This is a harmless, painless test that measures the electrical charges coming from brain cells. There are approximately 86 billion neurons in the human brain. The testing detects the patten of brain waves. Examples of abnormal patterns are abnormal slowing as seen in tumors, stroke, encephalitis as well as in dementias and encephalopathy. Another abnormal pattern is spike or spike and wave-wave or electroencephalographic seizure activity, all seen in epilepsy.
How is an EEG done?
The patient lies back on a bed or a couch and relaxes through the entire study, ideally falling asleep. EEG abnormalities are more commonly seen during sleep. Sleep deprivation before the test is usually helpful therefore. Metallic discs are applied to the surface of the scalp and these discs are connected by wires to a recording machine. Do not use hair conditioner, hair oil or hairspray before the test as it makes application of the electrodes more difficult. During the testing you may be asked to breathe fast and deeply for a few minutes or look at a flashing light and to sleep. The office routine study typically takes about 45 minutes.
Electromyogram (EMG) and Nerve Conduction (NCS)
Nerve conduction studies (NCS) and EMG (electromyograms) are very useful tests to diagnose the causes of weakness, numbness, tingling, cramp and pain coming from the peripheral nervous system or from muscles. There is minimal risk attached to this testing, it requires minimal effort from the patient and the testing provides immediate results. Persons with
pacemakers or cardiac defibrillators cannot have nerve conduction tests close to their devices, and those on anticoagulants need care at the EMG to ensure there is no active bleeding, particularly if muscles in the forearm or the anterior leg are to be tested. Do not use creams on the arms or legs on the day of the testing as this makes attachment of surface electrodes more difficult. Take your medications as usual. Taking an oral pain medication before the testing will not interfere with the results but is not necessary.
What is a nerve conduction study?
A nerve conduction study measures how fast nerves conduct and how much current they can conduct. The procedure is highly accurate and sensitive and is direct measure of nerve function. Rather like assessing traffic flow, it determines how fast traffic is moving down the nerve, and how many lanes are open. Discovering the sites of slowing or sites of lost traffic lanes localizes where the injury is along the nerve, and whichever of these two patterns is found correlates with injury either to the myelin sheath covering the axon or to the axon process of the nerve respectively. These 2 major patterns of neuropathy are called demyelinative and axonal
neuropathy respectively. Nerve conduction studies can also be used to test for fatigue of the neuromuscular junction – the place where the nerve meets the muscle to cause it to contract - and so detect disorders like myasthenia gravis, Lambert-Eaton syndrome and botulism.
How is a nerve conduction study done?
The doctor applies small thin sticky surface electrodes over the middle and end of the muscle belly to be tested. The patient lies relaxed on the examination couch. A small pulse of current is sent through stimulating electrodes delivered to the skin over the nerve. The stimulus travels through the scan then down the nerve to be received and recorded by the surface electrodes. This causes a mild tingling feeling. Knowing both the time it takes from the start of the pulse to the time it was received by the surface electrodes and the distance between these two sites allows a simple distance divided by time calculation to discover the nerve conduction velocity. Knowing the maximum voltage recorded by the surface electrodes is to measure the ability of the nerves to conduct current and so is a measure of the number of lanes or axons available, using the traffic metaphor for a nerve as explained above.
What is an EMG?
An EMG or electromyogram directly tests muscle contraction by measuring the electrical activity of the muscle at rest and during a contraction. Since muscles only contract after they have received a nerve stimulus, the EMG also indirectly assesses the nerves supplying that muscle. The EMG test is performed after the nerve conduction study, and both compliment each other in the analysis of the particular problem being investigated.
How is an EMG done?
The skin over the tested muscle is sterilized by wiping with an alcohol swab, then a tiny needle electrode is inserted into the muscle. When the patient contracts the muscle the monitor attached to the electrode records the electrical signals generated as both a visual display and as a sound. Deviations from normal provide information about the muscle fibers and the nerves conducting to the fibers.
Evoked Potential Testing
Evoked potentials are potentials generated in the brain by the technician providing a visual or sound or skin electrical stimulation. The test determines the presence or absence of potentials and the speed of conduction along the spinal cord and through the brain. This information is most helpful in determining the presence of demyelinating disease in these nerve pathways for example in multiple sclerosis.
How are evoked potentials done?
Visual evoked potentials are electrical potentials recorded over the scalp with surface electrodes while the patient looks at a checkerboard on a computer monitor screen. Somatosensory evoked responses are electrical potentials recorded by surface electrodes produced by a tiny electrical pulse applied to the wrist or ankle. Brainstem auditory evoked responses are electrical potentials recorded over the scalp with surface electrodes from a tone or clicking noise delivered through headphones. This testing takes approximately 1/2-hour to do.