Patients with dermatomyositis (DM) typically have muscle weakness affecting the thighs, shoulders, and sometimes the neck. The muscle enzymes, such as the creatinine kinase (CK) are usually elevated. In addition to the muscle inflammation, these patients characteristically have skin changes; with a rash that can affect the back of the hands, chest, back, face, scalp, sometimes the elbows and sides of the legs as well. Not uncommonly, patients will also note changes in their cuticles too, which can become sore and ragged. Whenever these symptoms happen in children and adolescents, we call this condition Juvenile Dermatomyositis (JDM). Sometimes blood tests can be found positive for specific autoantibodies that give us clues on other manifestations associated with the disease, such as the increased risk for cancer or lung inflammation, what we call interstitial lung disease (ILD). Interestingly, dermatomyositis can happen without muscle disease and we call this subtype amyopathic dermatomyositis (ADM).

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

Patients with antisynthetase syndrome usually have a particular combination of different 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 – interstitial lung disease or ILD -, and Raynaud’s phenomenon (change in colors of the digits when exposed to the cold). In addition to the presentation of all these possible symptoms, these patients also have an autoantibody directed against a protein called aminoacyl-transfer RNA (tRNA) synthetases, such as anti-Jo-1 and anti-PL-7 antibodies.

Patients with this subtype of myositis have muscle weakness and elevation of muscle enzymes in the blood tests, such as polymyositis, which also doesn’t present with rashes. However, the extent of muscle involvement among those with immune-mediated necrotizing myopathy (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.

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 can also have associated muscle weakness and elevation of muscle enzymes in the blood tests. Frequently, the muscle enzymes being elevated are the only signs of muscle inflammation, without any evidence of muscle weakness.

The most common subtype of myositis in individuals older than 50 years old, inclusion body myositis (IBM) characteristically presents more insidiously compared to the other types of myositis mentioned above, with the disease sometimes progressing 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.

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.