Trochlear Nerve Palsy (Fourth Nerve Palsy) Treatment & Management

Prisms may be used for patients with small deviations and diplopia without a torsional component. Incomitance of deviation often limits usefulness of this therapy.

Botulinum toxin also has been studied in the treatment of fourth nerve palsy. ^[32] It is a neuromuscular agent that acts presynaptically to block neurotransmitter release and results in muscle weakening. Use of this agent as primary therapy for fourth nerve palsy has been discouraging. However, it best may be used to correct residual deviation after strabismus surgery, to delay or avoid further surgery.

BOTOX® is purified botulinum toxin A, derived from a culture of the Hall strain of Clostridium botulinum. It binds to receptor sites on motor nerve terminals and inhibits the release of acetylcholine. BOTOX® may be used for the treatment of strabismus and blepharospasm in patients 12 years and older. It is pregnancy category C.

Side effects for use in strabismus include ptosis and vertical deviation by action at extraocular muscles close to the site of injection. Injection should be performed under direct visualization during a surgical procedure or with the aid of electromyography.

Each vial of BOTOX® contains 100 units of botulinum toxin A in a vacuum-dried form. It needs to be reconstituted using preservative free 0.9% sodium chloride as the diluent. Doses used in strabismus range from 1.25-5 units, depending on the amount of deviation.

In 1970s, Knapp developed a surgical approach for superior oblique palsy. ^[1] He classified superior oblique palsy by determining the field of gaze in which deviation was greatest. Based on this classification, he recommended operation on the muscle or muscles that acted in this direction of gaze.

A 2-year-old girl with compensatory left head tilt due to congenital right superior oblique palsy.

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Postoperative photo of same girl; note marked improvement of head tilt.

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Plager described a tailored treatment plan that evolved from Knapp's recommendations, with some additions based on more recent operative algorithms. ^[33] For a deviation of less than 15 prism diopters, single muscle surgery usually suffices. If there is any inferior oblique overaction, inferior oblique weakening by tenotomy, recession, disinsertion/disinsertion- myectomy, and anterior transposition all are acceptable choices. There is no consensus on which procedure is superior. ^[34] Without any evidence of inferior oblique overaction, another muscle may be chosen. In case of ipsilateral superior rectus restriction, a superior rectus recession would be indicated. Superior oblique tendon tuck is preferred if significant tendon laxity is present, as has been described in congenital cases. Contralateral inferior rectus recession is chosen if there is no evidence of superior rectus restriction or superior oblique tendon laxity. This is an especially useful procedure when deviation is greatest in downgaze.

A critical decision to make in the treatment of fourth nerve palsy is whether to perform a 1-muscle or 2-muscle surgery. Nash et al compared 1-muscle versus 2-muscle surgery for moderate-angle hyperdeviations (14-25 prism diopters) due to unilateral fourth nerve palsy in a retrospective chart review of 73 patients. They concluded no clear advantage of 2-muscle surgery for motor outcomes or for diplopia correction. Less-symptomatic diplopia undercorrections were more common with 1-muscle surgery, whereas 2-muscle surgery resulted in fewer more-symptomatic diplopia overcorrections. ^[35]

Two-muscle surgery generally includes weakening of the ipsilateral inferior oblique, as well as a procedure on the ipsilateral superior rectus, superior oblique, or contralateral inferior rectus. For large deviations, 3-muscle surgery may be considered. Inferior oblique and contralateral inferior rectus should be weakened. Then, the surgeon may choose to operate on the superior oblique or superior rectus, based on intraoperative findings.

A modified Harada-Ito procedure is useful for patients with large excyclotorsional deviation. This is likely to be the case for patients with bilateral superior oblique palsy, and bilateral surgery should be performed. ^[36] In this procedure, the superior oblique tendon is split, and anterior fibers are advanced anteriorly and laterally.

Careful assessment of deviation in all fields of gaze should be performed.

Multiple measurements should be taken to ensure that deviations are stable.

Ductions should be evaluated to determine if there is inferior oblique overaction. ^[37]

Presence of V-pattern esotropia is highly suggestive of bilateral superior oblique palsy.

It may not be possible to determine if there is superior rectus restriction in the clinic, and this test may be performed in the operating suite.

Photographs that show head position and ocular motility findings, including head tilts, are useful for documentation.

Infants presenting with torticollis may be suspected of having superior oblique palsy.

To differentiate true cases of strabismus from neuromuscular causes of torticollis, a patch test may be performed in the office. After 20 minutes of monocular occlusion, the child is reevaluated, still wearing the patch. If head tilt was adopted for fusional purposes, it will be reduced after patching.

There is a low risk for amblyopia in affected children, presumably because they can achieve intermittent fusion by using head tilt and large fusional amplitudes. Loss of compensatory head position by a child suggests loss of fusion and may be associated with development of amblyopia.

Patients with congenital superior oblique palsy often have an abnormally lax superior oblique tendon. An exaggerated, forced duction test described by Guyton can be performed intraoperatively to determine if there is any degree of tendon laxity relative to normal eye, as follows ^[38] :

• This test is performed by grasping the eye obliquely at 2- and 8-o'clock positions for the left eye and at 4- and 10-o'clock positions for the right eye.

• The eye is rotated superiorly and medially while simultaneously depressing the eye into the orbit. This places the superior oblique tendon on maximal tension.

• The eye is rolled back and forth over the tendon to ascertain its tension.

• Performing this test prior to the start of the case will guide the surgeon to determine eyes that will benefit from a superior oblique tendon tuck.

• Of equal importance is that it will identify those patients who are at risk of developing a postoperative Brown syndrome.

• Tendon tucks should be performed only for markedly lax tendons. The tuck should be enough to match tension of the normal eye's tendon.

Any surgeon who performs oblique muscle surgery should be familiar with anatomy, landmarks, and appropriate approaches to these muscles.

Visualization is more difficult than with rectus muscle surgery, and injury to adjacent nerves, blood vessels, and other extraocular muscles may occur. Use of a headlight can improve visualization.

Without careful preoperative assessment, bilateral asymmetric superior oblique palsy may be mistaken for unilateral palsy. After surgery for unilateral palsy, contralateral superior oblique weakness becomes unmasked; unfortunately, a second surgery then is required.

Patients with torsional complaints are among the most difficult to treat. Considerations are as follows:

• In general, patients can fuse up to about 8° of cyclotropia before becoming symptomatic.

• Patients with bilateral fourth nerve palsies from head trauma should be warned about the likelihood of persistent diplopia after surgery.

• Many of these patients also may have central disruption of trauma from severe head injury and will be unable to fuse even after excellent surgical alignment.

As with any strabismus surgery, undercorrections and overcorrections may occur. It generally is better to undercorrect a patient than to overcorrect a patient.

For patients with long-standing disease and large fusional amplitudes, a small residual deviation may be perfectly well controlled, but an overcorrection will be intolerable.

Adjustable suture surgery minimizes the risk for overcorrection and undercorrection.

Perhaps the most troublesome complication is that of iatrogenic Brown syndrome, resulting in severe limitation of elevation. ^[39] Assessing superior oblique tendon intraoperatively should make this less likely.

The prognosis of trochlear nerve palsy varies depending on etiology. The best information regarding outcome comes from cases collected at the Mayo Clinic, as follows:

• Rates of complete and spontaneous resolution of microvascular fourth nerve paresis are excellent, with 89% of patients achieving resolution within 10 months. ^[40]
• Idiopathic cases also have a greater than 50% likelihood of spontaneous recovery.
• Most cases resolve within weeks to months, with the vast majority completely recovering by 6 months.
• Some cases may resolve slowly over the course of a year.
• Patients with head trauma were less likely to recover, yet 44% of these patients experienced gradual and spontaneous recovery. ^[41]
• Cases due to aneurysm or neoplasm were least likely to have functional recovery.

Because patients have good fusional abilities, surgery generally produces excellent results. Wang et al reported an “excellent” surgical outcome in 74% of patients and a “good” surgical outcome in 23% when evaluating degree and pattern of vertical deviation and degree of oblique muscle dysfunction. ^[34] Plager reported a nearly 90% success rate with his surgical algorithm. ^[33] Mitchell and Parks also reported excellent results in correcting excyclotorsion using a modified Harada-Ito procedure. ^[36]