Published: 18 Jun 2025

ICD9: 710.3      ICD10: M33.90      ICD11: 4A41.0

Myositis, polymyositis, and dermatomyositis are a group of rare inflammatory diseases that affect the muscles.
They cause muscle weakness, pain, and fatigue. Here's a breakdown of each:

1. Myositis:

This is a general term meaning inflammation of the muscles. It's not a specific disease, but rather a description of a symptom. Many different conditions can cause myositis.

2. Polymyositis (PM):

What it is: A chronic inflammatory disease that causes muscle weakness, mainly affecting the muscles closest to the trunk of the body (shoulders, hips, thighs, upper arms).
Symptoms:
Progressive muscle weakness (difficulty climbing stairs, lifting arms, rising from a chair)
Muscle pain and tenderness (sometimes)
Fatigue
Difficulty swallowing (dysphagia)
Difficulty breathing (if respiratory muscles are affected)
Cause: The exact cause is unknown, but it's believed to be an autoimmune disorder, where the body's immune system mistakenly attacks its own muscle tissue.
Diagnosis:
Physical exam and medical history
Blood tests (elevated muscle enzymes like creatine kinase (CK))
Electromyography (EMG) - measures electrical activity in muscles
Muscle biopsy - examination of a small piece of muscle tissue
Treatment:
Corticosteroids (like prednisone) - to reduce inflammation
Immunosuppressants (like methotrexate, azathioprine) - to suppress the immune system
Physical therapy - to maintain muscle strength and function

3. Dermatomyositis (DM):

What it is: A type of myositis that also involves a characteristic skin rash in addition to muscle inflammation and weakness.
Symptoms: Includes all the symptoms of polymyositis *plus* a distinctive rash.
Rash:
Heliotrope rash: A purplish or bluish rash around the eyelids.
Gottron's papules: Raised, scaly, reddish or purplish bumps on the knuckles, elbows, and knees.
Other skin manifestations: Redness or scaling on the face, neck, upper chest ("V-neck sign"), and back ("shawl sign").
Cuticles can be inflamed or thickened
Cause: Similar to polymyositis, it's believed to be an autoimmune disorder. There is also a higher association with certain cancers in older adults with dermatomyositis.
Diagnosis: Similar to polymyositis, but with visual assessment of the rash.
Treatment:
Similar to polymyositis (corticosteroids, immunosuppressants)
Topical corticosteroids or other skin treatments for the rash
Sun protection (sunscreen, protective clothing) to minimize rash flares.

Key Differences and Considerations:

Skin involvement: The presence of the characteristic rash is the main difference between polymyositis and dermatomyositis. Polymyositis does *not* have a rash.
Cancer risk: Dermatomyositis has a slightly higher association with underlying cancers, particularly in older adults. Therefore, cancer screening may be recommended.
Age of onset: Polymyositis typically affects adults, while dermatomyositis can affect both adults and children.
Other organs: In some cases, polymyositis and dermatomyositis can affect other organs, such as the lungs, heart, and joints.

Important points to remember:

These are rare conditions.
Early diagnosis and treatment are important to manage symptoms and prevent complications.
Treatment is often long-term and requires ongoing monitoring.
The severity of the disease can vary from person to person.

Disclaimer: This information is for general knowledge and educational purposes only, and does not constitute medical advice. It is essential to consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment. They can properly diagnose your condition and recommend the most appropriate course of action.