Congenital paucity of secondary synaptic clefts (CPSC) syndrome in 2 adult sibs.

We studied 2 elderly sibs with a congenital form of myasthenia who had ptosis since early childhood. The extraocular muscles were weak and the proximal limb muscles became slowly weaker throughout life. Laboratory investigations of biopsies of intercostal muscle from these patients showed the following abnormalities: the amplitude of miniature end-plate potentials was small and the binding of 125I-alpha-bungarotoxin at the end-plate area was reduced, suggesting a considerable reduction of acetylcholine receptors (AChRs). Secondary synaptic clefts were scarce, whereas the number of end-plates per muscle fiber was increased. There was no indication of impaired transmitter release as the quantal content was within the range of controls. We conclude that these patients suffered from the congenital paucity of secondary synaptic clefts (CPSC) syndrome, described recently in 2 cases of myasthenic children, and suggest that the CPSC syndrome is a developmental disorder in which a deficiency of AChRs may be caused by a decreased clustering or insertion of AChRs. The increased number of end-plates per muscle fiber in both patients could serve as a compensatory mechanism.