Measurement of junctional acetylcholine receptors in myasthenia gravis: clinical correlates.

We measured the number of acetylcholine receptors (AChRs) at neuromuscular junctions (NMJs) in motor point biopsies from 76 patients with myasthenia gravis (MG) and 49 control subjects, using an alpha-bungarotoxin binding technique. The mean number of AChRs/NMJ was significantly lower (P less than 0.001) in muscles of MG patients (deltoid 0.7 +/- 0.1 X 10(7) than in controls (2.1 X 0.2 X 10(7). None of the control muscles had fewer than 0.9 X 10(7) AChRs/NMJ, and 75% of MG muscles were below that level. Conversely, 88% of controls had 1.5 X 10(7) or more AChRs/NMJ, whereas only one of the MG muscles (2.5%) was at or above that level. The reduction of AChRs correlated approximately with the clinical severity of weakness (P less than 0.02). Patients with focal weakness had reduced numbers of AChRs/NMJ in clinically strong muscles. The only other condition in which junctional AChRs were reduced was polymyositis. These findings in a large sample of patients confirm the reduction of junctional AChRs in MG, the relationship of available junctional AChRs to clinical manifestations, and the systemic nature of the disorder even in cases with focal weakness. Measurement of AChRs can be useful diagnostically in situations where other tests are inconclusive.