Proximal myotonic myopathy: Clinical, neuropathologic, and molecular genetic features

The primary genetic abnormality in myotonic dystrophy (DM) is an expansion of die CTG trinucleotide repeat on chromosome 19q. Recently, patients with similar clinical features, but without this genetic alteration, have been d esignated as proximal myotonic myopathy (PROMM). We describe two additional cases of PROMM, both of whom presented with clinical features suggestive o f myotonic dystrophy The patients had electromyographic (EMG) evidence of m yotonia, normal cardiac evaluation, and no cataracts. Genetic analysis of p eripheral blood leukocytes revealed no expansion of the trinucleotide repea t by polymerase chain reaction (PCR) and Southern blot analysis. Muscle bio psies in both cases were significant with features suggestive of myotonic d ystrophy, such as large numbers of fibers containing multiple internal nucl ei, occasional nuclear chains, and fiber atrophy although sarcoplasmic mass es and ring fibers were absent. These cases illustrate the clinical and neu ropathologic findings of PROMM and underline the importance of correlating these aspects with genetic studies in patients with myotonic muscle disorde rs. (received 15 November 2000, accepted 22 November 2000).