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

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.

Polymyositis is an inflammatory myopathy mediated by cytotoxic T cells with an as yet unknown autoantigen, while dermatomyositis is a humorally mediated angiopathy resulting in myositis and a typical dermatitis.

The cause of polymyositis is unknown and may involve viruses and autoimmune factors. Cancer may trigger polymyositis and dermatomyositis, possibly through an immune reaction against cancer that also attacks a component of muscles.

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.

Types of myositis include:

- myositis ossificans

- (idiopathic) inflammatory myopathies

- dermatomyositis

- juvenile dermatomyositis

- polymyositis

- inclusion body myositis

- pyomyositis

Eosinophilia can be idiopathic (primary) or, more commonly, secondary to another disease. In the Western World, allergic or atopic diseases are the most common causes, especially those of the respiratory or integumentary systems. In the developing world, parasites are the most common cause. A parasitic infection of nearly any bodily tissue can cause eosinophilia.

Diseases that feature eosinophilia as a sign include:

- Allergic disorders

- Asthma

- Hay fever

- Drug allergies

- Allergic skin diseases

- Pemphigus

- Dermatitis herpetiformis

- IgG4-related disease

- Parasitic infections

- Addison's disease and stress-induced suppression of adrenal gland function

- Some forms of malignancy

- Acute lymphoblastic leukemia

- Chronic myelogenous leukemia

- Eosinophilic leukemia

- Clonal eosinophilia

- Hodgkin lymphoma

- Some forms of non-Hodgkin lymphoma

- Lymphocyte-variant hypereosinophilia

- Systemic mastocytosis

- Systemic autoimmune diseases

- Systemic lupus erythematosus

- Kimura disease

- Eosinophilic granulomatosis with polyangiitis

- Eosinophilic fasciitis

- Eosinophilic myositis

- Eosinophilic esophagitis

- Eosinophilic gastroenteritis

- Cholesterol embolism (transiently)

- Coccidioidomycosis (Valley fever), a fungal disease prominent in the US Southwest.

- Human immunodeficiency virus infection

- Interstitial nephropathy

- Hyperimmunoglobulin E syndrome, an immune disorder characterized by high levels of serum IgE

- Idiopathic hypereosinophilic syndrome.

- Congenital disorders

- Hyperimmunoglobulin E syndrome

- Omenn syndrome

- Familial eosinophilia

The underlying cause of JDM is unknown. It most likely has a genetic component, as other autoimmune disease tend to run in the families of patients. The disease is usually triggered by a condition that causes immune system activity that does not stop as it should, but the trigger is almost certainly not the cause in most cases. Common triggers include immunizations, infections, injuries, and sunburn.

The cause is unknown, but it may result from an initial viral infection or cancer, either of which could raise an autoimmune response.

Between 7 and 30% of dermatomyositis arise from cancer, probably as an autoimmune response. The most common associated cancers are ovarian cancer, breast cancer, and lung cancer. 18 to 25% of people with amyopathic DM also have cancer.

Some cases are inherited, and HLA subtypes HLA-DR3, HLA-DR52, and HLA-DR6 seem to create a disposition to dermatomyositis.

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.

Dermatomyositis (DM) is a long term inflammatory disorder which affects muscles. Symptoms generally a skin rash and worsening muscle weakness over time. Symptoms may occur suddenly or develop over months. Other symptoms may include weight loss, fever, lung inflammation, or light sensitivity. Complications may include calcium deposits in muscles or skin.

The cause is unknown. Theories include that it is an autoimmune disease or a result of a viral infection. It is a type of inflammatory myopathy. Diagnosis is typically based on some combination of symptoms, blood tests, electromyography, and muscle biopsies.

While there is no cure for the condition, treatments generally improve symptoms. Treatments may include medication, physical therapy, exercise, heat therapy, orthotics, and assistive devices, and rest. Medications in the corticosteroids family are typically used with other agents like methotrexate or azathioprine recommended if steroids are not working well. Intravenous immunoglobulin may also improve outcomes. Most people improve with treatment and in some the condition resolves completely.

About 1 per 100,000 people per year are newly affected. The condition usually occurs in those in their 40s and 50s with women being affected more often than men. People of any age, however, may be affected. The condition was first described in the 1800s.

The exact cause of IOI is unknown, but infectious and immune-mediated mechanisms have been proposed. Several studies have described cases where onset of orbital pseudotumor was seen simultaneously or several weeks after upper respiratory infections. Another study by Wirostko et al. proposes that organisms resembling Mollicutes cause orbital inflammation by destroying the cytoplasmic organelles of parasitized cells.

Orbital pseudotumor has also been observed in association with Crohn’s disease, systemic lupus erythematosus, rheumatoid arthritis, diabetes mellitus, myasthenia gravis, and ankylosing spondylitis all of which strengthen the basis of IOI being an immune-mediated disease. Response to corticosteroid treatment and immunosuppressive agents also support this idea.

Trauma has also been seen to precede some cases of orbital pseudotumor. However, one study by Mottow-Lippe, Jakobiec, and Smith suggests that the release of circulating antigens caused by local vascular permeability triggers an inflammatory cascade in the affected tissues.

Although these mechanisms have been postulated as possible causes of IOI, their exact nature and relationships to the condition still remain unclear.

Allergic reactions to drugs are a common cause of eosinophilia, with manifestations ranging from diffuse maculopapular rash, to severe life-threatening drug reactions with eosinophilia and systemic symptoms (DRESS). Drugs that have been shown to cause DRESS are aromatic anticonvulsants and other antiepileptics, sulfonamides, allopurinol, nonsteroidal anti-inflammatory drugs (NSAIDs), some antipsychotics such as risperidone, and certain antibiotics. Phenibut, an analogue of the neurotransmitter GABA, has also been implicated in high doses. The reaction which has been shown to be T-cell mediated may also cause eosinophilia-myalgia syndrome.

There are many causes of eosinophilia that may underlie eosinophilic myocarditis. These causes are classified as primary (i.e. a defect intrinsic to the eosinophil cell line), secondary (induced by an underlying disorder that stimulates the proliferation and activation of eosinophils), or idiopathic (i.e. unknown cause). Non-idiopathic causes of the disorder are sub-classified into various forms of allergic, autoimmune, infectious, or malignant diseases and hypersensitivity reactions to drugs, vaccines, or transplanted hearts. While virtually any cause for the elevation and activation of blood eosinophils must be considered as a potential cause for eosinophilic myocarditis, the follow list gives the principal types of eosinophilia known or thought to underlie the disorder.

Primary conditions that may lead to eosinophilic myocarditis are:

- Clonal hypereosinophilia.

- Chronic eosinophilic leukemia.

- The idiopathic hypereosinophilic syndrome.

Secondary conditions that may lead to eosinophilic myocarditis are:

- Infections agents:

- Parasitic worms: various "Ascaris, Strongyloides, Schistosoma, filaria, Trematoda", and "Nematode" species. Parasitic infestations often cause significant heart valve disease along with myocarditis and the disorder in this setting is sometimes termed Tropical endomyocardial fibrosis. While commonly considered to be due to the cited parasites, this particular form of eosinophilic myocarditis may more often develop in individuals with other disorders, e.g. malnutrition, dietary toxins, and genetic predisposition, in addition to or place of round worm infestation.

- Infections by protozoa: various "Toxoplasma gondii, Trypanosoma cruzi, trichinella spiralis, Entamoeba", and "Echinococcus" species.

- Viruses: While some viral infections (e.g. HIV) have been considered causes of eosinophilic endocarditis, a study of 20 patients concluded that viral myocarditis lacks the characteristic of eosinophil-induced damage in hearts taken during cardiac transplantation.

- Allergic and autoimmune diseases such as severe asthma, rhinitis, or urticarial, chronic sinusitis, aspirin-induced asthma, allergic bronchopulmonary aspergillosis, chronic eosinophilic pneumonia, Kimura's disease, polyarteritis nodosa, eosinophilic granulomatosis with polyangiitis (i.e. Churg-Strauss syndrome), and rejection of transplanted hearts.

- Malignancies and/or premalignant hematologic conditions not due to a primary disorder in eosinophils such as Gleich's syndrome, Lymphocyte-variant hypereosinophilia Hodgkin disease, certain T-cell lymphomas, acute myeloid leukemia, the myelodysplastic syndromes, systemic mastocytosis, chronic myeloid leukemia, polycythemia vera, essential thrombocythemia, myelofibrosis, chronic myelomonocytic leukemia, and T-lymphoblastic leukemia/lymphoma-associated or myelodysplastic–myeloproliferative syndrome-associated eosinophilias; IgG4-related disease and Angiolymphoid hyperplasia with eosinophilia as well as non-hematologic cancers such as solid tumors of the lung, gastrointestinal tract, and genitourinary tract.

- Hypersensitivity reactions to agents include:

- Antibiotics/anti-viral agents: various penicillins (e.g. penicillin, ampicillin), cephalosporins (e.g. cephalosporin), tetracyclins (e.g. tetracycline), sulfonamides (e.g. sulfadiazine, sulfafurazole), sulfonylureas, antituburcular drugs (e.g. isoniazid, 4-aminosalicylic acid), linezolid, amphotericin B, chloramphenicol, streptomycin, dapsone, nitrofurantoin, metronidazole, nevirapine, efavirenz, abacavir, nevirapine.

- Anticonvulsants/Antipsychotics/antidepressants: phenindione, phenytoin, phenobarbital, lamotrigine, lamotrigine, clozapine, valproic acid, carbamazepine, desipramine, fluoxetine, amitriptyline, olanzapine.

- Anti-inflammatory agents: ibuprofen, indomethacin, phenylbutazone, oxyphenbutazone, acetazolamide, piroxicam, diclofenac.

- Diuretics: hydrochlorothiazide, spironolactone, chlortalidone.

- ACE inhibitors: captopril, enalapril.

- Other drugs: digoxin, ranitidine, lenalidomide, methyldopa, interleukin 2, dobutamine, acetazolamide.

- Contaminants: Unidentified contaminants inrapeseed oil cause the toxic oil syndrome and in commercial batches of the amino acid, L-tryptophan, cause the eosinophilia–myalgia syndrome.

- Vaccinations: Tetanus toxoid, smallpox, and diphtheria/pertussis/tetanus vaccinations.

Every year between 2.18 and 7.7 people per million receive a diagnosis of PM or DM. 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.

IOI or orbital pseudotumor is the second most common cause of exophthalmos following Grave’s orbitopathy and the third most common orbital disorder following thyroid orbitopathy and lymphoproliferative disease accounting for 5–17.6% of orbital disorders, There is no age, sex, or race predilection, but it is most frequently seen in middle-aged individuals. Pediatric cases account for about 17% of all cases of IOI.

Typical age of onset is around 40 to 50 years. It is not clear whether it is more common in women than men - patient numbers are small and some studies report a preponderance of men and others women. It is also found in children.

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).

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.

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

Mixed connective tissue disease (also known as Sharp's syndrome), commonly abbreviated as MCTD, is an autoimmune disease characterized by the presence of high blood levels of a specific autoantibody, now called anti-U1 ribonucleoprotein (RNP). The idea behind the "mixed" disease is that this specific autoantibody is also present in other autoimmune diseases such as systemic lupus erythematosus, polymyositis, scleroderma, etc. It was characterized in 1972, and the term was introduced by Leroy in 1980.

It is sometimes said to be the same as undifferentiated connective tissue disease, but other experts specifically reject this idea because undifferentiated connective tissue disease is not necessarily associated with serum antibodies directed against the U1-RNP, and MCTD is associated with a more clearly defined set of signs/symptoms.

Hypereosiophilia or eosinophilia may be associated with the following autoimmune diseases: systemic lupus erythematosus eosinophilic fasciitis, eosinophilic granulomatosis with polyangiitis, dermatomyositis, severe rheumatoid arthritis, progressive systemic sclerosis, Sjogren syndrome, thromboangiitis obliterans, Behcet syndrome, IgG4-related disease, inflammatory bowel diseases, sarcoidosis, bullous pemphigoid, and dermatitis herpetiformis.

Eosinophilic granulomatosis with polyangiitis consists of three stages, but not all patients develop all three stages or progress from one stage to the next in the same order; whereas some patients may develop severe or life-threatening complications such as gastrointestinal involvement and heart disease, some patients are only mildly affected, "e.g." with skin lesions and nasal polyps. EGPA is consequently considered a highly variable condition in terms of its presentation and its course.

Eosinophilic granulomatosis with polyangiitis (EGPA; also known as Churg-Strauss syndrome [CSS] or allergic granulomatosis) is an extremely rare autoimmune condition that causes inflammation of small and medium-sized blood vessels (vasculitis) in persons with a history of airway allergic hypersensitivity (atopy).

It usually manifests in three stages. The early (prodromal) stage is marked by airway inflammation; almost all patients experience asthma and/or allergic rhinitis. The second stage is characterized by abnormally high numbers of eosinophils (hypereosinophilia), which causes tissue damage, most commonly to the lungs and the digestive tract. The third stage consists of vasculitis, which can eventually lead to cell death and can be life-threatening.

This condition is now called "eosinophilic granulomatosis with polyangiitis" to remove all eponyms from the vasculitides. To facilitate the transition, it was referred to as "eosinophilic granulomatosis with polyangiitis (Churg-Strauss)" for a period of time starting in 2012. Prior to this it was known as "Churg-Strauss syndrome", named after Drs. Jacob Churg and Lotte Strauss who, in 1951, first published about the syndrome using the term "allergic granulomatosis" to describe it. It is a type of systemic necrotizing vasculitis.

Effective treatment of EGPA requires suppression of the immune system with medication. This is typically glucocorticoids, followed by other agents such as cyclophosphamide or azathioprine.

Helminths are common causes of hypereosiophilia and eosinophilia in areas endemic to these parasites. Helminths infections causing increased blood eosinophil counts include: 1) nematodes, (i.e. "Angiostrongylus cantonensis" and Hookworm infections), ascariasis, strongyloidiasis trichinosis, visceral larva migrans, Gnathostomiasis, cysticercosis, and echinococcosis; 2) filarioidea, i.e. tropical pulmonary eosinophilia, loiasis, and onchocerciasis; and 3) flukes, i.e. shistosomiasis, fascioliasis, clonorchiasis, paragonimiasis, and fasciolopsiasis. Other infections associated with increased eosinophil blood counts include: protozoan infections, i.e. "Isospora belli" and "Dientamoeba fragilis") and sarcocystis); fungal infections (i.e. disseminated histoplasmosis, cryptococcosis especially in cases with [[central nervous system]] involvement), and coccidioides); and viral infections, i.e. Human T-lymphotropic virus 1 and HIV.

Common treatments include corticosteroids such as prednisone, though other medications such as hydroxychloroquine have also been used.

The prognosis is usually good in the case of an early treatment if there is no visceral involvement.