Clinical Approach to Muscle Diseases

Carlayne E. Jackson, M.D.

Disclosures

Semin Neurol. 2008;28(2):228-240. 

In This Article

Other Tests

In addition to CK determinations, other blood tests that can be extremely helpful in the evaluation of a patient with a suspected myopathy include serum electrolytes, thyroid function tests, parathyroid hormone levels, and HIV tests. In patients with an inflammatory myopathy, serological determinations for systemic lupus erythematosus, rheumatoid arthritis, and other immunological markers (e.g., Jo-1 antibodies) can occasionally be useful. A urine analysis can also be performed to detect the presence of myoglobinuria. This should be suspected if the urine tests positive for blood but no red blood cells are identified.

Forearm exercise testing can be a critical part of the evaluation of a patient with a suspected metabolic myopathy. The exercise test should be performed without the blood pressure cuff because ischemic exercise may be hazardous in patients with defects in the glycolytic enzyme pathway. The test is performed by asking the patient to do isometric contractions using a hand grip dynamometer for 1.5 seconds separated by rest periods of 0.5 second for 1 minute. A resting blood sample for venous lactate and ammonia is obtained at baseline and subsequently at 1, 2, 4, 6, and 10 minutes following the completion of exercise. A threefold increase in lactate level represents a normal response. The characteristic elevation of serum lactate after exercise is absent (phosphofructokinase deficiency, myophosphorylase deficiency) or reduced (phosphoglycerate mutase deficiency). Conversely, myoadenylate deaminase deficiency results in a normal lactate rise but little or no increase in ammonia. Forearm testing is normal in all disorders of fat metabolism and also in some glycolytic disorders with fixed muscle weakness such as acid maltase deficiency. With submaximal effort, neither lactate nor ammonia will increase.[23]

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