Calcification from<unk>  X-Ray of the knee in a person with<unk> . Micrograph of<unk> , muscle biopsy, H&E stain The diagnosis of<unk>  is based on five criteria, which are also used to differentially diagnose with respect to polymyositis: # Muscle weakness in both thighs or both upper arms # Using a blood test, finding higher levels of enzymes found in skeletal muscle, including creatinine kinase, aldolase, and glutamate oxaloacetate, pyruvate transaminases and lactate dehydrogenase # Using testing of electric signalling in muscles, finding all three of: erratic, repetitive, high-frequency signals; short, low-energy signals between skeletal muscles and motor neurons that have multiple phases; and sharp activity when a needle is inserted into the muscle # Examining a muscle biopsy under a microscope and finding mononuclear white blood cells between the muscle cells, and finding abnormal muscle cell degeneration and regeneration, dying muscle cells, and muscle cells being consumed by other cells (phagocytosis) # Rashes typical of<unk> , which include heliotrope rash, Gottron sign, and Gottron papules The fifth criterion is what differentiates<unk>  from polymyositis; the diagnosis is considered definite for<unk>  if three of items 1 through 4 are present in addition to 5, probable with any two in addition to 5, and possible if just one is present in addition to 5.<unk>  is associated with autoantibodies, especially antinuclear antibodies (ANA). Around 80% of people with<unk>  test positive for ANA and around 30% of people have myositis-specific autoantibodies which include antibodies to aminoacyl-tRNA synthetases (anti-synthetase antibodies), including antibodies against histidine—tRNA ligase (also called Jo-1); antibodies to signal recognition particle (SRP); and anti-Mi-2 antibodies. Magnetic resonance imaging may be useful to guide muscle biopsy and to investigate involvement of internal organs; X-ray may be used to investigate joint involvement and calcifications. A given case of<unk>  may be classified as amyopathic<unk>  if only skin is affected and no muscle weakness for longer than 6 months is seen according to one 2016 review, or two years according to another. Classification<unk>  is a form of systemic connective tissue disorder, a class of diseases that often involve autoimmune dysfunction. It has also been classified as an idiopathic inflammatory myopathy along with polymyositis, necrotizing autoimmune myositis, cancer-associated myositis, and sporadic inclusion body myositis. A form of this disorder that strikes children is known as juvenile<unk> .
Dermatomyositis
Scleroderma
Inclusion body myositis
Polymyositis
Scleromyositis
Dermatopolymyositis
Trichinosis
Juvenile dermatomyositis