A rash usually accompanies or precedes the muscle weakness in DM.
in 1975 a set of five criteria were established to aid in the diagnosis of the disease: progressive proximal symmetrical muscle weakness, elevated muscle enzymes, abnormality on an electromyogram (EMG), abnormal muscle biopsy, and cutaneous skin disease.
High magnification micrograph of dermatomyositis from a H&E stain of a muscle biopsy.
Image by Nephron
Gottron's papules in dermatomyositis. Discrete erythematous papules.
Photos above & below from E. M. Dugan, A. M. Huber, F. W. Miller, L. G. Rider
Heliotrope. Confluent macular erythema confined to the upper eyelid, with associated periorbital edema.
Heliotrope. Confluent macular erythema - upper & lower eyelid, with associated periorbital edema.