Inflammatory myopathies: Clinical sciences

Inflammatory myopathies are a group of autoimmune conditions associated with muscle inflammation and subsequent progressive muscle wasting and weakness. These conditions occur when immune cells like lymphocytes and macrophages infiltrate skeletal muscle fibers, causing widespread inflammation. Inflammatory myopathies include five conditions: dermatomyositis, antisynthetase syndrome, immune-mediated necrotizing myositis, inclusion body myositis, and polymyositis.

Now, if your patient presents with chief concerns suggesting inflammatory myopathy, your first step is to perform a focused history and physical. Patients usually report symptoms of proximal muscle weakness, commonly affecting the shoulder and pelvic girdle muscles. So, they might find it difficult to do activities like combing their hair, standing up from a sitting position, or climbing stairs. To remember this, think hair, chairs, and stairs! They might also report difficulty swallowing or a skin rash. Additionally, some individuals have a positive personal or family history of autoimmune or inflammatory diseases, like lupus, scleroderma, or Sjogren syndrome.

Finally, the physical examination can reveal symmetric proximal muscle weakness, with or without skin abnormalities like a rash.

At this point, you should suspect inflammatory myopathy, so be sure to order serum creatine kinase and electromyography, or EMG. Now, If your patient’s creatine kinase levels are normal and EMG reveals normal findings or evidence of neuropathy, consider an alternative diagnosis, like motor neuron disease, peripheral polyneuropathy, or myasthenia gravis. On the other hand, if labs reveal elevated creatine kinase, usually ten times the upper limit of normal or more, and the EMG shows a myopathic process, such as fibrillations and early recruitment of muscle fibers, diagnose inflammatory myopathy!

Here’s a clinical pearl to keep in mind! Non-inflammatory conditions, such as electrolyte imbalances, hypothyroidism, Cushing syndrome or adrenal insufficiency, can also result in myopathy, so you should always check serum electrolytes, magnesium, TSH and morning cortisol screen for these conditions

Alright, now once you confirm the presence of an inflammatory myopathy, your next step is to identify the underlying cause of muscle inflammation. First, order myositis-specific antibodies, such as anti-Mi-2 and anti-Jo-1 antibodies. However, keep in mind that these antibodies can be associated with more than one disease, so you will need to interpret the results along with history and physical exam findings. Also, don’t forget to obtain a muscle biopsy, which is the gold standard for diagnosis.

And here’s another clinical pearl! Once you confirm the diagnosis of inflammatory myositis, it's important to screen your patient for extramuscular manifestations and various associated malignancies. This might involve lung imaging and pulmonary function tests to detect interstitial lung disease; electrocardiography and echocardiography to detect cardiac disease; and cancer screening tests, like a mammogram and prostate examination, as well as an endoscopy or a colonoscopy.

Now, let’s have a look at various underlying causes of inflammatory myopathies, starting with dermatomyositis. As the name implies, it affects both the skin and the muscles. In this case, a physical exam typically reveals Gottron papules, which are raised red lesions over the knuckles; or Gottron sign, which is a red, flat, scaly rash over extensor surfaces of joints. Additionally, these patients often develop a Heliotrope rash, which refers to red-to-purple discoloration and edema of the eyelids.

Other findings include a V-sign, or v-shaped redness on the anterior neck and chest, and a shawl sign, which refers to a red discoloration on the back of the neck and shoulders. You might also notice nail bed telangiectasias and subcutaneous calcium deposits.

Now, one physical exam finding that distinguishes dermatomyositis from similar conditions like scleroderma and systemic lupus erythematosus is poikiloderma, which refers to areas of skin affected by skin atrophy, hyper- and hypopigmentation, that are typically seen in chronic disease; and telangiectasias. Also, in patients with dermatomyositis, the anti-Mi-2 antibodies are typically positive.

Finally, the muscle biopsy will reveal inflammation of the perimysium, which is a thin layer of fibrous tissue surrounding a muscle fascicle; as well as degeneration of muscle fibers. With these findings, diagnose dermatomyositis!