Type 2 Fiber Atrophy

This is associated with a number of drug-induced chronic painless proximal myopathies. Clinically these myopathies are usually slowly progressive, with symmetrical proximal weakness and wasting. Several of the drugs that can cause acute or subacute painful myopathy more often produce myopathy of this type. While the more acute forms are characterized by myalgia and increased CK levels, in the chronic painless forms of myopathy CK is usually normal or only mildly increased. Electromyography may reveal myopathic changes but without the increased insertional and spontaneous activity often seen in the acute situation. Stopping the offending agent usually allows some recovery but the time course and progress mirror the chronicity of the presentation.

The two commonest causes of chronic painless myopathy causing type 2 fibre atrophy are alcohol and steroids. The type 2B fibres are particularly affected . Chronic alcohol-related neuromyopathy is the most common neuromuscular disorder in the world (Rajendram et al 2003). Chronic myotoxicity often occurs on a background of alcohol-related neuropathy. The incidence of alcoholic myopathy (40–60% of alcoholics) is about  twice as high as cirrhosis, polyneuropathy or cardiomyopathy. However, the condition evolves insidiously and the symptoms are often relatively mild, so many cases go unrecognized. Although the CK level is typically normal in the chronic form of alcohol-related myopathy, ingestion of alcohol by the patient will usually result in a rapid increase in CK level (Lane and Radoff 1981).

Steroids act at the nuclear level, interfering with protein metabolism, with an overall catabolic effect. A common difficulty is in distinguishing steroid myopathy from an underlying myopathy related to the condition being treated, e.g. inflammatory myopathy. In the latter, inflammatory cells in the muscle biopsy may still be present if the response to treatment has been poor, and sarcolemmal expression of MHC class I increased, although the effect of steroid therapy on MHC class I expression is not clear. Typical EMG changes of fiber injury and increased CK are also more likely in an inflammatory myopathy.

Causes: Type 2 muscle atrophy is seen in disuse atrophy (physical deconditioning), aging related sarcopenia, corticosteroid myopathy, Cushing syndrome, paraneoplastic myopathy, and some congenital myopathies associated with hyptonia.