Diagnosis of Myopathy
There is a national network of Muscular Dystrophy Association (MDA) clinics that specialize in diagnosing and treating myopathies. Neuromuscular specialists at these centers are familiar with current diagnostic tests and treatments for myopathy.
Often a specific myopathy diagnosis is unnecessary, unless a patient wants to know for his or her own peace of mind. The specific diagnosis is more important if the myopathy is inheritable and children are or would be at risk for the inheriting disease.
Generally, a diagnosis involves several outpatient tests to determine the specific type of myopathy. In some cases, it is necessary to wait until the disease progresses to a point at which the syndrome can be identified.
Medical History to Diagnose Myopathy
A complete family history can help determine whether the disease is an inheritable myopathy.
Clinical and Neurological Evaluation to Diagnose Myopathy
Some early symptoms of myopathy include muscle weakness, muscle pain or tenderness, muscle pain during exercise, and muscle fatigue. In some cases, patients with myopathy fall often, have trouble walking, or experience difficulty getting out of a chair.
Various signs and symptoms that can help the physician diagnose myopathy 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
- Deep tendon reflexes (the knee jerk reaction)
In some cases, the neurologist tests for the inability of a muscle to relax after it contracts (called myotonia). In this test, the doctor may have the patient squeeze his or her hand muscles very hard and then watches for signs of failure to relax.
Blood Tests to Diagnose Myopathy
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.
Types of muscle proteins include the following:
- Creatine kinase (CK)
- Lactic dehydrogenase (LDH)
- Pyruvate kinase (PK)
As the disease progresses and 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 reveals 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 to Diagnose Myopathy
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 to Diagnose Myopathy
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.