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.

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.

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.

Pauci-immune (pauci- Latin: few, little) vasculitis is a form of vasculitis that is associated with minimal evidence of hypersensitivity upon immunofluorescent staining for IgG.

Normally a kidney sample will be subjected to immunofluorescence if rapidly progressive glomerulonephritis is a concern. In this case, the immunofluorescence can show three patterns: linear, granular and negative (pauci-immune). The linear and granular patterns are examples of positive immunofluorescence and are associated with Goodpasture syndrome and post-streptococcal glomerulonephritis accordingly. A negative pattern or pauci-immune pattern can be associated with systemic vasculitis such as microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis (EGPA) or granulomatosis with polyangiitis (GPA). In many cases however, it is limited to the kidney and it is thus called idiopathic.

Since it can be associated with the three systemic vasculitides mentioned above, a pauci-immune pattern finding can be associated with antineutrophil cytoplasmic antibodies (ANCA). Therefore, an ANCA test should follow a negative immunofluorescence result in order to distinguish between the above-mentioned systemic vasculitis.

Peak incidences in 50- to 60-year-olds symptoms include intermittent fever / weight loss / shortness of breath / joint pain.

Diagnosis is by complete blood count (CBC). However, in some cases, a more accurate absolute eosinophil count may be needed. Medical history is taken, with emphasis on travel, allergies and drug use. Specific test for causative conditions are performed, often including chest x-ray, urinalysis, liver and kidney function tests, and serologic tests for parasitic and connective tissue diseases. The stool is often examined for traces of parasites (i.e. eggs, larvae, etc.) though a negative test does not rule out parasitic infection; for example, trichinosis requires a muscle biopsy. Elevated serum B or low white blood cell alkaline phosphatase, or leukocytic abnormalities in a peripheral smear indicates a disorder of myeloproliferation. In cases of idiopathic eosinophilia, the patient is followed for complications. A brief trial of corticosteroids can be diagnostic for allergic causes, as the eosinophilia should resolve with suppression of the immune over-response. Neoplastic disorders are diagnosed through the usual methods, such as bone marrow aspiration and biopsy for the leukemias, MRI/CT to look for solid tumors, and tests for serum LDH and other tumor markers.

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).

Diagnostic markers include eosinophil granulocytes and granulomas in affected tissue, and antineutrophil cytoplasmic antibodies (ANCA) against neutrophil granulocytes. The American College of Rheumatology 1990 criteria for diagnosis of Churg–Strauss syndrome lists these criteria:

- Asthma

- Eosinophilia, i.e. eosinophil blood count greater than 500/microliter, or hypereosinophilia, i.e. eosinophil blood count greater than 1,500/microliter

- Presence of mononeuropathy or polyneuropathy

- Unfixed pulmonary infiltrates

- Presence of paranasal sinus abnormalities

- Histological evidence of extravascular eosinophils

For classification purposes, a patient shall be said to have Churg–Strauss syndrome (CSS) if at least four of these six criteria are positive. The presence of any four or more of the six criteria yields a sensitivity of 85% and a specificity of 99.7%.

In eosinophilic myocarditis, echocardiography typically gives non-specific and only occasional findings of endocardium thickening, left ventricular hypertrophy, left ventricle dilation, and involvement of the mitral and/or tricuspid valves. However, in acute necrotizing eosinophilic myocarditis, echocardiography usually gives diagnostically helpful evidence of a non-enlarged heart with a thickened and poorly contracting left ventricle. Gadolinium-based cardiac magnetic resonance imaging is the most useful non-invasive procedure for diagnosing eosinophilic myocarditis. It supports this diagnosis if it shows at least two of the following abnormalities: a) an increased signal in T2-weighted images; b) an increased global myocardial early enhancement ratio between myocardial and skeletal muscle in enhanced T1 images and c) one or more focal enhancements distributed in a non-vascular pattern in late enhanced T1-weighted images. Additionally, and unlike in other forms of myocarditis, eosinophilic myocarditis may also show enhanced gadolinium uptake in the sub-endocardium. However, the only definitive test for eosinophilic myocarditis in cardiac muscle biopsy showing the presence of eosinophilic infiltration. Since the disorder may be patchy, multiple tissue samples taken during the procedure improve the chances of uncovering the pathology but in any case negative results do not exclude the diagnosis.

Myositis is inflammation or swelling of the muscles. Injury, medicines, infection, or an immune disorder can lead to myositis. It is a documented side effect of the lipid-lowering drugs statins and fibrates.

The key to diagnosis is skin changes combined with blood eosinophilia but the most accurate test is a skin, fascia and muscle biopsy.

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

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.

The best imaging modality for idiopathic orbital inflammatory disease is contrast-enhanced thin section magnetic resonance with fat suppression. The best diagnostic clue is a poorly marginated, mass-like enhancing soft tissue involving any area of the orbit.

Overall, radiographic features for idiopathic orbital inflammatory syndrome vary widely. They include inflammation of the extraocular muscles (myositis) with tendinous involvement, orbital fat stranding, lacrimal gland inflammation and enlargement (dacryoadenitis), involvement of the optic sheath complex, uvea, and sclera, a focal intraorbital mass or even diffuse orbital involvement. Bone destruction and intracranial extension is rare, but has been reported. Depending on the area of involvement, IOI may be categorized as:

- Myositic

- Lacrimal

- Anterior – Involvement of the globe, retrobulbar orbit

- Diffuse – Multifocal intraconal involvement with or without an extraconal component

- Apical – Involving the orbital apex and with intracranial involvement

Tolosa–Hunt syndrome is a variant of orbital pseudotumor in which there is extension into the cavernous sinus through the superior orbital fissure. Another disease variant is Sclerosing pseudotumor, which more often presents bilaterally and may extend into the sinuses.

CT findings

In non-enhanced CT one may observe a lacrimal, extra-ocular muscle, or other orbital mass. It may be focal or infiltrative and will have poorly circumscribed soft tissue. In contrast-enhanced CT there is moderate diffuse irregularity and enhancement of the involved structures. A dynamic CT will show an attenuation increase in the late phase, contrary to lymphoma where there is an attenuation decrease. Bone CT will rarely show bone remodeling or erosion, as mentioned above.

MR findings

On MR examination there is hypointensity in T1 weighted imaging (WI), particularly in sclerosing disease. T1WI with contrast will show moderate to marked diffuse irregularity and enhancement of involved structures. T2 weighted imaging with fat suppression will show iso- or slight hyperintensity compared to muscle. There is also decreased signal intensity compared to most orbital lesions due to cellular infiltrate and fibrosis. In chronic disease or sclerosing variant, T2WI with FS will show hypointensity (due to fibrosis). Findings on STIR (Short T1 Inversion Recovery) are similar to those on T2WI FS. In Tolosa–Hunt syndrome, findings include enhancement and fullness of the anterior cavernous sinus and superior orbital fissure in T1WI with contrast, while MRA may show narrowing of cavernous sinus internal carotid artery (ICA).

Ultrasonographic findings

On grayscale ultrasound there is reduced reflectivity, regular internal echoes, and weak attenuation, in a way, similar to lymphoproliferative lesions.

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.

The French Vasculitis Study Group has developed a five-point system ("five-factor score") that predicts the risk of death in Churg–Strauss syndrome using clinical presentations. These factors are:

- Reduced renal function (creatinine >1.58 mg/dl or 140 µmol/l)

- Proteinuria (>1 g/24h)

- Gastrointestinal hemorrhage, infarction, or pancreatitis

- Involvement of the central nervous system

- Cardiomyopathy

The lack of any of these factors indicates milder case, with a five-year mortality rate of 11.9%. The presence of one factor indicates severe disease, with a five-year mortality rate of 26%, and two or more indicate very severe disease: 46% five-year mortality rate.

Diagnosis requires ruling out other potential causes. This includes ruling out vasculitis on skin biopsy.

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.

Treatment is directed toward the underlying cause. However, in primary eosinophilia, or if the eosinophil count must be lowered, corticosteroids such as prednisone may be used. However, immune suppression, the mechanism of action of corticosteroids, can be fatal in patients with parasitosis.

The prognosis of eosinophilic myocarditis is anywhere from rapidly fatal to extremely chronic or non-fatal. Progression at a moderate rate over many months to years is the most common prognosis. In addition to the speed of inflammation-based heart muscle injury, the prognosis of eosinophilc myocarditis may be dominated by that of its underlying cause. For example, an underlying malignant cause for the eosinophilia may be survival-limiting.

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.

In this table: ANA = Antinuclear antibodies, CRP = C-reactive protein, ESR = Erythrocyte Sedimentation Rate, "ds"DNA = double-stranded DNA, ENA = extractable nuclear antigens, RNP = ribonucleoproteins; VDRL = Venereal Disease Research Laboratory

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).