Diagnosing Inflammatory Myopathies

Inflammatory myopathies are diseases involving chronic muscle inflammation and weakness. The causes of these conditions are unknown. One theory holds that they are autoimmune disorders, meaning that the body’s immune system turns against itself, in this case attacking muscles and damaging tissues.

Experts at NYU Langone have extensive experience diagnosing and treating diseases of the muscles, joints, and bones. A correct diagnosis is vital to managing pain and muscle weakness associated with inflammatory myopathies.

Types of Inflammatory Myopathies

There are three main types of inflammatory myopathy: polymyositis, dermatomyositis, and inclusion body myositis. Inflammatory myopathies are chronic, or persistent, conditions. They are not life threatening, but their symptoms must be managed over the course of a lifetime.

Polymyositis

Polymyositis involves progressive weakness in the muscles of the hips, thighs, shoulders, upper arms, and neck on both sides of the body. Polymyositis occurs most often in men and women between ages 30 and 60.

Common symptoms include difficulty swallowing, speaking, lifting objects, reaching overhead, and climbing stairs. The condition usually develops slowly, over the course of a person’s lifetime. Rarely, polymyositis can develop over a short period of time.

Dermatomyositis

Dermatomyositis is similar to polymyositis, but a skin rash accompanies the muscle weakness. The rash is patchy with purple or red discoloration. It usually develops on the eyelids and over the muscles used to extend or straighten joints, such as the knuckles, elbows, and knees. It can also develop on the face, back, shoulders, chest, and other places on the body. Calcium deposits may also form under the skin or in the muscle tissue, causing hard bumps.

As with polymyositis, people with dermatomyositis may have weakness in the muscles of the hips, thighs, shoulders, upper arms, and neck—on both sides of the body—and have difficulty swallowing and speaking.

Dermatomyositis develops gradually over the course of a lifetime and can begin in either childhood or adulthood. It tends to affect women more often than men.

Inclusion Body Myositis

The third main type of inflammatory myopathy is inclusion body myositis, which affects the muscles of the wrists, fingers, and thighs and those that lift the front of the foot. Certain proteins may stick together and form clumps, known as inclusion bodies.

The weakness may affect one side of the body more than the other and can also cause difficulty swallowing. The disease usually begins after age 50 and occurs more frequently in men than in women. Although some people with inclusion body myositis are able to walk—with or without the assistance of a cane or walker—others require a wheelchair within 10 or 15 years of diagnosis.

Diagnostic Tests

To diagnose an inflammatory myopathy, an NYU Langone doctor takes a thorough medical history, performs a physical exam, and tests your muscle strength. Several other tools, like the ones described below, may also be used to help a doctor confirm the diagnosis and determine what type of inflammatory myopathy is causing your symptoms.

Blood Test

Doctors use a blood test to look for elevated levels of a substance called creatine kinase, which is released into the bloodstream when muscle fibers deteriorate. Elevated levels may mean you have an inflammatory myopathy. Creatine kinase levels are usually very high in people with polymyositis and dermatomyositis but only slightly elevated or even normal in people with inclusion body myositis.

A blood test alone doesn’t provide a definitive diagnosis, and additional tests are necessary. Doctors may also send blood to be tested for antibodies that are associated with polymyositis.

Muscle Biopsy

In this outpatient procedure, a doctor numbs the area to be tested with a local anesthetic and surgically removes a small piece of muscle. This is examined under a microscope for signs of chronic inflammation or muscle fiber death, which may signal an inflammatory disease.

A biopsy can also help a doctor rule out a more serious cause of muscle weakness, such as muscular dystrophy.

Electromyography

Testing of the body’s nerve function can help confirm a diagnosis and determine the best treatment for you. Electromyography shows how the nerves and muscles work together by measuring the electrical impulses along nerves, nerve roots, and muscular tissue. The results can help doctors distinguish inflammatory myopathies from other neuromuscular diseases, such as muscular dystrophy.

To perform this outpatient test, the doctor inserts a tiny needle electrode—a conductor for an electrical current—through the skin and into the muscle. This allows your doctor to measure the amount of electricity generated by muscle cells when they become activated by a nerve impulse.

In people with inflammatory myopathy, the muscle fibers do not respond as well to repeated electrical stimulation as do normally functioning muscles.

NYU Langone doctors use the results of these tests to create a treatment plan to help you manage symptoms.