Electromyography and Nerve Conduction Studies

Electromyography (EMG) and nerve conduction studies (NCS) diagnose different causes of arm or leg pain, numbness or weakness. They are called electrodiagnostic studies because they use a mild electrical current to test muscles and nerves.  

You might have these tests if an MRI does not give the doctor enough information about your symptoms or if your doctor suspects you could have more than one health problem. Electrodiagnostic studies can also help your doctor diagnose general nerve problems such as polyneuropathies, which can be caused by conditions such as diabetes. With spine problems, doctors often use these studies to evaluate a pinched nerve in the neck or lower back (radiculopathy).

The tests are done on your arms, legs or both. It takes about 25 to 30 minutes to test one arm or leg. Your doctor will review your medical history and do a short physical examination to identify the nerves and muscles that need testing. The electromyography (EMG) portion of the nerve study only takes a few minutes. Before this, your doctor will tape small electrodes over different muscles to test individual nerves. 

Once the electrodes are in place, your doctor will use a stimulator to send a mild electrical current to the nerves that connect to the muscle. It feels like a small static electricity shock. Some patients say it feels like a "strong tap."

For the EMG test, your doctor will insert a very small needle, about the size of an acupuncture needle, one-eighth to one-quarter inch under your skin and into specific muscles. Your doctor can test three to seven muscles at a time, so you might have several small needles. Your doctor will ask you to relax and then gently contract (squeeze) the muscle. The muscle sends signals that the electrode picks up and the EMG equipment records on a graph. This part of the test only takes a minute or two. Your doctor looks at the graph to help diagnose your condition.  

The EMG test is similar to an electrocardiogram (EKG), where the electrode records the electrical signals your heart sends when it beats. With EMG and NCS, the recording electrode shows any nerve damage or muscle weakness. Your doctor can tell if your symptoms are coming from a certain nerve in your back or one in your arm or leg (such as carpal tunnel syndrome from a pinched nerve in your arm).