Unusual type of benign x-linked muscular dystrophy.

Duchenne muscular dystrophy affects mainly young boys and is usually inherited as an X-linked recessive trait. The disease begins in infancy or early childhood and is characterized by weakness of the lower limbs and pelvic girdle musculature almost invariably associated with swollen calves ('pseudohypertrophy'). The weakness gradually progresses and ultimately the child becomes confined to a wheelchair by about the age of 10 and he dies at about 20 (Walton and Nattrass, 1954). In recent years a more benign form of X-linked muscular dystrophy has been recognized by Becker (Becker and Kiener, 1955; Becker, 1957; Becker, 1962). In this type of muscular dystrophy the onset is later than in the Duchenne type and the course of the disease is also slower so that affected persons may become chair ridden only after 25 to 30 years and the average life expectancy is only slightly decreased. However, apart from the late onset and benign course of the disease, the clinical picture is similar to that of Duchenne muscular dystrophy: weakness begins in the lower limbs and pelvic girdle musculature and only later affects the upper limbs and there is pseudo-hypertrophy of the calf muscles. Others have also described families in which the disease began later and the progression was slower than in the 'classical' Duchenne type of muscular dystrophy (Levison, 1951; Lamy and de Grouchy, 1954; Walton, 1955, 1956; Blyth and Pugh, 1959; Emery, 1964). Blyth and Pugh dismissed as unlikely the possibility of modifier genes being responsible for the different clinical manifestations in the two types of muscular dystrophy for the reason that they never found severe and mild cases of X-linked muscular dystrophy in the same family. In 1961, Dreifuss and Hogan described a large family in Virginia in which eight male members were affected with a benign form of muscular dystrophy which was inherited as an X-linked recessive trait. This family has been studied again and evidence is presented which suggests that the muscular dystrophy in this family is distinct from that described by Becker and represents yet another type of X-linked muscular dystrophy. METHODS