Myotonic dystrophy. Predictive value of normal results on clinical examination.

Myotonic dystrophy (DM) is well known for its highly variable clinical picture, including the age at which symptoms are first detected. In order to assess the proportion of asymptomatic gene carriers at different ages, we have used linked DNA markers to study individuals at 50% genetic risk of DM, in whom neurological examination, slit-lamp examination and electromyography (EMG) had failed to show diagnostic signs. A total of 139 asymptomatic offspring of DM patients were studied. Our analyses identified 11 out of these 139 as probable gene carriers. Our data show that penetrance of the DM gene increases with age. After correction for the possibility of genetic recombination between the DM gene and the DNA markers used, we calculated the residual chance of carrying the DM gene as 8.3% for clinically normal offspring aged between 20 and 39 yrs. We evaluated several factors that might influence this figure. Neither the sex of the propositus nor that of the affected parent modified the risk of carrying the DM gene. Presence of aspecific lens opacities also did not correlate with the risk of having inherited the DM gene. Since a significant proportion of DM gene carriers are not detected by neurological examination, including slit-lamp examination and EMG, these results confirm the need for DNA analysis in asymptomatic offspring of DM patients.