Contributor: Edward H. Bossen
Myasthenia gravis is a neuromuscular disease characterized by muscle weakness due to an impairment of the acetylcholine receptor function at the neuromuscular junction. Most cases are a manifestation of an autoimmune disease. Several inherited types result from genetic defects involving the acetylcholine receptor or XXXXXXXXX. The muscular weakness involves many muscles including the levator palpebrae superioris. Thymoma is present in 10% of patients with myasthenia gravis. Myasthenia gravis is a characterized clinically by increasing weakness with repetitive muscle use, but improved by anti-cholinesterase drugs.
Estimates of the incidence for myasthenia gravis range from 2-10/100,000, with 20% of the cases beginning in patients < 20 years old. It is most commonly a generalized disease, with 69-90% of the patients having extraocular muscle involvement, but in 13-14% of the cases there is only ptosis and/or ophthalmoplegia . Approximately one-half of patients with ocular myasthenia will become generalized in <2 years. Individuals who have pure ocular myasthenia who develop generalized myasthenia gravis do so within three years after onset of their ocular diseases.
In adults generalized myasthenia gravis affects females twice as often as males. The onset, usually between the ages of 20 and 35 years, may be gradual or sudden, and may be precipitated by an illness or emotional disturbance. The course is variable, with most deaths occurring during the first year and rarely after having the disease for 10 years.
Involvement of the extraocular muscles is manifest by ptosis and external ophthalmoplegia, which may be alternating and recurrent . Pure ocular myasthenia is slightly more common in males, but the female to male ratio in patients <20 years of age is 2:1. The extraocular muscles might be particularly vulnerable because of a lower safety factor for neuromuscular transmission which is due to the rapidity of firing and the simplified endplates normally present in these muscles. The fetal isoforms of ACR present in the multiterminal fibers of extraocular muscles may make the eye muscles more susceptible. Myasthenia gravis may be mistaken for progressive external ophthalmoplegia or a focal lesion of the medial longitudinal fasciculus. Internal ophthalmoplegia, characterized by dilated unequal pupils which respond to anticholinesterase agents, is very rare
Patients over the age of 40 years are most likely to have other autoimmune diseases. Patients treated with d-penicillamine for rheumatoid arthritis or Wilson disease may also develop myasthenia gravis.