Paralytic Strabismus

Congenital Third Nerve Palsies

Most studies have found that the most common cause of isolated pediatric third nerve palsies is congenital. Children with congenital third nerve palsies usually have unilateral involvement and no other neurological abnormalities; however, some of the latter have been reported. Congenital cases are generally considered nontraumatic in origin because excessive birth trauma is not consistently found.

Most often, in congenital palsies, all the extraocular muscles innervated by the third nerve are affected, resulting in exotropia, hypotropia, and ptosis of varying amounts. Pupillary involvement can result from either a primary manifestation of the palsy (a larger pupil from deficient sphincter innervation) or aberrant regeneration (pupillary constriction with adduction or downgaze). Pupillary sparing is not a reliable indicator of congenital origin because its presence in congenital cases is inconsistent. Most children suffer a loss of binocular function and amblyopia from ptosis or constant exotropia. When ptosis is either absent or incomplete, to optimize their binocularity, affected children may develop an abnormal head posture (torticollis) consisting of chin elevation and/or a contralateral face turn to neutralize the hypotropia, ptosis, or exotropia. Another amblyogenic factor is reduced accommodation in the affected eye, in which case a bifocal lens for near vision may be beneficial.

Congenital third nerve palsy should be distinguished from congenital fibrosis of the extraocular muscles (CFEOM). Absence or hypoplasia of the third cranial nerve has been described in CFEOM, which is a congenital cranial dysinnervation disorder. Presentation may be bilateral or unilateral, with variable degrees of ptosis, vertical movement deficits, reduction of horizontal eye movements, and pupillary findings. Three forms of CFEOM have been described based on phenotype—CFEOM1, CFEOM2, and CFEOM3—but the phenotypes can overlap, so identification of the underlying genetic mutations may allow more accurate diagnosis and classification.

Acquired Third Nerve Palsies

Acquired third nerve palsies, although more common than their congenital counterpart, still are unusual. They produce an incomitant exotropia and/or hypotropia and/or ptosis. In addition, visually mature individuals report diplopia and visual confusion unless they also have significant ptosis. Torticollis (contralateral face turn and/or chin-up posture) develops if the posture neutralizes the diplopia.

The most common causes of acquired third nerve palsies in children include trauma, infection, migraine, and neoplasia. Children with a slowly progressive acquired isolated oculomotor nerve palsy and normal neuroimaging may not require more invasive investigation but should undergo repeat imaging every 2 years with the expectation of eventually detecting a small tumor, frequently a schwannoma.

The incidence of acquired third nerve palsies increases with age, and the causes are age dependent. Patients older than 60 years have the highest incidence (12.5 per 100,000), compared with 1.7 per 100,000 per year in patients younger than 60 years, predominantly because of a large increase in microvascular third nerve palsies.

Patients with an acquired third nerve palsy require further neurological evaluation because the palsy may be an ominous sign, although exceptions occur. Intracranial aneurysms are a dreaded cause of pupil-involving third nerve palsy because of the risk of a potentially fatal subarachnoid hemorrhage. However, once thought indicative of benign and non–life-threatening microvascular disease, pupillary sparing can occur even in cases caused by intracranial aneurysms. Pain and pupil involvement are unreliable in differentiating a microvascular from aneurysmal compression (see Part 9, Neuro-Ophthalmology, for specific details and recommendations for evaluating these patients).

Isolated pareses of individual muscles innervated by either branch of the third nerve have been described and generally defy neuroanatomic localization. They almost never are indicative of serious pathology, and manifestations depend on the affected muscle. Brown’s syndrome must be considered in a patient with an apparent isolated inferior oblique muscle palsy; the three-step test (discussed later) and forced-duction testing can usually distinguish between these two conditions. In patients who have craniofacial disease, isolated pareses may be caused by abnormalities of the orbit, abnormal insertion of the muscles, and congenital absence of a rectus or oblique muscle.

Surgical Treatment

Restoration of normal motility generally is not attainable except in the mildest cases, so realistic surgical goals should be made clear to the patient in the early stages of treatment. Surgical treatment should be undertaken when little, if any, expectation of additional recovery exists, usually at least 6–12 months after onset. When some medial rectus muscle function is present, a large recess–resect procedure may produce acceptable results for the horizontal deviation; this has been advocated in some patients with complete paralysis if combined with a traction suture. The resected medial rectus can be supraplaced or infraplaced to improve an associated vertical deviation.

Resections of completely nonfunctional muscles have a limited long-term effect because the nonfunctioning muscle stretches with time; therefore when no demonstrable medial rectus function exists, some other method to generate passive adducting force is necessary in addition to functional inactivation of the lateral rectus muscle. The latter requires supramaximal recession of the lateral rectus muscle to at least 16 mm from the original insertion, fixation of the lateral rectus muscle to the periosteum of the lateral orbital wall, or extirpation or large myectomy of the lateral rectus muscle. Passive adducting force can be accomplished by transposition of the superior oblique tendon to a position between the superior and the medial rectus insertion, either with or without its removal from the trochlea. Additional alternatives include nasal orbital fixation and nasal transposition of the lateral rectus muscle. Nasal orbital fixation of the eye globe may be accomplished with the use of various materials, including an orbital bone periosteal flap, a nonabsorbable suture anchored to the medial orbital wall periosteum, or a T-plate titanium anchoring system fixated to the medial orbital wall. Transposition of the lateral rectus muscle to the superior and inferior borders of the medial rectus muscle, in combination with adjustable sutures or augmented fixation sutures, may generate adducting force and eliminate the lateral rectus opposing force. Adequate splitting of the lateral rectus muscle is critical when performing nasal transposition in order to avoid optic nerve compromise.

An accompanying ptosis generally is addressed after ocular alignment has been optimized because raising the lid of a hypotropic eye unable to utilize Bell’s phenomenon to protect the cornea may produce exposure keratitis and worsen diplopia. However, ptosis repair may be necessary before strabismus surgery in visually immature children to prevent the development or worsening of amblyopia.

With isolated muscle palsies, the prognosis for an acceptable outcome is greater because only one muscle is affected. Such pareses generally are treated by weakening the antagonist muscle along with strengthening (resection or plication) of the affected muscle, provided some of its function remains. In the case of isolated inferior oblique muscle paresis, weakening of the antagonist superior oblique muscle must be done with caution because of the resulting effects on ocular torsion. In such cases, contralateral superior rectus muscle weakening can be effective.

Whenever performing surgery on multiple rectus muscles of the same eye, anterior segment ischemia (ASI) is a possible complication. This condition may result when perfusion of the anterior segment is abruptly compromised by a sudden loss of the contribution made from the anterior ciliary arteries that normally accompany the rectus muscles and penetrate the sclera at the muscle insertions. The vertical rectus muscles generally contribute more than the horizontal rectus muscles. With time, perfusion to the anterior segment is restored by augmented circulation from the posterior ciliary circulation. This lends support to the strategy of staging surgery and waiting several months to operate on additional muscles of the same eye. ASI can occur years after the initial extraocular surgery or even after surgery on only two muscles. Acute manifestations of ASI include pain, corneal edema, Descemet’s membrane folds, and anterior chamber inflammation. Late effects include iris atrophy, an eccentric pupil, and infrequently, visual loss. Although ASI usually arises only with surgery on three or four rectus muscles, circumstances such as circulatory disorders or advanced age can increase the risk with surgery on fewer muscles. Preservation of the anterior ciliary circulation during rectus muscle surgery, either with or without the use of the operating microscope, has been proposed to reduce, if not eliminate, the risk of this complication (as discussed in Chapter 11.13).