Ischemic Forearm Exercise Test Dx

Definition: A common test used to diagnosis metabolic diseases of muscle.

Description: This test was first used by McArdle to describe the absence of elevated blood lactate levels during exercise in patients with a myophosphorylase deficiency (McArdle disease) and has subsequently been useful in detecting other storage disorders. In McArdle disease, the enzyme deficiency impairs the metabolism of glycogen (a source of energy) to lactate in muscle. Because this test is often poorly tolerated and may have hazardous consequences in patients with McArdle disease, some have advocated the use of nonischemic test methods (see Alternatives section).

Method: Ischemic forearm exercise testing requires that the patient’s non-dominant, exercised arm have the antecubital vein cannulated and then a blood pressure cuff on the same arm is inflated (>20 mm Hg above systolic pressure) to occlude venous return. The arm is then maximally exercised to exhaustion using intermittent anisometric maximal forearm contractions (i.e., squeezing a ball or inflated blood pressure cuff) to the point of ischemia or exhaustion. The blood pressure cuff is released. Blood samples are taken at baseline and then serially (1, 3, 5, and 10 minutes) after exercise.

Normal Values: Normal individuals will show at least a threefold increase in serum lactate and ammonia levels without an increase in CPK or potassium.

Abnormal Values: Patients with McArdle disease (myophosphorylase deficiency) show an increase in ammonia levels but not in serum lactate. Patients with a myoadenylate deaminase deficiency will show an increase in lactate but not ammonia levels. Although forearm ischemic testing is >90% sensitive with glycogen storage disorders, only 50% of patients with mitochondrial myopathies will show abnormalities with elevated baseline lactate levels that will normalize in response to exercise.

Confounding Factors: Results may be altered by catheter placement but not by preexercise dietary intake.

Adverse Effects: Cramping, myalgia, contractures, and, rarely, myoglobinuria or rhabdomyolysis are seen (usually in patients with glycogenolytic disorders such as McArdle disease). Compartment syndrome is rarely reported.

Indications: Useful in the evaluation of patients suspected of having McArdle disease and other like disorders of muscle metabolism.

Alternatives: A standardized nonischemic forearm exercise test has been proposed and shows promise. This uses a nonischemic grip test that can be used in patients with exercise intolerance. In this test, an isometric 70% maximum voluntary contraction of 30 seconds is followed by serial blood samples at 1, 2, 3, 4, 6, and 10 minutes post-contraction to measure lactate and ammonia levels. This alternative test takes nearly 30 minutes and was not associated without significant toxicity. Patients with McArdle disease failed to increase their blood lactate levels but did increase serum ammonia and, in some, CPK levels in response to isometric exercise.

Cost: Approximately $1400 (includes procedure, labs, interpretation)

BIBLIOGRAPHY

Hogrel JY, Laforet P, Yaou RB, et al. A non-ischemic forearm exercise test for the screening of patients with exercise intolerance. Neurology 2001;56:1733–1738. PMID:11425942

Wortmann RL, Vladutiu GD. The clinical laboratory evaluation of the patient with noninflammatory myopathy. Curr Rheumatol Rep 2001;3:310–316. PMID:11470050