Myotonic muscular dystrophy is the commonest disease of its kind with an incidence of 1 per 8000. It is characterized by slowly progressive weakness along with myotonia. It also involves various other body systems as described below.
It is a genetic disorder with autosomal dominant transmission and incomplete penetrance. Abnormal gene is located at chromosome 19q13.1 and has an abnormal expansion of CTG repeats. Phenomenon of anticipation is also present, which means this CTG expansion increases in successive generations leading to worsening of disease in each successive generation. There are up to 37 CTG repeats in normal individuals however affected individuals may have CTG repeats ranging from 50 to more than 1000. Longer the expansion lesser will be the life expectancy.
2. Clinical Features:
It has characteristic weakness pattern that progresses from distal to proximal, which may present as foot drop or weak and clumsy hand.
Myotonia is a state of delayed relaxation or prolonged contraction. Myotonia can be elicited by asking the patient to grip and release his fist as fast as he can, which he will not be able to do. It can also be elicited by percussing the thenar eminence muscles with tendon hammer, which will cause the thumb to flex into palm.
There is typical wasting pattern in longstanding myotonic muscular dystrophy. It leads to wasting of masseter and temporalis resulting in long thin face often known as lugubrious facies.