Fourth Nerve Palsy

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Etiology and Systemic Associations Of the many causes of trochlear palsy in childhood (Table 5-5), "congenital" and traumatic are by far the most common.191,209 The cause of most congenital trochlear palsies remains unknown, but aplasia of the trochlear nucleus has been reported to accompany the absence of other cranial nerve nuclei.10317464 The superior oblique tendon or muscle is often the primary

TABLE 5-5. Etiology of Fourth Nerve Palsy.

Location/signs

Etiologies

Nucleus and fascicle

Trauma; tumor; demyelination; after

Contralateral Horner's syndrome

neurosurgery; nuclear aplasia;

arteriovenous malformation;

hemorrhage; infarction

Subarachnoid space

Trauma; tumor; increased intracranial

Papilledema; other cranial nerve

pressure; after lumbar puncture or

palsies

shunt for hydrocephalus; spinal

anesthesia; meningitis; mastoiditis

Cavernous sinus/superior orbital fissure

Tumor; internal carotid aneurysm;

Ipsilateral Horner's syndrome,

Tolosa-Hunt syndrome

ipsilateral IlIrd, Vth, Vlth nerve

involvement; proptosis; disc

edema; orbital pain

Orbit

Tumor; trauma; inflammation

Ipsilateral IIIrd, VIth nerve

involvement; proptosis;

enophthalmos; disc edema; orbital

pain; conjunctival/episcleral

injection

Uncertain location

Congenital; idiopathic

problem. Laxity of this tendon has been described on forced duction testing381 and correlates well with the presence of attenuated superior oblique muscles on orbital MRI.432 Therefore, congenital cases may be more correctly termed congenital superior oblique palsy/underaction instead of fourth nerve palsy. Absence of the superior oblique muscle altogether is also in the differential of an apparent congenital superior oblique palsy.87

The trochlear nerves are particularly vulnerable to closed head trauma when there may be contrecoup of the tectum of the midbrain against the edge of the tentorium.292 In this way, the nucleus or fascicle may be injured within the substance of the midbrain, or the nerve itself may be contused as it exits the brain-stem dorsally and passes laterally around the midbrain (see Fig. 53). The proximity of the two trochlear nerves to each other at the site of their crossing in the anterior medullary velum (roof of the Sylvian aqueduct; see Fig. 5-4) explains the high incidence of bilateral involvement after coup-contrecoup, closed head trauma.286

Clinical Features and Assessment Vertical deviations may also result from other processes, such as abnormal neuromuscular transmission, restriction, inflam-

handbook of pediatric neuro-ophthalmology mation, skew deviation, dissociated vertical divergence, small nonparalytic vertical deviations associated with horizontal strabismus, and paresis of other cyclovertical muscles. The clinical assessment of a vertical deviation is carefully executed to exclude these various possibilities.

It is important to ask about previous extraocular muscle surgery or orbital trauma and to obtain any history that suggests myasthenia gravis or skew deviation. The examiner notes any anomalous head position (Figs. 5-8, 5-9), versions, ductions, cover test measurements in cardinal fields of gaze, any secondary deviation, forced (Bielschowsky) head tilt test measurements, presence or absence of both subjective and objective torsion, and presence or absence of dissociated vertical deviation. Forced ductions, Tensilon testing, and other supplemental tests are performed as appropriate.

FIGURE 5-8. (A) Unilateral congenital cranial nerve palsy, right eye. The photograpph demonstrates a right hypertropia that increases in left gaze. There is slight underaction of the right superior oblique nad significant overaction of the right inferior oblique muscle. (B) The photograph of head tilt test, with right hypertropia increasing on tilt right and diminishing on tilt left. Positive head tilt with the right hyper increasing in left gaze indicates a right superior oblique palsy.

FIGURE 5-8. (A) Unilateral congenital cranial nerve palsy, right eye. The photograpph demonstrates a right hypertropia that increases in left gaze. There is slight underaction of the right superior oblique nad significant overaction of the right inferior oblique muscle. (B) The photograph of head tilt test, with right hypertropia increasing on tilt right and diminishing on tilt left. Positive head tilt with the right hyper increasing in left gaze indicates a right superior oblique palsy.

Congenital Fourth Nerve Palsy

FIGURE 5-9. Bilateral asymmetric congenital fourth nerve palsy and esotropia. Note that the right superior oblique palsy is more severe than the left, and there is a right hypertropia in primary position. There is significant superior oblique underaction, right side more than left side. A significant V-pattern is present. There is a right hypertropia in right gaze and a left hypertropia in left gaze.

FIGURE 5-9. Bilateral asymmetric congenital fourth nerve palsy and esotropia. Note that the right superior oblique palsy is more severe than the left, and there is a right hypertropia in primary position. There is significant superior oblique underaction, right side more than left side. A significant V-pattern is present. There is a right hypertropia in right gaze and a left hypertropia in left gaze.

Several other comments regarding the clinical evaluation are crucial.

1. The familiar "Parks-Bielschowsky three-step" test helps to combine information from cover test measurements and the Bielschowsky head tilt phenomenon.59,370 This test is only useful when there is a palsy of a single cyclovertical muscle and can therefore only be applied after the careful assessment just described.281 A fourth nerve palsy would reveal hypertropia, worsening on horizontal gaze in the direction contralateral to the hypertropic eye, and worsening on head tilt ipsilateral to the hypertropic eye. Infants with congenital superior oblique palsies present with a head tilt, whereas older children and adults with decompensated congenital palsies complain of vertical and/or torsional diplopia.323

To diagnose a congenital superior oblique palsy, old photographs are helpful, often revealing a long-standing head tilt. Also, vertical fusional amplitudes frequently exceed the normal range of 3 to 4 prism diopters. The presence of a suppression scotoma when assessing diplopia or the presence of fusion also aids in establishing the chronicity of the condition as suppression is usually a childhood adaptation mechanism. Moreover, the presence of facial asymmetry may be associated with a longstanding head tilt from early childhood.176,202,338,528 The presence of facial asymmetry may not be a specific sign for congenital superior oblique palsy, however, because patients with acquired superior oblique palsy and heterotopic rectus muscles exhibited similar features of facial asymmetry.502 The causal relationship of the head tilt due to an abnormal superior oblique is not established.373 Hemifacial changes are often associated with plagio-cephaly as a craniofacial anomaly, and craniofacial anomalies are commonly associated with anomalous extraocular muscles.124

2. The examiner also checks for bilateral and asymmetrical superior oblique palsies, because the larger paresis may "mask" the smaller until unilateral surgery is performed.274,280 Bilateral involvement should particularly be suspected after closed head injury. Findings that suggest bilaterality include alternation of hypertropia with fixation, gaze, or head tilt: excyclotorsion of 10° or more: and V-pattern esotropia.286

3. Excyclodeviations usually occur with trochlear palsies, may accompany restrictions and myasthenia gravis, and are less commonly seen with skew deviations.494 The triad of skew deviation, head tilt, and incyclotorsion of the hypertropic eye is termed the ocular tilt reaction, an entity that can mimic fourth nerve palsy on the traditional three-step test.128 Therefore, examination for torsion, by double-maddox rod or simple fundoscopy, is essential in distinguishing a fourth nerve palsy from ocular tilt reaction.

Inheritance

Rarely, congenital superior oblique palsy may be familial.28,198 The mode of inheritance in the described families has not been determined.

Natural History Long-standing congenital superior oblique palsy may decom-pensate in adulthood for a variety of reasons, including trauma, with the presenting symptom of vertical diplopia. As for traumatic cases, most cases of unilateral injury do resolve (see following). Also, after long-standing fourth nerve palsy, a "spread of concomitance" may be observed where the deviation in rightgaze and leftgaze are nearly equal, although the differential deviation in right versus left head tilt persists. This spread of concomitance has been attributed to a "contraction" of the ipsi-lateral superior rectus muscle.26

Treatment

Most surgeons wait 6 to 12 months before deciding on strabismus surgery for traumatic cases, to await spontaneous resolu tion of the deviation or stability in measurements. For congenital cases presenting with head tilt in infancy, surgery may be performed as soon as possible to correct the head posture and thus to aid in normal development of the neck muscles and the alignment of cervical vertebrae. It is unknown, however, whether early strabismus surgery can prevent or reverse facial asymmetry. For the large head tilts in infancy, a superior oblique tuck may treat the head tilt quickly; the benefit of normalizing head posture with this procedure may outweigh the resultant iatrogenic Brown's syndrome.

For long-standing fourth nerve or superior oblique palsy, a variety of options exist. One approach is to operate on one muscle for vertical deviations of up to 15 prism diopters and to consider two-muscle surgery in deviations above 15 prism diopters. The first choice of procedure is often ipsilateral inferior oblique muscle weakening. The second procedure often performed when the deviation is greater than 15 prism diopters is either ipsilateral superior rectus recession,26 when the vertical deviation is worse in upgaze, or contralateral inferior rectus recession, when the deviation is worse in downgaze.202

A fast and easy approach to deciding which muscle to weaken first is to perform a "modified Parks three-step test"205 to determine which muscle is overacting and then to weaken that muscle first. This modified three-step test is performed in the same manner as the traditional one, except for the first step, in which the overacting vertical muscles are circled in each eye (instead of the traditional method of circling the presumed weak vertical muscles).

In the case of bilateral palsy, bilateral inferior rectus recession and Harada-Ito procedures are recommended, both able to treat excyclotorsional diplopia.

Prognosis

When a child presents with a postinfectious, isolated trochlear palsy that cannot be explained as congenital, traumatic, restrictive, myasthenic, or neoplastic, the prognosis is good and observation alone is sufficient.

Overall prognosis for recovery of isolated fourth nerve palsies in adults and children was reported to be 53.5% combined (1000 total patients from 2 months to 91 years of age, 90% of whom were over 19 years and 75% of whom were over 35 years of age).424 Unilateral traumatic fourth nerve palsies in a series of 24 pediatric and adult patients (ages 7-78 years; mean, 35.4 years),

46% of whom sustained minor head trauma, resulted in 75% resolution.483 Another series reported 65% resolution in unilateral but 25% in bilateral cases of traumatic fourth nerve palsy.479

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