EEG Glossary and Polarity 2024-2025

EEG GLOSSARY FOR INTERPRETATION AZRA ZAFAR FCPS MEDICINE, FCPS NEUROLOGY ASSOCIATE PROFESSOR, DIVISION OF NEUROLOGY, COLLEGE OF MEDICINE/ IAU...

EEG GLOSSARY FOR INTERPRETATION AZRA ZAFAR FCPS MEDICINE, FCPS NEUROLOGY ASSOCIATE PROFESSOR, DIVISION OF NEUROLOGY, COLLEGE OF MEDICINE/ IAU CONSULTANT NEUROLOGIST KFHU EEG GLOSSARY (TERMS WILL BE DISCUSSED ACCORDING TO THE ALPHABETICAL ORDER). This glossary includes the terms most commonly used in clinical EEG. It is based on the previous proposals (Chatrian et al., 1974, Noachtar et al., 1999, Noachtar et al., 1999) and includes terms necessary to describe the EEG and to generate the EEG report. Revision based on 2017 guidelines are also mentioned. Keynote; All EEG phenomena should be described as precisely as possible in terms of frequency, amplitude, phase relation, waveform, localization, quantity, and variability of these parameters. Chatrian, G.E., Bergamini, L., Dondey, M., Klass, D.W., Lennox Buchthal, M., Petersén, I., 1974. A glossary of terms most commonly used by clinicalelectroencephalographers. Electroenceph. Clin. Neurophysiol. 37, 538–548. Kane N, Acharya J, Benickzy S, Caboclo L, Finnigan S, Kaplan PW, Shibasaki H, Pressler R, van Putten MJAM. A revised glossary of terms most commonly used by clinical electroencephalographers and updated proposal for the report format of the EEG findings. Revision 2017. Clin Neurophysiol Pract. 2017 Aug 4;2:170-185. doi: 10.1016/j.cnp.2017.07.002. Erratum in: Clin Neurophysiol Pract. 2019 Jun 15;4:133. PMID: 30214992; PMCID: PMC6123891. BASIC PRINCIPLE The EEG report should follow a standard format that includes a factual description and a clinical interpretation of the EEG record. The interpretation of the EEG requires knowledge of the patient’s age, past medical and medication history, their clinical condition during the EEG, particularly level of consciousness and ability to co-operate. The EEG interpretation summarizes the results of the EEG and gives a clinical interpretation in light of the diagnosis and the questions posed by the referring physician. The terminology of the EEG interpretation should follow common neurological and clinical practice and use terms understandable to other physicians not specialized in EEG. ABSENCE SEIZURE: Absence seizure: A generalized seizure type 3 Hz spike-slow wave ACTIVATION PROCEDURE: Any procedure designed to modulate EEG activity, for instance to enhance physiological waveforms or elicit abnormal paroxysmal activity. Examples include: eye closing, hyperventilation, photic stimulation, natural or drug-induced sleep, sensory stimulation (acoustic, somatosensory or pain).  Activity, EEG An EEG wave or sequence of waves of cerebral origin. Active sleep Normal sleep stage in neonates characterized by eye closure, intermittent periods of rapid eye movements, irregular respirations and scant body movements. ALIASING: Distortion of the EEG signal leading to misidentification of frequency, , which occurs when the signal is sampled at less than twice the highest frequency present. ALPHA BAND Frequency band of 8–13 Hz inclusive. Alpha wave Wave with duration of 1/8–1/13 s (77– 125 ms) ALPHA RHYTHM: Rhythm at 8–13 Hz inclusive occurring during wakefulness over the posterior regions of the head, generally with maximum amplitudes over the occipital areas. Amplitude varies but is mostly below 50 micV in the adult, but often much higher in children. Best seen with the eyes closed, during physical relaxation and relative mental inactivity. Blocked or attenuated by attention, especially visual, and mental effort. AMPLITUDE, EEG: It is the height of a waveform representing the voltage of the change of EEG signals, usually measured in microvolts (µV). The amplitude of an electroencephalogram (EEG) wave is the measure of voltage across the brain at any given time. ARRHYTHMIC ACTIVITY: A sequence of EEG waves with an inconstant periodicity. AROUSAL: Change from a lower to a higher level of alertness as manifest in EEG activity. Abrupt change to a lighter sleep with the possibility of awakening as the final outcome ARRAY, ELECTRODE: A regular arrangement of electrodes over the scalp or brain, or within the brain substance. Synonym: electrode montage. ARTIFACT: (1) A physiological potential difference due to an extracerebral source present in EEG recordings, such as eye blinks and movements, electrocardiogram (ECG) or muscle contractions (EMG). (2) A modification of the EEG caused by extracerebral factors, such as instrumental distortion or malfunction, movement of the patient, or ambient electrical noise. ASYMMETRY: Unequal amplitude, frequency or morphology of EEG activity in channels over homologous areas of the two hemispheres.  ASYNCHRONY The non-coherent occurrence of the EEG activities over regions on the same or opposite sides (hemispheres) of the head. For example, two similar waveforms occurring at separate electrodes or channels, but not simultaneously due to a time lag between the channels.  Attenuation: Reduction in the amplitude of EEG activity (for example, the alpha rhythm is usually attenuated or blocked on eye opening). Augmentation: Increase in the amplitude of EEG activity (for example the alpha rhythm is characteristically augmented upon closing the eyes). BACKGROUND ACTIVITY: Any underlying EEG activity representing the setting in which focal or transient activity, either normal or abnormal, appears and from which such underlying pattern is distinguished. BASAL ELECTRODE: Any electrode located in proximity to the base of the skull (see foramen ovale electrode, nasopharyngeal electrode, sphenoidal electrode).  Beta band: Frequency band of 14–30 Hz inclusive. Beta rhythm or activity: Any EEG rhythm between 14 and 30 Hz (wave duration 33–72 ms). Most characteristically recorded over the fronto-central regions of the head during wakefulness. Amplitude of fronto-central beta rhythm varies but is mostly below 30 µV. BILATERAL INDEPENDENT PERIODIC DISCHARGES (BIPDS): BIPDs are 2 bilateral independent (i.e. asynchronous) surface-negative bi- or di-, tri- or poly-phasic complexes consisting of spike, sharp, or polyspike components, with variably following slow waves, lasting 60–600 ms (typically 200 ms) that occupy at least 50% of a standard 20 minute EEG. Amplitude ranges from 50 to 150 µV (occasionally up to 300 µV), which may be asymmetric, Usually recurring at 0.5–2 c/s (but very occasionally with intervals of upto 10 s). BILATERAL INDEPENDENT PERIODIC DISCHARGES (BIPDS): BIPHASIC WAVE: Complex consisting of two wave components developed on alternate sides of the baseline. Synonym: diphasic wave. BLOCKING Apparent, temporary obliteration or attenuation of EEG rhythms in response to physiological or other stimuli, such as eye opening on alpha rhythm, or a change in state/ attenuation BUILD-UP: to describe progressive increase in voltage of EEG activity or the appearance of slow waves of increasing amplitude. For example during hyperventilation when often associated with decrease in frequency. Sometimes applied to seizure pattern. BUILD UP PHENOMENON BUILD UP PHENOMENON DURING HYPERVENTILATION Britton JW, Frey LC, Hopp JLet al., authors; St. Louis EK, Frey LC, editors. Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants [Internet]. Chicago: American Epilepsy Society; 2016. Figure 46. [Pediatric EEG: build-up response becomes...]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK390356/figure/f46/ BURST A group of waves with a minimum of four phases and duration longer than 500 ms which appear and disappear abruptly and are distinguished from background activity by differences in frequency, form and/or amplitude. BURST SUPPRESSION: Pattern characterized by paroxysmal bursts of theta and/or delta waves, at times intermixed with sharp and faster waves, alternating with intervening periods of attenuation or suppression (below 10 mV) lasting more than 50% of the record. CAP, HEAD A cap that is fitted over the head to hold electrodes in position. Synonym: electrode cap. CHANNEL: Complete system for the detection, amplification and display of potential differences between a pair of electrodes, or a computed reference (for example common average reference). Digital EEG machines simulate a multichannel display by tracing several voltage time plots on a visual display. COMPLEX: A sequence of two or more waveforms having a characteristic composite morphology; and when recurring are seen with a fairly consistent form, distinguished from background activity. (For example see spike-and-slow-wave complex). COMPLEX CONTINUOUS EEG (CEEG): Prolonged EEG recording and often analysis for monitoring of electrical brain activity. Data may be collected with analog or digital systems, the latter enabling post- acquisition processing with a number of quantitative EEG techniques. cEEG is generally practiced in the intensive care unit with application varying according to the clinical situation CORTICAL ELECTRODE Electrode applied directly upon or inserted in to the cerebral cortex. CYCLES PER SECOND (C/S) Unit of frequency defined as the number of complete cycles in one second. Synonym: hertz (Hz).  Delta band: Frequency band of 0.1–30 to 80 Hz. Gamma rhythm or activity: An EEG rhythm above >30–80 Hz (wave duration 12.5–33 ms). Comment: most commonly recorded with intracranial electrodes from actively engaged or driven neural networks. GENERALIZED PAROXYSMAL FAST ACTIVITY (GPFA): Bilateral synchronous bursts of spikes of 2–10 s duration, with frequency between 10 and 25 Hz (typically around 10 Hz) and maximum in the frontal regions. GPFA is considered a feature of Lennox-Gastaut syndrome. GENERALIZED PAROXYSMAL FAST ACTIVITY (GPFA): GENERALIZED PERIODIC DISCHARGES (GPDS): Previously termed as generalized periodic epileptiform discharges (GPEDs). generalized, synchronous, periodic or quasi-periodic complexes that occupy at least 50% of the record. They are high amplitude (typically >100 µV) and have duration of about 0.5 s, with an intervening background activity amplitude usually not more than 35 µV. GENERALIZED PERIODIC DISCHARGES (GPDS): GRAPHOELEMENTS Any EEG pattern (transient, potential or rhythm) that is distinguished from the ongoing background activity, which may be physiological or pathological.  Hyperventilation: Deep and regular respiration performed for a period of several minutes. Used as an activation procedure. Intermittent photic stimulation: Delivery of intermittent flashes of light to the eyes of a subject. Used as EEG activation procedure. Synonym: photic stimulation (PS). LATERALIZED PERIODIC DISCHARGES (LPDS) LPDs are unilateral surface negative discharges of spike, sharp or sharp slow- wave polyphasic morphology, usually lasting from 100 to 300 ms that typically recur at quasiperiodic intervals of up to 3/s. LATERALIZED PERIODIC DISCHARGES (LPDS):  Montage: The arrangement or array of channels on the EEG machine display, defined by the exploring and reference electrodes Morphology: Refers to the form of EEG waves (i.e. their shape and physical characteristics). Multifocal: Three or more spatially separated independent foci :Monomorphic : Existing only in one form Monorhythmic: Having only one rhythm MULTIFOCAL EPILEPTIC DISCHARGES PAROXYSM Phenomenon with abrupt onset, rapid attainment of maximum and sudden termination, distinguished from background.  Periodic discharges (PDs): Repetition of a waveform with relatively uniform morphology and duration, with a quantifiable inter-discharge interval between consecutive waveforms, and recurrence of the waveform at nearly regular intervals. PDs may be generalized (GPDs), lateralized (LPDs), bilateral independent (BIPDs). PHASE REVERSAL Phase reversal: Simultaneous trace deflections in opposite directions from two or more channels in a bipolar recording montage. POLYGRAPHIC RECORDING Polygraphic recording: Simultaneous monitoring of multiple physiological parameters such as the EEG, respiration, electrocardiogram, electromyogram, eye movements (electrooculogram), oxygen saturation, and leg movements, etc.  Polymorphic activity: Irregular EEG waves having multiple forms, which may also vary in frequency and amplitude. Polyphasic wave: Wave consisting of more than two phases developed on alternating sides of the baseline (for example triphasic wave). POLYSOMNOGRAPHY (PSG) Polygraphic recording of sleep including EEG, electrooculogram, electromyogram (chin and leg), airflow parameters and oxygen saturation, along with video.  Polyspike-and-slow-wave complex: An epileptiform pattern consisting of two or more spikes associated with one or more slow waves Polyspike complex: A sequence of two or more spikes. Posterior dominant rhythm (PDR): Rhythmic activity seen at the occipital or parietal regions predominantly during wakefulness while the eyes are kept closed. QUASIPERIODIC Quasiperiodic: applies to EEG waves or complexes that occur at random intervals, which only approach regularity and that are not an exact repeating frequency. SECONDARY BILATERAL SYNCHRONY Secondary bilateral synchrony: Spreading by propagation of an initially focal or regional epileptiform discharge to become generalized SEIZURE PATTERN, EEG: Seizure pattern, EEG: Phenomenon consisting of repetitive epileptiform EEG discharges at >2 c/s.  Sharp wave: An epileptiform transient clearly distinguished from the background activity, although amplitude varies. A pointed peak and duration of 70–200 ms, usually with a steeper ascending phase when compared to the descending phase. Sharp-and-slow-wave complex: An epileptiform pattern consisting of a sharp wave and an associated following slow wave, clearly distinguished from background activity.  Spike: A transient, clearly distinguished from background activity, with pointed peak and duration from 20 to less than 70 ms. Spike-and-slow-wave complex: An epileptiform pattern consisting of a spike and an associated following slow wave, clearly distinguished from background activity.  Suppression: Entirety of an EEG record showing activity below10 µV. Symmetry: Approximately equal amplitude, frequency and form of EEG activities over homologous areas on opposite sides of the head. Synchrony: The simultaneous occurrence of EEG waves over distinct regions on the same or opposite sides of the head with the same speed and phase.  Theta band: Frequency band from 4