

L ID F ISSURE C HANGES ON E YE M OVEMENT

Ocular restriction caused by a tight rectus muscle or a restric- tive adhesion to the globe will cause globe retraction and lid fissure narrowing as the agonist rectus muscle attempts to pull the eye away from the restriction [see Duane’s syndrome (Fig.

10-12), later in this chapter]. These movements occur because

the eye is restricted from rotating; therefore, the contracting

agonist muscle pulls the eye posteriorly and causes globe

retraction and lid fissure narrowing. A rectus muscle paresis

will cause the opposite: lid fissure widening and relative prop-

tosis. As the patient looks into the field of action of the paretic

rectus muscle, the agonist muscle relaxes secondary to

the palsy. The antagonist muscle also relaxes because of

Sherrington’s law, and pressure from orbital fat pushes the eye forward. A patient with a sixth nerve palsy, for example, will show lid fissure widening on attempted abduction (see Fig. 10-10, later in this chapter). This change occurs because the medial rectus muscle relaxes on attempted abduction (Sherrington’s law) and, along with the paretic lateral rectus, it is loose; therefore, the posterior pressure of the orbital fat pushes the eye forward.

MANAGEMENT OF INCOMITANT STRABISMUS: GENERAL PRINCIPLES

Management begins with understanding why the deviation is incomitant. For example, if an incomitant strabismus is associ- ated with severe limitation of ductions, determine whether the limitation is caused by restriction or paresis. If a significant restriction is the cause of limited adduction, then one must release the restriction. If severe limitation of ocular rotations is secondary to poor rectus muscle function, then one has to address the muscle weakness.

In cases in which the incomitance is associated with little or no limitation of eye movements, the incomitance can be managed by operating on the good eye to match ocular rotations of the deviated eye. Determine where the deviation is greatest and operate to achieve alignment in primary position while reducing the incomitance. Use this strategy: recession proce- dures have their greatest effect in the field of action of the recessed muscle, and resections produce a leash with the great- est effect occurring when the eye rotates away from the resected muscle (see Chapter 11). Recessing the right medial rectus muscle will produce an exodeviation greater in leftgaze and almost no effect in rightgaze, and resecting the right lateral rectus muscle produces an exodeviation that increases in left- gaze. With this strategy in mind, determine what surgery would best correct the following strabismus.

Example 1. Trace limitation of abduction of unknown etiology, left eye; negative forced ductions.

Right Primary Left

ET2 ET 8 ET 16

ET, estropia.

The surgical plan is to recess the right medial rectus muscle 4.0 to 5.0 mm, as this will match the right medial rectus muscle to its underacting yoke muscle, the left lateral rectus muscle.

Weakening the right medial rectus muscle will slightly reduce adduction but will not affect abduction; this reduces the large esotropia in leftgaze without causing an exotropia in rightgaze.

Do not recess the left medial rectus muscle because this surgery has little effect in leftgaze where the esotropia is largest and will produce an exo-deviation in rightgaze. Also, avoid a left lateral rectus resection as this will not strengthen the weak lateral rectus. Instead, it will cause a tight lateral rectus muscle that also has little effect in leftgaze where the esotropia is greatest and will cause an exodeviation in rightgaze. For an incomitant esodeviation that is greater than 10 to 15 prism diopters (PD) in primary position and increases in leftgaze, two-muscle surgery will be required to correct the deviation in primary position.

Consider asymmetrical bilateral medial rectus recessions, with a larger recession on the right medial rectus muscle.

The Faden operation has also been suggested to reduce incomitance. Adding a Faden to a recession of the medial rectus muscle increases the weakening effect of the recession in adduc- tion and improves the incomitance. The use of the Faden is con- troversial. If it is used, it is most effective on the medial rectus muscle, as the medial rectus has the shortest arc of contact. The- oretically, the Faden weakens the muscle mostly in the field of action of the muscle, with little effect in primary position; there- fore, it may be helpful in reducing incomitance (see Chapter 11).

A report on the effect of the Faden procedure on the medial rectus muscles in reducing the AC/A ratio concluded there was a beneficial effect; however, the table of data in this study showed no change of the AC/A ratio. It is likely the Faden pro- cedure has little effect, except in extreme fields of gaze.

If the limitation is severe, recessing the yoke muscle to match the limitation will not work, as operating on the good eye will not improve the ability of an eye with limited ductions to come to midline. In these cases of moderate to severe limi- tation of ductions, one must release the restriction or, in the case of a palsy, transpose muscle forces to bring the eye to midline.

Recessing the contralateral yoke muscle only works if the lim- itation is slight, such as a trace to 1 limitation of ductions.

Vertical incomitance can be treated with the same strategy

as described previously for horizontal strabismus. One special

situation that occurs with Grave’s disease and floor fractures is

that of a patient with orthotropia in primary position and a hypotropia in upgaze secondary to a tight inferior rectus muscle.

In this case, recess both inferior rectus muscles, with a larger recession on the side with the restriction. The diagnosis and management of specific types of restrictive and paralytic stra- bismus follow.

SPECIFIC TYPES OF RESTRICTIVE STRABISMUS

Fat Adherence

Fat adherence is a restrictive form of strabismus occurring after periocular surgery or accidental trauma. Marshall Parks was the first to describe the clinical characteristics and etiology of the fat adherence syndrome or, as it is also called, the adhesive syn- drome.

Normally, Tenon’s capsule and muscle sleeve act as an elastic barrier separating the globe from the surrounding orbital fat. Fat adherence is caused by violation of the posterior Tenon’s capsule, allowing exposure and manipulation of extraconal fat and fascia, which produces an adhesion of these tissues to the sclera. Because the septae within the extraconal fat connect to the periorbita, fibrosis associated with fat adherence can extend from the orbital bone to the sclera (Fig. 10-4). In severe cases, the eye is virtually scarred to the orbital bone, immobilizing ocular rotations. Violation of the muscle sleeve can also result in fat adherence to a rectus muscle causing a tight muscle. Fat adherence most frequently occurs after strabismus surgery involving posterior exposure (especially oblique muscle surgery) and retinal buckle surgery, but can also occur after any perioc- ular surgery, even after blepharoplasty.

Fat adherence is difficult to surgically correct, as recurrence of fat adherence after removal of adhesions is very common.

Once Tenon’s capsule is violated and a scar established, it is

almost impossible to reestablish the delicate fascial barrier to

prevent recurrence of scarring. Teflon or silicone sheaths have

been used as an artificial barrier, but they become encapsulated

in scar and often make the restriction worse. Amniotic mem-

brane transplantation has been used to create a barrier separat-

ing periocular fat from the sclera, but the technique is difficult,

at best, and remains investigational.

Surgical correction of fat

adherence consists of releasing the scar by dissecting close to

sclera and removing the adhesions without repenetrating the orbital fat. (Perform forced ductions after freeing adhesions to evaluate improvement of the restriction.) Dissect carefully with direct visualization, as posterior dissections can be dangerous.

A

FIGURE 10-4A,B. Fat adherence syndrome. (A) Diagram on the right

shows the normal anatomy of the periocular fascia with Tenon’s capsule

as the barrier separating orbital fat from the sclera and muscle. Diagram

to the left shows fat adherence (after violation of Tenon’s capsule) over-

lying the rectus muscle in an area away from the rectus muscle over

sclera. Note that a fibrous scar extends throughout the fat septae attach-

ing periosteum to the muscle and sclera. This scar causes a restrictive

leash that limits eye movements. (B) Photograph of fat adherence to the

inferior rectus muscle. (Modified from Parks and Mitchell, 1978, with

permission.)

Cases of inadvertent optic nerve transection have occurred, although they are rarely reported. If fat and scar are adherent to a rectus muscle, remove a small amount of the anterior scar, then recess the tight muscle en bloc with the scar rather than trying to dissect all the scar off the muscle. Avoid extensive dis- section of scar off the muscle, as this usually results in further fat manipulation and worsening of the adherence. Medical treat- ment with mitomycin-C has not been effective in reducing post- operative fibrosis and may even increase scarring.

Injection of peribulbar corticosteroids also fails to prevent postoperative scarring. The best treatment for fat adherence syndrome is prevention: avoid penetration of posterior Tenon’s capsule during the initial surgery. During strabismus surgery, perform minimal dissection of muscle fascia and, when dissecting, dissect close to the muscle to stay away from surrounding orbital fat. If Tenon’s capsule is inadvertently torn so fat is exposed, cover the exposed fat by repairing the Tenon’s tear with 7-0 vicryl suture.

Grave’s Ophthalmopathy

Grave’s ophthalmopathy is an autoimmune disease associated with inflammation of the extraocular muscles. Initially, there is an acute phase during which there is a lymphocytic infiltration of the extraocular muscles, resulting in extraocular muscle enlargement and proptosis. This active phase usually lasts several months to more than a year. Orbital imaging studies show thickened extraocular muscles, especially posteriorly. The second phase is a cicatricial phase with quiescence of inflam- mation and secondary contracture of the muscles. All muscles are usually involved, but the inferior rectus and medial rectus are most severely affected.

Strabismus is caused by tight fibrotic muscles and can develop in both phases but is most pro- nounced in the cicatricial phase. A restrictive hypotropia caused by tight inferior rectus muscles is the most common type of strabismus, followed by esotropia associated with tight medial rectus muscles.

The management of Grave’s ophthalmopathy is careful

observation during the acute inflammatory phase. Treatment

with systemic steroids and even external beam radiation may be

indicated for severe disease; however, radiation therapy is not

effective for treatment of the strabismus.

Orbital decompres-

sion is indicated for severe proptosis and visual loss associated

with optic nerve compression from inflamed extraocular muscles. In most cases, it is better to perform strabismus surgery after the active phase has subsided and strabismus measure- ments have stabilized. A report on eight patients whose eyes were operated on during the active phase of thyroid ophthal- mopathy noted that all eight patients achieved successful long- term alignment ( 16 months follow-up); however, half the patients required more than one operation.

Regarding the timing of surgery, strabismus surgery is usually performed after orbital decompression surgery, because orbital surgery can alter eye alignment.

The strategy for the treatment of Grave’s ophthalmopathy strabismus is to release the restriction from the tight rectus muscle, with a rectus muscle recession being the procedure of choice. It is not advis- able to use rectus muscle resections, as this tightens an already stiff, inelastic muscle. A right hypotropia less than 15 PD with a tight right inferior rectus muscle can be surgically addressed with a right inferior rectus recession, with or without an adjustable suture technique (Fig. 10-5).

If the deviation in primary position is greater than 18 to 20 PD with severe restric- tion, recess the tight inferior rectus muscle more than 5.0 mm and add a recession of the contralateral superior rectus muscle.

As a rule, expect 3 PD of vertical correction for each millimeter of vertical rectus muscle recession.

One common problem with correcting thyroid strabismus has been late overcorrection after inferior rectus recession, which occurs in up to 50% of cases.

Initially after surgery, there is a successful result. Then, at 4 to 6 weeks after the infe- rior rectus recession, a consecutive hypertropia on the side of the recession occurs, with underaction of the recessed inferior rectus muscle and ipsilateral lower eyelid retraction.

R.

Friedman suggested that performing asymmetrical bilateral inferior rectus recessions avoids late overcorrection. A report by Cruz and Davitt on eight patients who underwent asymmetri- cal bilateral inferior rectus recessions showed no overcorrec- tions; however, 25% of these patients were undercorrected.

Ludwig has suggested that a stretched scar at the new insertion is the cause of the overcorrection. It is hypothesized that, at 4 to 6 weeks after surgery, the absorbable suture loses its strength.

The muscle–scleral attachment stretches and causes the tight

muscle to retract posteriorly. This author has now switched to

nonabsorbable sutures (6-0 Mersiline), and preliminary results

have been good, even when using an adjustable suture.

A

B

FIGURE 10-5A,B. Thyroid-associated strabismus. (A) Patient with

Graves’ disease and limited elevation, right eye, secondary to a tight right

inferior rectus muscle. (B) CT scan shows thyroid-associated changes; the

medial inferior and superior rectus muscles are enlarged bilaterally.

Congenital Fibrosis of the Extraocular Muscles

Congenital fibrosis of the extraocular muscles (CFEOM) is an autosomal dominant, nonprogressive disorder usually charac- terized by bilateral congenital ptosis and restrictive external ophthalmoplegia

; however, rare unilateral cases have been described (CFEOM 8, 21, 26, 29, 30, 31).

Systemic diseases reported to be associated with CFEOM include Prader–Willi syn- drome (CFEOM 25),

Joubert syndrome (CFEOM 23),

and cor- tical and basal ganglia dysplasia (CFEOM 2).

CFEOM has been mapped to chromosomes 12, 11, and 16 (CFEOM 3, 5, 6, 7, 9, 18, 16).

There can be significant phenotypic heterogeneity with a variety of subtypes of CFEOM found in the same family (CFEOM 6 and 8).

The clinical features of CFEOM have been classified into five groups: (1) generalized fibrosis syndrome,

(2) fibrosis of inferior rectus with blepharophimosis, (3) strabismus fixus, (4) vertical retraction syndrome,

and (5) unilateral fibrosis blepharoptosis and enophthalmos (CFEOM 17).

The medial rectus muscle is one of the most commonly involved, causing a strabismus fixus esotropia with extreme restriction to abduction (Fig. 10-6). Strabismus fixus is a term for an eye that is fixed and cannot move, usually secondary to severe restriction or a com- bination of restriction and paresis. The strabismus associated with CFEOM is mostly caused by tight fibrotic muscles, but a component of paresis can also be a factor. As with thyroid- related strabismus, the surgical procedure of choice is a reces- sion of the tight rectus muscle. Resections should be avoided.

These CFEOM cases can be technically difficult because expo- sure of the muscle is limited, especially in cases with a fibrotic medial rectus muscle.

The etiology of CFEOM is unknown, but the syndrome is

associated with atrophic and fibrotic changes of the extraocular

muscles.

Light and electron microscopy demonstrated replace-

ment of normal muscle with collagen, dense fibrous tissue, and

areas of degenerated skeletal muscle (CFEOM 29, 30, 31).

Research suggests that the cause of congenital fibrosis of the

extraocular muscles is an abnormality in the development of

the extraocular muscle lower motor neurons, with agenesis of

the third nerve being most common (CFEOM 1, 14, 11, 10).

Nakano et al. reported finding three mutations in ARIX gene

(also known as PHOX2A) in four pedigrees of congenital fibro-

sis of the extraocular muscles type 2 (CFEOM 2).

ARIX

encodes a homeodomain transcription factor protein shown to be required for development of cranial nerves III and IV in mouse and zebrafish. These findings confirm the hypothesis that CFEOM 2 results from the abnormal development of cranial nerves III and IV and emphasize a critical role for ARIX in the development of these midbrain motor nuclei.

Double Elevator Palsy or Monocular Elevation Deficit Syndrome

Double elevator palsy is classically defined as a congenital inability to elevate one eye, with the limitation occurring in adduction and abduction (Fig. 10-7). One might question why double elevator palsy is included under restrictive strabismus.

The term double elevator palsy is a misnomer because, in most

cases, the cause for the limited elevation is not a palsy of both

elevators but is a tight inferior rectus muscle. Studies using sac-

cadic velocity measurements and forced ductions showed that

approximately 70% of patients diagnosed as having a double ele-

vator palsy actually had limited elevation as a result of inferior

rectus restriction, not a palsy of the superior rectus and inferior

FIGURE 10-6. Patient with congenital fibrosis syndrome and a large

angle esotropia. There was severe limitation to abduction, bilaterally, and

forced ductions at the time of surgery show severe restriction to abduc-

tion in both eyes. Bilateral medial rectus recessions (7.0 mm) resulted in

good alignment with improved abduction.

oblique muscles.

A more descriptive term now used is monocular elevation deficit syndrome (MED). MED may be mistaken for Brown’s syndrome, although the limited elevation is worse in adduction than abduction in the latter. Patients with MED present with a hypotropia, a chin elevation, and, often, an ipsilateral ptosis. True congenital ptosis is present in 25% of cases whereas pseudo-ptosis may occur in almost all patients with a large hypotropia.

In those cases with a true double ele- vator palsy and a lack of an upgaze saccade, forced ductions at time of surgery usually reveal a tight inferior rectus muscle coexisting with the superior rectus palsy.

An interesting finding in approximately 25% of patients with double elevator palsy and congenital ptosis is the Marcus Gunn jaw-winking phenomenon.

This association indicates a congenital misdirection syndrome involving the oculomotor nerve. It is possible that, as with congenital fibrosis syndrome, the cause of the tight inferior rectus and, in some cases, supe- rior rectus and inferior oblique palsy, is abnormal development of cranial nerves (including the oculomotor nerve) with second- ary muscle fibrosis.

Surgery for MED is indicated if a significant hypotropia is present in primary position with an associated chin elevation.

The type of surgery depends on the cause of the elevation deficit (Table 10-3). If the etiology is a tight inferior rectus muscle and the upgaze saccade is normal, recess the ipsilateral inferior rectus muscle, usually around 5 to 6 mm depending on the size of the hypotropia. It is important to evaluate preoperatively for the presence of an upgaze saccade and to perform forced duc- tions at the time of surgery to make the correct procedural choice. Lack of upgaze saccades, combined with a weak supe- rior rectus muscle on forced generation testing, indicates a true

A B C

FIGURE 10-7A–C. Double elevator palsy (monocular deficit syndrome).

Child has had limited elevation of the right eye since birth. Note that elevation of right eye is worse in abduction (A) than it is in adduction (C).

Patient is fixing with the involved right eye so the left eye is hypertropic

as per Hering’s law of yoke muscles (B). Preoperatively, this patient had

intact upgaze saccades and a tight inferior rectus muscle on forced-

duction testing at the time of surgery. The elevation deficit was success-

fully treated with a right inferior rectus muscle recession of 6.5 mm.

double elevator palsy. In these cases, a recession of the ipsilat- eral inferior rectus will not correct the hypotropia. Treatment of a true double elevator palsy with weak superior rectus muscle is to perform a transposition of the ipsilateral medial and lateral rectus muscles up to the superior rectus muscle. In patients with the superior rectus palsy type of MED, forced ductions are often positive, and the ipsilateral inferior rectus muscle should be recessed. This author prefers the partial tendon transfer (Hummelsheim) instead of the full-tendon transposition (Knapp) to avoid the possible complication of anterior segment ischemia that can occur up to 20 years after strabismus surgery. In severe cases of hypotropia over 15 PD, consider adding a recession of the contralateral superior rectus muscle.

Orbital Floor Fracture

Signs of a blowout fracture include diplopia secondary to restricted vertical eye movement, enophthalmos, and numbness of face below the traumatized orbit and along the upper teeth.

Restrictive strabismus with limited elevation in orbital floor fractures is caused by entrapment of fat and the inferior rectus muscle at the fracture site (Fig. 10-8). Repair of the floor fracture in most cases will improve ductions. In addition to limited ele- vation, there can be limited depression on the side of the frac- ture, often associated with a posterior fracture.

The cause of the limited depression could be contributed to scarring of the inferior rectus to the floor, thus preventing the inferior rectus muscle from transmitting its contractual pull to the globe.

Adherence of the inferior rectus to the floor would also isolate the muscle anterior to the fracture and cause the anterior muscle to slacken on attempted downgaze, producing pseudoinferior rectus palsy. These patients characteristically have a small

TABLE 10-3. Treatment of Double Elevator Palsy (Monocular Elevation Deficit Syndrome).

• Tight inferior rectus muscle: good superior rectus function Recess ipsilateral inferior rectus muscle (5–6 mm)

• Superior rectus palsy

Recess ipsilateral inferior rectus muscle and ipsilateral transposition of half the medial and lateral rectus muscles up to the superior rectus insertion (preferred by author)

or

Knapp procedure: full-tendon transfer up to the superior rectus muscle

hypertropia in primary position, underaction of the inferior rectus muscle, and a large hypertropia in downgaze.

The key to the diagnosis of a pseudoinferior rectus palsy is normal inferior rectus muscle function and normal saccades when the eye moves from upgaze to primary position, with infe- rior rectus muscle weakness and slow ocular movements from primary position to downgaze. Treatment of pseudoinferior rectus palsy is to repair the floor fracture. If this does not relieve symptoms, then strabismus surgery is indicated. This author has found that a small (3–4 mm) ipsilateral inferior rectus muscle tightening procedure (Wright plication or resection) helps to eliminate the anterior muscle slack. A contralateral inferior rectus recession works well and produces only a slight limita- tion of elevation. If the muscle is captured in a trap-door frac- ture, direct damage to the inferior rectus muscle occurs and can truly weaken the inferior rectus muscles. Small trap-door floor fractures can pinch and strangle the inferior rectus muscle, causing necrosis and muscle damage.

Because of the potential for permanent damage, some advocate immediate repair within the first few days if there is imaging evidence that the inferior rectus is entrapped.

Strabismus surgery should be performed after reconstructive orbital surgery. If orbital reconstruction is not indicated, and the patient has persistent diplopia 4 to 8 weeks after the trauma, then strabismus surgery is indicated.

The strabismus surgical plan depends on the pattern of the strabismus. Table 10-4 lists patterns of strabismus and their associated treatment.

Myotoxic Effect of Local Anesthetics

Injection of local anesthetics such as lidocaine and marcaine into an extraocular muscle can result in myotoxic damage to the muscle and cause strabismus.

Elderly patients are espe- cially susceptible to the myotoxic effects of local anesthetics.

Immediately after the injection of a local anesthetic into an

extraocular muscle, there is an acute paresis of the muscle that

lasts for one to several days. Over the next few weeks, localized

segmental intramuscular fibrosis occurs secondary to local

myotoxicity of the anesthetic. The fibrosis results in a tight and

contracted muscle. What is particularly interesting is that, in

some cases, the injected muscle overacts, producing a deviation

that increases in the field of action of the injected muscle.

This deviation is in contrast to the restriction pattern usually

A

B

FIGURE 10-8A–B. Orbital floor fracture left eye with entrapment of fat

and the inferior rectus muscle. (A) In primary gaze, there is no significant

deviation. (B) Restricted elevation of left eye in upgaze causes a large right

hypertropia.

expected with a tight muscle, where the deviation is greatest in the gaze opposite to the field of the muscle’s action. The cause of the muscle overaction is thought to be secondary to intra- muscular fibrosis, with stretching of the Z-bands and enhancing

C

FIGURE 10-8C. (C) CT scan shows herniation and entrapment of infe- rior orbital fat into the maxillary antrum. Note that, after removal of the fat and repair of the fracture, the restriction resolved.

TABLE 10-4. Orbital Floor Fracture: Surgical Plans.

Tight inferior rectus muscle (hypotropia)

• Small hypotropia ( 8 PD) in primary position, no deviation in downgaze, and larger hypotropia in upgaze (tight inferior rectus muscle):

Asymmetrical bilateral inferior rectus muscle recessions, with a larger ipsilateral recession

Add a contralateral superior rectus recession for a large hypotropia in upgaze

• Large hypotropia in primary position, worse in upgaze (tight inferior rectus muscle):

Hypotropia 8 to 15 PD: recess ipsilateral inferior rectus muscle (3.5–5.0 mm) Hypotropia 15 PD: recess ipsilateral inferior rectus muscle (5–6 mm) PLUS

a contralateral superior rectus recession (4–6 mm) Pseudoinferior rectus muscle palsy (hypertropia)

• Hypertropia in primary position increases in downgaze with ipsilateral limited depression; intact saccades from upgaze to primary position:

Plication of the ipsilateral inferior rectus (3 mm) PLUS contralateral inferior

rectus recession (4–5 mm)

the action and myosin interaction.

The fibrosis acts to stretch the muscle fibers that subsequently increases their force, per the Starling’s length tension curve.

For example, inadvertent injec- tion of the inferior rectus muscle associated with a retrobulbar injection of anesthetic initially results in an ipsilateral hyper- tropia because of an inferior rectus paresis. Over a few weeks, this changes into an ipsilateral hypotropia with overaction of the inferior rectus muscle, resulting in the hypertropia being greatest in downgaze.

Any of the extraocular muscles can be infiltrated during a retrobulbar or peribulbar injection of local anesthetics, with the superior and inferior rectus muscles most commonly affected.

One of the findings is segmental enlargement of the injected muscle seen on orbital imaging. Hamed and Mancuso

reported on eight patients with an ipsilateral hypotropia after a retrobul- bar injection of anesthetic, with three patients showing seg- mental enlargement of the inferior rectus muscle. The treatment is to recess the tight or overacting muscle. This method has pro- duced excellent results, especially in the cases involving an overacting injected muscle, with the deviation larger in the field of action of the muscle. One can help prevent intramuscular injection injury by injecting into the orbital quadrant away from the extraocular muscles, using a blunt cannula and limiting anesthetic volume. The incidence of strabismus after cataract surgery has diminished dramatically since the widespread use of topical anesthesia during surgery.

Strabismus After Retinal Surgery

Strabismus can occur virtually after every known retinal surgi-

cal procedure.

The strabismus is usually tran-

sient; however, persistent strabismus occurs in approximately

7% of scleral buckling procedures.

Common causes of stra-

bismus after retinal detachment surgery include fat adherence

and restriction, a lost or slipped muscle, a displaced superior

oblique tendon, a large explant under a rectus muscle, and

ectopic fovea.

Other causes of strabismus after retinal

surgery include patients with preexisting strabismus before the

retinal surgery who then experience sensory strabismus sec-

ondary to loss of vision.

Of all the causes of persistent

restriction after retinal detachment surgery, fat adherence and

periocular scarring is by far the most common and most diffi-

cult to treat.

Fat adherence is difficult to treat because there

is no synthetic substitute to recreate the natural boundary between the orbital fat and the eye and muscle once Tenon’s capsule is violated.

Occasionally, a lost muscle is associated with postretinal surgery, as can occur when the traction sutures around the muscle are pulled to gain posterior exposure during the retinal surgery. In elderly patients, the muscle is relatively weak, and overzealous traction on the rectus muscle can result in a split- ting of the muscle; this has been termed pulled-in-two syn- drome (PITS). Spontaneous disinsertion and posterior slippage of a rectus muscle behind an encircling buckle can also occur, without removal of the muscle at the time of retinal surgery.

In these cases, the silicone band will cheese-wire through the muscle insertion over several months postoperatively, resulting in late slippage of the muscle behind the buckle and causing an underaction of the slipped muscle. The slipped rectus muscle can almost always be found attached to sclera at the posterior edge of the encircling buckle or connected to sclera by a pseudo- tendon. Appropriate treatment is to advance the muscle and reattach the muscle with nonabsorbable suture.

Another cause for strabismus after retinal surgery is an oblique muscle that has been displaced anteriorly by an encir- cling band.

Placement of the band behind the superior oblique tendon pulls the superior oblique tendon anteriorly to the nasal aspect of the superior rectus insertion. The superior oblique tendon now inserts at the nasal side of the superior rectus insertion, anterior to the equator. The new anterior inser- tion of the superior oblique tendon changes the action of the superior oblique muscle from a depressor to an elevator. These patients typically present with a hypertropia and limitation of depression of the involved eye. Forced ductions, however, show relatively mild restriction to depression as compared to the lim- itation on ductions and versions. Treatment is to release the entrapped superior oblique tendon from the buckle or, if there is severe scarring, perform a superior oblique tenotomy. If the hypertropia is greater than 5 PD in primary position, also perform a recession of the contralateral inferior rectus muscle (consider adjustable suture). The inferior oblique muscle can also be entrapped by an encircling element.

In this case, the element is passed behind, or splits, the inferior oblique muscle.

When the band is tied in place, the muscle is pulled anteriorly,

resulting in a hypotropia and excyclotropia. The hypotropia

occurs because the inferior oblique is displaced anteriorly to the

equator, pulling the front of the eye down. The excyclotropia is caused by the increased tension on the inferior oblique muscle.

Torsional diplopia after retinal surgery is not always associated with an entrapped oblique muscle.

Metz and Norris found two of four patients with torsional diplopia after retinal surgery to have no identifiable abnormality of the oblique muscle.

The complications of oblique muscle entrapments can be diminished by passing the encircling elements anteriorly, just behind the rectus insertions. Extreme posterior passage of the muscle hook may result in inadvertent hooking of an oblique muscle, espe- cially when working on the superior rectus and lateral rectus muscles.

The placement of a retinal explant sponge or buckle is often identified as a primary cause for strabismus after retinal surgery.

Transient strabismus after a retinal encircling procedure is fre- quent, occurring in approximately 20% of cases. In our experi- ence, however, a retinal encircling element by itself rarely causes persistent strabismus. Persistent strabismus after retinal surgery usually results from secondary scarring or a displaced muscle, as stated previously.

Infrequently, however, a retinal explant may be the primary cause of restriction; this occurs when a large explant is placed directly under a rectus muscle.

The explant causes the muscle to deviate from its normal course, thus tightening the muscle. For example, a large retinal sponge placed directly under the medial rectus will cause a tight- ening of the medial rectus, as the medial rectus courses over the large sponge and produces an esotropia. Low-profile encircling elements, such as 240 bands that indent the sclera, do not inter- fere with the course of the rectus muscle and, therefore, do not produce strabismus.

Foveal ectopia occurs in association with macular pucker,

peeling of the epiretinal membrane, and retinal translocation

surgery. Acquired foveal ectopia produces an interesting type of

strabismus and diplopia. These patients will observe that objects

in the central visual field appear double, with one image being

distorted by metamorphosia. Objects in the peripheral field,

however, will often be fused, as the peripheral retina may not

be involved with the ectopia. Thus, patients who undergo mem-

brane peeling for a macular pucker may experience postopera-

tive diplopia because of foveal ectopia. The image disparities

tend to be small with this condition, and prism glasses have been

found to be effective in treating this problem.

Retinal translocation surgery can result in severe torsional diplopia that prisms cannot correct. Instead, oblique muscle surgery is required to treat the problem.

Extorsion is induced from macular inferior translocation, and intorsion is secondary to superior macular translocation. Extorsion can be corrected by a large Harada–Ito procedure, possibly with an inferior oblique weakening procedure, whereas intorsion can be corrected with a weakening surgery of the superior oblique muscle, perhaps with a tuck of the inferior oblique muscle. Vertical offset of the rectus muscle can also change torsion, but one must consider the risk of anterior segment ischemia in this group of patients.

Glaucoma Explants and Strabismus

The incidence of strabismus after glaucoma explant surgery ranges from 10% to 70%, depending on the study.

The cause of the strabismus is, for the most part, the large bleb created by the glaucoma explant. Strabismus associated with a large filtering bleb may be caused by the following mechanisms:

(1) orbital mass, which displaces the eye (Fig. 10-9); (2) a mass directly under a muscle or tendon; or (3) scarring or adhesions secondary to the surgical dissection during placement of the glaucoma explant. The old Baerveldt implant had been associ- ated with the highest incidence of strabismus; however, modi- fications of the Baerveldt implant (fenestrated Baerveldt) have reduced the bleb size and subsequently reduced the incidence of strabismus. Valved implants have also reduced the size of the filtering blebs and have subsequently produced the lowest inci- dence of strabismus.

A large explant in the superior nasal quadrant may cause a pseudo-Brown’s syndrome with restricted elevation in adduc- tion, as the bleb displaces and tightens the superior oblique tendon.

Placement of glaucoma explants should be super- otemporal rather than superonasal to avoid the problem of a secondary Brown’s syndrome. The treatment of a bleb-induced strabismus is to reduce the size of the bleb by suturing the bleb wall to the explant so it cannot expand. Additionally, the old explant can be replaced with a newer valved explant.

An interesting observation of some patients with strabismus

and severe glaucoma is that they do not experience diplopia but,

instead, have visual confusion.

Visual confusion is the simul-

taneous perception of two different foveal images in a patient

A

B

FIGURE 10-9A,B. Patient with a glaucoma explant in the left superior

temporal quadrant. The glaucoma was controlled; however, it produced

a large bleb that limited abduction. (A) Patient is looking left, and the left

eye shows severe restriction (4) to abduction. (B) Large temporal bleb is

causing a mass effect and restricting abduction of the left eye.

with strabismus. These patients see the superimposed images from each fovea. Patients with end-stage glaucoma have tunnel vision and lose their peripheral visual field. If these patients acquire strabismus, they may experience confusion rather than a true diplopia, as they only have central vision and are forced to use the fovea of each eye.

High Myopia and Esotropia (Myopic Strabismus Fixus)

High myopia, usually greater than 20 diopters, can be associated with an acquired large-angle esotropia along with limited abduc- tion and a hypotropia

; this is a form of acquired stra- bismus fixus and can be either monocular or binocular. Another term for the high myopia esotropia syndrome is heavy eye syndrome, with hypotropia and limited eye movement.

Restricted abduction is dramatic, and there is limited elevation of the hypotropic eye. Orbital imaging shows an extremely large globe with a posterior staphyloma that fills the orbit, a large infe- rior displacement of the lateral rectus muscle, and a mild nasal displacement of the superior rectus muscle. The cause of the esotropia and hypotropia is a combination of restriction, because of the massive expansion of the posterior globe against a tight medial rectus muscle, and displaced lateral and superior rectus muscles that change the normal vector forces. Displacement of the lateral rectus muscle inferiorly and superior rectus muscles nasally is most likely caused by the massive expansion of the posterior aspect of the globe into the superior temporal quad- rant.

The lateral rectus muscle shows the most displacement, probably due to the laxity of its pulley system. Slippage of the lateral rectus muscle below the globe weakens the abduction vector and pulls the eye down, thus contributing to the esotropia and hypotropia. The nasally displaced superior rectus muscle also contributes to the esotropia and hypotropia by pulling the eye nasally and diminishing the elevation vector force.

Treatment is aimed at realigning the lateral rectus muscle

and releasing the medial rectus muscle, which is inevitably

tight. This author prefers a large recession of the medial rectus

muscle, at least 7 to 8 mm on a hang-back suture, and a supe-

rior transposition of the lateral rectus muscle with a small resec-

tion. The posterior sclera is thin in these cases, and access to

the posterior globe is difficult because of the large eye. The hang-

back suture of the medial rectus allows for a large recession

without passing a posterior suture. Union of the superior and lateral rectus has also been described.

SPECIFIC TYPES OF PARALYTIC STRABISMUS

Sixth Nerve Palsy

A persistent, isolated, congenital sixth nerve palsy is extremely rare; however, newborns may have a transient sixth nerve palsy that resolves spontaneously over a few days to a few weeks. A common cause of isolated acquired sixth nerve palsy in early childhood is postviral inflammatory neuropathy, which may occur 1 to 3 weeks after a viral illness or immunization or spon- taneously without obvious cause. These patients should be fol- lowed closely to monitor their improvement and watch for the development of amblyopia. Improvement usually occurs within 6 to 10 weeks. After viral or idiopathic causes, the next most common causes of acquired sixth nerve palsy in children and young adults include closed head trauma and intracranial neo- plasms. Neuroimaging is indicated for acquired sixth nerve palsy if the palsy does not improve rapidly or if other neurological signs are present. Other causes of an acquired sixth nerve palsy include Gradenigo’s syndrome (mastoiditis and sixth nerve palsy), meningitis, myasthenia gravis, and cavernous sinus disease.

Sixth nerve palsy is typically associated with limited abduc- tion and an esotropia that increases upon gaze to the side of the palsy (Fig. 10-10). On attempted abduction, there is relative lid fissure widening because both the medial and lateral rectus muscles are relaxed on attempted adduction and the posterior orbital pressure proptoses the eye. Remember that, on attempted abduction, the medial rectus muscle is inhibited (Sherrington’s law). Mild sixth nerve paresis may allow relatively good lateral rectus function and show only a trace limitation of abduction.

These patients, however, will have a pattern of divergence paresis with an esotropia that is greater in the distance than at near. The divergence paresis pattern should alert the examiner to the possibility of a sixth nerve paresis.

Initial therapy of a traumatic or vascular sixth nerve palsy

is observation for 6 months while monitoring the patient for

spontaneous recovery. Spontaneous recovery of traumatic sixth

nerve palsy is approximately 80% for unilateral cases and 40%

for bilateral cases.

A complete palsy at the initial presentation and bilateral involvement indicate a poor prognosis for recov- ery.

During the observation period, alternate monocular occlu- sion or press-on prisms can be used to eliminate diplopia if a face turn does not allow fusion. To prevent secondary contrac- ture of the medial rectus muscle and increase the chances for recovery, some advocate the use of botulinum injection into the ipsilateral medial rectus muscle.

Botulinum paralyzes the muscle for 3 to 6 months, thus preventing contracture. The hope is that preventing secondary contracture of the medial rectus muscle will increase the chances of recovery without strabis- mus surgery. The use of botulinum remains controversial, however. Studies comparing botulinum to conservative treat- ment for the management of nerve palsy have shown no sig- nificant difference in recovery rates.

Holmes et al., in a prospective multicenter study of acute traumatic sixth nerve palsy or paresis, reported that patients treated either with botu- linum or conservatively had similarly high recovery rates.

It should be noted that, after a botulinum injection into the medial rectus muscle for a complete sixth nerve palsy, both the medial and lateral rectus will be paralyzed, resulting in essentially no horizontal movement of the paretic eye. Therefore, the patient A

B

FIGURE 10-10A,B. (A) Photographs of a child with a traumatic right sixth nerve palsy and poor lateral rectus function, evidenced by absent abduc- tion saccades and severe limitation of abduction of the right eye. There is 4 limitation of abduction as the right eye does not go past midline.

(B) Results after surgery consisting of a right Hummelsheim transposi-

tion and a right medial rectus recession of 6.0 mm. Note the eyes are

orthotropic in primary position. There is improved abduction, but abduc-

tion remains limited.

should be warned that the paretic eye may have decreased move- ment after the injection. In addition, the surgeon should be aware that the effects of botulinum can last more than 6 months, and surgery should be delayed until the botulinum has dissipated.

After the 6-month observation period, lateral rectus muscle function should be evaluated, as this is critical for determining the surgical plan. Lateral rectus muscle function can be assessed by saccadic velocity testing and the active forced-generation test. If the saccadic velocities are less than 60% of normal or the active forced-generation test is estimated to be half of the normal fellow eye, a vertical rectus muscle transposition proce- dure is indicated.

Transposition procedures act by moving innervated vertical rectus muscles to the lateral rectus insertion to provide lateral force. The lateral force of the transposition does not appropri- ately activate on attempted abduction but, instead, provides a constant lateral force. Transposition of vertical rectus muscles can involve the full muscle (full-tendon transfer) or the muscle can be split longitudinally and only half the muscle is transferred (partial-tendon transfer). In addition to a transposi- tion, patients with significant residual paresis almost always require an ipsilateral medial rectus recession to reduce adduc- tion forces.

The vertical rectus muscles provide substantial circulation to the anterior segment. Older adult patients, especially those with arteriosclerotic disease or hyperviscosity syndromes, are at risk for developing anterior segment ischemia after vertical recti transposition, particularly those receiving full-tendon transfers.

A partial-tendon transfer procedure should be considered in

these patients to maintain anterior circulation and prevent

anterior segment ischemia. Modifications of the Hummelsheim

partial-tendon transposition include suturing the transposed

vertical muscle to the lateral and resecting a few millimeters of

the transposed vertical muscle halves.

An important aspect of

the partial-tendon transfer is to fully mobilize the muscle being

transferred by splitting the vertical rectus muscles for at least

14 mm posterior to their insertions.

If carefully performed, a

partial-tendon transfer procedure results in long-term good post-

operative eye alignment while reducing the risk of anterior

segment ischemia. Other options include full-tendon transposi-

tion with injection of botulinum toxin to the medial rectus

muscle.

This treatment, however, may not provide a stable outcome, as an esotropia may recur after 4 to 6 months when the effect of the botulinum dissipates and medial rectus func- tion returns. This author’s recommendations for the surgical treatment of sixth nerve palsy are listed in Table 10-5.

Duane’s Retraction Syndrome

The cause of Duane’s retraction syndrome (DRS) has been iden- tified to be an agenesis of the sixth nerve and nucleus, with the inferior division of the oculomotor nerve (nerve to the medial rectus muscle) splitting to innervate both the medial and lateral rectus muscles.

Because both the medial and lateral rectus muscles are innervated by the nerve to the medial rectus muscle, both muscles fire and contract simultaneously on attempted adduction. This cocontraction of the medial and lateral rectus muscles on adduction gives rise to the term

TABLE 10-5. Surgical Treatment for Sixth Nerve Palsy.

Clinical presentation Surgery

Excellent lateral rectus function Recess contralateral medial rectus

(90%–100%): 5–6 mm (adjustable suture optional)

Ductions  trace limitation ET in primary position  2 to 8 PD Diplopia to the side of the palsy

Good lateral rectus function (80%–90%) Bilateral medial rectus recessions, but Ductions  1 recess the contralateral medial ET in primary position  10 to 20 PD rectus muscle 6 mm and the

ipsilateral medial rectus muscle 3–5 mm (adjustable suture advised) Fair lateral rectus function (60%–80%) Ipsilateral medial rectus recession

Ductions  2 6 mm (adjustable suture advised);

lateral rectus resection or Wright plication 5 mm and contralateral medial rectus recession ET in primary position  20 to 30 PD 3–5 mm (with optional Faden) Poor lateral rectus function (⬍60%) Ipsilateral medial rectus recession

Ductions  3 to 4 6–7 mm (adjustable suture in adults

or cooperative children), and vertical

rectus partial-tendon transposition

ET in primary position  30
PD to the lateral rectus muscle (either

Jensen or Hummelsheim); author

prefers modified Hummelsheim

Duane’s cocontraction syndrome. Cocontraction of the lateral rectus muscle against the medial rectus muscle on adduction causes globe retraction, producing relative enophthalmos and lid fissure narrowing.

There are various patterns of innerva- tion that account for the four types of Duane’s syndrome. Figure 10-11 shows a diagram of various patterns of abnormal innerva- tion possible in DRS.

Table 10-6 explains the various types of DRS as they cor- relate to the innervation patterns noted in Figure 10-11. In Duane’s type I, there is agenesis of the sixth nerve and the sixth nerve nucleus, with part of the medial rectus branch of the third nerve going to the lateral rectus muscle. Because most of the medial rectus branch of the third nerve appropriately goes to the medial rectus muscle, the eye will adduct with cocontraction by the aberrantly innervated lateral rectus muscle. This contrac- tion causes lid fissure narrowing; however, because of the absent

A B C D

FIGURE 10-11A–D. Diagrammatic representation of misdirection of

nerve fibers in Duane’s syndrome. The aberrant nerve pathway is shown

in red, and the dotted lines represent nerve hypoplasia or agenesis. (A)

type I: poor abduction and good adduction. Agenesis of the sixth nerve

and part of the third nerve splits to innervate both the medial and the

lateral rectus muscles, but most of the medial rectus nerve goes to the

medial rectus muscle so adduction is intact. (B) Type II: poor adduction

and good abduction. Sixth nerve is intact and innervates the lateral rectus

muscle, but the medial rectus nerve splits to innervate the medial and

lateral rectus muscles. There is poor adduction because the lateral rectus

contracts against the medial rectus muscle. (C) Type III: poor adduction

and poor abduction. Agenesis of the sixth nerve and part of the third nerve

splits to innervate both the medial and the lateral rectus muscles. The

split is equal so the eye does not move in or out. (D) Synergistic diver-

gence and paradoxical abduction on attempted adduction. Agenesis of the

sixth nerve and part of the third nerve splits to innervate both the medial

and the lateral rectus muscles, but most of the medial rectus innervation

goes to the lateral rectus muscle. When the eye attempts to adduct, it

abducts because the medial rectus nerve innervates the lateral rectus

muscle.