The disorder is usually inherited as an autosomal recessive trait that is caused by mutations in perlecan, a heparin-sulfate proteoglycan that is bound to the basement membranes of skeletal muscle and cartilage. Loss of function of the protein perturbs the organization of the basement membrane leading to an altered clustering of ACh esterase and abnormal expression of ion channels. Electron microscopic studies of muscle have yielded inconsistent findings: dilated T system, Z-band streaming, and dilatation of mitochondria; in addition, in the patient reported by Fariello and colleagues, muscle biopsy disclosed signs of denervation (group atrophy). In the latter case, treatment with procainamide, phenytoin, diazepam, and barbiturates was ineffective.
The condition, described by Aberfeld and coworkers, of two siblings in whom myotonia was combined with dwarfism, diffuse bone disease, and unusual ocular and facial abnormalities, and thought by them to represent a unique disorder, is probably a variant of the Schwartz-Jampel syndrome.
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