Among drug-induced myopathy, steroids are probably the most common cause. The risk of steroid myopathy(SM)increases with the dose and duration of use. It is typically a proximal myopathy, preferentially affecting the hip girdlemuscles. Motor and sensory nerve conduction studies are normal. The needle EMG is usually within the normal rangeor may be minimally abnormal. Occasionally, low-amplitude, short-duration MUAPs may be seen in the proximal muscles.Of note, abnormal spontaneous activity is not seen. This point is often very useful in differentiatingpolymyositis(PM) from SM. It is common for patients with PM to be treated with steroids, respond well, and then havethe steroids tapered. If muscle weakness then returns, it may be very difficult to differentiate recurrent PM from SM onclinical grounds. The presence of abundant abnormal spontaneous activity strongly suggests PM rather than SM.
