Elevated creatine kinase (CK) levels in the blood (at most ~10 times normal) are typical in sIBM but affected individuals can also present with normal CK levels. Electromyography (EMG) studies usually display abnormalities. Muscle biopsy may display several common findings including; inflammatory cells invading muscle cells, vacuolar degeneration, inclusions or plaques of abnormal proteins. sIBM is a challenge to the pathologist and even with a biopsy, diagnosis can be ambiguous.

A diagnosis of inclusion body myositis was historically dependent on muscle biopsy results. Antibodies to cytoplasmic 5'-nucleotidase (cN1A; NT5C1A) have been strongly associated with the condition. In the clinical context of a classic history and positive antibodies, a muscle biopsy might be unnecessary.

IBM is often initially misdiagnosed as polymyositis. A course of prednisone is typically completed with no improvement and eventually sIBM is confirmed. sIBM weakness comes on over months or years and progresses steadily, whereas polymyositis has an onset of weeks or months. Other forms of muscular dystrophy (e.g. limb girdle) must be considered as well.

Diagnosis is fourfold: History and physical examination, elevation of creatine kinase, electromyograph (EMG) alteration, and a positive muscle biopsy.

The hallmark clinical feature of polymyositis is proximal muscle weakness, with less important findings being muscle pain and dysphagia. Cardiac and pulmonary findings will be present in approximately 25% of cases of patients with polymyositis.

Sporadic inclusion body myositis (sIBM): IBM is often confused with (misdiagnosed as) polymyositis or dermatomyositis that does not respond to treatment is likely IBM. sIBM comes on over months to years; polymyositis comes on over weeks to months. Polymyositis tends to respond well to treatment, at least initially; IBM does not.

Proximal muscle weakness, characteristic skin rash and elevated muscle enzymes are routinely used to identify JDM. Typical magnetic resonance imaging and muscle biopsy changes are considered the next most useful diagnostic criteria, followed by myopathic changes on electromyogram, calcinosis, dysphonia and nailfold capillaroscopy. Other useful criteria include myositis-specific or -related antibodies, nailfold capillaroscopy, factor VIII-related antigen, muscle ultrasound, calcinosis and neopterin.

Polymyositis, like dermatomyositis, strikes females with greater frequency than males.

Early diagnosis and treatment by a paediatric rheumatologist or a rheumatologist can help manage inflammation, relieve pain, and prevent joint damage. Careful examination, laboratory tests (blood and urine), and various forms of imaging like X-rays may be some of the tests conducted by a doctor.

Polymyositis and dermatomyositis are first treated with high doses of a corticosteroids

The diagnosis of dermatomyositis is based on five criteria which are also used to differentially diagnose with respect to polymyositis:

1. Muscle weakness in both thighs or both upper arms

2. Using a blood test, finding higher levels of enzymes found in skeletal muscle, including creatinine kinase, aldolase, as well as glutamate oxaloacetate, pyruvate transaminases and lactate dehydrogenase

3. Using testing of electric signalling in muscles, finding all three of the following: 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

4. 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)

5. Rashes typical of dermatomyositis, which include heliotrope rash, Gottron sign and Gottron papules

The fifth criterion is what differentiates dermatomyositis from polymyositis; the diagnosis is considered definite for dermatomyositis 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.

Dermatomyositis is associated with autoantibodies, especially antinuclear antibodies (ANA). Around 80% of people with DM 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 dermatomyositis may be classified as amyopathic dermatomyositis if only skin is affected and there is no muscle weakness for longer than 6 months according to one 2016 review, or two years according to another.

Rheumatoid factor and ANA tests are generally negative in systemic JIA.

Lab findings: anemia of chronic disease, neutrophilia, thrombocytosis, elevated acute phase reactants (ESR, CRP, ferritin).

There are a number of known causes of myopathy, and it is only once these have been ruled out that a clinician will assign an idiopathic inflammatory myopathy (IIM) syndrome to a case. 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. 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.

Of the children diagnosed with and treated for JDM, about half will recover completely. Close to 30 percent will have weakness after the disease resolves. Most children will go into remission and have their medications eliminated within two years, while others may take longer to respond or have more severe symptoms that take longer to clear up.

A common lasting effect of JDM is childhood arthritis.

Dermatomyositis 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.

There is a form of this disorder that strikes children, known as juvenile dermatomyositis (JDM).

The radiological features of myositis ossificans are ‘faint soft tissue calcification within 2–6 weeks, (may have well-defined

bony margins by 8 weeks) separated from periosteum by lucent zone and on CT, the characteristic feature is peripheral ossification’.

25% of cases progress to severe destructive arthritis. In the United States and Canada, mortality is estimated at about 4% and in Europe, mortality is estimated at 21.7%.

Despite its very similar clinical presentation to PM, IBM does not respond to the drugs that effectively treat PM, and there is no proven effective therapy for IBM. Alemtuzumab is being studied but as of May 2013 it had not demonstrated clinical effectiveness in IBM. Dysphagia (difficulty swallowing) may be improved by intravenous immunoglobulin, though more trials are needed. Non-fatiguing, systematic strength-building exercise has demonstrated benefit. Occupational and rehabilitation therapists can offer good advice on walking without falling and performing fine motor tasks, and can provide appropriate canes, braces and wheelchairs. Speech pathologists can provide advice on preventing choking episodes and reducing the anxiety of an immanent aspiration for both patients and carers.

Patient should seek a physician for skin tests. Typically, after a consultation with rheumatologist, the disease will be diagnosed. A dermatologist is also another specialist that can diagnose.

Blood studies and numerous other specialized tests depending upon which organs are affected.

Outbreaks may be measurable clinically by elevated levels of alkaline phosphatase and bone-specific alkaline phosphatase.

The treatment of juvenile arthritis includes medications, physical therapy, splints and in severe cases surgery. These treatments are focused on reducing swelling, relieving pain and maintaining full movement of joints. Children are encouraged to be involved in extra-curricular activities, physical activity when possible, and to live a "normal" life.

Distinguishing laboratory characteristics are a positive, speckled anti-nuclear antibody and an anti-U1-RNP antibody.

The differential diagnosis of Kikuchi disease includes systemic lupus erythematosus (SLE), disseminated tuberculosis, lymphoma, sarcoidosis, and viral lymphadenitis. Clinical findings sometimes may include positive results for IgM/IgG/IgA antibodies.

For other causes of lymph node enlargement, see lymphadenopathy.

Types of myositis include:

- myositis ossificans

- (idiopathic) inflammatory myopathies

- dermatomyositis

- juvenile dermatomyositis

- polymyositis

- inclusion body myositis

- pyomyositis

Calcification is typically depicted 2 weeks earlier by ultrasound (US) when compared to radiographs. The lesion develops in two distinct stages with different presentations at US. In the early stage, termed immature, it is depicts a non-specific soft tissue mass that ranges from a hypoechoic area with an outer sheet-like hyperechoic peripheral rim to a highly echogenic area with variable shadowing. In the late stage, termed mature, myositis ossificans is depicted as an elongated calcific deposit that is aligned with the long-axis of the muscle, exhibits acoustic shadowing, and has no soft tissue mass associated. US may suggest the diagnosis at early stage, but imaging features need to evolve with successive maturation of the lesion and formation of the characteristic late stage changes before they become pathognomonic.

The differential diagnosis includes many tumoral and nontumoral pathologies. A main concern is to differentiate early myositis ossificans from malignant soft-tissue tumors, and the latter is suggested by a fast-growing process. If clinical or sonographic findings are dubious and extraosseous sarcoma is suspected, biopsy should be performed. At histology, detection of the typical zonal phenomenon is diagnostic of myositis ossificans, though microscopic findings may be misleading during the early stage.

It is diagnosed by lymph node excision biopsy.

Kikuchi disease is a self-limiting illness which has symptoms which may overlap with Hodgkin's lymphoma leading to misdiagnosis in some patients.

Antinuclear antibodies, antiphospholipid antibodies, anti-dsDNA, and rheumatoid factor are usually negative, and may help in differentiation from systemic lupus erythematosus.

Dermatopolymyositis (also called PM/DM) is a family of myositis disorders that includes polymyositis and dermatomyositis.

The prognosis of mixed connective tissue disease is in one third of cases worse than that of systemic lupus erythematosus (SLE). In spite of prednisone treatment, this disease is progressive and may in many cases evolve into a progressive systemic sclerosis (PSS), also referred to as diffuse cutaneous systemic scleroderma (dcSSc) which has a poor outcome. In some cases though the disease is mild and may only need aspirin as a treatment and may go into remission where no Anti-U1-RNP antibodies are detected, but that is rare or within 30% of cases. Most deaths from MCTD are due to heart failure caused by pulmonary arterial hypertension (PAH).