Disorders of metabolism disrupt energy supply to the muscle for its proper functioning hence leading to various types of myopathies. Important metabolic disorders affecting muscles include disorders of carbohydrate metabolism, disorders of lipid metabolism and mitochondrial dysfunction. Here we have described the disorders of carbohydrate metabolism (glycogen storage diseases) that affect functioning of muscles.
A. Types of Carbohydrate Metabolism Disorders:
There are various enzymes that participate in carbohydrate metabolism. Dysfunction or absence of these enzymes leads to disordered carbohydrate metabolism. These enzyme defects can be divided into two broad categories including those enzyme defects that cause exercise intolerance and secondly those, which cause muscle weakness. Two main muscle diseases under this category are McArdle’s disease and Pompe’s disease.
B. McArdle’s Disease:
It was described by Dr Brian McArdle for the first time in 1952. Underlying defect is myophosphorylase deficiency resulting from defect in myophosphorylase gene encoded on 11q13 and usually inherited as autosomal recessive. It is also known as type V glycogenosis.
1. Presenting Complaints:
Most common presenting complaint for McArdle’s disease is exercise intolerance. During initial stage of exercise carbohydrates stored in muscles is utilized as source of energy until lipids have started to metabolize to provide energy. In this disease due to faltered carbohydrate metabolism it is not possible to carry on exercise beyond initial stage hence exercise intolerance. However there is a second wind phenomenon according to which if patient exercises slowly and gently through the initial stage until blood flow id diverted to the muscles and lipid metabolism has started then the patient can carry on with the exercise for longer periods of time.
Although there usually is childhood history of exercise intolerance but majority of the cases present in 2nd or 3rd decade of life. These patients can present with muscle pain after minimal exercise, painful muscle contractures or episodes of passing dark colored urine (myoglobinuria, as a result of muscle damage).
There usually are no notable signs and these patients develop myopathy and its signs later in life.
Provocative Exercise Test is the first test to perform among patients suspected of having this disease. This leads to a rise in serum creatine kinase (CK) levels.
Ischemic Lactate Test shows that there is no increase in lactate and is potentially dangerous in that it can lead to muscle necrosis.
Electromyography is usually normal because painful muscle contractures are silent on electromyography.
Muscle Biopsy done in a routine way is normal or may show some necrosis and elevated glycogen. Specific muscle biopsy reveals enzyme deficiency.
Treatment is supportive with physiotherapy, appropriate exercises, gait aids or wheel chair. Treatment is aimed at minimizing complications, optimizing mobility and establishing a satisfied, gratifying life.