Myositis: Definition, Associated Diseases, Causes, Symptoms, Diagnosis and Treatments

It is the inflammation and degeneration of skeletal muscle tissue, which is used to move the body.

Myositis can cause muscle diseases known as myopathies .


Myositis is usually caused by an underlying disease, such as an injury, infection, or autoimmune disease.

Many times the cause of the condition commonly known as idiopathic inflammatory myositis is unknown.

Among them, bacterial infections cause an acute intramuscular infection with abscess formation.

Some rare forms of myositis are focal nodular myositis, giant cell myositis, and eosinophilic myositis, which can be diagnosed by muscle biopsy.

Two most common types of idiopathic inflammatory myositis are polymyositis and dermatomyositis, which affects approximately 60 to 80 per million of the population.

There can be multiple causes of myositis, including:

  • Bacteria such as staphylococci and streptococci.
  • Viruses such as influenza a and b, hepatitis b, among others.
  • Parasites
  • Some medications and toxins such as statins, rifampin, sulfonamide, cyclosporine, alcohol.


It can be defined as an inflammatory myopathy with gradual onset (weeks to months) and constant progression.


There is a debate about triggers, but the exact cause is not yet clear.

The main theories behind what could be behind the disease process are an autoimmune mechanism or an as yet unidentified infectious cause.


In this condition, systemic inflammation affects the endomysium of the muscles. This differentiates it from dermatomyositis, where perimysial inflammation is the focus of inflammation.

There is a T cell-mediated response in this condition against the surface of the muscle fibers, leading to necrosis.


The condition affects people of African or Caribbean descent at nearly twice the rate that it affects Caucasians.

The disease affects people of all ages, but it does not usually occur in middle-aged adults, aged 50 to 70 years, or in children aged 5 to 15 years.

The disease affects the female population more likely than the male sex.

Signs and symptoms

Common signs include proximal muscle weakness, that is, weakness of the shoulders and hips.

It is difficult for the individual to get up from a chair, climb stairs, lift objects, or even comb their hair.

In the later stage of the disease, some fine motor movements that depend on the strength of the distal muscles, such as buttoning a shirt, sewing, knitting or writing, are also affected.

Muscle atrophy can also appear at this stage. Polymyositis never affects the eye muscles, even in the advanced stage of the disease.

The facial and respiratory muscles are rarely affected by this condition. Pain and tenderness in the muscles can occur early in the disease.

This condition generally occurs in association with some connective tissue disorders, systemic autoimmune diseases, or viral infections.

Heart disease and interstitial lung disease can occur in some patients affected with this condition.


Some muscular dystrophies, conditions, and diseases can mimic polymyositis and should be excluded. They include:

  • Beamescapulohumeral dystrophy.
  • Distrofia muscular de Becker.
  • Metabolic myopathies.
  • Neurogenic muscle atrophies.
  • Biochemical muscle diseases.

Laboratory investigations reveal an increase in serum muscle enzymes in this condition, such as:

  • Creatine kinase.
  • Serum glutamic-oxaloacetic transaminase.
  • Serum glutamic-pyruvic transaminase.
  • Lactate dehydrogenase.

The definitive diagnosis is based on the histopathological examination of the muscle biopsy. Inflammation is observed with foci of endomysial infiltrates around the muscle fibers.

The presence of the main histocompatibility complex of class 1 and cluster of differentiation 8 is considered specific for the diagnosis of polymyositis.


Treatment of acute flare-ups is intravenous corticosteroids and intravenous immunoglobulin.

Treatment consists of corticosteroid medications that will be gradually reduced from a month to a month and a half.

Maintenance of treatment is usually a long-term low dose of steroids.

Since long-term use of steroids can lead to fractures, these patients are also likely to take bone-protecting medications, such as bisphosphonates and vitamin D supplements, to ensure calcium intake.

Response to treatment is variable, but about 20% of patients with the disease die within 5 years of diagnosis.

Immunosuppressive therapy is also an option for serious and unmanageable conditions, such as mycophenolate.

Some patients may also benefit from regular physical therapy and strengthening exercises.

Cancer and lung disease are the most frequent causes of death in these patients.


It is an inflammatory myopathy with skin manifestations.


Like the other inflammatory myopathies, the disease is more likely to affect women than men.

It most commonly affects adults between the ages of 40 and 60, and is up to three times more common in those of African Caribbean descent.

There is also a juvenile form of the disease that affects girls more than boys and usually occurs between the ages of 5 and 14.


The exact cause of dermatomyositis is not yet known, and there are several theories as to what the underlying mechanism of the disease may be. These include:

  • Genetic dysfunctions.
  • Immune dysfunction.
  • An environmental trigger such as ultraviolet radiation.
  • A virus or bacterial infection.
  • Medicines, such as statins and penicillamine (a chelating agent).


As mentioned above, this condition does not yet have a clear or known mechanism or trigger.

Tumor necrosis factor promoter polymorphism and an HLA-DQA gene allele are recognized as risk factors in genetic studies.

There is a proliferation of autoantibodies and this overactivation of the immune system causes tissue damage.

More specifically, this condition affects the perimysium of the muscle.

Myositis-specific autoantibodies have been found in 50 to 70% of dermatomyositis.

In the early stage of the disease, complex membrane-attacking cells are there as well, which can cause cell death and attack endothelial cells to mediate vascular damage.

Agents such as interferon alpha and beta also play a role in the disease process.

Signs and symptoms

The distinctive skin features in this disease make early recognition possible.

Classic skin symptoms include six characteristics including:

  • Poikiloderma depigmentation.
  • Heliotrope rash (this is a dark purplish periorbital rash).
  • Scale alopecia.
  • Gottron’s papules (flat purplish red papules that form on the back of the elbow, knees, knuckles, and ankles) on the back of the hands.
  • Itchy rash mainly in areas exposed to the sun.
  • Telargiectasia in dystrophic folds and cuticles.

Other non-classical signs and symptoms may include:

  • Proximal muscle weakness.
  • Photosensitivity.
  • Itch.
  • Weightloss.
  • Difficulty breathing due to the involvement of the respiratory muscles.
  • Muscle and joint pain.
  • Palpitations (due to heart involvement).
  • Raynaud’s phenomenon, although this is not specific to dermatomyositis only.

The condition is also associated with an increased risk of malignancy, interstitial lung disease, and heart problems.

The disease may overlap with rheumatoid arthritis and systemic lupus erythematosus, among other connective tissue diseases.


Polymyositis is often confused with dermatomyositis, but the skin manifestation is the differentiating factor.

Other skin diseases such as systemic lupus erythematosus, dermatitis, ringworm of the body, psoriasis or rosacea should be ruled out.

The diagnosis of dermatomyositis is generally clinical, based on the examination and the discovery of a symmetric proximal myopathy.

There are also tests that can confirm and support the diagnosis. These include:

  • Elevated muscle enzymes (creatine kinase and aldolase).
  • Anti-mi2 antibodies.
  • Myopathic changes in electromyography.
  • Positive anti-nuclear antibody.
  • Reduced left ventricular ejection fraction.
  • Elevated troponin (due to cardiac involvement).
  • Inflammatory infiltrate, revealed by muscle biopsy.


Treatment of acute flare-ups is intravenous corticosteroids and intravenous immunoglobulin.

Sun protection with sunscreen is also highly recommended, as are antipruritic drugs, such as mild topical paraffin.

Immunosuppressive therapy is also an option for severe and unmanageable disease, for example topical tacrolimus or oral methotrexate.

Inclusion body myositis

Inclusion body myositis is characterized by distinctive inclusions, containing tubular filaments in a subset of patients with polymyositis.

It is classified as “sporadic inclusion body myositis,” showing inflammation, and “inherited inclusion body myositis.”

It is a group of disorders that includes both autosomal dominant and autosomal recessive conditions.

They result in myopathy, which causes muscle weakness that progresses gradually over 10 to 20 years.


This group of disorders is rare in the general population. It occurs before some of the other myopathies, that is, polymyositis or dermatomyositis.

The autosomal form of the disease results in preservation of the quadriceps, but they are affected in advanced disease.

The autosomal recessive form of the disease affects people of Middle Eastern and Jewish descent more often than the general population.


The cause of inclusion body myositis has been traced to the underlying genetic inheritance, but the element that malfunctions in the muscle tissue pathway is not yet known.


On muscle biopsy, muscle cells show small border vacuoles and there is no evidence of inflammation.

This separates it from inflammatory myopathies such as polymyositis and dermatomyositis.

Signs and symptoms

Early symptoms include balance problems, making running or walking on heels difficult or impossible.

Index finger weakness can also present as an early feature.

The unique feature of this disorder is the partial or complete preservation of the quadriceps muscles, even in the advanced stages of the disease.


A complete history and physical examination of the patient help detect the disease.

Muscle biopsy reveals inclusion bodies, that is, rimmed vacuoles formed due to the collection of abnormal proteins.

Magnetic resonance imaging shows fatty replacement of the hamstring muscles.

Nerve biopsy and a buccal swab for genetic testing will support and confirm the diagnosis.

The diagnosis of Hereditary Body Inclusion Myositis is based on both clinical symptoms and the histopathology of a muscle biopsy.


There is no curative treatment for this chronic and progressive disease. Lower limb orthoses can help if the patient suffers a foot drop.

Dietary modifications are proposed, including avoiding excess selenium, copper, and zinc.

Miositis granulomatosa

This form of myositis occurs in an individual with a chronic granulomatous disease such as sarcoidosis. It usually impacts the proximal muscles.

Miositis eosinofílica

This is characterized by eosinophilic disorders, and muscle biopsy is a cornerstone of diagnosis. The main causes of eosinophilic myositis are believed to be:

  • Parasitic infections.
  • Drugs
  • Toxic substances.
  • Hematological disorders.

Miositis focal

This is a rare disorder that presents with an inflammatory mass or “tumor.” As such, it is commonly confused with inflammatory disorders and neoplastic disorders.

It is important to exclude trauma as a potential cause of the mass and other causes of inflammation.

Miositis orbital

This is the inflammation of the extra eye muscles and it is a rare disease.

It usually occurs at young and confused ages. Like the other disorders, women are more affected than men.

It is associated with other disorders such as systemic lupus erythematosus, rheumatoid arthritis, Crohn’s disease, other autoimmune diseases, such as diabetes mellitus, and myasthenia gravis .

Signs include drooping eyelids and bulging eyes that can be mistaken for Grave’s disease.

There is often swelling around the eye (may be confused with periorbital edema.

Symptoms include eye pain that worsens with movement and double vision.