How Do Dermatomyositis and Polymyositis Differ?

Both polymyositis and dermatomyositis are autoimmune disorders that produce weakening and inflammation in the muscles. However, dermatomyositis is a skin condition that is more prone to appear in children.

Polymyositis and dermatomyositis are among the uncommon muscular conditions known as inflammatory myopathies (myositis).

Dermatomyositis and polymyositis result in chronic muscular weakening and other muscle-related problems, just like the other myopathies. Nonetheless, these two circumstances differ significantly in a few important ways.

Continue reading to find out which particular muscles each one affects, what symptoms they produce, and how different therapies and prognoses work for each.

What are the symptoms of dermatomyositis and polymyositis?

The term "myositis" refers to inflammation of the muscles (myos).

The types of muscle inflammation are further described by the terms "dermato" (skin) and "poly" (many).

Dermatomyositis symptoms

In addition to causing gradual, chronic muscular weakening, dermatomyositis is also recognised for its cutaneous manifestations. The reason for this is that it has an impact on the blood arteries that carry blood to your skin and muscles.

The most typical sign of early dermatomyositis is a rash that can range in colour from red to purple, depending on your skin tone.

You may have this spotty rash on your knees, ankles, shoulders, and face. It could manifest concurrently with or before muscle weakness.

As your dermatomyositis worsens, you can encounter:
  • low-grade fever
  • Calcinosis: painful nodules under your skin's surface that contain calcium; usually appears 1-3 years after beginning
  • inflammation of subcutaneous fat (panniculitis)
  • muscle tissue loss
  • muscle and joint pain
  • weakening and atrophy of the muscles, especially in the:
  • neck
  • shoulders
  • arms
  • hips
  • thighs
  • unintentional weight loss
  • sensitivity to light
As people mature, dermatomyositis can begin in childhood and get worse.

Polymyositis symptoms

Particularly affecting the skeletal muscles involved in daily motions is polymyositis. The following muscles are most commonly affected by the ensuing weakness:
  • neck
  • hips
  • thighs
  • shoulders
  • upper arms
In addition to muscle weakness, individuals with polymyositis may encounter:
  • finger or toe weakness
  • difficulty walking
  • falls
  • chronic dry cough
  • difficulty with swallowing
Polymyositis affects muscles symmetrically, in contrast to dermatomyositis. This indicates that the same muscles on both sides of your body are affected.

How do doctors diagnose dermatomyositis and polymyositis?

A muscle biopsy is usually required to diagnose myositis. For this test, a tiny sample of muscle is taken, and it is inspected under a microscope. A skin biopsy is another tool that doctors may use to diagnose dermatomyositis.

In addition to the symptoms you mention, a doctor may potentially diagnose you based on symptoms they see during a physical examination. They will also inquire about any family history of autoimmune diseases as well as your own.

A patchy rash on your eyelids or over the skin above the muscles that support the extension of your body's joints can be indicative of dermatomyositis. A physician may notice muscle symptoms around your wrists, forearms, and ankles as the illness worsens.

In addition, a physician may look for symptoms of polymyositis that indicate lung involvement, such as dry cough or dyspnea.

How do treatments for dermatomyositis and polymyositis differ?

Both polymyositis and dermatomyositis presently have no known cure. Treatments, however, may be able to effectively control symptoms and perhaps stop them from getting worse.

The goal of treatment for these kinds of myositis is to reduce excessive immune system activity, just like in many other autoimmune diseases. Combining immune modulators with oral steroids may be necessary for this.

A physician might also suggest skin therapies for dermatomyositis. These could consist of:
  • antihistamines
  • topical steroids
  • antimalarial drugs, such as hydroxychloroquine (Plaquenil)

What is the outlook for people with dermatomyositis vs. polymyositis?

As your muscles are affected by dermatomyositis and polymyositis, the condition can become unpleasant. Additionally, treatment can enhance quality of life and stop the progression of disease.

In contrast to dermatomyositis, polymyositis may recover partially or completely.

If treatment for either ailment is not received, serious consequences may result, such as:
  • heart disease
  • disability
  • respiratory problems
In older persons, complications are more likely.

What causes dermatomyositis and polymyositis?

It's unclear what specifically causes polymyositis and dermatomyositis. As with many inflammatory myopathies, these disorders may have an autoimmune origin, according to doctors.

When you have an autoimmune disease, your body's defences wrongly target healthy tissues and cells. Your muscles may be affected if you have polymyositis or dermatomyositis.

These two myopathies may contain hereditary components, like other autoimmune disorders, even though specialists do not think of them as genetic diseases. Infections and other environmental variables may also cause them.

There may be a connection between lupus and other autoimmune diseases and dermatomyositis.

Experts believe that an aberrant activation of CD8 cells—a subset of white blood cells—may be connected to polymyositis.

Who gets dermatomyositis or polymyositis?

Rare diseases that can appear at any age are polymyositis and dermatomyositis.

The onset of dermatomyositis usually occurs in infancy or early adulthood. It primarily affects girls over boys and typically begins in kids between the ages of 5 and 14.

Most cases of polymyositis occur in people over the age of 20. Polymyositis is more frequent in females, similar to dermatomyositis. Additionally, 0.5–8.4 out of every 100,000 persons are thought to be affected.


Under the general category of inflammatory myopathies are two forms of immune-mediated muscle disorders: polymyositis and dermatomyositis. Childhood is the prime time for the development of dermatomyositis.

Despite the rarity of these disorders, early diagnosis and treatment are crucial to halting the spread of the illness and its consequences.

See a doctor for an assessment if you're having problems with daily motions, rashes, or unusual muscle weakness. They can assist in determining if your symptoms are caused by myositis or by another illness.

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