Rheumatic syndromes caused by antirheumatic drugs

Muscle Toxicity of Drugs: When Drugs Turn Physiology into Pathophysiology

Drugs are prescribed to manage or prevent symptoms and diseases, but may sometimes cause unexpected toxicity to muscles. The symptomatology and clinical manifestations of the myotoxic reaction can vary significantly between drugs and between patients on the same drug. This poses a challenge on how to recognize and prevent the occurrence of drug-induced muscle toxicity. The key to appropriate management of myotoxicity is prompt recognition that symptoms of patients may be drug related and to be aware that inter-individual differences in susceptibility to drug-induced toxicity exist. The most prevalent and well-documented drug class with unintended myotoxicity are the statins, but even today new classes of drugs with unintended myotoxicity are being discovered. This review will start off by explaining the principles of drug-induced myotoxicity and the different terminologies used to distinguish between grades of toxicity. The main part of the review will focus on the most important pathogenic mechanisms by which drugs can cause muscle toxicity, which will be exemplified by drugs with high risk of muscle toxicity. This will be done by providing information on key clinical and laboratory aspects, muscle electromyography patterns and biopsy results, and pathological mechanism and management for a specific drug from each pathogenic classification. In addition, rather new classes of drugs with unintended myotoxicity will be highlighted. Furthermore, we will explain why it is so difficult to diagnose drug-induced myotoxicity, and which tests can be used as a diagnostic aid. Lastly, a brief description will be given of how to manage and treat drug-induced myotoxicity.

Drug-induced myopathy and neuropathy

We review the drugs that are known to induce muscle and/or nerve toxicity, describe the commonly encountered signs and symptoms, and discuss the mechanisms believed to be behind these iatrogenic effects. The diagnostic approach to patients suspected of having drug-induced neuromuscular disorders includes a comprehensive history and physical examination, proper blood workup, electrophysiological studies, and, if necessary, muscle or nerve biopsy. A lot of controversial issues continue to prevail when reviewing such a topic, secondary to the greatly variable levels of certainty for the association between drugs and their neuromuscular toxicity. We focus on drugs that are proven to cause these adverse effects, with less emphasis on the anecdotally suspected agents for which coincidence has not been eliminated.

Drug-induced myopathies

Drug-induced muscle disorders represent a broad clinical spectrum, from asymptomatic elevated serum creatine kinase levels to life-threatening myopathies. An increasing number of drugs have been suspected or identified as myotoxic. It would be unrealistic to attempt to list them. In clinical practice, an iatrogenic origin must be discussed in any patient presenting with muscle symptoms in as much as drug-induced myopathies are usually reversible after discontinuation of the offending compound. The clinical and histopathological features depend on the causative agent and individual susceptibility to a given compound. Apart from isolated myalgias, drug-induced myopathies can be divided into five major categories: necrotising myopathies mainly due to lipid-lowering drugs; inflammatory myopathies, including polymyositis (especially associated with thiol compounds) and maphrophagic myofasciitis; mitochondrial myopathies, mainly due to antiretroviral nucleoside analogues; corticosteroid myopathy; and various forms of painless neuromyopathies. In some cases (e.g., statin-induced myopathies), risk factors have been clearly identified and preventive measures have been recommended.

The effects of toxins on muscle

Substances known to be myotoxic are reviewed, including descriptions of the resultant clinical syndromes and the mechanisms important in their development. Certain host characteristics such as altered immune and metabolic regulation are known to modulate the effects of individual myotoxins, producing varied clinical syndromes. Evaluation procedures important in identifying a toxin-induced myopathy are described.