Author: Brigo, Francesco
Date published: June 1, 2011
Lambda waves (LWs) (Figure 1) are physiological sharp transients occurring over the occipital regions in awake subjects during visual exploration. Initially described in an oral presentation by Evans in 1949 (Evans 1953), lambda waves were written about by Yves Gastaut in 1951 (Gastaut 1951).
Lambda waves are biphasic or triphasic sharp-contoured waves, with a predominant surface positive component usually preceded and followed by a negative one. They are triangular or sawtooth shaped, with an amplitude generally below 20 µ?, although in referential montages they may sometimes exceed 50 µ V. The amplitude of lambda waves is higher in infants and young children; this may be due to the presence of open ectocranial sutures with resultant larger surface potentials (Shih and Thompson 1998). The overall duration of lambda waves is roughly 100 to 250 msec. These waves usually repeat at intervals from 200 to 500 msec (Kellaway 1990, Noachtar et al. 1999, Niedermeyer 2005, Blume et al. 201 1).
Lambda waves are best elicited when the subject visually scans a complex image with fast saccadic eye movements (Figure 2). Different type of visual stimuli may increase the sensitivity of detecting these waves (Shih and Thompson 1998). Lambda waves disappear on eye closure, in darkness, when the subject views a blank background, or during fixation of gaze. Unlike positive occipital sharp transients of sleep (POSTS), lambda waves are recorded exclusively in awake subjects. The presence or absence of lambda waves is of little clinical significance, being normal EEG phenomena.
Lambda waves are confined to occipital regions, usually bilaterally synchronous, although sometimes quite sharply contoured in morphology and with a shifting asymmetry (Figure 3) (Stern and Engel 2004), thus potentially misinterpreted as interictal epileptiform abnormalities. A marked and consistent asymmetry points to a brain lesion on the side lacking lambda waves.
In the most extensive review on lambda waves available in the literature (Chatrian 1976), the incidence of lambda waves in routine EEGs of adults and adolescents has been found to range between 2 and 88%, being unmistakably present in some records and not readily demonstrable in others. Lambda waves prevalence depends, however, on several factors: stimuli parameters and laboratory procedures (lighting, patterned ceiling tile, use of routine picture viewing) (MacDonald 2003), cooperation of subjects, size of studied population, thoroughness in EEG evaluation, and the emphasis placed on the demonstration of this EEG pattern (Shih and Thompson 1998). Lambda waves are infrequently recorded in routine EEGs, probably because visual scanning of complex images is not a common routine recording procedure (Fisch 1999).
An age-dependent effect has been reported; lambda waves seem to occur more frequently in children aged 3 to 12 years (82.3%); their prevalence declines to 72% between 18 and 30 years and to 36.4% between ages 31 and 50 years (Chatrian 1976). No significant gender difference in the expression of lambda waves has been reported (Shih and Thompson 1998).
Lambda waves occur with saccadic eye movements (so called "exploratory saccades") during visual scanning, disappearing when eyes are closed and in sleep. They are best elicited when patients visually scan a complex picture, thus are seen during visual exploration of a brightly illuminated object or room (Kellaway 1990, Noachtar 1999, Niedermeyer 2005, Blume et al. 2011). Placing a white sheet of paper in front of the subject may eliminate the visual input which is considered essential in the genesis of lambda waves.
The origin of lambda waves has been related to the occipital driving response, given their occipital location and relationship to visual stimuli. Although they are considerably independent with respect to posterior alpha rhythm, marked similarities between lambda waves and visual evoked responses have been reported by Remond et al. (1965) and Lesèvre (1967). As a matter of fact, lambda waves resemble the occipital driving response evoked by intermittent flash stimulation, in terms of distribution, morphology, and latency between the visual input and the wave peak (Fisch 1999).
A common generator for both lambda waves and occipital photic driving was initially presumed by Gastaut (1951) and Evans (1953), but was questioned by subsequent intracranial depth electrode recordings (Perez-Borja et al. 1962, Chatrian et al. 1976) who reported different locations within the parietal and occipital lobes (either within or near the calcarme region, or more laterally) giving rise to occipital photic driving response and lambda waves.
Using averaging studies, Billings (1989) investigated the relationship between lambda waves, saccadic eye movements, and pattern reversal visual evoked potentials in normal control subjects and in patients with suspected multiple sclerosis. Lambda waves and photic driving response had significantly different latencies, although three patients with suspected multiple sclerosis had unilateral delay of both responses from the same eye. Billings suggested a hypothesis for the generation and transmission of lambda waves, proposing the existence of two different types of lambda wave. The first type of lambda wave would be generated as a result of retinal afferent activity due to the return of normal visual sensibility in the peripheral visual field, following its suppression during the saccadic eye movement, and transmitted via fast-conducting optic nerve fibers subserving peripheral vision. The second type of lambda wave would result from the retinal afferent volley caused by the release of suppression of the central visual field and conducted via slower-conducting fibers subserving central vision.
More recently, Shih and Thompson (1998) prospectively examined the incidence of lambda waves in 367 subjects (aged 4 months to 88 years) evaluating for a statistical correlation between lambda waves and occipital photic driving. The incidence of lambda waves, photic driving, and epileptiform discharges was 76%, 85%, and 23%, respectively. Lambda waves and photic driving were found to be strongly related because of a significant statistical correlation between them (P < 0.001) and because both were age-dependent (the maximum incidence in the 6- to 20-year-old range, without patient demographic profile or testing condition variability explaining such a result). Shih and Thompson concluded that the strong correlation between lambda waves and the occipital photic driving response suggests the possibility that the expression of these waveforms is due to a functional activation of a common region in the brain, probably localized within parieto-occipital region.
Recently Radhakrishnan et al. (2007) described an EEG response to visual scanning of geometric patterns, which was called pattern-induced negative occipital potentials (PINOP). Like lambda waves, PINOP are distributed over occipital regions and disappear on eye closure. In contrast to lambda waves, PINOP are surface negative, briefer in duration, and sharper, thus being more likely than lambda waves to be confused with epileptiform abnormalities. Radhakrishnan et al. concluded that PINOP represent an unusual benign physiologic phenomenon, because the prevalence of epilepsy was not significantly different among subjects with PINOP than among age- and sex-matched control subjects without PINOP, and because none of the patients with PINOP exhibited paroxysmal response to intermittent photic stimulation. Both lambda waves and PINOP are therefore physiologic responses to pattern and picture viewing. Radhakrishnan et al. critically examined the relation of PINOP with lambda waves, stating that the electrophysiologic basis of PINOP and the reasons why they differ in morphology from lambda waves are uncertain. They hypothesized that differential areas of activation in the occipital lobe during pattern viewing versus picture viewing and the orientation of these activated areas to the scalp EEG electrodes may account for the differences in the morphology of the lambda waves and PINOP.
DISTINGUISHING EEG FEATURES
Lambda waves resemble other EEG patterns, although they can be easily differentiated from them. POSTS have been previously called "lambdoid waves" because they resemble lambda waves in shape and distribution. However, lambda waves are exclusively recorded in awake subjects, whereas POSTS are positive sharp transients occurring over occipital regions (Figure 4) seen exclusively during non-rapid eye movement (NREM) sleep, thus with eyes closed, and associated with other sleep EEG figures (K-complex, spindles).
Lambda waves may be differentiated by interictal epileptiform abnormalities because of their characteristic triangular shape and occurrence only during visual exploration. Interictal epileptiform abnormalities are usually sharper than lambda waves, are rarely affected by eye opening or closure, and are more frequently encountered during sleep, in contrast to lambda waves which occur only during wakefulness (Stern and Engel 2005). The lack of prominent after-following slow waves is another aspect which points to lambda waves. Disappearing with blank card viewing, darkness, eye closure, drowsiness, and sleep is another aspect to be considered in the differential diagnosis. In doubtful cases (which are nevertheless exceptional) oculographic monitoring may assist proper identification (MacDonald 2003).
Subjects who have lambda waves are more likely to have a marked photic stimulation driving response (Stern and Engel 2005). Lambda waves resemble the intermittent photic stimulation driving response, because of distribution, morphology and latency between the change of visual input and the wave peak (Fisch 1999). However, the time-locked relationship (1:1) between flash stimulation frequency and occipital positive potentials (Figure 5), and the persistence of such a response at different stimulus frequency easily differentiate lambda waves from occipital photic driving.
It is noteworthy that lambda waves are often temporally related to ocular artifacts such as blinking, lateral gaze, and lateral rectus muscle spike, because lambda waves occur in the context of saccades during visual exploration, and may be time-locked to saccades with an average delay of 75 msec (Stern and Engel 2005).
Although, like lambda waves, PINOP are distributed over the occipital regions and disappear on eye closure and while the subject views a blank background, unlike lambda waves, PINOP are surface negative, briefer in duration (usually less than 200 msec), and sharper. The restricted distribution over the posterior head region and absence of associated, more widely distributed epileptiform responses during photic stimulation and pattern testing are clues to PINOP (Radhakrishnan et al. 2007).
Lambda waves are a normal phenomenon representing a physiologic response to visual exploration. Although the presence of lambda waves is associated with a marked occipital photic driving response, the two phenomena may have different cerebral generators. The EEG features and clinical states differentiate lambda waves from POSTS, PINOP, and interictal epileptiform abnormalities.
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Francesco Brigo, M.D.
Department of Neurological, Neuropsychological, Morphological,
and Movement Sciences
Section of Neurology
University of Verona