The aim was to describe the natural history of adults with hereditary muscu
lar dystrophies, including myotonic dystrophy, with respect to muscular fun
ction, ventilation and electrocardiogram. In a prospective study, 46 subjec
ts were followed over a period of five years. In 1991 and 1996, their muscl
e function was assessed according to an observation scheme and their lung v
ital capacity was measured by spirometer, Electrocardiograms were obtained
in 1991, 1993 and 1996, Deterioration of muscular function was seen with re
gard to both the functional muscle tests and the vital capacity, The propor
tion of pathological electrocardiograms increased from 38% in 1991 to 54% i
n 1996 in the 26 patients with myotonic dystrophy without an increase in cl
inically detected cardiac abnormalities. Timely examinations using standard
methods can reveal medically important information on deterioration, which
often passes clinically unnoticed because of the insidious progress of the
diseases.