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).