Dermatomyositis is a relatively rare disease marked by muscle weakness and a distinctive skin rash. Because of similarities in signs, symptoms and treatment, it's often discussed in conjunction with polymyositis. Both conditions fall into the category of inflammatory muscle diseases — myo means "muscles" in Greek; itis means "inflamed." Derma, which means "skin," implies the skin-related signs and symptoms that accompany the muscle inflammation.
Although dermatomyositis may occur at any age, it mostly affects adults in their late 40s to early 60s or children between 5 and 15 years of age. Women have it more often than men do. Dermatomyositis in children is distinct from the adult form. The disease usually develops over weeks or months.
Globally, dermatomyositis and polymyositis together affect about 5 to 10 people out of 100,000. Other inflammatory muscle diseases include inclusion body myositis, which progresses more slowly than other forms; myositis associated with other connective tissue diseases, such as lupus or scleroderma; and myositis associated with cancer (malignancy).
Periods of remission, during which signs and symptoms improve spontaneously, may occur. Treatment can improve your skin and your muscle strength and function.
The skin rash usually occurs at the same time as muscle weakness, but may precede it by a few weeks. Sometimes, the skin rash alone determines the diagnosis. In some children with dermatomyositis, the skin may become thick and hard in a way similar to scleroderma. When this happens, the condition is called sclerodermatomyositis.
Weakness in muscles such as your hips and shoulders can lead to difficulty in getting out of chairs, climbing stairs, brushing your hair or working with your arms over your head. Weakness in your neck muscles can make it hard to hold your head up.