Electroencephalogram (EEG)

The many nerve cells that make up the brain produce continuous electrical activity when a person is awake, asleep or even in a coma. This can be recorded using small metal discs electrodes, which are placed on the scalp. The electrical signals are then amplified by specialised equipment to produce EEG tracing as wavy lines, representing the fluctuations in electrical activity from moment to moment.

The EEG of someone sitting quietly with their eyes closed doing nothing in particular have a characteristic appearance. In this state, the alpha rhythm will commonly be recorded from the back of the head. This is one of the brain’s ‘resting rhythms’. It may become slowed or disappear altogether in many conditions affecting the brain, such as infections, coma or dementia. When the eyes are opened, the alpha rhythm may either disappear altogether or become less prominent.

During the EEG recording activation procedure such as hyperventilation may be carried out. This will commonly produce a change in the brain’s electrical activity and may bring out abnormalities not otherwise seen in the EEG. In children with absence seizures, hyperventilation may well provoke a fit. Observing the EEG during the fit will allow the diagnosis to be confirmed.

The second activation procedure used routinely is photic stimulation. Photic lights are known to produce a standard response in the brain, but in some people they provoke abnormal responses, including epileptic fits. People with photosensitive epilepsy may find that things such as disco strobe lighting or sitting too close to a flickering TV screen bring on fits. Photic stimulation during the EEG will help identify people who are photosensitive.

An EEG examination provides the only objective sign to show that an epileptic seizure is primarily due to the disturbance of brain function and also resulting from any one of the epilepsies.  An ictal EEG will diagnose epilepsies at the highest probability rate possible.  Interictal EEG would provide sub-clinical signs of epilepsy.  A negative EEG does not exclude the possibility of epilepsy.

Sleep EEG

If a routine EEG has been normal or has shown features which are suspicious but not definite enough to support a diagnosis of epilepsy, a Sleep EEG with sedative agents like melatonin or sleep deprived EEG may be carried out. Abnormalities may become more apparent in the Sleep EEG during drowsiness and light sleep, particularly the abnormal brain activity seen in epilepsy.

Ambulatory EEG

There are limitations of routine EEG recordings in detecting the often intermittent abnormalities seen in epilepsy. One way to improve the information gained is to perform what is known as an Ambulatory EEG recording. This involves recording the brainwaves of someone who is walking around, freely mobile and not confined to the testing room in an EEG department.

Standard EEG electrodes are placed on the scalp, but a smaller number than with a routine recording. The wires from these electrodes are plugged into a small portable relay box and the EEG signals are recorded on a hard drive or a memory card in a battery-operated recorder worn around the chest or waist.

This technique is used with both adults and children. There are two advantages to Ambulatory EEG recording. First of all, it allows the EEG to be recorded over a much longer period than a routine EEG – for several days if need be. This greatly increases the chances of detecting abnormalities. The second advantage is that it may be possible to actually record the EEG during an attack, particularly if these are occurring quite often. This may allow doctors to distinguish whether the attacks are epileptic or pseudoseizures.

Video-Telemetry or Long Term Video EEG Monitoring (LTEM)

This is a more sophisticated method of recording the EEG over long periods, which makes use of both EEG and video recording. This is done in hospital, either on one of the general wards or in a specialised LTEM  unit. A standard set of EEG electrodes is placed on the scalp and these are connected to a small relay box worn around the chest or waist. This in turn is connected by a long cable to the EEG recording machine, which stands at the bedside. The machine also has a video camera, which is directed at the person having the test.

The video image and the EEG can be viewed together on a split-screen display, with the EEG on one side and the video image on the other or separate monitors can be used for the two sets of information. This method has the advantage over Ambulatory EEG that the doctor is able to study exactly what happened during any attacks on the video and match this up with the simultaneous EEG recording.

Video-telemetry is valuable in trying to establish whether attacks are epileptic or not and in determining the type of epilepsy. It also has a more specialised application in assessing whether people whose epilepsy has been difficult to control with drug treatment might benefit from an operation to treat their seizures.

How long a person will need to be in hospital for video-telemetry will depend on how often attacks are occurring, but it is usually between 2 and 5 days. If anti-epileptic drugs are being taken, the dose will often be reduced or the drugs may even be stopped altogether to maximise the chances of recording fits over a reasonable period of time.

Evoked Potentials

Evoked Potentials (EP) are electrical responses produced in the nervous system by stimulation of some kind. Clinical neurophysiology is most commonly concerned with stimulation of one of the senses - usually vision, hearing or the sensory nerves in the limbs. A single EP resulting from one stimulus is normally quite small and difficult or impossible to distinguish from the background EEG activity.

The evoked responses, to many stimuli, rather than just one, are collected and averaged the signal emerges clearly. All the electrical activity not directly related to the stimulus is averaged out and disappears from the recording, allowing the response caused by the stimulus itself to be seen quite easily. Somatosensory Evoked Potentials (SEPs or SSEPs) measure the electrical response in the brain and spinal cord when a nerve in an arm or leg is stimulated with a small electrical pulse. There may be a delay in the passage of the signals up the spinal cord or through the brainstem, this will show up in the test, which may also give information about where in the nervous system this delay has occurred.

The test is also used to help with the diagnosis of a range of other disorders known to produce abnormal SEPs, including multiple sclerosis,  being used during spinal surgery to monitor the spinal cord function. This can reduce the risk of serious injury to the spinal cord during this type of operation.

Somato sensory evoked potentials (SSEPs) are usually recorded by stimulating the median and tibial nerves which assess the dorsal column  or medial leminiscal functions.

Electodiagnosis of Visual Pathways & Retina

Visual Evoked Potentials (VEPs)

Visual evoked potentials (VEPs), also sometimes referred to as visual evoked responses (VERs), are the electrical responses recorded from the occipital area of the brain.

Although a traversing checkerboard pattern is most commonly used to record VEPs, in some circumstances a simple flash of diffuse light may be used instead (flash VEPs). This is commonly done in assessing visual function in babies and young children, who are not able to focus on a normal checkerboard pattern. Flash VEPs may also be used in the further assessment of people who have not shown any response to the standard checkerboard pattern.