A 20-year-old woman complains of weakness that is worse in the afternoon, worse during prolonged activity, and improved by rest. When fatigued, the patient is unable to hold her head up or chew her food. On physical examination, she has no loss of reflexes, sensation, or coordination.
Which of the following is the likely pathogenesis of this disease?
A. Antiacetylcholine receptor antibodies causing neuromuscular transmission failureThe disease process described is myasthenia gravis, a neuromuscular disease marked by muscle weakness and fatigability. Myasthenia gravis results from a reduction in the number of junctional acetylcholine receptors as a result of autoantibodies. Antibodies cross-link these receptors, causing increased endocytosis and degradation in lysosomes. A decreased number of available acetylcholine receptors results in decreased efficiency of neuromuscular transmission. Successive nerve impulses result in the activation of fewer muscle fibers and produce fatigue. Myasthenia presents with weakness and fatigability, particularly of cranial muscles, causing diplopia, ptosis, nasal speech, and dysarthria. Asymmetric limb weakness also occurs. Diseases of the central nervous system (poliomyelitis, Friedreich ataxia, or multiple sclerosis, as in choices b, c, and d) cause changes in reflexes, sensation, or coordination. ALS, a pure motor disorder, causes fasciculations and muscle atrophy as a result of lower motor neuron involvement. McArdle disease, a glycogen storage disease, causes muscle cramping and occasionally rhabdomyolysis with heavy exertion but only very rarely with usual daily activities. Ten percent of myasthenia patients have thymic tumors. Surgical removal of a thymoma is necessary because of local tumor spread. Even in the absence of tumor, 85% of patients clinically improve after thymectomy. It is now consensus that thymectomy be performed in all patients with generalized MG who are between puberty and age 55.