EEG and neurophysiological examinations — the brain's electrical activity.
10-20 system, preparation and the artefacts you most often encounter
Where EKG shows the heart's electrical rhythm, EEG (electroencephalography) shows the cerebral cortex's electrical activity. As a neurophysiology assistant, EEG is one of your core tasks, and the competence objectives for the speciality demand that you can independently perform and explain EEG on both adults and children, cf. the curriculum for hospital technician assistant education from the Professional Committee for Hospital Technician Assistant Education (FUHA), which is administered by SEVU.
§10-20 system — the map of the scalp
To be able to compare EEG recordings between patients, clinics and countries the electrodes must sit in exactly the same place each time. This is ensured by the international 10-20 system which was defined in 1958 after discussions at the first international EEG congress in London in 1947. The system is based on four fixed anatomical points on the head — nasion (root of nose), inion (occipital bump) and the two preauricular points in front of the ears — and then distributes the electrodes at intervals of 10% and 20% of the distance between these points. That is what gives the system its name.
| Letter | Area | Example |
|---|---|---|
| Fp | Frontal (forehead) | Fp1, Fp2 |
| F | Frontal | F3, F4, Fz |
| C | Central | C3, C4, Cz |
| T | Temporal | T3/T7, T4/T8 |
| P | Paris Agreement | P3, P4, Pz |
| O | Occipital (back of neck) | O1, O2 |
Odd numbers (1 3 5 7) sit on the left side even numbers (2 4 6 8) on the right side and the letter 'z' (zero) marks electrodes on the center line such as Cz and Pz. Since 1985 there has been a further developed version the 10-10 system with more electrodes for research use but the 10-20 system is still the standard in clinical EEG.
§Why take an EEG?
- 01Clarification of epilepsy and seizure-like conditions.
- 02Assessment of consciousness level e.g. after brain injury.
- 03Sleep examinations, where EEG is included along with other measurements.
- 04Follow-up on known brain disease.
§Preparation and electrode application
Before recording you measure head circumference and mark points according to the 10-20 system. Skin under each electrode is gently cleaned to remove oil, and a conductive electrode paste or gel is applied so the contact resistance (impedance) is low enough for a clean signal — most EEG machines have built-in impedance measurement you check before recording starts. If the patient is a child, you should spend more time on reassurance and explanation, and adjust the pace so the child is not frightened of the electrodes.
§Artifacts in EEG
- 01Blink and eye movements cause large fluctuations in the frontal channels.
- 02Muscle tension in forehead or jaw gives rapid, irregular noise.
- 03Sweat can change the electrodes' impedance during work.
- 04Mains hum (50 Hz) from power in the room.
“A good aftertreatment is systematic. If you follow the same sequence every time you work faster and forget less.”