Clinical neurophysiology is an area of medical practice that primarily focuses on measuring the function of the central and peripheral nervous systems, including the autonomic nervous system and muscles.
Clinical neurophysiology is an area of medical practice that primarily focuses on measuring the function of the central and peripheral nervous systems, including the autonomic nervous system and muscles. Physiologists who work in the discipline of Neurophysiology carry out tests that identify and characterise diseases of these areas. This field relies on these measurements of the ongoing function whether it is spontaneous or in response to a defined stimulus (EEG, EP). Each of these tests measure function by recording alterations in physiology as manifested by changes in electrical waveforms, force or electromagnetic fields. These measurements are then obtained by measuring a waveform that changes over time.
An Electroencephalogram (EEG) is one of the principle tools in clinical neurophysiology that records the ongoing electrical activity of the brain. It is used to assist in the diagnosis of many illnesses including seizure disorders such as epilepsy, infections affecting the brain known as encephalopathies and a variety of other neurological symptoms.
The Physiologist applies the EEG by using a specific measurement system, the most common being the International 10-20 System or the Modified Maudsley. It is a non-invasive and painless test that is obtained by placing electrodes on the scalp with a conductive paste. A video is recorded alongside the ongoing EEG to further assist in diagnosis in relation to clinical features.
A routine EEG generally lasts for 20-30minutes whereas as a sleep EEG can last for 60-90minutes. Overnight and prolonged recordings have a longer duration and are typically known as Long Term monitoring (LTM)
Long Term Monitoring
Long Term Monitoring uses the same principles as a routine EEG recording only it is recorded over a longer period of time, sometimes up to 2 weeks. This test is used as a more thorough evaluation of the person’s illness. It can be requested in the context of pre-surgical workup for epilepsy or to characterise a person’s events. It may also be used in the intensive care setting to continuously monitor a patient over time.
Invasive monitoring is a specialized and detailed way of confirming the exact area of the brain that is causing seizures and the exact areas of the brain being used for important tasks such as speech or movement. Invasive monitoring involves an operation to place electroencephalogram (EEG) electrodes, directly over the surface of the brain (grids or strips), or into the brain itself (depth electrodes). These electrodes are then connected to an EEG monitor so that the Physiologist can record the brain activity over several days.
Evoked Potentials (EPs) are recordings of electrical activity from the brain, spinal nerves, or sensory receptors in response to specific external stimulation. The range of routine evoked potential tests carried out are known as; Somatosensory Evoked Potentials (SSEPs), Visual Evoked Potentials (VEPs) and Brainstem Auditory Evoked Potentials (BAEPs).
SSEP: The Physiologist places electrodes over specific areas of the body on the upper or lower limbs. A small generator (electrode/probe) is used to create tiny electrical impulses that stimulate nerves in the limb such as the wrist or ankle. Impulses are usually not painful but may cause corresponding twitches (thumb) which is normal.
VEP: The Physiologist places electrodes on the scalp to obtain a recording in response to a stimulus such a checkerboard pattern on a computer screen or a flash light. These responses originate from the occipital cortex which is the area of the brain involved in receiving and interpreting visual signals. An adjunct to the VEP test is an electroretinogram (ERG). The ERG is a measurement of the physiology of the retina. ERG’s are used to evaluate potential causes of night/colour blindness.
BAEP: The Physiologist places electrodes on the head to obtain a recording in response to an aural stimulus. This is usually in the form of head phones that plays a series of ‘clicks’. The responses originate from relay structures within the brainstem.
These tests are helpful in evaluating a number of different neurological problems, including spinal cord injuries, visual disturbances and hearing loss. These test are carried out in the lab, on the ward or in the ICU setting.
Nerve conduction Studies (NCS)
Nerve Conduction Studies evaluate electrical potentials from peripheral nerves. Neurophysiologists use a small generator to stimulate the nerve with an electrical current and then record how long it takes the nerve impulse to reach the muscle. The patient may feel a mild and brief electrical shock from the stimulator. This test is generally carried out on patient’s that suffer from nerve conditions with symptoms such as numbness, tingling, muscle pain, muscle weakness, muscle cramping, or pain/loss of sensation. The areas usually studied are the face and the upper and lower limbs.
Intraoperative Neuromonitoring (IONM) involves the use of neurophysiological recordings for detecting changes in the function of the nervous system during orthopaedic and neurosurgical procedures. It is an effective method that provides real time monitoring of function that can reduce the risk of postoperative deficits in procedures where the nervous tissue is being manipulated, which can be of great value to the surgeon.
Two of the main types of testing carried out by the Neuro-physiologists are somatosensory evoked potentials and motor evoked potentials. One of the differences between clinical EPs and intraoperative EPs is the use of needle electrodes.
Examples of the neurosurgical and orthopaedic procedures that may use IONM are scoliosis corrective surgeries, tumour resections and treatment for aneurysms.
In a Neurophysiology Department, the Neurophysiologist performs the following tests:
- Electroencephalography (EEG)
- Sleep Deprivation Studies, using EEG
- Electromyography (EMG)
- Nerve Conduction Studies (NCS)
- Visual Evoked Responses (VERs)