The following are some of the common problems that arise in the diagnosis of muscular dystrophy: 1. The diagnosis of muscular dystrophy in a child who has just begun to walk or in whom walking is delayed. Tests of peak power on command cannot be used with reliability in small children. The most helpful points in identifying Duchenne dystrophy are (1) unusual difficulty in climbing stairs or arising from a crouch or from a recumbent position on the floor, showing greater weakness at the hips and knees than at the ankles; (2) unusually large, firm calves; (3) male sex; (4) high serum CK, aldolase, and myoglobin levels; (5) myopathic EMG; (6) biopsy findings; and (7) special methods of testing for dystrophin protein (see previous discussion).
2. The adult patient with diffuse or proximal muscle weakness of several months’ duration, raising the question of polymyositis versus dystrophy. Even biopsy may be misleading in showing a few inflammatory foci in an otherwise dystrophic picture. As a rule, polymyositis evolves more rapidly than dystrophy. It may be associated with higher CK and aldolase values than most of the dystrophies except the Duchenne and distal Miyoshi types. With these points in mind, if immunostaining of a muscle biopsy fails to reveal the diagnosis of a dystrophy, there may still be uncertainty, in which instance a trial of prednisone may be indicated for a period of 6 months. Unmistakable improvement favors polymyositis; questionable improvement (physician’s and patient’s judgment not in accord) leaves the diagnosis unsettled but suggests inclusion of body myositis or a dystrophy. Pompe disease, a treatable metabolic glycogen storage myopathy discussed elsewhere in this chapter, may simulate Becker or limb-girdle dystrophy in an adult or child. Clues to the diagnosis are early respiratory involvement, myotonic or pseudomyotonic discharges in the EMG. Muscle biopsy with immunohistochemistry establishes the diagnosis, but the standard stains may be unrevealing.
3. An adult with a very slowly evolving proximal weakness. In addition to facioscapular and limb-girdle dystrophies, myositis and inclusion body myopathy, several of the congenital polymyopathies may begin to cause symptoms or to worsen in adult years. These include central core and nemaline myopathy. Examples have been reported in the adult of mild forms of acid maltase or debrancher enzyme deficiency with glycogenosis, progressive late-stage hypokalemic polymyopathy, mitochondrial myopathy, the abovementioned Pompe disease, and carnitine myopathy. Muscle biopsy and histochemical staining of the muscle usually provide the correct diagnosis.
4. The occurrence of subacute or chronic symmetrical proximal weakness in a child or adolescent that raises the question of spinal muscular atrophy as well as of polymyositis and muscular dystrophy. EMG and muscle biopsy settle the matter by distinguishing neuropathic from myopathic changes. Some of the same problems arise in an adult with distal dystrophy.