Background: Previous investigations in three families have shown that proximal myotonic myopathy (PROMM) is not linked to the gene loci for myotonic dystrophy (DM) or to the loci of the genes of the muscle sodium and chloride channels associated with other myotonic disorders. It is important to extend our clinical knowledge of this interesting new disorder by studying other families.
Patients: Thirty-five patients in 14 new families; 27 patients were examined.
Methods: Clinical examination, electromyography, muscle biopsy, DNA analysis.
Results: The following findings were noted: proximal without distal weakness of the legs (n = 21); myotonia on electromyograms (n = 23); intermittent clinical myotonia (n = 17); cataracts (n = 24) and a number of the cataracts were identical to the type in DM (n = 11); and peculiar muscle pain (n = 14). A few patients had cardiac arrhythmias, and others had elevations in the concentrations of serum gamma-glutamyltransferase. None of the patients had significant muscle atrophy. Muscle biopsy specimens showed mild myopathic changes. All patients had normal trinucleotide (cytosine, thymine, and guanine) repeat size of the DM gene in leukocyte DNA. Muscle DNA probes from three patients showed findings identical to those of their leukocyte DNA probes.
Conclusions: Proximal myotonic myopathy is a new genetic disorder similar to, but distinct from, DM. Patients suspected of having DM but with negative DNA studies may have PROMM. The gene defect for PROMM awaits discovery. Because of the similarities between PROMM and DM, this discovery will not only shed light on the pathomechanism of PROMM, but it may also increase our understanding of DM.