Definition

Muscular dystrophy is a heterogeneous set of diseases that includes fascioscapulohumeral dystrophy, limb-girdle dystrophy, Becker muscular dystrophy (BMD), Duchenne muscular dystrophy (DMD), and others. DMD, also known as pseudohypertrophic muscular dystrophy, is the most common and most severe form.

Pathophysiology

Duchenne muscular dystrophy is an inherited, sex-linked recessive disease. The disease presents in early childhood between 2 and 6 years of age. It is clinically evident in boys and has an incidence of one in 3500 live male births. Girls and women are generally unaffected but are carriers of the disorder. Mental retardation, of varying degrees, occurs in about 30% of patients with DMD. Death often occurs in late adolescence or early adulthood and is usually caused by respiratory failure.

Clinical manifestations

Patients with DMD experience an infiltration of fibrous and fatty tissue into the muscle followed by a progressive and painless degeneration and necrosis of muscle fibers. Muscle weakness ends with muscle destruction.

Duchenne muscular dystrophy is characterized by an unremitting weakness and a steady deterioration of the proximal muscle groups of the pelvis and shoulders. The child exhibits a clumsy, waddling gait and falls frequently. Weakness of the pelvic girdle leads to the classic finding of the Gower sign, in which patients use their hands to climb up their legs to arise from the floor. A steady deterioration of muscle strength forces most of these boys to start using wheelchairs by the age of 8 to 12 years.

Skeletal muscle atrophy is usually preceded by fat and fibrous tissue infiltration, resulting in pseudohypertrophy. The infiltrative process is most apparent in the calf muscles, which become particularly enlarged.

Degeneration of respiratory muscles occurs and leads to a restrictive type of ventilatory impairment. Unopposed action by healthy, nondystrophic axial muscles predisposes these patients to kyphoscoliosis, which further decreases the pulmonary reserve. Decreasing muscle strength also results in ineffective cough, impaired swallowing, and an inability to mobilize secretions.

More progressive forms of the disease affect not only skeletal muscle but also smooth muscle of the alimentary tract and cardiac muscle. Alimentary tract involvement can lead to intestinal hypomotility, delayed gastric emptying, and gastric dilation.

Myocardial involvement occurs in almost all patients with progressive disease. Myocardial disease includes fibrotic changes localized primarily to the left ventricle. Echocardiography can effectively evaluate left ventricular function in patients with DMD. Clinical symptoms of heart failure do not usually appear unless the patient is severely stressed or until advanced stages of the disease.

Electrocardiographic changes characteristic of preclinical cardiomyopathy include a large or polyphasic R wave in lead V₁, deep Q waves in the lateral precordial leads (V₄ to V₆), premature beats (atrial and ventricular), and labile sinus or atrial tachycardia.

Although often severe, the compromised cardiac and respiratory conditions may be masked by the limited activity imposed by the patient’s skeletal myopathy. Added stress, such as that produced by surgery and anesthesia, may suddenly increase cardiorespiratory demand and uncover the weakened cardiac and respiratory states.