Neuromuscular, cognitive and metabolic implications of McArdle Syndrome: a global overview

Abstract

Carbohydrates are the main source of body energy and they can be stored in the organism in form of glycogen and degraded when there is a need for energy. However, McArdle Syndrome patients exhibit problems to degrade glycogen due to a deficiency in myophosphorylase enzyme, making these patients intolerant to high intensity exercises because a lack of ATP available in muscle cells. It has been found muscular weakness and subsarcolemmal accumulation of glycogen in muscle fibers and in neuronal cells in McArdle Syndrome patients. In its later-onset form, it is associated to muscular dysmorphia, myoglobinuria and rhabdomyolisis. Analyzing the biochemical aspects, it is possible to notice that these patients have a low rate of ATP production due to a reduction in the availability of glucose, reducing oxidative phosphorylation. However, the metabolic "second wind" effect allows the use of other energy sources. Excessively decreased exercise-induced lactate is also characteristic of patients with McArdle Syndrome. Electromyography studies describe alterations in nerve conduction and the necessity of recruiting more motor units to contract muscle in these patients. McArdle Syndrome produces several metabolic changes in patients due to absence of myophophorylase activity. The practice of aerobic exercise acts positively in these patients probably by increasing mitochondrial metabolism.