Heterogeneity in Schwartz-Jampel chondrodystrophic myotonia

The Schwartz-Jampel syndrome (SJS; chondrodystrophic myotonia; McK 255,800) is a recessively inherited condition defined by myotonia, short stature, and bone dysplasia. Genetic linkage between SJS and chromosomal region 1q36-34 has been observed in several families, but the gene has not yet been identified. We studied the clinical and radiological features in 81 patients from the literature and 5 own patients trying to identify distinct subgroups. In addition, we tested genetic linkage to the SJS locus on chromosome 1 in one family with two affected sibs. We found that a group of patients have mild skeletal changes which may be secondary consequences of myotonia, while another group of patients appear to have primary bone dysplasia with myotonia. Within this latter group, there are differences in age of manifestation, clinical course and pattern of bone changes. We tentatively isolate three different types of SJS: type 1A, usually recognized in childhood, with moderate bone dysplasia, corresponding to the original descriptions of Schwartz, Jampel and Aberfeld; type 1B, similar to type 1A but recognizable at birth, with more pronounced bone dysplasia resembling Kniest dysplasia; and type 2, manifest at birth, with increased mortality and bone dysplasia resembling Pyle disease. Genetic analysis of the family with two sibs affected by SJS type 2 showed evidence against linkage to chromosome 1p36-34.

SJS is clinically and radiologically heterogeneous. The causes of heterogeneity are not known yet but are likely to include both different mutations at the SJS locus on chromosome 1 and the presence of a second SJS locus. A tentative clinico-radiological classification can be useful for the characterization of patients and the development of genotype-phenotype correlations.