Myotonic Dystrophy Type 2, Proximal Myotonic Myopathy

Abstract

Myotonic dystrophy type 2 (DM2) or proximal myotonic myopathy (PROMM) is a multisystemic disease typically characterized by proximal muscle weakness with myotonia, cardiac manifestations, and cataract. The onset of symptoms usually occurs during adulthood, and congenital or childhood onset has never been observed. A variable clinical phenotype may be characterized by proximal limb-girdle muscle weakness, myalgia, myotonia, rare involvement of the facial muscles, tremors, cardiac arrhythmia, conduction defects, cardiomyopathy (making cardiac monitoring necessary in all patients), posterior capsular opacification, endocrinal anomalies, testicular atrophy, insulin resistance and diabetes, and central nervous system abnormalities (visuospatial defects).

The disease is caused by the expansion of a quadruplet CCTG repeat in intron 1 of the ZNF9 gene (Table 40.1). Anticipation has been reported in some families, but it is not a constant feature. There is no correlation between the number of CCTG repeats and the age of onset of the disease.