The presentation of muscle weakness is similar to that of polymyositis, but the denominative feature is the skin changes. Most often, the skin changes precede the muscle syndrome and take the form of a localized or diffuse erythema, maculopapular eruption, scaling eczematoid dermatitis, or exfoliative dermatitis. Sometimes, skin and muscle changes evolve together over a period of 3 weeks or less. A characteristic form of the skin lesions are patches of a scaly roughness over the extensor surfaces of joints (elbows, knuckles, and knees) with varying degrees of pink-purple coloration. Red, raised papules may be present over exposed surfaces such as the elbows, knuckles, and distal and proximal interphalangeal joints (Gottron papules—applied in some writings to all the skin changes over the knuckles and extensor prominences); these are particularly prominent in DM of childhood. Also typical is a lilac- colored (heliotrope) change in the skin over the eyelids, on the bridge of the nose, on the cheeks, and over the forehead; it may have a scaly component. Itching may be a troublesome symptom in regions of the other skin eruptions. A predominance of rash over the neck and upper shoulders has been termed the V sign, while rash over the shoulders and upper arms, the shawl sign. This distribution suggests that the skin changes reflect heightened photosensitivity (a feature shared with pellagra). Periorbital and perioral edema are additional findings but mainly in fulminant cases. Skin changes may be transient and in some instances are restricted to 1 or more patches of dermatitis; they are difficult to appreciate in dark-skinned individuals. Evanescent and restricted skin manifestations are emphasized because they are frequently overlooked and provide clues to diagnosis. In the healing stage, the skin lesions leave whitened atrophic scars with a flat, scaly base. Dilated capillary loops at the cuticular nail beds may be seen but are more characteristic of the childhood type discussed further on.
In contrast to PM, DM affects children and adults about equally. Among adults, DM is more frequent in women, whereas in childhood males and females are affected equally.
Other physical signs include periarticular and subcutaneous calcifications that are common in the childhood form. Signs of associated connective tissue disease are more frequent than in pure PM (see further on). The Raynaud phenomenon has been reported in nearly one-third of the patients and a similar number have dilated or thrombosed nail fold capillaries. Whether this signifies the presence of a systemic autoimmune tissue disease has not been clarified. Others subsequently develop a mild form of scleroderma, and an associated esophageal weakness is demonstrated by fluoroscopy in up to 30 percent of all patients. The superior constrictors of the pharynx may be involved, but cinefluoroscopy may be necessary to demonstrate the abnormality.
In the strictest sense, this is an idiopathic subacute or chronic and symmetrical weakness of proximal limb and trunk muscles without dermatitis. The pattern of weakness is similar and most comments pertaining to DM given above also apply to PM. The difference is the rash and associated skin changes, which by definition are absent in this disorder. In question is the frequency of this disorder as an independent entity, with some authorities questioning its existence.