Myopathy

Myopathies are conditions affecting the muscles that lead to weakness and/or atrophy. They may be caused by congenital factors (as in the muscular dystrophies), by viral infection or by acute damage due to anoxia, infections, toxins or drugs. Muscle denervation is a major cause of myopathy. Muscle weakness can occur due to a lack of energy-producing molecules or a failure in the balance of electrolytes within and surrounding the muscle cell necessary for neuromuscular function.

Normal muscle that is overused will end up weak or in spasm until rested. In severe cases of overuse, especially where movements are strong and erratic as might occur during convulsions, damage to muscle cells may result. Severely damaged muscle cells release their contents, e.g. myoglobin, a condition known as rhabdomyolysis.

Muscle weakness

Muscle weakness, which may or may not progress to rhabdomyolysis, has many causes (Fig 73.1). Diagnosis of the condition will depend on the clinical picture and will include investigation of genetic disorders by enzymic or chromosomal analysis, endocrine investigations and the search for drug effects. Infective causes may be diagnosed by isolation of the relevant organism or its related antibody, but often no organism is detected. These cases, known as myalgic encephalitis (ME), post-viral syndrome or chronic fatigue syndrome, are relatively common and are now regarded as true diseases, whereas formally they were thought to be psychosomatic.