surgery until decompression has been accomplished. Likewise, eyelid surgery is usually postponed until the strabismus is treated because upper eyelid retraction may improve once the patient no longer strains to elevate the eye.
Large recessions of very tight inferior rectus muscles can cause lower eyelid retraction severe enough to require subsequent correction. Severing the lower eyelid retractors as part of the strabismus surgery has led to some success at preventing this complication. If necessary, a spacer of banked sclera or synthetic material can be placed to vertically lengthen the lower eyelid tarsus.
Thomas SM, Cruz OA. Comparison of two different surgical techniques for the treatment of strabismus in dysthyroid ophthalmopathy. J AAPOS. 2007;11(3):258–261.
Chronic Progressive External Ophthalmoplegia
Clinical features
Chronic progressive external ophthalmoplegia (CPEO) is a rare form of mitochondrial cytopathy that can affect various body systems. It usually begins in childhood with ptosis and slowly progresses to total paralysis of the eyelids and EOMs. CPEO may be sporadic or familial. Although a true pigmentary retinal dystrophy is usually absent, constricted visual fields and electrodiagnostic abnormalities can occur. The diagnosis of CPEO is confirmed when muscle biopsy results show ragged red fibers or specific alterations of mitochondrial DNA are detected. Kearns-Sayre syndrome consists of retinal pigmentary changes, CPEO, and cardiomyopathy (especially heart block).
See BCSC Section 2, Fundamentals and Principles of Ophthalmology, Section 5, NeuroOphthalmology, and Section 12, Retina and Vitreous, for additional information on these and other mitochondrial disorders.
Management
It is important to ensure that the patient’s cardiac status is evaluated, because life-threatening arrhythmias can occur in Kearns-Sayre syndrome. Treatment options for the ocular motility disorder are limited; small surgical series report a high rate of long-term undercorrections. Cautious surgical elevation (suspension) of the upper eyelids can lessen a severe chin-up head posture.
Myasthenia Gravis
Myasthenia gravis is a disorder in which antibodies directed against acetylcholine receptors cause muscle dysfunction. Onset in childhood is uncommon. A transient neonatal form, caused by the placental transfer of acetylcholine receptor antibodies of mothers with myasthenia gravis, exists but usually subsides rapidly. Another variety is not immune mediated and exhibits a familial predisposition.
The disease may be purely ocular. In its most severe form, it frequently occurs as part of a major systemic disorder that involves other skeletal muscles, especially in patients who have not received immunosuppressive therapy. Generalization to systemic myasthenia is less common in childhoodonset ocular myasthenia than in the adult-onset form.
BCSC Section 5, Neuro-Ophthalmology, discusses both the ocular and the systemic aspects of myasthenia gravis in depth. Additional information is available on the website of the Myasthenia Gravis Foundation of America, Inc (www.myasthenia.org).
Clinical features
The principal ocular manifestation of myasthenia gravis is weakening of the EOMs, including the levator muscle. Most cases (90%) exhibit both ptosis and limited ocular rotations. The ocular signs can resemble any unilateral or bilateral ophthalmoplegia, even the extremely rare isolated paralysis
of the inferior rectus muscle.
Affected muscles fatigue rapidly, so ptosis typically worsens when the patient is required to look upward for 30 seconds. In the sleep test, the patient rests in a dark room with the eyelids closed for 20–30 minutes; ptosis often subsequently resolves in patients with myasthenia gravis. The presence of the Cogan lid-twitch sign, an overshoot of the eyelid when the patient looks straight ahead after looking down for several minutes, is also highly suggestive.
External application of ice over the involved eyelids for 2–5 minutes improves function of the levator palpebrae superioris and other affected EOMs, providing a rapid and reliable method of establishing this diagnosis without the need for drug administration.
The classic edrophonium test can establish the diagnosis (Fig 12-8). Neostigmine, an alternative test medication, has the advantage of allowing more time to measure changes in alignment, as the effect of this agent begins later than that of edrophonium and is prolonged.
Figure 12-8 Myasthenia gravis. A, Bilateral ptosis (right more than left) with right hypotropia and exotropia. B, Following edrophonium injection, the eyes show orthotropia, normal eyelid position, and the lacrimation that frequently accompanies edrophonium injection.
Electromyography shows decreased electrical activity of involved muscles after prolonged voluntary innervations and increased activity (including faster saccadic velocity) after administration of edrophonium or neostigmine. Documentation of abnormalities by single-fiber electromyography or the presence of circulating anti–acetylcholine receptor or anti–muscle-specific kinase antibodies confirms the diagnosis, but a negative result does not rule it out.
Table 12-1 compares the features of thyroid eye disease with those of CPEO and myasthenia gravis.
Ortiz S, Borchert M. Long-term outcomes of pediatric ocular myasthenia gravis. Ophthalmology. 2008;115(7):1245–1248. Epub 2007 Dec 26.
Table 12-1
Management
A full discussion of treatment of the various forms of myasthenia gravis is beyond the scope of this chapter. In adults, the ocular manifestations are frequently resistant to the usual systemic myasthenia treatment. However, pediatric ocular myasthenia is often successfully managed with pyridostigmine alone. In adults and children in whom the ocular deviation has stabilized, standard eye muscle surgery can help restore binocular function in at least some gaze positions. Ptosis occasionally requires upper eyelid surgery.
Esotropia and Hypotropia Associated With High Myopia
In highly myopic patients, extremely increased axial length can cause the elongated globe to herniate between the superior and lateral rectus muscles. High-resolution MRI studies have shown stretching and dehiscence of the intermuscular septum between these 2 muscles. They also demonstrated inferior slippage of the lateral rectus pulley and other supporting tissues, along with medial displacement of the inferior rectus. These anomalies cause a progressively worsening hypotropia and esotropia. The medial rectus is often tight, exacerbating the severity of the esotropia.
Various surgical procedures have been devised to overcome the defect by stabilizing the position of the lateral rectus muscle. An effective option is a joining of the superior and lateral rectus muscles, usually with a nonabsorbable suture, to reposition the globe. Recession of the medial rectus muscle may also be needed if the muscle is tight.
Yamaguchi M, Yokoyama T, Shiraki K. Surgical procedure for correcting globe dislocation in highly myopic strabismus. Am J Ophthalmol. 2010;149(2):341–346. Epub 2009 Nov 24.
Internuclear Ophthalmoplegia
The anatomical and functional features of the medial longitudinal fasciculus (MLF) are discussed in BCSC Section 5, Neuro-Ophthalmology. The MLF integrates the nuclei of the cranial nerves governing ocular motility and has major connections with the vestibular nuclei. An intact MLF is essential for the production of conjugate eye movements. Lesions of the MLF result in a typical pattern of disconjugate movement called internuclear ophthalmoplegia (INO). Abnormalities of this pathway are frequently seen in patients with demyelinating disease, but they may also occur in patients who have had cerebrovascular accidents or brain tumors.
Clinical features
On horizontal versions, the eye ipsilateral to the MLF lesion adducts slowly and incompletely or not
at all, whereas the abducting eye exhibits a characteristic horizontal jerk nystagmus (see Chapter 13). Both eyes adduct normally on convergence. Skew deviation may be present, in addition to exotropia.
Management
If exotropia persists, medial rectus muscle resection and unilateral or contralateral lateral rectus muscle recession (to limit exotropia in lateral gaze) can help eliminate diplopia, particularly in bilateral cases.
Ocular Motor Apraxia
Ocular motor apraxia, also known as saccadic initiation failure, is a rare supranuclear disorder of ocular motility, sometimes including strabismus. The congenital form may be familial, most commonly autosomal dominant.
This condition has been associated with premature birth and developmental delay. Bilateral lesions of the frontoparietal cortex, agenesis of the corpus callosum, hydrocephalus, and Joubert syndrome (abnormal eye movements, developmental delay, microcephaly, hypoplasia of the cerebellar vermis, and retinal dysplasia, among several anomalies) also have been associated with the condition, as have type 3 Gaucher disease and ataxia-telangiectasia. Several case reports have identified mass lesions of the cerebellum that compress the rostral part of the brainstem. Neurodevelopmental evaluation and imaging of the brain are advisable for assessment of children with ocular motor apraxia, especially if there is an associated vertical apraxia.
Clinical features
In ocular motor apraxia, normal voluntary horizontal saccades cannot be generated. Instead, changes in horizontal fixation are accomplished by a head thrust that overshoots the target, followed by a rotation of the head back in the opposite direction once fixation is established. The initial thrust serves to break fixation; an associated blink serves the same purpose. Vertical saccades and random eye movements are intact, but horizontal vestibular and optokinetic nystagmus are impaired. The head thrust may improve in late childhood.
The differential diagnosis of acquired ocular motor apraxia includes conditions that affect the generation of voluntary saccades, including metabolic and degenerative diseases such as Huntington chorea. (See also BCSC Section 5, Neuro-Ophthalmology.)
Superior Oblique Myokymia
Superior oblique myokymia is a rare entity whose cause is poorly understood. Some evidence indicates that it is caused by aberrant regeneration of fourth cranial nerve fibers. Another suggested etiology is vascular compression of the nerve, which can be confirmed by MRI.
Clinical features
In superior oblique myokymia, there are abnormal torsional movements of the eye that cause diplopia and monocular oscillopsia. Usually, patients are otherwise neurologically normal. Recurrences may persist indefinitely.
Management
Treatment is not necessary if the patient is not disturbed by the visual symptoms. Various systemic medications (such as carbamazepine, phenytoin, propranolol, baclofen, gabapentin) and topical timolol have produced inconsistent results but have been advocated as first-line treatment, since some patients will benefit, at least in the short term. Effective surgical treatment requires that the superior oblique muscle be disconnected from the globe by generous tenectomy. This typically results in a