Myopathy

Published on 02/03/2015 by admin

Last modified 22/04/2025

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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.

Fig 73.1 Causes of myopathy, with associated changes in serum creatine kinase (CK).

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.

Table 73.1

Metabolic causes of myopathy

Fatty acid oxidation defects

Glycogen storage disorders

Pompe’s and Fabry’s diseases

Myoadenalate deaminase deficiency

Carnitine palmitoyl transferase deficiency

Respiratory chain (electrolyte transport) defects

Rhabdomyolysis

Muscle cells that are damaged will leak creatine kinase into the plasma. This enzyme exists in different isoforms. CK-MM or total CK is used as an index of skeletal muscle damage. Very high serum levels may be expected in patients who have been convulsing or have muscular damage due to electrical shock or crush injury. Creatine kinase concentrations may also be high in acute spells in muscular dystrophy.

The damaged muscle cells will also leak myoglobin. This compound stores oxygen in the muscle cells for release under conditions of hypoxia, as occurs during severe exercise. The dissociation curve of myoglobin is compared with haemoglobin in Figure 73.2. It delivers up its oxygen only when the PO₂ falls to around 3 kPa. When muscle cells become anoxic or are damaged by trauma, myoglobin is released into the plasma. It is filtered at the glomerulus and excreted in the urine, which appears orange or brown coloured; on urine dipstick testing myoglobin gives a false positive reaction for the presence of blood, which can lead to the mistaken diagnosis of haematuria. The damaged muscle cells also release large amounts of potassium and phosphate ions giving rise to hyperkalaemia and hyperphosphataemia; potentially serious hypocalcaemia may develop due to the binding of calcium by released intracellular organic and fatty acids.