For the recording of EEG we need
- Electrodes—To pick up the brain electrical wave activity and their attached wire to connect with the EEG machines.
- Amplifiers—To amplify these micro volts amplitude waves.
- Filters—To filter out unwanted (artifacts) waves.
- Writing units—To record these waves on paper.
Duration of EEG recording: EEG should be recorded for minimum of 30 minutes in normal routine hours and 60 minute for neonatal cases.
Electrodes usually consist of flat disc or cup (often gold or silver) connected to silver wires. In modern digital EEG machine amplifiers, filters and writing unit are within the computerised system.
Position of Electrodes
The 10–20 International System of Electrode Placement, where each electrode is 10–20% away from a neighboring electrode, is mostly used.
By convention electrodes placed on the left side of cerebral hemisphere are given odd numbers (e.g., Fp1, T3) while right sided electrodes have even numbers (e.g., Fp2, T4). The name includes the first letter of the general area of skull where electrode is placed and reveals lobe of the brain being recorded.
Fp1, 2—prefrontal
F3, 4—frontal
C3, 4—central
P3, 4—parietal
O1, 2—occipital
F7, 8—anterior temporal (records activity from anterior temporal region but placed on frontal bone)
T3, 4—mid-temporal
T5, 6—posterior temporal
A1, 2—ear
Fz—front vertex
Cz—central vertex
Pz—parietal vertex
Measurement for International 10–20 System of Electrode Placement
Four skull landmarks are used in the 10-20 system. The nasion—a point just above nasal bridge, inion—prominent part of occiput and right and left preauricular points, indentations just above tragus (Figs 1.1A to D).
Neonatal Electrode Placement
11 channels due to small head size (Fig. 1.1F)
Frontal and parietal channels are usually omitted.
Montages
EEG records brain electrical potential by recording the difference between the activity picked up by two electrodes. Montage refers to the different ways to connect electrodes (Fig. 1.1E).
Commonly two types of montages are used:
- Referential (monopolar)
- Bipolar
Referential montage: Involves recording the difference between an active electrode on the scalp and an inactive electrode placed over ear or nose or chin (called reference electrodes).
Bipolar montage: Involves recording the difference between two active scalp electrodes. Each channel of the EEG machine is connected to two different electrodes and the difference in activity picked up by each of these two electrodes is recorded on that one channel.
Montages should be arranged in an orderly sequence. Like for referential montage, anteriorly placed electrodes are placed on the first channel and the posteriorly placed electrodes on the later channel. Bipolar montages have electrodes in chains usually from front to back (anteroposterior) or side to side (transverse), from one electrode to its neighbour and then onto the next one, etc.
Paper Speed
Slow paper speed of 15 mm/sec is used for monitoring repetitive/periodic discharges as in SSPE.
- Periods of attenuation/ rhythmicity.
Faster paper speed of 60 mm/sec is used for study of asymmetrical onset of seemingly synchronous activity.5
Filters
A circuit which permits only the necessary signal to pass by varying the frequency response of an electric current is called filter. It excludes very slow and very fast activity and select the spectrum of frequencies which has the greatest clinical significance. Filters should be used in such a way that they do not eliminate spike/ sharp wave activity. It should not be used to clean up the artifacts and produce a pretty record. Usually, a low frequency is set at 1 Hz and high frequency filter at 70 Hz (so it does not allow waves below 1 Hz and above 70 Hz to pass through it).
Sensitivity
Generally, sensitivity in pediatric EEG is kept at 7 microvolt/mm (7–10 uV/mm). At this sensitivity, a voltage of 7 uV causes a pen deflection of 1 mm.The sensitivity is adjusted to see appropriate waveforms (Figs 1.4 and 1.6). If waveforms are of high amplitude, sensitivity should be kept at 10 uV/mm. Higher numeric value of sensitivity mean lower sensitivity and vice versa.
Activation Procedures
Activation procedures are used to enhance epileptiform activity in recording and thus improve diagnostic yield.
Hyperventilation—For 3–5 minutes are very important for provoking epileptiform discharges in Idiopathic Generalized Epilepsy (IGE). It should not be done in patients with significant cardiac and respiratory disease, suspected cerebral aneurysm, raised intracranial pressure.
Photic stimulation—From 3 Hz to 24–30 Hz is used routinely to induce photic sensitive discharges and for diagnosis of photosensitive epilepsy.
Sleep deprivation—Maximum sleep deprivation before EEG recording is advised. It may be difficult in very young patients, but is very useful in enhancing EEG abnormalities especially in focal seizures, rolandic epilepsy and Juvenile Myoclonic Epilepsy. It also helps to induce sleep in children without sedatives for sleep record in EEG. EEGs should have both sleep and awake record. The diagnostic yields of sleep deprived EEG in patients with normal awake EEG is enhanced to 40%.