Proximal myotonic myopathy: mini-review of a recently delineated clinical disorder

This mini-review describes proximal myotonic myopathy, a recently delineated, dominantly inherited disorder that is similar to but distinct from myotonic dystrophy. Proximal myotonic myopathy is not linked to the gene locus for myotonic dystrophy or to the loci of the genes of the muscle sodium and chloride channels associated with other myotonic disorders. Patients often present with myotonia and peculiar muscle pain in early adulthood and develop weakness of the thigh muscles later in life. Cataracts that are indistinguishable from those in myotonic dystrophy also occur commonly. The gene defect responsible for proximal myotonic myopathy awaits discovery. Because of the clinical similarities between proximal myotonic myopathy and myotonic dystrophy, clarification of the genetic differences will not only shed light on the pathomechanism of proximal myotonic myopathy, but may also increase our understanding of myotonic dystrophy.