What common symptom helps distinguish between myopathy and polymyositis?

Muscle weakness with tenderness versus without tenderness is a key distinguishing symptom between myopathy and polymyositis. In polymyositis, which is an inflammatory condition that affects the muscles, patients typically experience muscle weakness that is often accompanied by tenderness and can be associated with systemic symptoms such as fatigue and malaise. This tenderness results from the inflammation affecting the muscle fibers.

In contrast, myopathy may refer to a range of muscle diseases that can cause weakness but do not typically include the same degree of tenderness due to inflammation. For example, in non-inflammatory myopathies, individuals often report muscle weakness without the associated tenderness, which leads clinicians to differentiate these conditions based on the presence or absence of tenderness alongside the muscle weakness.

The other symptoms listed, such as severe pain, muscle swelling, and rapid muscle fatigue, can occur in either condition but are not as definitive for distinguishing between them in the way that the presence of tenderness is. Thus, the symptom of muscle weakness accompanied by tenderness is particularly pivotal for making an accurate diagnosis.