Profiles of neuromuscular diseases. Myotonic dystrophy

Ninety-two individuals with myotonic dystrophy (MD) were evaluated prospectively over a 10-yr period and separated into two types, 75 noncongenital (NC-MD) and 17 congenital (C-MD) MD. Muscle weakness was relatively mild and similar in both types, 4.0 +/- 0.7 manual muscle test (MMT) scores for NC-MD and 3.8 +/- 0.7 in C-MD. However, weakness was progressive in the former, -0.36 MMT units per decade, and nonprogressive in C-MD. Weakness was usually generalized in both types, with no significant differences between upper and lower extremities or the proximal and distal muscles. Flexor and extensor differences were variable. Quantitative strength measurements showed a similar pattern but were more sensitive showing marked strength losses of 40-50% in muscle groups with MMT scores of four or more. There was a high frequency (47%) of relatively mild, nonprogressive scoliosis in C-MD, whereas spine deformity was unusual in NC-MD. Contractures, usually at the ankles, were also more common in C-MD. In NC-MD and C-MD, respectively, there was a low frequency of severe restrictive lung disease (14 and 20%) but a high percentage of significant electrocardiographic (ECG) abnormalities (75 and 81%), including conduction defects. There was a marked difference between the two types of MD in intellectual and cognitive function. Seventy-five percent of C-MD subjects showed impairment, frequently severe, compared with 35% impairment, usually mild, for NC-MD individuals. Functional evaluation was not markedly affected, but timed motor performance showed significant disability especially for individuals with C-MD.