The exact cause of dermatomyositis is unknown in modern medicine, but the disease shares many characteristics with autoimmune disorders, in which your immune system mistakenly attacks your own body tissues. Small blood vessels in muscular tissue are particularly affected in dermatomyositis. Inflammatory cells surround the blood vessels and eventually lead to degeneration of muscle fibers.

It is believed in Chinese medicine that the causative factor of this group of the diseases is "insects". Pathogenic insects and other factors of wind, dampness and heat invade the skin, fight with Wei-Defensive Qi and body fluid and gather for a long time to cause this disease. Excessive wind leads to serious pruritus. Excessive dampness causes obvious suppurative fluid. Excessive heat results in obvious red rash. When blood fails its moistening ability after a long duration of illness, the hair becomes dry and lustresless and the condition becomes lingering.