Myopathies are not an unusual complication of
drug therapy. The major symptoms in
drug-induced myopathies are proximal
muscle weakness, increased muscle
enzyme levels, electromyographic changes and histological lesions. Some
drug-induced myopathies are associated with neuropathy.
Drug-induced myopathies can be classified according to the presence or absence of muscular
pain and associated neuropathy. Among painless
myopathies, we can distinguish
myopathies without neuropathy (
corticosteroids),
myopathies with neuropathy (
colchicine,
chloroquine and
hydroxychloroquine) and myasthenic syndromes (
D-penicillamine,
antibiotics, beta-blockers). Among painful
myopathies, the classification is similar: painful
myopathies may or may not be associated with neuropathies. Painful
myopathies include
polymyositis (
D-penicillamine,
cimetidine,
zidovudine) and other
myopathies without
polymyositis (
clofibrate, statines,
cyclosporin). Among the painful neuromyopathies,
eosinophilia-myalgia syndrome is a recently described disorder associated with the use of
L-tryptophan. Combinations of drugs (for example, a
fibrate and a
statine or
cyclosporin and
colchicine) can induce severe
myopathies. If such drugs are used together a vigorous surveillance to detect any sign of
myopathy is warranted. Instead of classifying
drug-induced myopathies according to clinical features, a histological classification can be proposed. Many drugs can induce
vacuolar myopathy (
colchicine,
chloroquine,
amiodarone,
cyclosporin, drugs causing hypokalaemia and
lipid-lowering agents), some others cause a
mitochondrial myopathy (
zidovudine) or a necrotizing
myopathy as seen with
vincristine. Overall, several criteria for reporting
drug-induced myopathy can be recommended: lack of pre-existent muscular symptoms, a free period between the beginning of the treatment and the appearance of symptoms, lack of another cause accounting for the
myopathy, and complete or incomplete resolution after withdrawal of the treatment. Rechallenge of the treatment is not advisable because of the risk of a serious relapse. The exact mechanisms by which drugs cause
myopathies are unknown. Some cases may be due to metabolic changes, whereas others may be immune mediated. Nevertheless, the aspect these conditions have in common is the regression of the
myopathy with the discontinuation of the
drug.