Symptoms of glycogen and glucose metabolism
How do disorders of glycogen and glucose metabolism present clinically?
Patients who develop muscle cramps, spasms, and/or myoglobinuria with short bursts of high-intensity exercise (weight lifting or sprinting) may have a defect of glycogen metabolism known as GSD. There are 16 types of GSD (glycogenoses, types 0–15), with 11 having muscle symptoms. Most are autosomal recessive. Predominantly, these patients are well at rest and perform low-intensity exercise without difficulty because free fatty acids are the major source of energy under these conditions. The enzymatic block that interferes with the use of carbohydrates to generate ATP causes problems only when exercise reaches a level that produces anaerobic conditions. Typically, the patient starts exercising and within a few minutes the muscle fatigues (patients describe this as “hitting the wall”). In some cases (McArdle and Tarui diseases), continued exertion can result in improved exercise tolerance, referred as the “ second wind ” phenomenon. However, in many patients if they continue exercising, the muscle becomes painful and may develop a firm cramp. This can result in muscle damage and myoglobinuria. Over time, four of the GSDs (acid maltase deficiency, aldolase A deficiency, branching enzyme deficiency, and debrancher deficiency) may result in fixed weakness.
