Myasthenia gravis (MG) is an autoimmune disorder characterized by weakness and fatigability of skeletal muscles. Muscle weakness due to dysfunction of the neuromuscular junction (myasthenia) may be an acquired disorder, and the vast majority of patients who develop generalized myasthenia in adolescence or adulthood have autoantibodies that play a pathogenetically important role. Such antibody-mediated disease is referred to as MG.
When the symptoms of MG are isolated to the levator palpebrae superioris, orbicularis oculi, and the oculomotor muscles, it is referred to as ocular MG (OMG). Over one-half of all patients with MG initially present with isolated ptosis, diplopia, or both, and no signs or symptoms of weakness elsewhere [1-3]. Approximately 15 percent of all patients with MG have isolated OMG as the only manifestation of their disease. Although the disease pathology at the neuromuscular junction is not known to be distinct from generalized MG (GMG), there are several important and unique differences between OMG and GMG with regard to diagnosis and treatment.
This topic will discuss OMG. Other aspects of MG are discussed separately. (See "Pathogenesis of myasthenia gravis" and "Clinical manifestations of myasthenia gravis" and "Differential diagnosis of myasthenia gravis" and "Diagnosis of myasthenia gravis" and "Treatment of myasthenia gravis".)
The pathophysiology of ocular myasthenia gravis (OMG) is not known to be different than that for generalized MG (GMG). (See "Pathogenesis of myasthenia gravis".)
It is uncertain why ocular muscles are frequently involved in myasthenia and why the disease stays localized to the ocular muscles in 15 percent of cases. It has been proposed that subtle alterations in the function of extraocular muscles are more likely to produce symptoms than in limb muscles, and that the levator palpebrae, under constant activation during eye opening, may be more susceptible to fatigue. Other factors may contribute to this phenomenon [4-6]: