Myopathy- McArdle type
Overview
McArdle's disease is caused by deficiency of the muscle-specific isoform of phophorylase enzyme. The enzyme splits glucose-1-phosphate from glycogen to fuel the glycolytic pathway needed for muscle activity. McArdle's disease, although being the commonest form of glycogenosis affecting skeletal muscle, is a rare disease. The disease is typically manifested in childhood/adolescence by exercise intolerance, muscle cramps/pain with the classic "second wind" phenomenon (improvement of symptoms after a short period of slowing down or rest). McArdle's disease with late-onset presentation is very rare and clinically more variable in its presentation than the early onset disorder. The disease may escape clinical diagnosis until progressive or persistent muscle weakness or atrophy occurs at advanced age. We report an atypical case of McArdle's disease diagnosed at the age of 65 years in a patient presenting with progressive fixed proximal muscle weakness with no previous history of episodic muscle dysfunction and discuss the clinical-pathological aspects of the disease
Symptoms
McArdle's disease (glycogenosis type V) is caused by deficiency of the enzyme alpha-1,4-glucan orthophosphate glycosyl transferase (myophosphorylase ). The enzyme plays a vital role in the liberation of glucose-1-phosphate from glycogen in skeletal muscles to create energy (1). There is currently no reliable data about the frequency of McArdle's disease, however the prevalence of the disease has been estimated to be 1:100,000 (2). The disease usually presents in adolescent and young adults with exercise intolerance and muscle cramps that improve after a short period of slowing down or rest, a characteristic feature known as “second wind” phenomenon (3). Late presentation of the disease in older age is rare (4) and the clinical diagnosis become difficult due to the atypical presenting symptoms (5)
Diagnosis
The clinical features depend of the phase of the disease. In childhood and adolescence the main symptom is easy fatigability. The classical features of the disease are usually manifested in the second or third decade of life by muscle pain, cramps and weakness with tachycardia and breathlessness in the first few minutes of exercise with or without transient myoglobinuria. The patient experiences a “second wind phenomenon” in which the level of perceived exertion drop after 5-10 minutes of exercise or after a short period of rest or slowing down of exercise allowing significant improvement in exercise tolerance. The second wind phenomenon is a characteristic feature of McArdle disease and is due to increased availability of extramuscular fuels, such as hepatic glucose and fatty acids to partially compensate for the blocked muscle glycogenolysis. However, some patients deny ever experiencing a second wind, but when exercised in a laboratory, the “second wind phenomenon “ can be elucidated (8). Myoglobinuria from rhabdomyolysis is a serious complication of the disease and, if severe, can cause acute renal failure requiring dialysis (12). Fixed muscle weakness become more common with increased age as a consequence of repeated episodic muscle damage (1). The diagnosis is suspected from the classical clinical history and supportive laboratory data including increased serum CK activity and flat lactate response to the forearm ischemic test. Electromyography may show myopathic changes (13). The diagnosis is confirmed by biochemical assay or absence of histochemical reaction for the enzyme activity in the muscle biopsy and or by molecular genetic testing. Regarding the wide range of the reported mutations in the PYGM gene, the muscle biopsy is a more practical method for the diagnosis. The histochemical reaction for myophosphorylase used routinely in the muscle biopsy is a valuable diagnostic tool to demonstrate the absence of myophosphorylase activity and establishes the diagnosis. However, a biopsy taken shortly after an episode of rhabdomyolysis may show enzyme activity in the regenerating fibers which contain a fetal isoform of phosphorylase encoded by a different gene expressed transiently during myogenesis (1,8). Atypical cases are rare and include fatal infantile form (14), a form associated with congenital myopathy (15) and late-onset form as in our case (13) . McArdle's disease with late-onset of symptoms is rare and is clinically more variable than the early onset disease. In those patients who do not recall exercise intolerance earlier in life, the diagnosis is obscured due to the atypical symptoms and may need several biopsies to establish the diagnosis (1,5). In our case the patient presented with fixed proximal muscle weakness of upper and lower extremities at the age of 65 years. The exact onset of his symptoms was unclear, as it had been slowly progressive with no obvious episodic muscle dysfunction earlier in his life. The atypical clinical picture of the patient and the electrophysiologic studies were not suggestive of the diagnosis. The clinical differential diagnosis was centered around an inflammatory myopathy versus cholesterol lowering agent myopathy. The diagnosis was established from the demonstration of total absence of myophosphorylase activity in two repeat muscle biopsies