Ocular Aspects of Myasthenia Gravis

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.