Fig. 6.11  Axial MR scan demonstrating a trochlear neurinoma in child with right superior oblique palsy and NF2. Used with permission from Brodsky and Boop75

Rare Causes of Trochlear Nerve Palsy

Rarely, trochlear nerve palsy can occur in patients who have elevated intracranial pressure without ventriculomegaly (pseudotumor cerebri).27 In this context, trochlear nerve palsy must be distinguished from skew deviation, which can also rarely accompany pseudotumor cerebri.27 These vertical deviations are usually seen in conjunction with sixth nerve palsy and resolve with normalization of intracranial pressure.

Differential Diagnosis

The differential diagnostic considerations of trochlear nerve palsy in childhood are listed in Table 6.4. Isolated superior oblique palsies are generally straightforward in older children. In children who have sustained head trauma, however, careful attention should be paid to ruling out skew deviation.213

Table 6.4  Differential diagnostic considerations of trochlear nerve palsy in childhood

Dissociated vertical divergence

Congenital muscular torticollis

Synostotic plagiocephaly

Double elevator palsy

Ocular tilt reaction

Incomitant skew deviation

The major differential diagnostic considerations in infants with a head tilt are dissociated strabismus and congenital muscular torticollis. Dissociated vertical divergence may present with a hyperdeviation and a contralateral head tilt, which simulates trochlear nerve palsy, but is not associated with diplopia.115 Congenital muscular torticollis can be distinguished from congenital trochlear nerve palsy by (1) the examiner’s inability to passively tilt the head in the opposite direction, (2) the palpation of a tight sternocleidomastoid muscle on the side of the tilt, (3) the persistence of the tilt when the infant is reclined, and (4) the persistence of the tilt when either eye is patched. Synostotic plagiocephaly can also present with a hypertropia, contralateral head tilt, and facial asymmetry that can resemble a congenital trochlear nerve palsy. The diagnosis of trochlear nerve palsy must also be considered in older children who appear to have a monocular elevation deficiency (“double elevator palsy”). This situation arises in children who habitually fixate with the paretic, hypertropic eye and develop a contracture in the contralateral inferior rectus muscle (fallen eye syndrome). The ocular tilt reaction is a rare ocular motility disturbance of central vestibular origin characterized by vertical divergence of the eyes (skew deviation), a head tilt toward the side of the lower eye, and bilateral torsion of the eyes with the superior poles directed toward the side of the lower eye.

The combination of vertical strabismus and a head tilt may initially suggest the diagnosis of trochlear nerve palsy, however, the finding of subjective and objective intorsion of the higher eye and extorsion of the lower eye establishes the diagnosis.147 The hypertropia is usually comitant, but cases of incomitant hypertropia with a Bielschowsky Three Step test corresponding to that seen in trochlear nerve palsy have been described.147 Heterotopic positioning of the extraocular muscles can simulate trochlear nerve palsy when the lateral rectus muscle in the contralateral eye is inferiorly positioned, causing it to descend in abduction to a greater degree than the adducting eye descends (Fig. 6.13).562 The finding of facial asymmetry is not useful in distinguishing these two conditions.562 Finally, trochlear nerve palsy can evolve to simulate an idiopathic hypertropia when the superior rectus muscle of the chronically hyperdeviating eye develops a secondary contracture. This contracture can erase the preexisting torsion and enhance the Bielschowsky Head Tilt difference.278

Fig. 6.13  Coronal MR image demonstrating excyclorotation of extraocular muscles within left orbit. Inferior displacement of left lateral rectus caused overdepression of left eye during abduction and simulated right superior oblique palsy. Courtesy of Joseph Demer, M.D.

Treatment

Traumatic or other forms of acquired trochlear nerve palsy should be observed for a minimum of 6 months before considering surgical correction. During this period, occlusion therapy is often unnecessary for unilateral cases, because most children can fuse with a compensatory head tilt. The development of amblyopia in this setting suggests a coexistent motility disorder, an associated traumatic disruption of the fusional mechanism, or the inability to obtain fusion by adopting an anomalous head position.

The surgical treatment of unilateral trochlear nerve palsy should be individualized, but some general guidelines will be summarized. The goal of surgery is to obtain single binocular vision within a functional field of gaze and to normalize the head position. Kraft and Clarke298 found that patients with trochlear nerve palsy who underwent surgery to eliminate a compensatory head posture had a 75.6% incidence of successful restoration of normal head posture. However, the Bielschowsky head tilt test difference usually persists after successful surgery, at least in congenital cases.400 The measured postoperative improvement in saccadic conjugacy is greater in patients with congenital trochlear nerve palsy and correlates with preoperative vertical vergence.331

Using the example of a right trochlear nerve palsy, the important fields of gaze to consider in planning surgical strategy are primary gaze, right gaze, and downgaze. Because there is always a hyperdeviation in left gaze, this finding does not enter into the surgical decision. The following list is a general guideline to surgical strategy:

Most cases of trochlear nerve palsy manifest with isolated overaction of the ipsilateral inferior oblique muscle, with little or no underaction of the superior oblique muscle. These cases can be managed by weakening the antagonist inferior oblique muscle surgically (i.e., recession or myectomy). Inferior oblique surgery can neutralize up to 15 prism diopters of hypertropia in primary gaze and has the advantage of being self-titrating.220,238,351,385,409

Numerous inferior oblique weakening procedures have been used to treat unilateral trochlear nerve palsy, but properly performed recession or myectomy seem to work equally well.154,157,241,391 However, it is important to remember the caveat that inferior oblique anteriorization192,382 runs the risk of inducing diplopia in upgaze, secondary to limitation to elevation of the affected eye. Anterior and nasal transposition of the inferior oblique muscle has been advocated for treatment of severe or recurrent trochlear nerve palsy to convert the inferior oblique muscle into an intorter or a depressor.528

A significant (>10 diopter) right hypertropia in right gaze suggests a secondary right superior rectus contracture, which does not respond to inferior oblique weakening alone. In this setting, the ipsilateral superior rectus muscle must also be recessed (usually 3 mm or less) to eliminate the hyperdeviation in right gaze.278 Pseudo-overaction of the contralateral superior oblique muscle serves as a useful clinical clue to the presence of a superior rectus contracture.582,596 Superior rectus contracture can be confirmed by forced duction testing at surgery, and small (3 mm) recessions can eliminate them.

A significant (>10 diopter) right hyperdeviation in downgaze suggests a contralateral inferior rectus contracture, as occurs in children who habitually fixate with the paretic eye. Because inferior oblique surgery does not produce a significant effect in downgaze, a small (3 mm or less) recession of the contralateral inferior rectus muscle (with or without a posterior fixation suture) should be considered. In our hands, combined inferior oblique and contralateral inferior rectus weakening produce a high incidence of surgical overcorrection. Contrary to the recommendation of combined surgery, we and others now treat such cases with isolated inferior oblique recession220,238,351,385,409 and add small inferior rectus recessions (no more than 3 mm) only in the most severe cases. Numerous inferior oblique weakening procedures, ranging from recession, resection, disinsertion, and anteroplacement, and orbital wall fixation have been used to treat unilateral trochlear nerve palsy successfully. Except for the aforementioned problems with inferior oblique muscle anteroplacement, all of these procedures seem to work equally well.

We reserve superior oblique tucks for cases in which there is superior oblique underaction, symptomatic torsion, and minimal ipsilateral inferior oblique overaction.50 Unlike the inferior oblique weakening procedure, a superior oblique tuck must be carefully titrated.488 A tuck that is too small

produces a negligible treatment effect, while one that is too large produces a problematic Brown syndrome. Parents must be warned preoperatively that even a successful tuck will produce a mild iatrogenic Brown syndrome that may be associated with vertical diplopia in gaze up and to the opposite side. Some authors recommend that this surgical strategy be modified for cases of congenital trochlear nerve palsy, which are often associated with a lax or anomalous superior oblique tendon.229,436

The absence of normal superior oblique tendon tension can be demonstrated prior to surgical exploration by one of several exaggerated forced duction tests.206,436 von Noorden573 has outlined a surgical strategy for treatment of congenital absence of the superior oblique tendon that relies on inferior oblique weakening as the central procedure, with other muscles added with minor modifications to the general rules previously stated. Plager has argued that decreased superior oblique tendon tension suggests a lax or absent tendon that should be explored and tucked if present, while a child with normal tendon tension is at high risk of iatrogenic Brown syndrome if the superior oblique tendon is tucked, but responds favorably to inferior oblique recession with recession of additional muscles as indicated.436

Bilateral trochlear nerve palsy is more difficult to treat, and parents should be warned preoperatively that comfortable binocular vision in all fields of gaze may not be possible.216,285,378 In children with alternating hyperdeviations in side gaze and a large esotropia in downgaze, bilateral superior oblique tucks can often restore single binocular vision in primary gaze, but diplopia near the downgaze position usually persists due to esotropia and residual torsion in this position of gaze. Kushner311 has utilized a procedure described by Forrest Ellis, MD, and Carlos Souza-Dias, MD, consisting of bilateral inferior rectus recession (5 mm OU) to successfully restore single binocular vision in downgaze. This procedure produces a “fixation duress,” requiring excess downgaze innervation that recruits the paretic superior oblique muscles, thereby enhancing abduction and intorsion of the eyes in downgaze. It, therefore, requires that some residual superior oblique function be present. Jampolsky has advocated bilateral superior rectus and inferior oblique weakening to “scroll down” the field of single binocular vision from upgaze to primary position. This procedure carries the relative disadvantage of crippling upgaze.

Children with bilateral superior oblique palsies may have complaints that are predominantly torsional in nature with minimal alternating hyperdeviations in sidegaze or V-pattern in downgaze. In such cases, an alternative to a superior oblique tuck is the dissection and anterior–inferior transposition of the anterior portion of the superior oblique tendon that is primarily involved in torsional movements (Harada–Ito procedure).216,365 In addition to decreasing or eliminating the excyclodeviation, this procedure augments abduction in downgaze, thereby reducing the associated V-pattern esotropia.