Proximal myotonic myopathy is a multisystemic disease characterised by the association of proximal muscle weakness with myotonia, cardiac manifestations and cataract. Prevalence is estimated at around 1 in 100,000 but precise values are not yet available. The disease appears to be more prevalent in Germany and the USA, suggesting a founder effect originating from North European families. Patients usually present during adulthood (between 40 and 50 years of age). There are no reports of congenital or childhood onset but a rare juvenile form of the disease has been described. Expression is variable and is characterised by: 1) a proximal motor deficit (involving the pelvic and scapular girdles) associated with frequent myalgia which is often indicative of the diagnosis (myotonia is reported in 75% of cases and involvement of the facial muscles is rare, occurring in only 12% of patients); 2) tremors that are present in between 20 and 30% of cases; 3) cardiac manifestations with arrhythmia and conduction anomalies, and in some cases cardiomyopathy, making cardiac monitoring necessary in all patients; 4) posterior capsular opacification; 5) endocrinal anomalies with hyperhidrosis, testicular atrophy, insulin resistance and diabetes; 6) in rare cases, central nervous system anomalies (visuospatial defects); 7) biochemical anomalies (hypogammaglobulinemia, cholestasis). The disease is transmitted in an autosomal dominant manner and is caused by expansion of a CCTG repeat in intron 1 of the CNBP gene (3q21). Anticipation has been reported in some families but is not a constant feature. There is no correlation between the number of CCTG repeats and the age of onset of the disease. Molecular diagnosis relies on PCR and Southern blot analysis. Proximal myotonic myopathy can be distinguished from Steinert myotonic dystrophy (see this term) by the following criteria: 1) absence of a congenital form; 2) only minimal involvement of the central nervous system (absence of hypersomnia); 3) the topography of the motor deficit: proximal deficiency without facial or bulbar involvement; 4) frequent myalgia; and 5) less severe cardiac involvement. Management and follow up should be multidisciplinary and cardiac monitoring is recommended. Several therapeutic approaches are currently being investigated in model cell systems. The prognosis depends on the extent of the cardiac involvement. The respiratory insufficiency is rarely severe. The ability to walk is often retained until 60 years of age.
Last update: July 2007