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Acquired slow–channel syndrome: A form of myasthenia gravis with prolonged open time of the acetylcholine receptor channel
Author(s) -
Wintzen Axel R.,
Plomp Jaap J.,
Molenaar Peter C.,
van Dijk J. Gert,
van Kempen Gertrudis T. H.,
Vos Renske M.,
Wokke John H. J.,
Vincent Angela
Publication year - 1998
Publication title -
annals of neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.764
H-Index - 296
eISSN - 1531-8249
pISSN - 0364-5134
DOI - 10.1002/ana.410440412
Subject(s) - myasthenia gravis , repetitive nerve stimulation , acetylcholine receptor , thymectomy , curare , acetylcholine , stimulation , chemistry , congenital myasthenic syndrome , neuromuscular junction , neuromuscular transmission , medicine , edrophonium , endocrinology , receptor , anesthesia , neuroscience , psychology
A 32‐year‐old female presented with a 2‐year history of fluctuating generalized weakness including extraocular, bulbar, and limb muscles, suggesting myasthenia gravis, but with poor response to pyridostigmine and unusual electromyographic findings. After rest, power increased on repeated maximal contractions, followed by progressive weakness. There were decremental responses at low‐frequency stimulation, but incremental responses at high frequencies, and single stimuli evoked repetitive compound muscle action potentials. Plasmapheresis was ineffective. In a conventional assay, antibodies against acetylcholine receptors (AChRs) were borderline. However, in an assay using cells expressing mainly adult‐type human AChRs, the patient's serum was positive. Thymectomy revealed a hyperplastic thymus. An intercostal muscle specimen revealed small miniature end‐plate potentials, 0.22 ± 0.02 mV instead of 0.56 ± 0.05 mV in controls. The number of 125 I‐α‐bungarotoxin binding sites was normal. The decay time constant of end‐plate potentials was increased from 5.3 ± 0.6 msec in controls to 23 ± 3.6 msec in the patient. Ultrastructurally, there was no destruction of the end plate. Transfer of the patient's plasma to mice in vivo produced similar physiological changes in their diaphragms. We conclude that the patient has an immune‐mediated disorder, in which an antibody specific to the adult form of the AChRs alters the channel properties, reducing total current and slowing the closure. We propose the name “acquired slow‐channel syndrome” for this variant of myasthenia gravis.