Myopathies

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Causes and Risk Factors

Inheritable myopathies are caused by a genetic defect. The most common muscular dystrophies, Duchenne and Becker MD, result from a genetic defect on the X chromosome.

Risk factors for other myopathies include the following:

• Autoimmune disorders (e.g., myasthenia gravis, scleroderma, thyroiditis)
• Endocrine disorders (e.g., Cushing syndrome, hypothyroidism, hyperthyroidism, Addison disease)
• Exposure to toxins (e.g., herbicides, insecticides, flame retardant chemicals)
• Infection (e.g., HIV, Lyme disease, trichinosis)
• Vitamin D deficiency, vitamin E or A toxicity
• Medication (e.g., some antihistamines, long-term corticosteroid use)
• Metabolic disorder (e.g., glycogen and lipid storage diseases)

Signs and Symptoms

Although symptoms depend on the type of myopathy, some generalizations can be made. Skeletal muscle weakness is the hallmark of most myopathies, with some noticeable exceptions, such as myotonia and paramyotonia congenita. In these two inheritable muscular disorders the muscles become enlarged, rather than weakened and atrophied, and do not relax after contracting.

In most myopathies, weakness occurs primarily in the muscles of the shoulders, upper arms, thighs, and pelvis (proximal muscles). In some cases, the distal muscles of the hands and feet may be involved during the advanced stage of disease.

Other typical symptoms of muscle disease include the following:

• Aching
• Cramping
• Pain
• Stiffness
• Tenderness
• Tightness

Initially, individuals may feel fatigued doing very light physical activity. Walking and climbing stairs may be difficult because of weakness in the pelvic and leg muscles that stabilize the trunk. Patients often find it difficult to rise from a chair. As the myopathy progresses, there may be muscle wasting.

Complications

If heart (cardiac) muscle is affected in later stages of disease, abnormal heart rhythms or weakness of the heart muscle (cardiomyopathy) may develop. A patient with cardiomyopathy is at risk for congestive heart failure.

When the muscles involved in breathing weaken, there may be significant breathing difficulties and increased risk for pneumonia, flu, and other respiratory infections. In severe cases, patients may require a machine that assists breathing (respirator). When swallowing muscles are affected, patients are at increased risk for choking and malnutrition.

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