Electrophysiologic tests provide information about the nerves and muscles of the spine, and probe for the effects of disease on neuromuscular function of the spine. Their purpose is to identify a neurologic deficit that can explain a patients pain.
The spinal cord and nerves run the length of the spine behind the vertebral bodies and discs. The spinal nerves pass out of the spine into the arms from the cervical region, to the chest and abdominal wall from the thoracic region, and into the legs from the lumbosacral region of the spine. Three electrodiagnostic tests are available to examine the spinal cord, its nerves, and its muscles: electromyography (EMG), nerve conduction studies (NCS), and somatosensory evoked potentials (SSEP) testing.
Electromyography tests the functions of the peripheral nerves and the way that the nerves affect muscle function. During an EMG study, fine needle electrodes (like accupuncture needles) are placed into muscles of the arms and/or legs being tested and along the muscles surrounding the spine. Sometimes skin surface electrodes are used. The nerve is stimulated nearer the spine and impulses are recorded from the muscle electrodes. Muscle twitching and contraction is felt by the patient; some patients report a level of discomfort with this test. EMG testing may be used to diagnose a pinched nerve in the spine (radiculopathy), and can also be used to differentiate radiculopathy from other causes of arm and/or leg pain such as peripheral neuropathy (primary disease of the nerve endings, most commonly seen in patients with diabetes) and peripheral entrapment neuropathy (a pinched nerve outside of the spine such as carpal tunnel syndrome).
Nerve conduction studies make use of the nerves ability to conduct an electrical signal. A nerve is stimulated by an electrical signal from a skin electrode placed over the nerve, and a transmitted signal is recorded at a selected site by another skin electrode somewhere along the same nerve. If the signal travels normally, the nerve is healthy. Often more than one nerve is tested. In NCS tests, the patient will feel an electrical impulse; muscles may twitch. The patients role is passive, to relax as much as possible.
Somatosensory evoked potentials are of most use when there is the suspicion of spinal cord dysfunction, whereas NCV and EMG testing assess the neuromuscular system outside the spinal cord. Evoked potentials are tiny electrical signals recorded from the spine and head following stimulation of nerves in the limbs or brain.
These tests, typically in combination, tell the physician if there is demonstrable dysfunction of the nervous system below the brain. They are often able to localize at what level that dysfunction is occurring and how severe the dysfunction is. They may also be able to provide some information about the likelihood of neurologic recovery with treatment.