Belonging to the same group of autoimmune diseases, both dermatomyositis and polymyositis cause muscle inflammation and weakness. But dermatomyositis also affects your skin and is more likely to develop in childhood.

Dermatomyositis and polymyositis belong to a group of rare muscle disorders called inflammatory myopathies (myositis).

Like all myopathies, dermatomyositis and polymyositis cause chronic muscle weakness and other muscle-related issues. However, there are some key differences between these two conditions.

Read on to learn which specific muscles each one affects, the symptoms they cause, and how their treatments and outlooks differ.

The word “myositis” means muscle (“myos”) inflammation (“itis”).

The terms “dermato” (skin) and “poly” (many) further refer to the type of muscle inflammation.

Dermatomyositis symptoms

Dermatomyositis causes progressive yet chronic muscle weakness but is also known for its skin symptoms. This is because it affects the blood vessels that supply blood to both your skin and muscles.

Some of the most common symptoms of early dermatomyositis include a rash that is red to purple in color, depending on your natural skin tone.

This patchy rash may cover your face, shoulder, knees, and ankles. It may occur before or at the same time as muscle weakness.

As dermatomyositis progresses, you may experience:

Dermatomyositis can start in childhood and progressively worsen with age.

Polymyositis symptoms

Polymyositis specifically affects skeletal muscles responsible for day-to-day movements. The resulting weakness primarily affects muscles in the following areas:

  • neck
  • shoulders
  • upper arms
  • hips
  • thighs

Aside from muscle weakness, people with polymyositis may experience:

Unlike dermatomyositis, polymyositis affects muscles symmetrically. This means it affects the same muscles on both sides of your body.

Diagnosing myositis typically involves a muscle biopsy. This test involves taking a small sample of muscle and examined under a microscope. Doctors may also use a skin biopsy to help diagnose dermatomyositis.

A doctor may also make a diagnosis based on symptoms they observe in a physical exam and the symptoms you report. They’ll also ask about your personal and family history of autoimmune disorders.

In dermatomyositis, a patchy rash can appear on your eyelids or along the skin over muscles that help extend joints throughout your body. As the condition progresses, a doctor might detect muscular effects around your ankles, wrists, and forearms.

A doctor may also check for signs of lung involvement in polymyositis, such as shortness of breath or a dry cough.

There’s currently no cure for dermatomyositis or polymyositis. However, treatments can help effectively manage symptoms and possibly prevent further progression.

Like with many other autoimmune conditions, treatment for these types of myositis aims to stop immune system overactivity. This may involve a combination of oral steroids and immune modulators.

Additionally, a doctor may recommend skin treatments for dermatomyositis. These might include:

Dermatomyositis and polymyositis can be uncomfortable as the condition progresses and affects your muscles. Treatment can also help prevent disease progression and improve quality of life.

Unlike dermatomyositis, partial to complete recovery is possible with polymyositis.

Both conditions can lead to severe complications when left untreated, including:

Complications are more likely in older adults.

The exact causes of dermatomyositis and polymyositis aren’t clear. However, like all inflammatory myopathies, experts think these conditions may be autoimmune in nature.

With an autoimmune disease, your immune system mistakenly attacks healthy cells and tissues. In the cases of dermatomyositis and polymyositis, this may include your muscles.

While experts don’t consider them to be genetic diseases, these two myopathies may have genetic components, similar to other autoimmune diseases. Environmental factors, such as infections, may also trigger them.

Dermatomyositis may be linked to other autoimmune disorders, such as lupus.

With polymyositis, experts think an abnormal activation of CD8 cells (a subtype of white blood cells) may be involved.

Dermatomyositis and polymyositis are rare conditions that can develop at any age.

Dermatomyositis tends to develop in childhood or early adulthood. It most often starts in children between the ages of 5 and 14 and affects females more than males.

Polymyositis tends to develop in adults over age 20. Like dermatomyositis, polymyositis is more common in females. It also affects an estimated 0.5–8.4 out of every 100,000 people.

Dermatomyositis and polymyositis are two types of immune-mediated muscle disorders that fall under the umbrella of inflammatory myopathies. Dermatomyositis is more likely to develop during childhood.

While these are rare conditions, early detection and treatment are important to help prevent disease progression and complications.

If you’re experiencing unusual muscle weakness, rashes, and difficulty with everyday movements, see a doctor for an evaluation. They can help determine if your symptoms are related to myositis or another medical condition.