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Abstract
Ocular myasthenia gravis is a not uncommon autoimmune disorder causing diplopia, ptosis, and weakness of lid closure. The predilection of myasthenia for the ocular muscles may be related to differences between limb and extraocular muscles in either physiological function or antigenicity. Clinically, ocular myasthenia can mimic any form of pupil-sparing ocular motility disorder. Dynamic abnormalities of myasthenic eye movements may reflect the primary hallmarks of the disease, which are fatigability and variability in strength, or secondary adaptive effects by the central nervous system. Tests to confirm the diagnosis include edrophonium challenge, repetitive nerve stimulation, single-fiber electromyography (EMG) of the frontalis, and assays for antibody directed against the acetylcholine receptor: all are less sensitive for ocular myasthenia than for generalized myasthenia. There is a higher incidence of other autoimmune conditions in myasthenia, notably thymoma and thyroid dysfunction. The differential diagnosis includes other diseases of the neuromuscular junction, such as Lambert-Eaton syndrome and botulism. Treatment consists of symptomatic use of acetylcholinesterase inhibitors and immunosuppression with steroids or azathioprine. Between 50 and 70% of patients with ocular myasthenia will eventually develop generalized disease: there is some retrospective data that steroids or azathioprine may reduce this by about 75%. The role of thymectomy in ocular myasthenia remains unclear.
Introduction
Myasthenia gravis is a disorder of transmission at the neuromuscular junction. The two defining clinical characteristics of myasthenic weakness are variability and "fatigability," which is weakness that worsens with exertion and improves with rest. While rare congenital forms exist, caused by genetic anomalies or deficiencies in various components of the neuromuscular junction, acquired myasthenia is an autoimmune condition, marked by antibodies directed against the nicotinic acetylcholine receptor in the motor end-plate of striated muscles.
It has no racial or geographic predilection2 and may occur at any age. Neonatal forms are rarely encountered, and the clinical course in children and infants differs from that in adults. Before age 40 the disease is more common in women. Purely ocular myasthenia, which tends to start at a slightly later age than generalized myasthenia, is more common in men.
Myasthenic weakness may affect virtually any striated muscle, including the diaphragm, limb and bulbar musculature, but the extra-ocular muscles are particularly susceptible. Ptosis and diplopia are the initial complaint in 75% and eventually develop in 90% of all myasthenic patients. Also, in a substantial minority, the manifestations of the disease continue to be limited to the eyes over many years. Thus, awareness of the ocular manifestations of this disease is critical to clinical practice.
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