Muscular dystrophy is caused by various genetic mechanisms. The basic defect can be mapped genetically to band Xp 21. Duchenne’s and Becker’s muscular dystrophies are X-linked recessive, and Landouzy-Dejerine dystrophy is autosomal
dominant. Erb’s dystrophy may be inherited in several ways but is usually autosomal recessive.

Exactly how these inherited defects cause progressive muscle weakness isn’t known. They may create an abnormality in the intracellular metabolism of muscle cells. The abnormality may be related to an enzyme deficiency or dysfunction or to an inability to synthesize, absorb, or metabolize an unknown substance vital to muscle function.

Duchenne’s and Becker’s muscular dystrophies lead to crippling disability and contractures. Progressive skeletal deformity and thoracic muscle weakness inhibit pulmonary function, increasing the risk of pneumonia and other respiratory infections. These diseases can also lead to such cardiac problems as arrhythmias and hypertrophy; sudden heart failure may cause death. Most patients with Duchenne’s or Becker’s muscular dystrophy die from respiratory complications.

Complications from other types of dystrophy vary with the site and severity of muscle involvement.

The patient’s family history may point to evidence of genetic transmission. If another family member has muscular dystrophy, its clinical characteristics can indicate the type of dystrophy the patient has and how he may be affected.

The patient may complain of progressive muscle weakness. The onset and characteristics of the increasing weakness vary with the type of dystrophy involved.

Duchenne’s muscular dystrophy begins insidiously. Onset typically occurs when the child is between ages 3 and 5. Weakness begins in the pelvic muscles and interferes with the child’s ability to run, climb, and walk. The disease progresses rapidly; by age 12, the child usually can’t walk.

Signs and symptoms of Becker’s muscular dystrophy resemble those of Duchenne’s muscular dystrophy, but they progress more slowly. They start after age 5, but the patient can still walk well beyond age 15—sometimes into his 40s.

Landouzy-Dejerine dystrophy—a slowly progressive and relatively benign form of muscular dystrophy—typically begins before the child reaches age 10. However, symptoms may develop during adolescence. Early symptoms include weakness of eye, face, and shoulder muscles. The patient may complain that he can’t raise his arms over his head or close his eyes completely. The patient or the patient’s parents may notice other early signs, including an inability to pucker the lips or whistle, abnormal facial movements, and the absence of facial movements when laughing or crying. Pelvic muscles weaken as the disease progresses.

Erb’s dystrophy follows a similarly slow course and commonly causes only slight disability. Onset usually occurs when the child is between ages 6 and 10 but may occur in early adulthood. Muscle weakness first appears in the upper arm and pelvic muscles.

Inspection reveals the effects of muscle weakness and eventually muscle wasting. Findings vary according to the type of dystrophy. Early in Duchenne’s and Becker’s muscular dystrophies, you may notice that the patient has a wide stance and a waddling gait. He may also display Gowers’ sign when rising from a sitting or supine position.

During the initial stage, you may notice muscle hypertrophy. As the disease progresses, however, most muscles atrophy. The calves remain enlarged because of fat infiltration into the muscle. As abdominal and paravertebral muscles weaken, you may observe posture changes. The patient develops lordosis and a protuberant abdomen. Weakened thoracic muscles may cause scapular “winging” or flaring when the patient raises his arms. Bone outlines become prominent as surrounding muscles atrophy. In later stages, you may note contractures as well as pulmonary signs, such as tachypnea and shortness of breath.