Forearm ischemic exercise screening test
There is evidence that ischemic exercise test can be harmful in patients with myopathies. Therefore recently non-ischemic forearm exercise test has been suggested. However in view of the long tradition of ischemic testing and the lack of sensitivity in mitochondrial myopathy the protocol for ischemic testing is given.
While the patient is at rest, blood samples are taken for baseline levels of lactate and ammonia. A blood pressure cuff is then placed over the upper arm and inflated to a pressure higher than systolic, rendering the forearm ischemic. The patient begins immediately repetitive, rapid grip exercises. Normal individuals are able to tolerate ischemic exercise for as long as 180 seconds before pain and fatigue ensues. Patients with glycogen metabolism disorders seldom exercise more than 60 seconds.
When the patient fatigues, the blood pressure cuff is released and 1 minute later blood is drawn from the exercised arm. Similar samples are taken again at 2,4,6,10,and 14 minutes following the end of the exercise.
Normal subjects exhibit a 3 to 5 fold rise in lactate and ammonia levels within 5 minutes after the end of the exercise, with a full return to baseline level about 10-15 minutes after cessation of the test. Failed lactate production suggest a metabolic block (myophosphorylase deficiency (McArdle disease)). The venous level of ammonia also rises during the test. Failure of the lactate and ammonia to rise suggests an inadequate test. Normal lactate but impaired ammonia production suggests myoadenylate deaminase deficiency or a related disorder of purine nucleotide metabolism.