A Case of McArdle Disease: Efficacy of Vitamin B6 on Fatigability and Impaired Glycogenolysis

Abstract

McArdle disease is a glycogenetic myopathy caused by a deficit of myophosphorylase inherited in an autosomal recessive pattern. Here, we report a case of McArdle disease in which fatigability was the only subjective complaint. Objective neurological findings were normal except for very mild muscle weakness in limbs and an elevated serum creatine kinase level. Ischemic forearm exercise test showed deficient glycogenolysis. In the muscle biopsy specimen, periodic acid Schiff (PAS) stained subsarcolemmal glycogen was increased and the muscle phosphorylase A activity was decreased. After administration of vitamin B6, fatigability was diminished and ischemic forearm exercise test showed improved glycogenolysis. Vitamin B6 may be beneficial for McArdle disease, especially for its easy fatigability.

Confirmation of the efficacy of vitamin B6 supplementation for McArdle disease by follow‐up muscle biopsy

“…On the other hand, the withdrawal of vitamin B 6 supplementation from a patient after 2 years of daily administration resulted in decreased exercise tolerance and increased muscle cramps,10 which suggested the efficacy of therapy with vitamin B 6 supplements. In addition, a Japanese patient with a very mild case of McArdle disease was treated with vitamin B 6 supplementation (90 mg/day) for 3 months, and the forearm exercise test showed improved glycogenolysis, as in our patient 11…”

Myopathies Related to Glycogen Metabolism Disorders

“…Given the rarity of most of the metabolic GSDs, most of the research into therapies has been done with GSD V (McArdle disease) patients [96][97][98][99][100][101][102][103][104]; however, many of the general recommendations can be applied to other myopathic GSDs with some caveats. Prevention is the mainstay of therapy, with patients being educated on the signs and symptoms of a muscle energy crisis and taking steps to avoid triggering situations such as heavy snow shoveling, prolonged isometric contractions (i.e., moving furniture or carrying heavy items), stair climbing, etc.…”

Metabolic Myopathies

“…Regular moderate aerobic training (resulting in a heart rate of no more than 60-70 % of maximal) is effective because it optimizes alternate fuel delivery and utilization [27]. Another promising therapeutic agent, at least in cases with residual phosphorylase activity, is vitamin B6, because the overall body stores of pyridoxal phosphate (PLP) are depleted in McArdle's disease due to the frequent lack of enzyme protein (to which PLP is bound) [28][29][30]. Naturally, patients should be warned about the risks of strenuous exercise and advised to seek medical attention at the first appearance of pigmenturia, especially if accompanied by oliguria.…”