Myopathy
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.
Investigation
In all cases of muscle weakness, serum electrolytes should be checked along with creatine kinase (CK). A full drug history should be taken to exclude pharmacological and toxicological causes, and a history of alcohol abuse should be excluded. Neuromuscular electrophysiological studies should be performed to detect neuropathies. Where a genetic or metabolic cause is suspected (Table 73.1), specialist laboratories should be involved in the investigations at an early stage. Investigations include measurement of plasma (and CSF) lactate and specialist metabolic tests in blood, CSF and urine; muscle biopsy for histopatological studies and measurement of muscle enzymes may also be indicated. In contrast to rhabdomyolysis, serum CK may sometimes be normal in myopathic disorders, especially in the chronic setting, and if muscle mass decreases.
