Inflammatory myopathy

Idiopathic inflammatory myopathy is a diagnosis of exclusion. There are a number of known causes of myopathy, and it is only once these have been ruled out that a clinician should assign an idiopathic inflammatory myopathy. The usual criteria for a diagnosis of PM are weakness in muscles of the head, neck, trunk, upper arms or upper legs; raised blood serum concentrations of some muscle enzymes such as creatine kinase; unhealthy muscle changes on electromyography; and biopsy findings of (i) muscle cell degeneration and regeneration and (ii) chronic inflammatory infiltrates in muscle cells. If heliotrope (purple) rash or Gottron's papules are also present, then the diagnosis is DM. In DM, myositis may not be clinically apparent but detectable via biopsy or MRI.[2] If the criteria for PM are met but muscle weakness also affects the hands and feet or is not accompanied by pain IBM should be suspected, and confirmed when muscle cell biopsy reveals (i) cytoplasmic vacuoles fringed by basophilic granules and (ii) inflammatory infiltrate comprising mostly CD8 T lymphocytes and macrophages; and electron microscopy reveals filamentous inclusions in both cytoplasm and nucleus.[3]

There have been few randomized treatment trials, due to the relative rarity of inflammatory myopathies.[4] The goal of treatment is improvement in activities of daily living and muscle strength. Suppression of immune system activity (immunosuppression) is the treatment strategy. Patients with PM or DM almost always improve to some degree in response to treatment, at least initially, and many recover fully with maintenance therapy. (If there is no initial improvement from treatment of PM or DM, the diagnosis should be carefully re-examined.) There is no proven effective therapy for IBM, and most IBM patients will need assistive devices such as a cane, a walking frame or a wheelchair. The later in life IBM arises, the more aggressive it appears to be.[5]

Every year between 2.18 and 7.7 people per million receive a diagnosis of PM or DM.[8] Around 3.2 children per million per year are diagnosed with DM (termed juvenile dermatomyositis), with an average age of onset of seven years. Diagnosis of adult DM commonly occurs between 30 and 50 years of age. PM is an adult disease, usually emerging after the age of twenty. PM and DM are more common in females, more common in Caucasians, and least common in Asians. At any given time, about 35.5 people per million have IBM; it emerges after the age of 30 (usually after 50), and may be more common in males.[4]

Myositis (inflammation of the muscle) can be caused secondary to a number of primary diseases including: infection, muscle trauma, medications and toxins, inherited muscle diseases, and autoimmune disease such as lupus. Other autoimmune diseases can mimic the symptoms of IMM (muscle weakness and autoantibodies), including Lambert-Eaton myasthenic syndrome, myasthenia gravis, and the muscle form of sarcoidosis.[1]

Although idiopathic inflammatory myopathy (IIM) is a diagnosis of exclusion, it is often the initial misdiagnosis of several acquired non-inflammatory myopathies.[1][9][10] This is due to a number of factors, including:

• overlapping symptoms (such as muscle weakness, pain, elevated CK);
• that delaying treatment for an inflammatory myopathy, in order to exclude potential non-inflammatory myopathies, may cause irreversible damage (although administering immunosuppressants and glucocorticosteroids to non-inflammatory myopathies may also cause damage);
• that many of the testing methods for idiopathic inflammatory myopathy are non-specific (the same results could apply to many different myopathies);
• and the unfamiliarity of the many rare diseases that are outside of rheumatology.[1][10]

Acquired non-inflammatory myopathies include toxin-induced myopathies, endocrine myopathies, metabolic myopathies, muscular dystrophies, congenital myasthenic syndrome, nutritional imbalances, and infection.[11][1]

• Acquired non-inflammatory myopathy
• Myositis