This indicated an autosomal dominant inheritance pattern. Three groups of clinical disorders, each caused by a different genetic defect and transmitted by autosomal dominant inheritance, have myotonia as a prominent clinical feature. We initially considered myotonic dystrophy which is the most common myotonic disorder and the one most frequently associated with weakness. Although there was no atrophy or weakness of the distal muscles, which is frequently seen in myotonic dystrophy, the EMG showed both myotonia and myopathic features. Myopathy is more commonly associated with myotonic dystrophy than with the channelopathies.
Myotonic dystrophy is a multi-system disease with highly variable clinical manifestations.
However, the chest pain was suggestive of possible cardiac involvement. The cardiac conduction system is frequently affected resulting in sinus bradycardia, atrioventricular block, and bundle branch block. Also, the younger onset in the patient compared to his aunt and mother was suggestive of anticipation, a clinical phenomenon frequently observed in myotonic dystrophy where there is earlier onset of disease symptoms of greater severity in successive generations. The cardiac work up which consisted of an EKG, echocardiogram, holter monitor, and exercise stress test was completely normal. The DNA analysis did not demonstrate an expanded trinucleotide repeat sequence on chromosome 19 which is seen in myotonic dystrophy.