13081Journal ofNeurology, Neurosurgery, and Psychiatry 1994;57:1308-1319 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.57.11.1308 on 1 November 1994. Downloaded from NEUROLOGICAL INVESTIGATIONS Electroencephalography C D Binnie, P F Prior Genesis ofthe electroencephalogram cific diagnostic significance. Thus slowing The electroencephalogram (EEG) is a record- may arise from causes as diverse as cerebral ing of cerebral electrical potentials by elec- oedema or hypoxia, or systemic disorders trodes on the scalp. Cerebral electrical activity such as hepatic insufficiency. The most reli- includes action potentials that are brief and able abnormal EEG sign is reduction of nor- produce circumscribed electrical fields, and mal activity, ranging from reduced amplitude slower, more widespread, postsynaptic poten- over a past cerebral infarct or a subdural tials. The magnitude of the signal recorded haematoma, to electrocerebral silence in brain from a neural generator depends on the solid death. Spiky waveforms (epileptiform activity) angle subtended at the electrode. Conse- occur in epilepsy and in some patients with quently, the activity of a single neuron can be cerebral disorder but without seizures. recorded by an adjacent microelectrode, but Rhythmic slow activities may occur bilaterally not at a distant scalp electrode. Synchronous over the frontal or posterior temporal regions activity in a horizontal laminar aggregate of in patients with dysfunction of diencephalic or neurons with parallel orientation may, how- brainstem structures. ever, constitute a generator of sufficient extent The EEG is profoundly influenced by alter- to be detectable on the scalp. Thus the EEG ations in vigilance and also changes with age, is a spatiotemporal average of synchronous most noticeably during childhood. Interpre- postsynaptic potentials arising in radially ori- tation must take account of the range of nor- ented pyramidal cells in cortical gyri over the mal findings at different ages and in different cerebral convexity. It is estimated that the states of awareness. The slower components smallest detectable generator has an extent of diminish with maturation and increase in some 6 cm.2 Tangentially oriented generators sleep and drowsiness. As slowing is a common in the walls of sulci do not generally appear in EEG abnormality, it may be difficult to distin- the EEG, but are seen in recordings of the guish the effects of immaturity, drowsiness, brain's magnetic field (magnetoencephalo- and pathology. This similarity between the gram (MEG)).' immature and the abnormal EEG underlies Synchronous neuronal activity arises by an interesting approach to quantitative clinical http://jnnp.bmj.com/ various mechanisms. Isolated aggregates of EEG analysis by Matousek and Petersen.3 interconnected neurons spontaneously adopt They developed a method of computing the rhythmic synchronous firing patterns. patient's apparent age from spectral features Afferents-for instance, from the reticular for- and used the ratio of calculated to actual age mation-stimulate individual neurons into as a measure of EEG abnormality. independent asynchronous activity. Thus syn- chrony is reduced by arousal and cognitive on September 30, 2021 by guest. Protected copyright. activity and increases with reduced vigilance, Electroencephalography technology both in normal sleep and in pathological DEVELOPMENTS states, reflected in the EEG by increased Traditionally EEGs were written on electro- amplitude and slowing. Specific pacemakers mechanical chart recorders; these are now also exist that produce rhythmic synchronous being replaced by digital systems, which offer Department of Clinical activity.' There is, for example, an inhibitory improved reliability and compact, accessible Neurophysiology, feedback loop involving thalamocortical neu- archives on optical discs. Within a few years The Maudsley rons that produces oscillatory burst firing in clinical neurophysiology laboratories will be Hospital, Denmark Hill, London SE5 8AZ, drowsiness and sleep. Transitory synchronous based on a local computer network, probably UK activity can be elicited by afferent stimuli with generic data acquisition stations for C D Binnie (evoked potentials), spontaneous arousal recording EEG, EMG, and evoked potentials Department of (producing such phenomena as vertex sharp directly on to a file server, and workstations Clinical transients in and for reviewing the data and entering reports to Neurophysiology, light sleep), pathological St Bartholomew's neuronal discharges in epilepsy. form an integrated archive with the original Hospital, West signals. Smithfield, London These innovations have done little to EClA 7BE, UK Interpretative principles reduce the inherent technological difficulties P F Prior of the Correspondence to: Abnormalities on the EEG reflect general of obtaining satisfactory recordings Dr C D Binnie. pathological processes and are rarely of spe- EEG which, having an amplitude of only Electroencephalography 1 309 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.57.11.1308 on 1 November 1994. Downloaded from some 5 to 200,V, is very susceptible to arti- established and its promotion as a substitute facts, from both bioelectric and physical for conventional EEG can only be deplored.'2"1 sources. The problems can be largely over- come by good electrode technique, but this is particularly difficult to achieve in children and Cerebral lesions in others who may be distressed and uncoop- Electroencephalography provides information erative. Methods of constructing electrodes that primarily concerns disturbances of func- have changed little in recent decades, but a tion rather than structure. Whereas clinical significant advance has been the development studies in the 1930s showed localised changes of improved adhesive pastes, which achieve at the site of cerebral mass lesions, routine secure electrode fixation and a low contact referral for EEGs on suspicion of intracranial resistance without abrasion of the skin-an tumour is no longer appropriate. Modern important consideration given current con- imaging techniques, although somewhat more cerns with avoidance of cross infection. costly, provide more precise identification of Changes in vigilance may affect the occur- the presence, nature, and site of such lesions. It rence of pathological phenomena; particularly should also be noted that in this context the in epilepsy, clinically relevant abnormalities value of a negative EEG in excluding pathol- may be found in sleep but not in wakefulness. ogy may be somewhat illusory. The normal Sleep recording is generally underused and is EEG does not exclude intracranial disorders; not routinely available in many departments. a more appropriate approach to the investiga- tion is to recognise the significance of EEGs EPILEPSY MONITORING with positive findings. The EEG only plays a Arguably the most important recent develop- relevant part when patients cannot, for vari- ment in epileptology has been long term EEG ous reasons, undergo scanning or when and video monitoring (see Binnie4 and potential epileptogenicity,'4 possible postoper- Gotman et al I for reviews). As the manifesta- ative recurrence of a tumour, or toxic effects tions of epilepsy are intermittent, a routine of medical oncological drugs v metastatic dis- EEG often fails to show epileptiform activity, ease require evaluation. In these situations, which may occur only during seizures. clinical value accrues in the evolution of Moreover, interictal epileptiform activity may changes over serial recordings. be of doubtful clinical value, either for identi- Vascular lesions may be more rewarding to fying the site of onset of seizures or for deter- investigate than tumours. The changes after a mining whether particular clinical events are cerebral infarct will be most characteristic in epileptic. the first hours and days, before those on CT The EEG can be telemetered over days become evident. Typically the appearances through a cable or radio link, permitting lim- are of a localised reduction of normal cortical ited mobility in hospital, while behaviour is rhythms and a major surrounding slow wave documented by video. Alternatively, ambula- abnormality with individual waves of less than tory monitoring can be carried out in an 1 Hz. There is often a rapid evolution of the everyday environment with a portable cassette EEG abnormality that may resolve before the recorder, but behavioural documentation will scan becomes positive. be less reliable, depending on reports of car- Prognostic assessment of CT negative ers. These technologies have different applica- patients with transient or mild ischaemia http://jnnp.bmj.com/ tions; telemetry is generally preferred, unless depends on subtle abnormalities evident only it is essential to record in a particular environ- when quantitative EEG techniques are used. ment. These utilise computer analysis of the EEG frequency spectrum. A sensitivity of 50-70% BRAIN MAPPING and a specificity of 90-100% have been A technical development that has generated reported.'5-"7 The topic is thoroughly reviewed recent enthusiasm is brain electrical activity by van Huffelen,'8 who also reminds us of the mapping. Computer assisted EEG analysis value of quantitative EEG techniques and on September 30, 2021 by guest. Protected copyright. has been used in research for more than 30 somatosensory evoked potentials in monitor- years, but has few clinical uses beyond moni- ing patients at risk