Idiopathic inflammatory myopathies (IIMs) are connective tissue disorders characterized by inflammation of the muscles. These conditions are chronic and can occur at any age. There are many forms of IIMs, each with unique symptoms. 

Dermatomyositis and Polymyositis

Patients with dermatomyositis typically have muscle weakness affecting the thighs, shoulders and sometimes the neck, as well as elevated muscle enzymes, such as the creatinine kinase. In addition to muscle inflammation, these patients may have a rash that affects the back of the hands, chest, back, face, scalp, and sometimes the elbows and sides of the legs. Patients may also notice changes in their cuticles, which can become sore and ragged.

Whenever these symptoms happen in children and adolescents, we call this condition “juvenile dermatomyositis.” Sometimes blood tests can be positive for specific autoantibodies that give us clues on other conditions associated with the disease, such as the increased risk for cancer or lung inflammation, known as interstitial lung disease or ILD. When dermatomyositis happens without muscle disease, it is called subtype amyopathic dermatomyositis. 

Similar to dermatomyositis, polymyositis patients present with muscle weakness and elevated blood tests, which reflect muscle inflammation and breakdown. However, polymyositis patients do not have the typical rash that affects patients with dermatomyositis, and should be screened for other causes of muscle weakness first before being diagnosed with polymyositis.

Overlap Myositis Syndrome

Patients who have been diagnosed with other rheumatologic autoimmune conditions, such as lupus systemic erythematosus, mixed connective tissue disease, systemic sclerosis, Sjogren’s syndrome, and rheumatoid arthritis also can have associated muscle weakness and elevated muscle enzymes in their blood tests. Frequently, the elevated muscle enzymes are the only signs of muscle inflammation, without any evidence of muscle weakness.

Antisynthetase Syndrome

Patients with antisynthetase syndrome usually have a particular combination of clinical manifestations, such as:

  • Muscle weakness
  • Arthritis (inflammation in the joints)
  • “Mechanic’s hands” (rough overly dry skin, sometimes with cuts, in the skin of the hands)
  • Inflammation in the lungs, known as interstitial lung disease or ILD
  • Raynaud’s phenomenon (change in colors of the digits when exposed to the cold)

In addition to these possible symptoms, these patients have an autoantibody directed against a protein called aminoacyl-transfer RNA (tRNA) synthetases, such as anti-Jo-1 and anti-PL-7 antibodies.

Immune-Mediated Necrotizing Myopathy (IMNM)

Patients with this subtype of myositis have muscle weakness and elevated muscle enzymes in their blood tests, such as with polymyositis, which doesn’t present with rashes. However, the extent of muscle involvement among those with IMNM is usually greater compared to patients with polymyositis.

In some cases, IMNM can be associated with the antibody anti-HMGCR and history of statin use, a medication prescribed to lower cholesterol. An SRP antibody is another type of antibody that can be found in some patients with IMNM.

Inclusion–Body Myositis (IBM)

IBM, the most common subtype of myositis in individuals older than 50 years old, usually presents in a gradual, subtle way compared to other types of myositis. IBM sometimes progresses slowly over the course of years before being diagnosed. Patients with IBM do not have inflammation in their skin or lungs.

The presence of the antibody anti-NT5C1A in the blood tests and structures called rimmed vacuoles in the muscle biopsy can be helpful in the diagnosis of this condition, although these findings are not always present.

Interstitial Lung Disease

The lungs are commonly affected in the setting of myositis. In several cases, lung disease is the only or the first manifestation of an underlying diagnosis of myositis. Patients with interstitial lung disease present with cough and/or shortness of breath due to the different stages of inflammation or scarring in the lung tissue.