Utility and limitations of EEG in the diagnosis and management of ALDH7A1-related pyridoxine-dependent epilepsy. A retrospective observational study

The International League Against Epilepsy (ILAE) Task Force on Nosology and Definitions proposes a classification and definition of epilepsy syndromes in the neonate and infant with seizure onset up to 2 years of age. The incidence of epilepsy is high in this age group and epilepsy is frequently associated with significant comorbidities and mortality. The licensing of syndrome specific antiseizure medications following randomized controlled trials and the development of precision, gene-related therapies are two of the drivers defining the electroclinical phenotypes of syndromes with onset in infancy. The principal aim of this proposal, consistent with the 2017 ILAE Classification of the Epilepsies, is to support epilepsy diagnosis and emphasize the importance of classifying epilepsy in an individual both by syndrome and etiology. For each syndrome, we report epidemiology, clinical course, seizure types, electroencephalography (EEG), neuroimaging, genetics, and differential diagnosis. Syndromes are separated into self-limited syndromes, where there is likely to be spontaneous remission and developmental and epileptic encephalopathies, diseases where there is developmental impairment related to both the underlying etiology independent of epileptiform activity and the epileptic encephalopathy. The emerging class of etiology-specific epilepsy syndromes, where there is a specific etiology for the epilepsy that is associated with a clearly defined, relatively uniform, and distinct clinical phenotype in most affected individuals as well as consistent EEG, neuroimaging, and/or genetic correlates, is presented. The number of etiology-defined syndromes will continue to increase, and these newly described syndromes will in time be incorporated into this classification. The tables summarize mandatory features, cautionary alerts, and exclusionary features for the common syndromes. Guidance is given on the criteria for syndrome diagnosis in resource-limited regions where laboratory confirmation, including EEG, MRI, and genetic testing, might not be available.

We show here that children with pyridoxine-dependent seizures (PDS) have mutations in the ALDH7A1 gene, which encodes antiquitin; these mutations abolish the activity of antiquitin as a delta1-piperideine-6-carboxylate (P6C)-alpha-aminoadipic semialdehyde (alpha-AASA) dehydrogenase. The accumulating P6C inactivates pyridoxal 5'-phosphate (PLP) by forming a Knoevenagel condensation product. Measurement of urinary alpha-AASA provides a simple way of confirming the diagnosis of PDS and ALDH7A1 gene analysis provides a means for prenatal diagnosis.

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. 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 (Brazier et al., 1961). The description should be independent of the recording parameters such as amplification, montages, and computer program/display. Biological and technical artifacts that interfere with an adequate EEG interpretation should either be eliminated or, if this is not possible, be noted in the description. 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/vigilance 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. A proposal for the EEG report form is given in Appendix A. Glossary Absence: A generalized seizure type. Use of term discouraged when describing EEG patterns. Terms suggested, whenever appropriate: spike-and-slow-wave complex, 3 c/s spike-and-slow-wave complex, sharp-and-slow wave complex. 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). Active sleep: Normal sleep stage in neonates characterized by eye closure, intermittent periods of rapid eye movements, irregular respirations and scant body movements. The EEG shows activité moyenne in term and near term infants, and tracé discontinue (discontinuous pattern) in preterm infants 75 µV, measured over the frontal regions. Synonym: slow wave sleep. (See light sleep). Delta band: Frequency band of 0.1– 8 Hz; 10–60 µV). Localization changes with PMA. Synonyms: ripples of prematurity, spindle-delta bursts (use of terms discouraged). (See also extreme delta brush). Delta wave: Wave with duration of ¼-2 s (250–2000 ms). Depth electrode: Electrode (usually a multicontact electrode) implanted within the brain substance. Depth electroencephalogram: Recording of electrical activity of the brain by means of electrodes implanted within the brain substance, usually deep structures such as both hippocampi (see for example stereotactic [stereotaxic] depth electroencephalogram). Depth electroencephalography: Technique of recording intra-cranial depth electroencephalogram (see for example stereotactic [stereotaxic] depth electroencephalography). Derivation: (1) The process of recording from, or computing voltage differences between, a pair of electrodes in an EEG channel. (2) The EEG record obtained by this process. Desynchronization: Terms suggested: blocking or attenuation, depending on circumstance. The term desynchronization is acceptable when referring to the mechanisms presumably responsible for blocking or attenuation. It is also used in describing attenuation of a frequency band based on power spectra analysis of the EEG signal (for instance “event-related desynchronization”). (See blocking and attenuation). Diffuse: Colloquialism: an EEG activity spread over large areas of both sides of the head (see generalized). This does not imply abnormality as a normal rhythm may be diffusely distributed (for example alpha activity in some individuals, or slow waves in deep sleep). Comment: where possible the topographic distribution, symmetry and synchrony should be specified. Differential amplifier: An amplifier whose output is proportional to the voltage difference between its two input terminals. Comment: electroencephalographs make use of differential amplifiers in their input stages. Differential signal: Difference between two signals applied to the respective two input terminals of a differential EEG amplifier. Digital EEG: (1) The representation of an analog EEG signal by a series of numbers related to successive measurements of the magnitude of the signal at equal time intervals. (2) The practice of electroencephalography using digital representation of EEGs. Diphasic wave: Complex consisting of two wave components developed on alternate sides of the baseline. Synonym: Biphasic wave. Dipole: An EEG signal vector produced by a separation of negative (sink) and positive (source) potential poles (or current). A dipole is characterized by its strength, location and orientation. Depending on their orientation, dipoles can be radial (perpendicular to the surface), tangential (parallel to the surface) or a combination of these (oblique). Comment: an equivalent current dipole is a theoretical construct commonly used in source imaging to model a generator of an EEG signal located in the center of gravity of the source (for example an evoked potential, or an epileptiform discharge). Distributed source models are computed using a large number of small dipoles, distributed within the source space. Direct coupled amplifier: An amplifier in which successive stages are connected (coupled) by devices, the performance of which is not frequency dependent. Direct current (DC-) amplifier: An amplifier that is capable of recording DC (zero frequency) voltages and slowly varying voltages. Discharge: Waveforms with no more than 3 phases (i.e. crosses the baseline no more than twice) or any waveform lasting 0.5 s or less, regardless of the number of phases. Interpretive term of action potentials and post-synaptic potentials commonly used to designate interictal epileptiform and seizure patterns (see epileptiform pattern, seizure pattern). Disk electrode: Typically a metal disk attached to the scalp with an adhesive such as collodion or adherent conductive paste. Disorganization: Gross alteration in frequency, form, topography and/or quantity of physiologic EEG rhythms in: (1) an individual record, relative to previous records in the same subject or the rhythms of homologous regions on the opposite side of the head, or (2) relative to findings in normal subjects of similar age and similar state of vigilance. (See organization). Display Gain: Manipulation of the data after acquisition to change the visual size of waveforms in order to aid visual inspection. Comment: results of increase or decrease of the display gain are similar to changes of sensitivity during acquisition. Distortion: An instrument-induced alteration of waveform (see artifact, clipping). Duration: (1) The interval from beginning to end of an individual wave or complex. Comment: the duration of the cycle of individual components of a sequence of regularly repeating waves or complexes is referred to as the period of the wave or complex. (2) The time that a sequence of waves or complexes or any other distinguishable feature lasts in an EEG record. Electrical status epilepticus during sleep (ESES): an EEG pattern seen in childhood which consists of continuous or near continuous spike-and-slow-waves during sleep. Discharges may be seen in wakefulness, often with a frontal or temporal emphasis, but increase markedly in sleep and typically abate in REM sleep. Quantification of epileptiform activity is not standardized but some use spike-and-slow-wave index of >50% or >85%. Most children have or develop seizures and present with neurocognitive dysfunction. Comment: often used synonymously with continuous spike and waves during sleep (CSWS). (See index, slow wave sleep, continuous spike and waves during sleep). Electrocerebral inactivity: Absence over all regions of the head of identifiable electrical activity of cerebral origin, whether spontaneous or induced by physiological stimuli or pharmacological agents. Comment: strict technical recording standards should be observed in suspected cerebral death (Stecker et al., 2016). Tracings of electrocerebral inactivity should be clearly distinguished from low voltage EEGs (see low voltage EEG). Synonyms: electrocerebral silence, flat or isoelectric EEG (use of terms discouraged). Electrocorticogram (ECoG): Record of EEG activity obtained by means of electrodes applied directly over or inserted in to the cerebral cortex. Comment: electrocorticograms can be performed intraoperatively and extraoperatively after surgical implantation (see subdural electrode). Electrocorticography (ECoG): Technique of recording electrical activity of the brain by means of electrodes applied over or implanted in to the cerebral cortex. Comment: electrocorticography can be performed intraoperatively and extraoperatively after surgical implantation (see subdural electrode). Electrode, EEG: A conducting device applied over or inserted in a region of the scalp or brain. Electrodecrement: A period of amplitude attenuation usually with superimposed fast activity. Electrode impedance: Total effective resistance to alternating current (AC), arising from ohmic resistance and reactance. Measured between pairs of electrodes or, in some electroencephalographs, between each individual electrode and all the other electrodes connected in parallel. Expressed in ohms (generally kilo-ohms, kΩ). Comments: (1) over the EEG frequency range, because the capacitance factor is small, electrode impedance is usually equal to electrode resistance. (2) Not a synonym of input impedance of EEG amplifier (see electrode resistance, input impedance). Electrode resistance: Total effective resistance to direct current (DC), through the interface between an EEG electrode and the scalp or brain. Measured between pairs of electrodes or, in some electroencephalographs, between each individual electrode and all the other electrodes connected in parallel. Expressed in ohms (generally kilo-ohms, kΩ). Comment: measurement of electrode resistance with DC currents results in varying degrees of electrode polarization (see electrode impedance). Electroencephalogram (EEG): Record of electrical activity of the brain taken by means of electrodes placed on the surface of the head, unless otherwise specified. Electroencephalograph (EEG): Instrument employed to record electroencephalograms. Electroencephalographic: Appertaining to bioelectrical recording, irrespective of the method employed (in the present context EEG, ECoG, SEEG, etc.). Electroencephalography (EEG): (1) The science relating to the electrical activity of the brain. (2) The practice of recording and interpreting electroencephalograms. Encoche frontale: Normal neonatal graphoelement in term and near term infants, between 34 and 44 weeks post menstrual age. Frontal broad diphasic sharp waves (50–100 µV); typically bilateral, but may be unilateral. Usually seen in transition from active to quiet sleep. Synonym: anterior sharp transient, transient frontal sharp wave. (See active and quiet sleep). Epicortical electrode: Use of term discouraged. Synonym: subdural electrode (preferred term). Epidural electrode: Electrode located over the dural covering of the cerebrum. Epileptiform pattern: Describes transients distinguishable from background activity with a characteristic morphology typically, but neither exclusively nor invariably, found in interictal EEGs of people with epilepsy. Epileptiform patterns have to fulfill at least 4 of the following 6 criteria: (1) Di- or tri-phasic waves with sharp or spiky morphology (i.e. pointed peak). (2) Different wave-duration than the ongoing background activity, either shorter or longer. (3) Asymmetry of the waveform: a sharply rising ascending phase and a more slowly decaying descending phase, or vice versa. (4) The transient is followed by an associated slow after-wave. (5) The background activity surrounding epileptiform discharges is disrupted by the presence of the epileptiform discharges. (6) Distribution of the negative and positive potentials on the scalp suggests a source of the signal in the brain, corresponding to a radial, oblique or tangential orientation of the source (see dipole). This is best assessed by inspecting voltage maps constructed using common-average reference. Synonyms: interictal epileptiform discharge, epileptiform activity. Epoch: EEG segment with a defined duration. Duration of epochs is determined arbitrarily but should be specified. Equipotential: Applies to regions of the head or electrodes that are at the same potential at a given instant in time. Synonyms: isopotential line, isopotential. Event-related potential (ERP): Refer to long latency responses (>70 ms) associated with an event, such as a deviant stimulus (as in mismatch negativity, P3 or P300), anticipation of a response (as in Bereitschaftspotential), or anticipation of a stimulus demanding a response (as in contingent negative variation). Applied mainly to slow (on account of their lower frequency content) ‘endogenous’ evoked potentials elicited by controlled manipulation of the psychological context. Thought to reflect some aspect of higher sensory processing, and therefore sometimes referred to as “cognitive potentials”, such as: attention, expectancy, novelty detection, stimulus salience, target recognition, task relevance, information delivery, decision making, stimulus evaluation time, template matching, memory, and closure of cognitive epoch. (See evoked potential, contingent negative variation, mismatch negativity, P3 or P300). Evoked magnetic field: Magnetic counterpart of EEG evoked potential. (See evoked potential, magnetoencephalography). Evoked potential (EP): Wave or complex elicited by and time-locked to a physiological or non-physiological stimulus or event, the timing of which can be reliably assessed. Comment: computer summation (averaging) techniques are especially suitable for detecting these and other event-related potentials from the surface of the head. See event-related potential. Exploring electrode: An electrode that registers electrical potentials from excitable tissue of the nervous system, historically connected to the input terminal 1 of an EEG amplifier, against a reference electrode, connected to the input terminal 2. Synonym: active electrode (use discouraged as all recording electrodes may be considered ‘active’, including any reference electrode). (See reference electrode). Extracerebral potential: Any potential that does not originate in the brain, generally referred to as an artifact in EEG. May arise from electrical interference external to the subject and recording system, the subject, the electrodes and their connections to the subject and the electroencephalograph, and the electroencephalograph itself (see artifact). Extreme delta brush: A particular pattern characterized by near continuous widespread rhythmic delta activity (1–3 c/s) with superimposed bursts of beta frequency activity (20–30 Hz) on top of each delta wave. Mostly symmetric and synchronous; do not vary with sleep-wake cycles or significantly with stimulation. The pattern has been described in autoimmune encephalitis associated with anti-N-Methyl D-Aspartate receptor antibodies. Named after a resemblance to the delta brush seen in preterm infants (see delta brush). Far-field potential: A potential generated in a deep neural structure and recorded by electrodes on the head at a distance from the generator, as a result of volume conduction and not mediated by neuronal activity. (See volume conduction, and for example brainstem auditory evoked potentials). Fast activity: Activity of frequency higher than the alpha band, i.e. beta and gamma activity, and high frequency oscillations. Fast alpha variant rhythm: A normal variant. Characteristic rhythm at 14–20 Hz, detected most prominently over the posterior regions of the head. May alternate or be intermixed with alpha rhythm, of which it is usually a harmonic frequency. Blocked or attenuated by attention, especially visual, and mental effort. Fast ripples: Part of the high frequency oscillation (HFO) bandwidth, usually defined as being in the range 250–1000 Hz (see high frequency oscillations). Fast wave: Wave with duration shorter than alpha waves, i.e. under 1/13 s. Focal: Limited to a small area of the brain in one hemisphere (see regional, multifocal). Focal epileptic seizures are conceptualized as originating within networks limited to one hemisphere, and usually associated with an initially localized epileptiform EEG pattern (see epileptiform pattern). Focus: A limited region of the scalp, cerebral cortex, or depth of the brain displaying a given EEG activity, either normal or abnormal. Foramen ovale electrode: A multicontact electrode bundle inserted through the foramen ovale to lie in proximity to the mesial temporal cortex. Comment: used for presurgical assessment of epilepsy of suspected mesial temporal lobe origin. (See basal electrode). Fourteen and 6-Hz positive burst or spikes: A normal variant. Burst of arch-shaped waves or spikes at 13–17 c/s and/or 5–7 c/s, but most commonly at 14 and/or 6 c/s, usually seen bilaterally over the posterior temporal and adjacent areas, typically during drowsiness and light sleep, with incidence peaking in adolescence. The sharp peaks of its component waves are positive with respect to other regions. Amplitude varies but is generally below 75 µV. Comments: (1) best demonstrated by referential recording using contralateral earlobe or common average reference electrodes. (2) This pattern has been termed “pseudo-epileptiform” (i.e. not associated with a liability to epileptic seizures). Synonym: ctenoids (use of term discouraged). Frequency: Number of complete cycles of repetitive waves or complexes in 1 s. Measured in cycles per second (c/s) or Hertz (Hz). Comment: the term Hz seems appropriate when applied to sinusoidal waves such as alpha activity, but seems inappropriate when applied to complex waveforms such as spike-and-slow-waves, which may be more correctly quantified by c/s. This principle has been applied throughout this glossary. Frequency response: Characteristics of an amplifier showing the relative response to the activities of different frequencies with respect to the response of 10 Hz activity. The bandwidth of EEG channels is determined by the low and high frequency filters and the particular frequency response characteristics of the recording system. Frequency response curve: A graph depicting the relationships between output trace detection or amplifier output and input frequency, for a particular setting of low and high frequency filters. Frequency spectrum: The distribution of amplitude and phases of different frequency components against frequency. This is typically demonstrated by a Fourier transform of the EEG. Comment: In most applications the amplitude spectrum is presented only (for example in delta, theta, alpha, beta, gamma bands), and not phase information. (See power spectrum, quantitative EEG). Frontal intermittent rhythmic delta activity (FIRDA): Fairly regular, approximately sinusoidal or sawtooth waves, mostly occurring in bursts at 1.5–2.5 Hz synchronously over the frontal areas of both sides of the head (occasionally unilateral). Comment: most commonly associated with mild to moderate unspecified encephalopathy in responsive ambulant patients, often in association with cerebrovascular disease. Synonym: occasional frontally predominant brief 2/s GRDA. Fronto-central theta: A normal variant. Theta rhythm located in the midline, just anterior to the vertex, that occurs during psychological stress and cognitive tasks, particularly problem solving. It appears predominantly in young healthy adults ( 30 to 80 Hz. Greek letter: γ. Comment: The graphic resolution of computer displays may limit the visual appreciation of higher frequencies. However, this does not justify limiting unduly the high frequency response of the EEG channels; for EEG waves include transients such as spikes and sharp waves with components at frequencies above 50 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. Generalization: Bilateral propagation of EEG activity from limited areas to all regions of the head (see generalized). Generalized: Loosely: an EEG activity spread over all regions of the head, usually with a frontal, but rarely with an occipital, maximum (see diffuse). Strictly: bilateral EEG discharges appearing reasonably symmetrically and synchronously over homologous regions of the head (see symmetric and synchronous). For example generalized epileptic seizures are conceptualized as originating at some point within, and rapidly engaging (i.e. synchronizing), bilaterally distributed networks. Comment: “generalized” is still used as a term for describing seizure types and epilepsy syndromes, although no seizure pattern involves the whole brain simultaneously (see secondary bilateral synchrony). 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 that only occurs during sleep. GPFA is considered a feature of Lennox-Gastaut syndrome. Comment: when the bursts are longer than 5 s a tonic seizure is often recorded (this may be discrete, and only detected with surface EMG electrodes). Synonyms: bursts of fast rhythms, fast paroxysmal rhythms, runs of rapid spikes (use of terms discouraged). Generalized periodic discharges (GPDs): GPDs are 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. The morphology of GPDs is variable and consists of sharp or spike and slow wave complexes, triphasic-like waves, and slow wave complexes. Repetition rate usually lies between 0.5 and 2.0 c/s. They occur most commonly in coma, usually after severe cerebral anoxia following cardiac arrest, in Creutzfeldt-Jacob disease, and with toxicity (for example baclofen or lithium). With anoxic insults, the periodicity typically ranges from 1.5 to 3.5 c/s. Most patients have a poor neurological prognosis or die, although not invariably. Synonym: generalized periodic epileptiform discharges (use of term discouraged). (See periodic discharges). Generalized periodic epileptiform discharges (GPEDs): Use of term discouraged. See generalized periodic discharges (preferred term). Graphoelement: Any EEG pattern (transient, potential or rhythm) that is distinguished from the ongoing background activity, which may be physiological or pathological. It is characterized by its name, morphology, location, duration, frequency (when rhythmic), mode of appearance and the relationship to activating or modulating factors (for example hypnagogic hypersynchrony). Ground connection: Conducting path between the subject, the electroencephalograph and earth. Ground projection: Projection of an artifact, such as blink artifact, recorded from a ground electrode into an exploring electrode whose impedance is high. Harness, head: A combination of straps fitted over the head to hold pad electrodes in position. Commercial EEG recording electrodes caps are an alternative. Hertz (Hz): Unit of frequency. Synonym: cycles per second (c/s). High frequency filter (or low pass filter): A circuit that reduces the sensitivity of the EEG signals to relatively high frequencies (for example above 70 Hz). For each setting of the high frequency filter, this attenuation is expressed as percent reduction in signal amplitude at a given frequency, relative to frequencies unaffected by the filter, i.e. in the mid-frequency band of the signal. Synonym: low pass filter. Comment: at present high frequency filter designations and their significance are not yet standardized for all instruments of different manufactures. For instance, for a given instrument, a position of the high frequency filter control designated as 70 Hz may indicate a 30% (3 dB), or other stated percent, reduction in sensitivity at 70 Hz, compared to the sensitivity, for example, at 10 Hz. High frequency oscillations (HFOs): Transient bursts of EEG activity, spontaneous or evoked, with frequencies beyond 80 Hz. Divided into ripples (80–250 Hz) and fast ripples (250–500 Hz). (See ripples and fast ripples). High frequency response: Sensitivity of an EEG channel to relatively high frequencies. Determined by the high frequency response of the amplifier and the high frequency filter used. Expressed as percent reduction in output trace deflection at certain specific high frequencies, relative to other frequencies in the mid-frequency band of the channel. High pass filter: Synonym: low frequency filter. Hypersynchrony: When describing EEG patterns that are attributed to increased synchronization of neuronal activity (for example hypnagogic hypersynchrony). Hyperventilation: Deep and regular respiration performed for a period of several minutes. Used as an activation procedure. Synonym: overbreathing (see activation). Hypnagogic hypersynchrony: A normal variant. Paroxysmal bursts of 3–5 c/s, high amplitude (75–350 µV) diffuse, but maximal fronto-central, sinusoidal activity occurring at the onset of sleep in normal infants and children, aged 3 months to 13 years (but typically 4–9 years). Hypsarrhythmia: Characteristic interictal EEG pattern typically, but not invariably, seen in infants with Infantile Spasms (West syndrome). Consists of diffuse very high amplitude (>300 µV) irregular slow waves interspersed with multiregional spikes and sharp waves over both hemispheres, usually with a highly disorganized and asynchronous appearance. It is most frequent during Non-REM sleep, followed by waking and arousal, and is absent or minimal during REM sleep. Variations include asymmetry, predominant single focus (within widespread abnormalities), episodes of attenuation or fragmentation, increased periodicity and preservation of interhemispheric synchrony (all termed ‘modified hypsarrhythmia’). Comment: the ictal pattern of spasms usually consists of one or more of following: diffuse high amplitude slow wave, low to medium amplitude fast activity, or electrodecrement (see low amplitude fast activity, electrodecrement). Ictal EEG pattern: See seizure pattern, EEG. Impedance meter: An instrument used to measure impedance (see electrode impedance). Inactivity, record of electrocerebral: See electrocerebral inactivity. Incidence: Descriptor used to characterize how often a transient or isolated discharge is seen throughout the recording. Suggested ranges for describing this: ≥1/10 s are abundant, ≥1/min but less than 1/10 s are frequent, ≥1/h but less than 1/minute are occasional, and 100 µV). Intermittent slow activity varies by more than 50% or regresses completely between times of its appearance, and can be polymorphic, arrhythmic or rhythmical (see continuous slow activity). Intracerebral depth electroencephalogram: See: depth electroencephalogram. Intracerebral electrode: Various conducting devices for recording EEG from the surface or within the substance of the brain. Examples include epicortical/subdural, epidural, foramen ovale, and stereotactic [stereotaxic] implanted depth electrodes. Synonym: depth electrode. Irregular: Applies to EEG waves and complexes of inconstant period and/or uneven contour or morphology. Isoelectric: (1) The record obtained from a pair of equipotential electrodes (see equipotential). (2) Use of term discouraged when describing record of electrocerebral inactivity (see electrocerebral inactivity). Isolated: Occurring singly. Isopotential: See synonym equipotential. K complex: A normal graphoelement. A well delineated negative sharp wave followed by a positive component standing out from the background EEG, with total duration ≥0.5 s, usually maximal in amplitude when recorded from fronto-central derivations and often associated with a sleep spindle. (See vertex sharp transient or vertex sharp wave). Lambda wave: A normal graphoelement. Diphasic sharp transient occurring over the occipital regions of the head of awake subjects during visual exploration. The main component is positive relative to other areas. Time-locked to saccadic eye movements. Amplitude varies but is generally below 50 µV. Greek letter: λ (note morphology resembling the Greek capital letter lambda). Laplacian montage: Montage that consists of a mathematical transformation involving the second spatial derivative; the Laplacian source of the potential may be approximated by using the weighted average of all the neighbouring electrodes as a reference for each site or electrode. This montage may be used for localization of focal abnormalities on digital EEG (see common average reference). Lateralized: Independently involving the right and/or left side of the head (or body) (see unilateral). 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. The incidence of clinical or electrographic seizures associated with LPDs is high, ranging from 50 to 100%, but there is debate as to whether they represent seizures proper. When contralateral motor movements are time-locked to LPDs they are considered to represent seizure patterns. Most LPDs are ephemeral phenomena occurring with both acute focal destructive lesions (for example cerebral infarcts, tumors or herpes simplex encephalitis) and more subacute/chronic pathologies (for example epilepsy and vascular compromise). Synonym: periodic lateralised epileptiform discharges (use of term discouraged). (See discharge, periodic discharges). Lead: Strictly: wire connecting an electrode to the electroencephalograph. Loosely: synonym of electrode, its wire and connector. Light sleep: Non-REM (NREM) sleep stages N1 and N2, which are characterized by sinusoidal eye movements, low amplitude mixed frequency EEG activity, vertex sharp waves, K complexes and sleep spindles. (See deep sleep). Linkage: The connection of a pair of electrodes to the two respective input terminals of a differential EEG amplifier (see derivation). Longitudinal bipolar montage: A montage consisting of contiguous channels of electrode pairs along longitudinal, mainly antero-posterior, arrays (for example, Fp1-F3, F3-C3, C3-P3, P3-O1 etc). Synonym: “double-banana” montage. Low frequency filter (high pass filter): A circuit that reduces the sensitivity of the EEG signal to relatively low frequencies (for example below 0.5 Hz). For each position of the low frequency filter control, this attenuation is expressed as percent reduction of the signal at a given stated frequency, relative to frequencies unaffected by the filter, i.e. in the mid-frequency band of the channel. Comment: at present low frequency filter designations and their significance are not yet standardized for instruments of different manufacturers. For instance, in a given instrument a low frequency filter setting designated 1 Hz may indicate a 30% (3 dB), or other stated percent, reduction in sensitivity at 1 Hz, compared to the sensitivity for example at 10 Hz. The same position of the low frequency filter setting may also be designated by the time constant. Synonym: high pass filter. Low frequency response: Sensitivity of an EEG channel to relatively low frequencies. Determined by the low frequency response of the amplifier and by the low frequency filter (time constant) used. Expressed as percent reduction in output trace deflection at certain stated low frequencies, relative to other frequencies in the mid-frequency band of the channel (see low frequency filter, time constant). Low pass filter: Synonym: high frequency filter. Low voltage EEG: A normal variant. Waking record characterized by activity of amplitude not greater than 20 µV over all head regions. With appropriate instrumental sensitivities this activity can be shown to be composed primarily of beta, theta and, to a lesser degree, delta waves, with or without alpha activity over the posterior areas. Comments: (1) low voltage EEGs are susceptible to change under the influence of certain physiological stimuli, sleep, pharmacological agents and pathological processes. (2) They should be clearly distinguished from tracings of electrocerebral inactivity, suppression and low voltage fast activity (see electrocerebral inactivity, suppression and low voltage fast activity). Low voltage fast activity: Refers to fast activity (beta rhythm and above), often recruiting, which can be recorded at the onset of an ictal discharge, particularly in intra-cranial depth EEG recording of a seizure. Magnetoencephalography (MEG): Recording of magnetic fields generated from the cortical neurons. Map, voltage: Topographical display of the voltage distribution on the scalp, using equipotential lines and color-codes to express the steps of gradient changes between the peak negativity and the peak positivity. The voltage difference between peak negativity and peak positivity is 100%, and the fall-off of the potential is shown in arbitrary steps of, for example, 10% of the maximum amplitude. Usually blue color symbolizes negativity, and red color positivity. Inspecting voltage maps allows estimation of the location and orientation of the source. Comment: it is recommended to calculate voltage maps using common average reference (that include all electrodes on the scalp and preferably the inferior temporal electrode chain too). (See Quantitative EEG). Synonyms: diagram of equipotential lines, isopotential map or amplitude map. Mismatch negativity (MMN): Is an automatic (i.e. attention independent) event-related response to physically deviant auditory stimuli occurring among frequent (standard) stimuli (e.g. tones or phonetic stimuli). MMN is a surface negative potential with an onset latency of about 130 ms and lasting 250–300 ms, with maximal amplitude over the fronto-central region. (See event-related potential). Monorhythmic delta activity: A normal graphoelement in preterm infants (24–34 weeks of post menstrual age). Characterized by relatively stereotyped delta activity (up to 200 µV) predominantly over the posterior regions (occipital, temporal and central). Montage: The arrangement or array of channels on the EEG machine display, defined by the exploring and reference electrodes (for example see bipolar and referential montages). Morphology: Refers to the form of EEG waves (i.e. their shape and physical characteristics). Motor evoked potential (MEP): Evoked potential recorded from muscle following direct stimulation of the exposed motor cortex, or transcranial stimulation of the motor cortex, either magnetically or electrically. Mu rhythm: Rhythm at 7–11 Hz, composed of arch-shaped waves occurring over the central or centro-parietal regions of the scalp during wakefulness. Amplitude varies but is mostly below 50 µV. Blocked or attenuated most clearly by contralateral movement, thought of movement, readiness to move or tactile stimulation. Greek letter: μ. Synonyms: rhythm rolandique en arceau, comb rhythm (use of terms discouraged). Multifocal: Three or more spatially separated independent foci (see focal). Multiple spike-and-slow-wave complex: Use of term discouraged. An epileptiform graphoelement consisting of two or more spikes associated with one or more slow waves (see epileptiform pattern). Synonym: polyspike and-slow-wave complex (preferred term). Multiple spike complex: Use of term discouraged. A sequence of two or more spikes. Synonym: polyspike complex (preferred term). Multiregional: Three or more lobar foci (see regional). Nasopharyngeal electrode: Rod electrode introduced through the nose and placed against the nasopharyngeal wall with its tip lying near the body of the sphenoid bone. (See basal electrode). Needle electrode: Small needle inserted into the subdermal layer of the scalp. Noise, EEG channel: Small fluctuating output of an EEG channel recorded when high sensitivities are used, even if there is no input signal. Measured in microvolts (µV), referenced to the input. Non-cephalic reference: Reference electrode that is placed on body parts other than the head (for example sternospinal reference). Non-REM sleep (NREM): Term summarizing all sleep stages except REM sleep (see REM sleep). Notch filter: A filter that selectively attenuates a very narrow frequency band, thus producing a sharp notch in the frequency response of an EEG signal. Commonly applied to attenuate electrical noise from mains interference (the frequency of which differs between countries, 50 or 60 Hz), which may occur under unfavorable technical conditions. Nyquist theorem: Accurate digital representation of an EEG signal requires that the sampling rate is at least twice the highest frequency of the signal, i.e. a frequency component of 30 Hz requires at least a sampling rate of 60 Hz. Comment: sampling at twice the Nyquist frequency only ensures an accurate representation of frequency content. Tolerable reproduction of waveforms requires at least a sampling rate 5 times above the fastest frequency components present. Occipital intermittent rhythmic delta activity (OIRDA): Fairly regular or approximately sinusoidal waves, mostly occurring in bursts at 2–3 Hz over the occipital areas of one or both sides of the head. Frequently blocked or attenuated by eye opening. An abnormal pattern seen in children’s EEGs more frequently than adults, often but not exclusively in association with genetic generalized epilepsies. Ohmmeter: An instrument used to measure resistance (see electrode resistance). Ordinate period: Time in milliseconds (ms) elapsing between two successive sampling points in digital EEG. (See bin width). Organization: Degree to which the posterior dominant rhythm (PDR) conforms to certain characteristics displayed by a majority of subjects in the same age group, without personal or family history of neurologic and psychiatric diseases, or other illnesses that might be associated with dysfunction of the brain. Comments: the organization of PDR progresses from birth to adulthood. Out-of-phase signals: Two waves of opposite phases (see differential signal; phase reversal - not a synonym). Output voltage: The voltage across the trace display of an EEG channel. Overbreathing: Synonym: hyperventilation. Overload: Condition caused by applying voltage differences which are larger than the channel is designed for or set to handle by the input terminals of an EEG amplifier. Displays clipping of EEG waves and/or blocking of the amplifier depending on its magnitude (see clipping, blocking). P3 or P300: Is an event-related potential response usually elicited using the oddball paradigm, in which low-probability target stimuli are mixed with high-probability non-target (or standard) stimuli. P3 is a surface positive potential with an onset latency of about 250 to 500 ms and maximal amplitude over the centro-parietal region, with two subcomponents denoted P3a and P3b. Synonym: late positive component (LPC). (See event-related potential). Pad electrode: Metal electrode covered with a cotton or felt and gauze pad, held in position by a head cap or harness. Paper speed: Velocity of movement of paper through an analogue EEG machine. Expressed in centimeters per second (cm/s) or millimeters per second (mm/s). Synonym: time base (in digital EEG). Paroxysm: Graphoelement phenomenon with sudden onset, rapid attainment of a maximum, and abrupt termination; distinguished from background activity. Comment: commonly used to refer to epileptiform and seizure patterns (see epileptiform pattern and seizure pattern). Paroxysmal fast: Fast frequencies in the beta range or above occurring in trains (see paroxysm, low voltage fast activity). Pattern: Any characteristic regular or repetitive EEG activity of approximately constant period (see regular and rhythmic). Peak: Point of maximum amplitude of a wave. Period: Duration of complete cycle of individual graphoelement in a sequence of regularly repeated EEG waves or complexes. Comment: the period of the graphoelement of an EEG rhythm is the reciprocal of the frequency of the rhythm. (For example, the duration of a spike-and-slow-wave complex in 3 c/s spike-and-slow-waves is 1/3 = 0.333). Periodic: Applies to: (1) EEG waves or complexes occurring in a sequence at an approximately regular rate, (2) EEG waves or complexes occurring intermittently at approximately regular intervals, generally of one to several seconds. (See periodic discharges). 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. Comments: PDs may be generalized (GPDs), lateralized (LPDs), bilateral independent (BIPDs). Old nomenclature for these new terms are GPEDs (=GPDs), PLEDs (=LPDs) and BIPLEDs (=BIPDs). The use of “epileptiform” as an interpretative term is now avoided, since these periodic patterns may or may not be associated with clinical seizures (Hirsch et al., 2013). Periodic lateralized epileptiform discharges (PLEDs): Use of term discouraged. See lateralized periodic discharges (preferred term). Phase: (1) Time or polarity relationships between a point on a wave displayed in a derivation and the identical point on the same wave recorded simultaneously in another derivation. (2) Time or angular relationships between a point on a wave and the onset of the cycle of the same wave. Usually expressed in degrees or radians. Phase reversal: Simultaneous trace deflections in opposite directions from two or more channels in a bipolar recording montage. Assuming a single generator, phase reversal is due to the same signal being applied to the input terminal 2 of one differential amplifier and to the input terminal 1 of the other amplifier. Comment: When observed in two linked bipolar channels, phase reversal indicates that the potential field is maximal or minimal at or near the electrode common to such derivations. A phase reversal seen in a referential recording, when assessed using mapping of the potential fields, indicates that the dipole source is horizontally located in the sulcal wall across the borderline of the two fields of opposite polarity. (See bipolar and referential montages, dipole, input terminal). Photic driving: Physiologic response consisting of periodic activity elicited over the posterior regions of the head, usually induced by repetitive photic stimulation at frequencies of about 1–30 Hz. Comments: (1) term should be limited to activity time-locked to the stimulus and of frequency identical or harmonically related to the stimulus frequency. (2) Photic driving should be distinguished from the visual evoked potentials elicited by isolated flashes of light or flashes repeated at low frequencies ( 50%) of cycle pairs. (Synonym: pseudoperiodic). (Hirsch et al., 2013). Quiet sleep: Normal sleep stage in neonates characterized by eye closure, absence of rapid eye movements, and scant body movements, except for occasional sucking activity or myoclonic jerks. The EEG shows tracé alternant in term and near term infants and tracé discontinue (discontinuous pattern) in preterm infants; the interburst interval depend on the post menstrual age. (See active sleep, tracé alternant and tracé discontinue). Reactivity: A phenomena in which the EEG pattern clearly and reproducibly changes with sensory (visual, auditory or noxious) stimulation. Changes may occur in frequency, morphology and/or amplitude, including attenuation of activity after the stimulus. Comment: appearance of muscle activity or eye blink artifacts or heart rate does not qualify as reactive. In general terms reactivity of the EEG in comatose patients is a favorable prognostic sign. Record: The end product of the EEG recording process. Synonyms: recording, tracing. Recording: (1) The process of obtaining an EEG record. Synonym: tracing. (2) The end product of the EEG recording process, most commonly on to digital storage media. Synonyms: record, tracing. Record of electrocerebral inactivity: See electrocerebral inactivity. Reference electrode: (1) In general: any electrode against which the potential variations of another electrode are measured. (2) Specifically: a suitable reference electrode is historically connected to the input terminal 2 of an EEG amplifier and placed so as to minimize the likelihood of recording the same EEG activity as detected by an exploring electrode (connected to the input terminal 1 of the same amplifier), or of other activities. Comments: (1) Whatever the location of the reference electrode, the possibility that it might be affected by appreciable EEG potentials should always be considered. (2) A reference electrode connected to the input terminal 2 of all EEG amplifiers is referred to as a common reference electrode. (See exploring electrode). Referential derivation: Recording from a pair of electrodes consisting of an exploring electrode historically connected to the input terminal 1 and a reference electrode usually connected to the input terminal 2 of an EEG amplifier (see exploring and reference electrode, input terminal 1 and 2, referential montage). Referential montage: A montage consisting of referential derivations. Comment: a referential montage in which the reference electrode is common to multiple derivations is referred to as a common reference montage (see referential derivation). Reformatting: Transformation of digitized EEG into different montages. Reformatting requires that the raw EEG signal is recorded to a common reference electrode. Only those electrodes can be included in the reformatting montages which are connected to amplifier input 1. Regional: EEG activity that is limited to a region of the scalp overlying a lobe (i.e. frontal, temporal, parietal, occipital). (See focal, multiregional). Regular: Applies to waves or complexes of approximately constant period and relatively uniform appearance. Synonyms: rhythmic, monomorphic (use of latter term discouraged). REM: Rapid eye movements characterizing REM sleep. Conjugate, irregular, sharply peaked eye movements with an initial deflection lasting 2 c/s and/or characteristic pattern with quasi-rhythmic spatio-temporal evolution (i.e. gradual change in frequency, amplitude, morphology and location), lasting at least several seconds (usually >10 s). Two other short duration ( 125 ms). Slow wave sleep: Non-REM sleep stage N3. Synonym: deep sleep. (See deep sleep, REM sleep). Small sharp spikes (SSS): A normal variant. Small sharp spikes of very short duration ( 50 μV. Main component is generally negative relative to other areas. Comments: (1) term should be restricted to epileptiform discharges. EEG spikes should be differentiated from sharp waves, i.e. transients having similar characteristics but longer durations. However, it should be kept in mind that this distinction is largely arbitrary and primarily serves descriptive purposes. (2) EEG spikes should be clearly distinguished from the brief unit spikes recorded from single cells with microelectrode techniques. (See sharp wave). Spike-and-slow-wave complex: An epileptiform pattern consisting of a spike and an associated following slow wave, clearly distinguished from background activity. May be single or multiple. Spindle: Group of rhythmic waves characterized by a progressively increasing, then gradually decreasing, amplitude (see sleep spindle). Spread: Propagation of EEG waves from one region of the scalp and/or brain to another (see generalization, propagation). Standard electrode: Conventional scalp electrode (see disk electrode, needle electrode, pad electrode, special electrode). Standard electrode placement: Scalp electrode location(s) determined by the ten-twenty system (see ten-twenty system). Status epilepticus, EEG: The occurrence of virtually continuous or repetitive epileptiform seizure pattern in an EEG. Term should be distinguished from clinical status epilepticus, although they may co-exist. (See seizure pattern). Synonym: electrographic status epilepticus. Stereotactic (stereotaxic) electroencephalogram (SEEG): Intracerebral EEG recordings using electrodes implanted stereotactically, thus permitting the calculation of electrode coordinates that can be projected on a stereotactic brain atlas or magnetic resonance images to create three-dimensional pictures. Note the abbreviation SDEEG is also used for stereotactic depth electroencephalogram. Synonym: stereoelectroencephalography. Stereotactic (stereotaxic) electroencephalography (SEEG): Technique of recording stereotactic (stereotaxic) electroencephalograms. Synonym: stereoelectroencephalogram. Sternospinal reference: A non-cephalic reference achieved by interconnecting two electrodes placed over the right sternoclavicular junction and the spine of the seventh cervical vertebra, respectively, and balancing the voltage between them by means of a potentiometer to reduce ECG artifact. Stimulus-induced rhythmic, periodic or ictal discharges (SIRPIDs): Sharp transients that are rhythmic, periodic or ictal appearing discharges in comatose patients that are consistently induced by alerting stimuli including: auditory and other sensory stimulation, such as noxious (airway suctioning) and other patient-care activities. SIRPIDs may be regional or lateralized, bilateral or generalized, and of variable duration. Their pathophysiology and clinical significance is uncertain, but on occasions may be associated with clinical seizures. Subclinical rhythmic discharges of adults (SREDA): This paroxysmal pattern is usually seen in the adult age group (typically over 50 years) and consists of a mixture of frequencies, often predominantly in the theta range, lasting 40–80 s. It may resemble a seizure discharge but is not accompanied by any clinical signs or symptoms. The significance of this pattern is uncertain, but it should be distinguished from an epileptiform seizure pattern. Subdural electrode: Electrode inserted under the dural covering of the cerebrum for recording of electrocorticogram as a pre-surgical evaluation of medically intractable partial epilepsy, usually in the form of electrode strips. Synonym: epicortical electrode (use of term discouraged). Suppression: Entirety of an EEG record showing activity below 10 µV (reference derivation). (See burst suppression pattern). 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. Comment: term simultaneous only implies a lack of possible delay that is measurable with standard computer display. Certain electrodes are so close (for example Fp1-Fp2 and 01–02 respectively) that volume conduction can affect the signal on the other side, making these electrodes unsuitable for assessing synchrony). (See volume conduction). Temporal intermittent rhythmic delta activity (TIRDA): An EEG pattern characterized by short trains of intermittent and rhythmic delta activity (1–3.5 Hz), often with sawtooth morphology, recorded predominantly over the anterior temporal regions. TIRDA usually occurs during drowsiness and light sleep and may either be unilateral or bilateral, occurring independently. It is associated with temporal lobe epilepsy, and when unilateral is virtually indicative of ipsilateral pathology. Temporal slow activity of the elderly: Pattern of uncertain significance but usually not considered abnormal. Unilateral (most often on the left side) or bilateral, short runs of theta or delta activity intermixed with the background activity over the temporal region(s), in subjects >50 years of age, without clinical abnormalities. Often accentuates during drowsiness and hyperventilation. Ten-ten (10–10) system: System of standardized scalp electrode placement. According to this system, additional scalp electrodes are placed at half distance between the standard electrodes of the ten-twenty system, i.e. 10 percentile increments of the reference curve (see ten-twenty system, closely spaced electrodes). Comment: use of additional supplementary scalp electrodes is indicated for instance during epilepsy monitoring, theoretically in order localize epileptiform discharges more precisely (for example surface sphenoidal or anterior “cheek” electrode). (See special electrode). Ten-twenty (10–20) system: System of standardized scalp electrode placement recommended by the International Federation of Clinical Neurophysiology. According to this system, the placement of electrodes is determined by measuring the head from 4 external landmarks and taking 10 or 20 percentiles of these measurements. Comment: the use of additional supplementary scalp electrodes, such as electrodes in the inferior temporal chain of electrodes, is indicated in various circumstances (for example focal epilepsy investigation). Theta band: Frequency band from 4− 25 µV), replacing a previously markedly intermittent record during evolution of EEG in preterm infants. Present in active sleep (see active sleep). Tracé discontinu: Normal discontinuous pattern in preterm infants which is characterized by bursts of high amplitude (up to 300 µV) mixed frequency activity regularly interrupted by low amplitude interburst activity (<25 µV). The pattern can still be called discontinuous if there is a modest amount of EEG activity in a single channel or a single transient in multiple channels. The interburst interval depends on the post menstrual age (PMA). It is seen in wakefulness, active and quiet sleep until 30 weeks PMA, then only in quiet sleep, and is rarely seen in infants of 38 weeks PMA. (See active and quiet sleep). Tracing: Synonyms: record, recording. Transient, EEG: Any isolated wave or complex, distinguished from background activity. Transverse bipolar montage: A montage consisting of contiguous channels of electrode pairs along transverse/coronal (i.e. side-to-side) arrays (for example F7-F3, F3-Fz, Fz-F4, F4-F8, etc.). Synonym: coronal bipolar montage. Triangular bipolar montage: A historic montage consisting of derivations from pairs of electrodes in a group of 3 electrodes arranged in a triangular pattern. Use of this montage is discouraged, because false lateralization may occur. Triphasic wave (TW): High amplitude (over 70 µV) positive sharp transients (with respect to common average), which are preceded and followed by relatively low amplitude negative waves. The first negative wave generally has lower amplitude than the negative afterwave, and has a steeper slope, which on occasions may be sharp. Usually bilateral with an antero-posterior or a postero-anterior lag, and repetition rate of about 1−<2 c/s. TWs occur typically in runs, and can either regress or increase with arousal or noxious stimuli. Often there is little ongoing or low amplitude (<40 µV) slow background activity between TWs. They are seen in a range of conditions, often present concurrently, associated with metabolic encephalopathy. The patient is usually comatose, and TW may decrease with sleep and after intravenous benzodiazepines. Synonym: continuous 2/s GPDs (with triphasic morphology). Unilateral: Confined to one side of the head (or body). Comments: (1) unilateral EEG activities may be focal, regional or lateralized to one hemisphere. (2) They are said to be lateralized to the right or left side of the head. (See lateralized). Vertex sharp transient or vertex sharp wave (V wave): A normal graphoelement. Sharply contoured wave with duration <0.5 s, maximal at the vertex, negative relative to other areas, apparently occurring spontaneously during light sleep or in response to a sensory stimulus (usually auditory). Vertex sharp waves may be single or repetitive. Amplitude varies but rarely exceeds 250 µV. (See light sleep, K complex). Visual evoked potential: Cortical evoked potential generated in response to visual stimulus, either unstructured diffuse flashes or patterned (for example pattern-reversal stimulus). (See evoked potential). Voltage: The difference in electric potential between two points (units: volts). (See amplitude). Volume conduction: The passive process by which electrical activity, originating from a generator, spreads through a conductive medium to be detected quite widely by distant (i.e. far-field) recording electrodes, without being mediated by neural activity (for example, see Brainstem Auditory Evoked Potential). (See propagation). Wave: Any change of the potential difference between pairs of electrodes in EEG recording, which may arise in the brain (an EEG wave) or outside of it (i.e. extracerebral potential). Waveform (wave form): The shape or morphology of an EEG wave (see morphology). Wicket spikes or wicket waves: Spike-like monophasic surface negative single waves or trains of waves occurring over the temporal regions, typically unilateral, during drowsiness that have an arcuate or mu-like appearance. These are mainly seen in older individuals and represent a benign physiological variant, but may also be seen in patients with a clinical diagnosis of epilepsy. Writer: Historically a system for direct write-out of the output of an EEG channel. Most writers used ink delivered by a pen, but in certain instruments the ink was sprayed in a jet stream, and in others the pen writer used carbon paper. Used infrequently since the advent of digital EEG, where laser printers are used to write-out EEGs.