Ischemic exercise test: failure to detect partial expression of McArdle's disease

The forearm ischemic exercise test (FIET) is a very important clinical tool for evaluation of patients suspected to have a metabolic disorder of muscle function. Failure to elevate lactate in blood washed out from ischemically exercised muscles confirms an abnormality of glycolytic metabolism. A normal increase of lactate on FIET is assumed to rule out a disorder of glycolytic metabolism. Two patients with low levels of muscle phosphorylase are presented who produced a normal elevation of lactate on FIET. This suggests that, although the FIET is capable of identifying patients with absence of myophosphorylase, the test cannot identify patients with a partial expression of that disorder. Evaluation of the blood samples for ammonia in addition to lactate can be a significant help in confirming that the exercise performed by the patient was an adequate challenge for the glycolytic pathway. In addition, failure to elevate ammonia on ischemic exercise can identify a relatively new disorder of muscle metabolism.