The first symptom is mostly muscle weakness at the height of the pelvic muscles, which results in difficulties with climbing stairs, riding a bicycle, etc. At the same time or shortly afterwards, the same complaints occur in neck and shoulder girdle muscles. Strength loss in muscles may vary, ranging from mild to severe to paralysis. More or less significant muscle pain may be present. Skin manifestations are usually present in the early phase of the disease and they may even precede the muscle problems. In some cases the muscle problems are less important than the skin manifestations. Typical skin symptoms are erythema (redness) and swelling of the skin by oedema (accumulation of excessive amount of fluid in tissues). These symptoms occur on spots exposed to sunlight, but may also be limited to some skin rashes, e.g. on the face. At the height of the extensor side of the fingers and knuckles, a typical reddish to purplish swelling of the skin, sometimes scurfy, occurs in 1 patient out of 3. Another symptom is the red-purplish discoloration of the eyelids, as well as swelling and redness around the fingernail borders, which hurts when they tap a hard surface. Inflammations of the joints with swelling, but without damage to the cartilage or bone occur. Since the heart is also a muscle, it is often involved. In most cases this damage is limited and does not cause problems. Occasionally, it can lead to significant heart impairment. Shortness of breath may occur early, either due to lung involvement or due to involvement of the respiratory muscles. Swallowing difficulties occur sometimes. Kidney involvement is rare and certainly not typical.
This disorder is mostly more serious in children than in adults. The course of the disease is characterised by a less favourable outlook, a higher mortality rate, resistance to cortisone treatment, a generalised involvement not only of the muscles and skin, but also of other organs, and especially the gastrointestinal tract, the heart and lungs. Very typical are the intestinal perforations caused by vasculitis (inflammation of the blood vessels) in the intestinal wall. Very serious generalised types (type Banker and Victor) are different from the more chronic type (type Brunstig), which only causes muscle involvement. Both types often evolve into generalized muscle fibrosis (increased formation of connective tissue in the muscle) and muscle calcification, which lead to very significant limitation of motor functions and invalidity.
There is a difference between the acute and chronic (or subacute) form. The acute form is more prevalent in young people and is characterized by a more generalized involvement of muscles, skin, the typical red-purplish discoloration of knuckles and eyelids, with possibly heart and lung damage, swallowing difficulties, vasculitis, fever, a general feeling of being ill and weight loss. This type is less frequent than the chronic (or subacute) type. An intensive and early treatment determines the outcome of the disease. If the disease is not treated, the mortality rate is 7 to 8 out of 10. An early and intensive treatment has reduced this rate to 2 out of 10. The chronic type occurs especially in women between 40 and 60
Sometimes people with dermatomyositis have tumors that are related to their dermatomyositis. That is why screening for such tumours has to be performed in the early phase of the disease. Men mostly develop a tumor e.g. in the lungs, stomach or prostate, while women are more likely to develop breast- or ovary cancer. Other types of tumors may also occur. In this group more men than women are affected, which indicates that this type is a separate entity. In general, these types of DM are very serious and often the patient dies of his DM and not of the cancer. In most cases, removing the tumour does not improve the DM.