EEG- Artefacts and Benign Variants Dr.Roopchand.PS Senior Resident Academic Dept. of Neurology, TDMC, Alappuzha
Artefacts:• Recorded signals of non cerebral origin.• Poses great problem in EEG reading.• Recognition and elimination are therefore important.• Mechanical o Electromagnetic, electrostatic, radio frequency, mains born, external electrical interference. o Instrument artefact. o IV drip artefact o Respirator artefact.• Biological o Eye movement, cardiac/pulse, respiratory, electromyographic, movement, cutaneous, glossokinetic, Msic
Electro magnetic interference:• Due to AC current.• Induces fluctuating magnetic fields to EEG leads.• Opposite fields may cancel out.• Proper earth connection of apparatus.• Patient bed may be connected to earth socket.
Electrostatic Interference:• Due to capacitance property of objects.• Patient or electrode may pick up capacitance potentials from sources in their vicinity.• Reduced by moving the patient from the source.• Proper earth connection.
Radio frequency Interference:• Signal between 120-400Hz• Especially diathermy equipments.• Radio frequency filters may be used.
• Mains born interference: due to fluctuating power supply. o Stabilized power supply can avoid this artefact.• External electrical interference: fluorescent light , ac, refrigerator…
Instrument Artefact:• Electrode artefact: due to change in resistance, capacitance and inductive reactance of two electrodes compared to others. o Individual electrode impedance should be less than 5kΩ.• Bizarre potentials are seen o Confined to two adjacent channels in bipolar chain . o Confined to one channel or one hemisphere in reference recording.• Waves are markedly different from background activity.• Appear as spike like potentials.
• Ground electrode artefact: due to defective grounding. o Produce 60Hz AC artefact.• Machine fault: loss of main, blown fuse, selective malfunctioning of one system.• Intravenous drip artefact: may produce stereotyped spike like potentials at fixed intervals. o Infusion pumps produce stereotyped brief spike like transients.• Respirator artefact: single high voltage or multiple high/low voltage transients of 2 to 40 Hz.
Eye movement Artefacts:• From eye ball or muscles around orbit.• Eye ball act as a dipole.• Eye movements produce AC fields.• Detectable in electrodes near globe o Fp1, Fp2, F7, F8• Negative deflection when Fp1 and Fp2 are positive.• Eye ball movement monitoring.• Blinking produces upward movement of eye ball. o Fp1 and Fp2 are positive.• Repetitive blinking can mimic FRIDA(Frontal Intermittent Rhythmic Delta Activity) or triphasic wave.
• More frequent blinking can simulate theta activity.• Horizontal or vertical nystagmus can produce aretfacts simulating theta activity.• Asymmetric eyeball artifacts can be seen in unilateral ophthalmoplegia and enucleation.• Can be abolished by keeping the eye closed or simultaneously recording eye ball movements.• IPS may produce photomyoclonic repose from orbicularis occuli and frontalis.
Cardiac and respiratory artefacts:• Normally electrical field of heart extends up to base of the skull.• In short necked persons can extend up to vertex.• Normally can extend up to ear.• Cardiac artefacts are mainly due to QRS complexes.• Positive in A1 and negative in A2.• Recognized by characteristic from and regularity.• Interfere in diagnosing electro cerebral silence.• Respiration may cause change is electrical axis of the heart. o Produce fluctuation in amplitude of waves.
Pulse artefact:• Electrode near or overlying a small scalp artery.• Systolic pulse alter the impedance.• Waves are periodic• Smooth and sharply contoured• Time locked to ECG by 200msec delay in peak.
Electromyographic artefact:• Brief single or multiple myogenic potentials.• Located in the temporal, frontal and occipital areas.• Frontal epileptiform discharges can mimic them.• Avoiding jaw clenching and frowning will abolish the waves.• Essential tremor and Parkinsons tremor produce 4- 6Hz sinusoidal artefacts.• Hemi facial spasm can also produce EMG artefacts.
Movement Aretacts:• Due to combination of instrument and biologic factors.• Related to observed activity of the subject.• Difficult to differentiate from discharges during GTCS.• Significant reduction possible by proper electrode placement and use of self retaining electrodes.
Cutaneous Artefacts:• Perspiration artefact: perspiration causes slow shift of electrical baseline due to change in impedance.• Sweat gland produces slow changing electrical potentials recorded by electrodes.• Produce slow wave forms of more than 2 sec.• Perspiration artefact + background slowing : hypoglycemia.• Reduced by lowering the room temparature and wiping the brow with alcohol.
• Galvanic skin response: represent sympathetic skin response produced by sweat gland and changes in skin conductance in response to sensory or psychic stimuli.• Slow waves of 0.5 to 1 sec, lasting for 1.5 to 2 sec with two to three prominent phases.• Can be confirmed by simultaneous recording of sympathetic response of palm.
Glossokinetic Artefact:• Tongue has a DC potential. o Tip negative compared to base• Tongue movements produce artifacts o Bursts of diffuse delta like activity, accompanied by muscle artefact.• Artefacts confirmed by asking the patient to pronounce lah lah lah.• Sucking by infants can also produce such artefacts.• Hiccpus, dental fillings.
Rhythmic activities:• Rhythmic temporal Theta bursts of Drowsiness or psychomotor variant pattern.• Trains of rhythmic theta waves of 5-7Hz.• Flat top, sharp contour or notched appearance.• Temporal location, maximal in the mid temporal electrodes.• Simulates psychomotor seizure discharge.• Seen in0.5-2% adolescent and adults in wakeful or drowsy state.,
Sub clinical rhythmic theta discharge in adults:• Rhythmic sharp theta waves of 5-7hz.• Widespread with highest amplitude at parietal and post. Temporal regions.• Lasts for 20 sec to few minutes.• 50% adults spontaneously, > 50yrs and prominent during HV.• In 50% : single monophasic sharp or slow wave followed one to several seconds later by another sharp wave and progress to discharge at a shorter interval reaching up to 7Hz.• Seen in relaxed and drowsy states.
• Midline theta rhythm: 5-7Hz smooth sinusoidal , arciform waves.• Central and vertex leads.• Present during wakefulness and sleep.• Benign.• Frontal Arousal rhythm: seen in children with minimum brain dysfunction.• Seen during arousal from sleep.• Trains of 7-10HZ waves• Frontal location.• Lasting up to 20 sec.• Disappears when child is fully awake.
Benign Epileptiform Variants:• 14-16Hz positive bursts: seen in drowsiness and light sleep.• Rhythmic trains of arc shaped waveforms with alternating positive spiky components and a negative smooth rounded wave form.• Sharp phase 0,5 to 1 sec• Usually14Hz.• Maximum amplitude in the posterior temporal region.• Appears by 3-4yrs, peaks in adolescence, decrease in old age.• Seen in o Head ache , vertigo, emotional instability, thalamic and hypothalamic epilepsy etc..
• Small sharp spikes: benign epileptiform transients of sleep, benign sporadic sleep spikes.• During drowsiness and light sleep.• Monophasic or diphasic spikes, 15µV, <15msec, abrupt ascending and steep descending.• May have single after coming slow wave.• Seen best in temporal and ear leads.• Mimic temporal epileptiform discharges. o Does not distorts background o Not followed by rhythmic slow wave activity. o Disappear with deeper levels of slow wave sleep.
• 6Hz spike and wave: 5-7Hz bursts lasting for 1-2secs.• Called phantom spikes: brief, small amplitude.• Appear mainly in relaxed wakeful state, drowsiness o Disappears in deep sleep.• Diffuse bilateral, prominent in anterior and posterior locations.• Wicket spikes: intermittent trains in clusters of arciform/single spike.• Wicket like appearance.• 6-11Hz, temporal region B/L during drowsiness and light sleep.• No after coming slow component and back ground slowing