Electrodiagnostic testing encompasses a range of specialized tests, including nerve conduction studies (NCS) and needle electromyography (EMG), that are used to evaluate the conduction of electrical impulses down peripheral nerves. These tests should be considered and performed only after a careful history and physical examination, which are sometimes sufficient to establish a diagnosis of neuromuscular dysfunction without further testing. However, in some cases, the subtlety of sensory or motor deficits necessitates further workup for a conclusive diagnosis.
Nerve conduction studies and needle EMG are commonly performed by neurologists to assess the ability of the nervous system to conduct electrical impulses and to evaluate nerve/muscle function to determine if neuromuscular disease is present.
Nerve Conduction Studies
For this test, a series of surface electrodes are placed at different locations over specific muscles. The nerve is stimulated at one site and recorded at a different site to determine if the nerve is conducting appropriately.
Each electrical stimulation is recorded as a waveform on a computer and analyzed by the electromyographer/neurologist performing the test.
Standard nerve conduction studies typically include motor nerve conduction, sensory nerve conduction, F waves, and H reflexes.
Sensory and motor nerve conduction studies involve analysis of specific parameters, including latency, conduction velocity, and amplitude. Onset latency is the time it takes for the stimulus to initiate an evoked potential and reflects the conduction along the fastest fibers. Peak latency is the latency along the majority of axons and is measured at the peak amplitude. Both are affected by the state of the myelination of the nerve.
The conduction velocity along the nerve also depends on the state of myelination and is often decreased in disorders or trauma that affects nerve myelination, although it may be normal if a few myelinated axons remain intact. Reduction of amplitudes of recorded responses generally indicates a loss of axons.
These studies, in conjunction with the physical examination and correlation to a set of normative values, assist the electromyographer in diagnosing a multitude of nerve disorders, including entrapment neuropathies, brachial plexopathies, and polyneuropathies.
Electromyography records the electrical activity from various muscles. This involves inserting a small, disposable needle into various muscles within certain limbs and perhaps the paraspinal musculature. The electromyographer will ask you to relax or contract the specific muscle while the needle is in place. EMG is a low-risk procedure, and complications are rare. There's a small risk of bleeding, infection and nerve injury where a needle electrode is inserted. The Neurologist conducting the EMG will need to know if you have certain medical conditions. Tell the neurologist if you:
Have a pacemaker or any other electrical medical device
Take blood-thinning medications
Have any bleeding disorder
For preparation take a shower or bath shortly before your exam in order to remove oils from your skin. DO NOT apply lotions or creams before the exam.