There are four main types of chronic or persistent inflammatory myopathy: (i) polymyositis; (ii) dermatomyositis; (iii) necrotising autoimmune myopathy (NAM); and (iv) inclusion body myositis (IBM).
Muscle inflammation can be caused by an allergic or autoimmune reaction, exposure to a toxic substance, another disease such as cancer or a rheumatic disorder, or a virus or other infectious agent. The chronic inflammatory myopathies are thought to be autoimmune disorders, in which the body’s own immune system attacks the muscle fibres, blood vessels and connective tissues in the muscles, as well as the skin, joints and other organs. In polymyositis and IBM, the T-lymphocytes are the immune cells that cause muscle injury, while in dermatomyositis and NAM the muscle damage is caused by antibodies, in combination with proteins belonging to the complement system.
The inflammatory myopathies are a group of rare disorders that can affect both adults and children. Dermatomyositis is the most common form of myositis in children and adolescents (“Juvenile Dermatomyositis”), but a rare childhood-onset form of polymyositis can also occur in children. On the other hand, inclusion body myositis usually affects individuals over the age of 50, and is the most important muscle disease associated with aging.
General symptoms of inflammatory myopathy include slow but progressive loss of muscle strength that commences in the proximal muscles—the muscles around the shoulders and hips. Inflammation damages the muscle fibres, causing weakness, and may also affect the blood vessels that run through the muscle. Other symptoms include fatigue with walking or standing, tripping or falling, and difficulties in swallowing or breathing. Some individuals may also have muscle pain (myalgia) or tenderness, but this is uncommon. People with polymyositis may also develop arthritis, interstitial lung disease causing shortness of breath, and heart arrhythmias.
Dermatomyositis is characterised by a skin rash that precedes or accompanies the progressive muscle weakness. In some cases, the rash may be the only manifestation and muscle involvement is only mild or not apparent. The rash is patchy and characteristically develops on the face and eyelids and over the knuckles, elbows and knees, and in severe cases there may be swelling in the affected areas. Adults with dermatomyositis may experience weight loss or a low-grade fever, have inflamed lungs, and be sensitive to light. In adults dermatomyositis, unlike polymyositis, may be associated with malignant tumours of the breast, lung, ovaries, bowel or other organs, and thorough investigation for an underlying malignancy is important.
Juvenile myositis has some similarities to adult dermatomyositis and polymyositis. It typically affects children aged 2 to 15 years, with symptoms that include proximal muscle weakness and myalgia, fatigue, skin rashes, abdominal pain, fever, and contractures which prevent the joints from moving freely.
Necrotising autoimmune myopathy (NAM) can occur in adults or children and causes severe proximal or more generalised muscle weakness. It is often associated with the presence of serum antibodies of different types, and may occur in people taking statin drugs, and in some cases it may be a sign of an underlying malignant tumour of one of the internal organs.
Inclusion body myositis (IBM) is characterized by progressive muscle weakness and wasting, and often difficulty swallowing. The onset of muscle weakness is gradual – over months or years – and it affects both proximal and distal muscles, often being more severe on one side of the body than the other. Falling or weakness of the hands are often the earliest symptoms which usually begin after the age of 50, although the disease can rarely also occur in younger individuals. Unlike polymyositis and dermatomyositis, IBM is more common in men than in women.
Inflammatory myopathies can be treated. Most cases of polymyositis, dermatomyositis and NAM respond to treatment with steroids or immunosuppressive medications, but some cases are resistant and need a combination of medications, or other therapies such as intravenous immunoglobulins or monoclonal antibodies such as Rituximab. Patients with IBM do not usually respond to these treatments, or may have only transient improvement, but may benefit from an exercise program and physical therapy.