Sixth Nerve

Duane's Syndrome Duane's syndrome is characterized by retraction of the globe and narrowing of the lid fissure on attempted adduction as well as limited eye movements. Three forms of abnormal motility have been classified217:

Type I: limited abduction with intact adduction (Fig. 5-11) Type II: limited adduction with intact abduction Type III: limited abduction and limited adduction

Incidence Duane's syndrome has been reported to account for 1% to 4% of all strabismus cases.122

Etiology Electromyographic data indicate that the medial and lateral recti contract simultaneously, that is, they "co-contract," and may thereby produce both the retraction of the globe into the orbit and the limitation of eye movement.216,217,308 One can speculate as to how different distributions of inappropriate neural input from the oculomotor and abducens nerves to the lateral and medial recti could produce each of the three patterns of limited ocular motility seen in Duane's syndrome. This

FIGURE 5-11. Duane's syndrome, left eye. This montage demonstrates the limitation of abduction (middle, right photo), palpebral lid fissure narrowing on adduction (middle, left photo), upshoot in adduction (top, left photo), and "Y" pattern (middle, top photo) seen with Duane's syndrome.

aberrant innervation is thought to be a result of congenitally deficient innervation of the VIth nucleus, leading to a fibrotic lateral rectus muscle (Fig. 5-12).

Neuropathological investigations of three patients with Duane's syndrome have all revealed aplasia or hypoplasia of the abducens nucleus and nerve, and in two of these cases, branches of the third nerve "substituted" for the defective sixth nerve by supplying some of its fibers to the lateral rectus. The first case was unilateral and demonstrated a hypoplastic lateral rectus muscle in addition to hypoplasia of the abducens nucleus and nerve.310 In a second patient with bilateral type III Duane's syndrome, both abducens nuclei and nerves were absent; also, both lateral recti were found to be partially innervated by the inferior division of the oculomotor nerves and were histologically normal in innervated areas but fibrotic in areas not inner-vated.213 The third patient had unilateral, left type I Duane's syndrome and showed, as did the previous case, absence of the sixth nerve, partial innervation of the lateral rectus by the inferior division of the oculomotor nerve, and fibrosis of the lateral rectus muscle in areas not innervated. However, although the left abducens nucleus was hypoplastic, containing less than half the number of neurons seen in the right nucleus, both medial longitudinal fasciculi were normal and the remaining cell bodies in the nucleus were interpreted to be internuclear neurons. This

'Normal" LR

'Normal" LR

'Early" onset embryonic aberrant CN III innervation

'Early" onset embryonic aberrant CN III innervation

Mildly librotic LR

Mildly librotic LR

'Later" onset embryonic aberrant CM III innervation

Severely Fibrotic LR

Severely Fibrotic LR

'Late" onset embryonic aberrant CN III innervation

'Late" onset embryonic aberrant CN III innervation finding corroborates the clinical observation that, in unilateral type I Duane's syndrome, adducting saccades in the unaffected eye are usually normal.177,322,333 This finding also indicates an exquisitely specific neural deficit. Electrophysiological techniques such as auditory evoked potentials237 and eye movement recordings351,537 have suggested that there may be other associated brainstem dysfunction, but these studies have not produced conclusive evidence or have not been reproducible.403,481

"Upshoots" and/or "downshoots" on attempted adduction are common motility findings. Theoretically, the cause of the upshoots and downshoots may be mechanical, innervational, or a combination of the two. In most cases, the mechanics of the lateral rectus seem to be largely responsible because weakening or eradicating the action of a tight lateral rectus results in significant reduction or elimination of upshoots and downshoots. The "bridle-effect theory" postulates that vertical sideslip of a tight lateral rectus across the adducting globe produces these movements234,510; however, neuroimaging has not confirmed vertical displacement of the lateral rectus during upshoots and downshoots.62,511 In certain individuals, an innervational anomaly may account for upshoots and downshoots. For example, one of the authors (B.N.B.) has observed that continued severe upshoot on adduction in a patient whose lateral rectus was detached from the globe and allowed to retract far

FIGURE 5-12. Proposed embryonal etiopathogenesis of Duane's syndrome as a congenitally deficient innervation syndrome. The developing cranial nerves have a "trophic" function on the developing mesenchyme of the future extraocular muscles. If there is late or no innervation to the developing mesenchyme, the muscle becomes dysplastic, fibrotic, and inelastic. If there is early aberrant innervation of the developing mes-enchyme by cranial nerve III, the lateral rectus has a "normal" architecture but abnormal innervation, leading to limited abduction only (type I). The later during embryogenesis the innervation, the more dysplastic the lateral rectus, leading to limited adduction as well (type III). The balance between the quantitative amount of aberrant innervation and the degree of lateral rectus fibrosis creates relatively different patterns of abduction and adduction, leading to the different "types" of Duane's syndrome. Type II Duane's syndrome (not depicted) may be caused by more innervation from the third cranial nerve to the lateral rectus compared to the medial rectus. Dotted lines represent absent or hypoplastic innervation; dashed lines represent later onset of innervation; thickness of lines represents quantitative amounts of innervation. LR, lateral rectus; MR, medial rectus; III, oculomotor nucleus; vI, abducens nucleus.

into the orbit before suture adjustment. In addition, there is EMG evidence for cocontraction of appropriate cyclovertical muscles and the lateral rectus during upshoots and down-shoots217,322,443; such co-contraction could play a substantial role in some cases.

Clinical Features and Natural History Most large series indicate that females represent about two-thirds of cases and that the left eye is affected in about two-thirds of unilateral cases. Approximately 75% are type I; type III accounts for most of the rest, and type II is quite rare. Types I and II may occasionally coexist in the 10% to 20% of cases that are bilateral. Many Duane's syndrome patients are orthotropic in primary position or with a small head turn and have excellent binocular function.229,391,402 Although amblyopia can occur in the involved eye, the reported incidence of amblyopia as well as ani-sometropia varies widely.448,491 Most Duane's syndrome patients ignore or are unaware of sensory disturbances,300 but occasionally an older child presents with "acute" awareness of diplopia in the appropriate fields of gaze.

As mentioned, upshoots and downshoots on attempted adduction may occur and may be accompanied by A, V, or X patterns, giving the appearance of oblique muscle dysfunction.

Clinical Assessment Other diseases should be considered in the differential diagnosis. Rarely, acquired orbital disease may produce limitations of abduction and retraction, thereby mimicking Duane's syndrome. This effect has been observed with medial orbital wall fractures, fixation of muscle by orbital metastases, orbital myositis, and a variety of other conditions.165,266,367,469

Systemic Associations Although Duane's syndrome is usually an isolated finding, it may accompany any of a multitude of other congenital anomalies in 5% to 57% of cases (Table

inheritance Familial cases are not uncommon, and an autosomal dominant mode of inheritance best fits most, but not all, of the reported pedigrees.126 Duane's syndrome, sensorineural deafness, upper limb defects, facio-auriculo-vertebral anomalies, and genitourinary and cardiac malformations appear as isolated findings or in combination throughout certain families and may all, perhaps, be ascribed to a highly pleiotropic autosomal dominant gene that is frequently nonpenetrant.198a361 Studies of

TABLE 5-7. Congenital Anomalies Associated with Duane's Syndrome.

Structure

Associated anomalies

Ocular/external

Microphthalmos; coloboma; heterochromia iridis; flocculi iridis; congenital cataract33'34'363 Ptosis; nevus of Ota; hypertelorism; prominent epicanthus33'34'210'377'448 Epibulbar dermoid307'363'377'379

Neural

Optic nerve anomalies34'120'248'261'307'377; DeMorsier syndrome5

Sensorineural deafness261-264'307'448

Seventh nerve palsy106'307'377'428

Marcus Gunn jaw winking230'307

Gusto-lacrimal reflex58'307'393

Fourth nerve palsy307

Möbius syndrome307

Musculoskeletal

Craniofacial anomalies; skeletal anomalies; Klippel-Feil syndrome; Goldenhar's syndrome; Marfanoid hypermotility syndrome; cleft lip/palate; muscular dystrophy34'35'106'212'261-264'307'361'363'377'379'393'421'448

Miscellaneous

Cardiac anomalies35'307'377 Genitourinary anomalies106'377

Noonan syndrome Fetal alcohol syndrome211 Congenital panhypopituitarism107 Oculocutaneous albinism208

monozygotic twins have revealed both concordance and discordance in more than one family.207,247

Two recent reports of large families with autosomal dominant Duane's syndrome, one in the U.K. and the other in Mexico, have both found linkage to chromosome 2q31.20,148 Other reports have found deletions in chromosome 8q in patients with Duane's syndrome associated with other abnormalities such as mental retardation and hydrocephalus.79,505

Treatment A patient with unacceptable primary position deviation, head position, globe retraction, upshoot, or down-shoot may require surgery. All these factors as well as the relative contributions of mechanical and innervational factors are considered during surgical planning. As a general recommendation, resections of the horizontal recti of an affected eye is usually avoided because this may increase globe retraction. Otherwise, the surgical approach is individualized.275 Depending on the situation, a wide variety of techniques may prove helpful, including transposition of the vertical recti with or without medial rectus recession,169,331 Y-splitting of the lateral rectus,410 adjustable sutures,388 and posterior fixation sutures.293,510

Prognosis Recession of horizontal rectus muscle eliminates the face turn in 79% of cases and significantly reduces the face turn in most of the remaining patients.276,388 Undercorrec-tion of primary position esotropia may occur postoperatively as the amount of recession needs to be larger than indicated in the tradition tables for concomitant strabismus; rerecession is recommended for these cases if the initial recession was less than 8 mm or if forced duction testing still indicates restriction. The occasional overcorrections may be reversed by advancing the recessed muscle or recessing the antagonist horizontal rectus muscle if tight.171,348,353

Synergistic Divergence synergistic divergence is a striking motility pattern consisting of an adduction deficit with simultaneous bilateral abduction on attempted gaze into the field of action of the involved medial rectus.109,514,525 As with Duane's syndrome, cocontraction of the lateral and medial recti has been demonstrated on EMG,525 and it has therefore been suggested that synergistic divergence may be placed along the Duane's "spectrum" of congenital anomalous innervation. In this conceptual scheme, synergistic divergence is similar to type II Duane's syndrome, except that the larger part of the branch of the third nerve "intended" for the medial rectus is misdirected to the lateral rectus. The globe retraction characteristic of Duane's syndrome does not accompany synergistic divergence, presumably because there is so little innervation to the medial rectus. However, this hypothesis has not been verified by clinicopathological study, and sac-cadic velocity studies in two patients indicate that the abducens nerve may not necessarily be absent or severely hypoplastic.188

Synergistic divergence has been observed as early as 5 months of age,108 may be bilateral,187,188,486 and has been associated with other abnormalities including Marcus Gunn jaw-winking,72,73,187 ipsilateral congenital Horner's syndrome,238 arthrogryposis multiplex congenita,109 congenital fibrosis syndrome, and oculocutaneous albinism.72,73

Surgical crippling of the ipsilateral lateral rectus has been combined with a variety of other procedures such as medial rectus resection and superior oblique tenotomy and inferior oblique myectomy330 to eliminate the simultaneous abduction as well as to correct the exotropia in primary position.188

Other types of anomalous innervation that may involve the sixth nerve include congenital or acquired synkinesis of the levator and lateral rectus during abduction,242,343 acquired trigemino-abducens synkinesis with abduction accompanying jaw movements,312,349,453 congenital twitch abduction on attempted upgaze,271 or lateral gaze synkinesis on downward saccades.503

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