The two forms of the disorder are clinically
similar. Onset of the clinical manifestations usually occurs in the second
or third decade of life. The following triad characterizes the disorder. (1)
Early contractures affecting the Achilles tendons (resulting in toe
walking), elbows (biceps and triceps), and posterior cervical muscles
(initially resulting in limited cervical flexion, which later extends to the
whole spine) often occur before any muscle weakness is apparent. (2) Early
in the course of the disease, a slowly progressive muscle wasting and
weakness (which is rarely severe) with a distinctive humeroperoneal
distribution (i.e., proximal in the upper limbs and distal in the lower
limbs) can be detected. As the disease progresses, weakness extends to the
proximal limb girdle musculature. (3) Cardiac conduction defects may range
from sinus bradycardia to prolongation of the PR interval and complete heart
block. Atrial paralysis is almost considered pathognomonic of EDMD. A
dilated right atrium on echocardiography and isolated atrial standstill and
absent P waves on the electrocardiogram should prompt exclusion of EDMD. A
generalized cardiomyopathy has been described, resulting in progressive
cardiac failure or sudden cardiac death from heart block (which occurs in up
to 40% of affected patients). Cardiac involvement is by far the most
serious and important feature of this disorder, occurring almost exclusively
after muscle weakness has become obvious. Almost all patients show some
degree of cardiac involvement by age 30 years. Early diagnosis is crucial,
allowing pacemaker implantation before fatal arrhythmias occur. Some
patients develop a scoliosis. These patients are mentally normal.