Neurophysiology or electrodiagnostic testing refers to specialised investigations used in the diagnosis and prognosis of peripheral nervous system disorders. There are two main techniques:
1. Nerve Conduction Studies (NCS)
2. Electromyography (EMG)
Objectives of Performing Neurophysiology
• To localise a nerve lesion
• To characterise the nature of a nerve lesion
• To quantify the degree or extent of a nerve lesion
• To prognosticate the likely outcome of a nerve lesion
Nerve Conduction Studies (NCS)
The tests provide information on how fast and how well a particular nerve transmit impulses. It measures both the sensory and motor nerves. During the test, surface electrodes are attached to your skin. They are either stick-on or Velcro-on pads. The pulse-emitting electrode is placed directly over the nerve to be tested and brief electrical pulses are generated. It feels like brief buzzing sensation that is not too uncomfortable. The recording electrode would then pick up the impulses should the nerve conduct. By knowing the distance between the stimulator and recording electrodes and the time taken for the impulse to travel, conduction velocity can be calculated (this is generated by the machine). The machine would also measure the amplitude (size) of the transmission wave. NCS is very safe and is well tolerated.
EMG assesses the condition of the muscle to be tested as well as the motor neuron (nerve cell) which controls it. During a needle EMG, a fine needle electrode is inserted into a muscle to detect electrical signals within the muscle. The neurophysiologist ascertains if there is any spontaneous activity when the muscle is at rest, and records the extent of electrical activity when you slightly or maximally contract the muscle. There may be minor bruising where the needle has been inserted but this should resolve within several days.
Preparing for NCS/EMG
- Tell your neurophysiologist if you have a cardiac pacemaker or are on any blood-thinning medications (aspirin, clopidogrel, warfarin)
- Inform your neurophysiologist if you suffer from any bleeding disorder
- Take a bath or shower beforehand and do not apply lotions or creams
Interpretation of Results
It is important to appreciate that neurophysiology report is not absolute and the tests are subject to false positives and false negatives. A normal result in a symptomatic patient or an abnormal result in an asymptomatic patient is frequently encountered in clinical practice. For instance, a patient with convincing clinical signs of cubital tunnel syndrome may have a normal NCS (i.e. the values are within normal limits). Clinical and published evidence have shown us that decompression in such situation could still be beneficial. One possible explanation is that there has not been significant ulnar nerve damage that is detectable by the testing.
Conversely, it has been well established that the incidence of abnormal NCS increases with advancing age, even though the patients may have no clinical symptom. This becomes particularly important as NCS is now easily accessible. Neurophysiology should only be requested with a clear clinical question in mind. Any abnormal results can then be interpreted in the relevant clinical context.
• Neurophysiology is an extension of clinical assessment and should NOT substitute careful history-taking and clinical examination.
• Communication between the surgeon and the neurophysiologist is absolutely crucial to making meaningful interpretation of the test results.