Electrophysiology is the science of measuring the electrical potentials generated by the body. Neurons in the brain communicate with each other by generating small electrical signals. Similarly, contracting muscles also generate electrical energy that is used to signal muscle fibers to shorten. Electrophysiology measures the electrical signals from brain or muscle to determine whether there is a problem and what type of problem may be present. Some particular electrophysiologic tests include:
EEG: Voltages in the brain are measured using surface electrodes applied to the scalp. This is referred to as electroencephalography, or EEG. The voltages can be measured while the child is resting or, in some cases (using special techniques), while the child is moving. EEG is used for many purposes, including the detection of seizures, analysis of general brain functioning, and measurement of voltages associated with movement or sensation. Between 16 and 40 electrodes are attached to the scalp using temporary glue. The voltages generated by the brain are recorded over 30 minutes or longer.
SEP: Somatosensory-evoked potentials use EEG electrodes to record the response of the brain to a sensory stimulus. Usually a small electrical pulse is given at the wrist or behind the knee. The response in the EEG is then measured. The pulse needs to be repeated at least several hundred times in order to have enough of a signal to analyze. From this information, the evaluator may determine whether there is a delay in conduction to the brain, a blockage at any point, or abnormally low or high activity in the brain.
VEP: Visual-evoked potentials are similar to somatosensory potentials; however, the stimulation is applied as patterns or flashes of light seen by the eye, and the brain's response to the visual stimulus is then assessed with EEG electrodes.
ERG: The electroretinogram measures electrical activity of the cells in the retina. If there is a loss of retinal cells, the ERG may show abnormalities. This may be a helpful clue to diagnosis.
AEP/BAER: Auditory-evoked potentials, or "brainstem auditory-evoked response") are the responses of the brainstem and auditory regions of the cortex to brief sounds in the ear. In addition to determining abnormalities of hearing, AEPs may be helpful in finding subtle problems with the function of the brainstem.
EMG: Muscle voltages are measured using electromyography. EMG uses surface electrodes that measure the voltages on the skin, or needle electrodes that are inserted through the skin and directly into the muscle. The two different types of electrodes give different information; often both types are used in a particular child.
Surface EMG can tell about the strength of contraction and pattern of contraction in different muscles. Needle EMG can tell if there is injury to the muscle or an abnormal timing of activation, as occurs in the bursts of activity with dystonia or myoclonus. In some cases, an electrical pulse is applied to a nerve and the nearby EMG signal is measured. This is done to determine the speed of transmission along the nerve. It also helps to determine if there is a blockage in the nerve or where the nerve connects to the muscle. These latter tests are called "nerve conduction studies" (NCS).
TMS: Transcranial magnetic stimulation is a research technique that uses a magnet to generate a pulse that goes through the skull and directly stimulates the brain beneath it. This activates the nerve fibers controlling muscles, and a surface EMG electrode may be used to measure the response in the muscles. Just as stimulating a nerve may give information about conduction through the nerve, stimulating the motor areas of the brain gives information about conduction from the brain to the spinal cord and muscles. TMS is currently used only for research; however, in the future, it may become a more widely used clinical technique.
