How are polymyositis and dermatomyositis diagnosed?

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Your doctor will talk to you about your symptoms and examine you. Because the symptoms of polymyositis and dermatomyositis are similar to many other conditions you’ll probably need some tests.

What tests are there?

Blood tests

Creatine phosphokinase

Creatine phosphokinase is an enzyme that leaks out of damaged muscle cells. It’s likely that you’ll have a high level of this in your blood if you have polymyositis or dermatomyositis, but the level should fall as the condition is treated and brought under control.

You’ll need to have repeat tests to tell your doctor how well your condition is responding to treatment.

Erythrocyte sedimentation rate (ESR)

ESR detects and measures inflammation by assessing how quickly blood cells settle at the bottom of a test tube. Inflammation occurs in other conditions, so a high ESR alone won’t confirm polymyositis or dermatomyositis.

Autoantibodies

Antibodies are part of the immune system and usually help us to fight off infections by recognising and attacking viruses or bacteria. Some people’s immune systems develop autoantibodies, antibodies that attack their body’s own tissues. This can contribute to causing autoimmune diseases like polymyositis and dermatomyositis.

An anti-nuclear antibody test (ANA) tests whether your blood contains antibodies to the nucleus (the central part) of cells. About 80–90% of people with polymyositis and dermatomyositis test positive for ANA, but people without either condition can also test positive so it’s only helpful alongside all the other information about your symptoms.

Other autoantibody tests include ENA (extractable nuclear antigens) and the anti-Jo-1 test. These tests are sometimes positive in patients with polymyositis and dermatomyositis.

Other tests

These tests may not be able to diagnose polymyositis or dermatomyositis, but they can help to rule out other conditions.

Electromyography (EMG)

A thin electrode is inserted into your muscle to record electrical discharges from nerve endings that cause your muscles to move. An unusual pattern of electrical activity in a number of different muscles suggests you have polymyositis or dermatomyositis. This test isn’t painful but may be slightly uncomfortable.

Muscle biopsy

A small sample is taken out of one of your larger muscles (for example at the front of your thigh) and examined under a microscope. The site of the biopsy might be chosen using an MRI scan to assess the area. The part of the muscle that looks most badly affected on the scan will be the site of the biopsy. You’ll be given a local anaesthetic to numb the area while the sample is taken, but you may feel some discomfort for a few days afterwards.

The biopsy may show white blood cells sticking to the covering of your muscle and damaging the proteins that make your muscles contract. This causes weakness and fatigue.

You may need to have a repeat biopsy if your condition doesn’t improve with standard treatments. This is to check for the proteins found in people with inclusion body myositis (IBM).

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