Myopathies

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Diagnosis

There is a national network of Muscular Dystrophy Association (MDA) clinics that specialize in diagnosing and treating myopathies. Anyone who suspects that they or their child may have a myopathy should visit one of these centers or find a neuromuscular specialist who is familiar with current diagnostic tests and treatments.

Often a specific diagnosis is unnecessary, unless a patient wants to know for their own peace of mind. They may want to know if the myopathy is inheritable and if their children are or would be at risk for the disease.

Generally, diagnosis involves several outpatient tests to determine the type of myopathy. Sometimes it is necessary to wait until the disease progresses to a point at which the syndrome can be identified.

Medical History
A complete family history will help determine whether the disease is an inheritable myopathy.

Clinical and Neurological Evaluation
Some of the early symptoms associated with myopathies include muscle weakness, muscle pain or tenderness, muscle pain during exercise, and muscle fatigue. Some may fall a lot, have trouble walking, or may experience difficulty getting out of a chair.

Various signs and symptoms the physician looks for include the following:

• Endocrine abnormalities
• Heart problems
• Mental dysfunction
• Muscle weakness that occurs in any particular pattern
• Muscular shrinkage (atrophy)
• Skin rash

The neurological exam involves testing the following:

• Ability to rise from sitting
• Ability to walk
• Coordination
• Deep tendon reflexes (the knee jerk reaction)

In some cases the neurologist tests for the inability of a muscle to relax after it contracts (myotonia) by having a patient squeeze their hand muscles very hard and watching for signs of failure to relax.

Blood Tests
A serum enzyme test measures how much muscle protein is circulating in the blood. Usually, a serum enzyme test is helpful only at the early stages of the disease, when the sudden increase of protein level in the blood is conspicuous.

These proteins include

• creatine kinase (CK),
• lactic dehydrogenase (LDH),
• and pyruvate kinase (PK).

Later, as muscle tissue wastes away, there is less and less protein to circulate and the amount in the blood drops to a normal level. The CK level is especially important in diagnosing Duchenne MD and the metabolic myopathies.

The level of potassium in the blood helps diagnose periodic paralysis.

When an endocrine myopathy is suspected, appropriate blood tests are performed to detect hormone excesses or deficiencies. For example, thyroid function testing would reveal hyper- or hypothyroidism.

Antibodies found in the blood might indicate an inflammatory myopathy.

DNA may be collected from the blood to evaluate whether one of the known genetic defects is present.

Electromyogram
An electromyogram (EMG) measures the electrical activity of the muscle. It involves placing a tiny needle into the muscle and recording the muscular activity on a TV monitor (oscilloscope). This helps identify which muscles are weakened. It is especially helpful for diagnosing myotonia and paramyotonia congenita.

Muscle Tissue Biopsy
A muscle biopsy involves surgically removing a very small amount of tissue to be examined under the microscope and analyzed for cellular and protein abnormalities. Biopsy is especially helpful for diagnosing central core disease, nemaline myopathy, and myotubular myopathy.

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