Basic principles and interpretations of electroencephalography [Workshop 1]

ABSTRACT

At the end of the workshop, the participant will be able to 1. Understand the principles of EEG recording. 2. Identify commonly encountered EEG artifacts. 3. Recognize normal awake and sleep EEGs in children and adults. 4. Interpret some common abnormal EEG patterns. Electroencephalography [EEG] is the science relating to the electrical activity of the brain. The background electrical activity of the human brain was first analyzed in a systematic fashion by the German Psychiatrist Hans Berger [1929]. Since then, EEG has been used in clinical practice to disclose non-epileptiform and epileptiform cortical dysfunction. EEG recording. EEG is recorded with scalp electrodes. The recordings may be bipolar or unipolar. Bipolar records show fluctuations in potentials between 2 electrodes and the unipolar record show potential difference between cortical electrodes and theoretically in-different electrodes. A digital EEG machine allows any section of the record to be completely reformatted namely, viewed with any Montage, Gain, Filter or Timebase. The standard placement recommended by the American EEG society for use in the International 10-20% system is for 21 electrodes [Figure 1]. The standard numbering system in the 10-20 system places odd-numbered electrodes on the left and even numbered electrodes on the right, with the letter designating the anatomic area. Montage. The term montage refers to the particular combination of electrodes examined at a particular point of time. In most instances, multiple montages are more useful than a single montage for long periods. The function of the montage is to record from all areas of the scalp and also to record activity in such a manner that it is easily perceived by the reader. The principal montages are longitudinal bipolar "Double Banana", coronal bipolar, circumferential bipolar, laplacian, common average referencial and ear [A1, A2] referential. It should be mentioned here that bipolar runs provide better localized focal or regional features, while the morphology of the widespread phenomena appear best on referential montages. Physiological basis of EEG [EEG generator]. The activity recorded in the EEG is mostly that of the most superficial layers of the cortical gray matter. The potential changes in the cortical EEG are due to current flow in the fluctuating dipoles formed on the dendrites of the cortical cells and the cell bodies [Figure 2] namely current to flow through the volume conductor between "source" at the soma and basal dendrites and the "sink" at the apical dendrites sustaining excitatory postsynaptic potential [EPSPs]. Figure 2 illustrates current paths taking increasingly remote curving routes. The zero potential surface is located halfway between the positive and negative poles of the dipole. To understand how electrical potentials are recordable on the scalp generated by populations of the pyramidal neurons, it can be easily understandable by the solid angle concept of the volume conduction theory. In this, the potential generated by a dipole layer in a volume conductor [brain and its environs] is proportional to the solid angle subtended by the dipole layer at the point of the measurement [Figure 3]. EEG interpretations. EEG interpretation requires a structured approach to ensure that important information is not lost or data misinterpreted. Prior to the interpretation, the only clinical data known to the reader should be the age of the patient. In evaluating an EEG pattern or event, one should answer the following Is it an artifact analysis. EEG analysis. The initial analysis of EEG includes the frequency or wave length of the background activity [Delta below 3.5/sec, Theta 4-7/sec, Alpha 8-13/sec, Beta above 13/sec and Gamma above 30 ' unlimited range/ sec], its voltage [micro-volts], waveform, manner of occurrence, locus, reactivity on eye opening or sensory stimuli, interhemisphericcoherence [Symmetry and synchrony]. [Figure 4 and Figure 5]. A continuous waveform recorded from living tissue are usually sinusoidal. Sine waves of several frequencies usually appear together in clinical EEG to produce complex waveforms [Figure 6]. This feature should be recognized in assessing such phenomena. Epileptic and paroxysmal pattern. The term " Epileptic discharge" has been attacked and condemned on the ground that such discharges may occur in the absence of the clinical seizure. The same is true for the term " seizure discharge" or even "Paroxysmal discharge". However, in interpreting interictal paroxysmal pattern, the interpretator will frequently encounter abnormal waves like apikes [pointed peak with 20-<70 msec duration], sharp waves [pointed peak with 70-200 msec duration], polyspikes or multiple spikes [mostly bilateral/generalized synchronous discharges encounter in myoclonus, primary generalized epilepsy, photosensitive individual and children with LGS], runs of rapid spikes, spike wave complex [classical 3 Hz], slow spike wave [1- 2.5 Hz] and periodic or quasiperiodic discharges. Activation methods. Hyperventilation [HV], intermittent photic stimulation [IPS] or sleep are used in almost all clinical EEG laboratories to enhance pre-existing abnormalities or induce abnormal findings in an otherwise normal EEG. These methods are called activation procedures. Hyperventilation should be performed for a minimum of 3 minutes with continued recording for at least one minute after the cessation of over-breathing. Aside from diffuse slowing, it is effective in eliciting bilaterally synchronous spike wave discharges in a patient with generalized epilepsy. In IPS a strobe lamp is placed at a distance of 20-30 cm in front of the subject's eyes. Flashes at frequencies of 1,3,6,9,10,15,20 and 40 Hz are given in trains of 5 second duration with eyes open and closed in a room with reduced illumination. The EEG changes induced by IPS are photic driving [a physiological response consisting of rhythmic activity elicited over posterior region of the head], photomyoclonic response [brief repetitive muscle spikes over the anterior region of the head] and photoconvulsive response [characterized by spike-and-slow and multiple spike-and 'slow wave complexes that are bilaterally synchronous, symmetrical and may out last the stimulus for a few seconds]. Localization and polarity. Localization of potentials plays a critical role in the clinical interpretations of EEGs. Accurate localization of an epileptiform discharge may help in identifying the epileptogenic zone. Most reviews of techniques of EEG localization have stressed the significance of a PHASE REVERSAL in bipolar montage and of AMPLITUDES in referential montages. Amplifiers for EEG recording are differential amplifiers namely, each channel records the difference in voltage between electrode selected for input 1 and that for input 2 [Figure 7]. When the input 1 is more negative than input 2, the output "pen" deflects upwards. Other options of the pen deflections are shown in the figures [Figures 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17]. However, in referential recording an indifferent scalp electrode is connected to input 1 of each amplifier but a single common electrode is connected to input 2 of each amplifier. Artifacts. Artifacts in EEG refers to any electrical signal that is not generated directly by the brain. Artifacts can mimic almost every kind of EEG pattern. The technologist and electroencephalographer need to be constantly alert to the possibility of artifact. Clinical report consists of I] A technical description of the findings of the visual analysis. II] Technical Impression Specifying general character of abnormality [focal, regional, diffuse, lateralized or combination of all]. III] Clinical impression Assess significance of EEG findings in terms of clinical history and findings. Concluding remarks. Paroxysmal discharges in the EEG are indicators of deviant neuronal behavior. A solid experience in clinical EEG and familiarity with the magnitude of epileptic seizure disorders are the indispensable pre- requisites for a truthful and clinically useful interpretation of paroxysmal EEG events. Moreover, detection and interpretation of the EEG data from visual analysis involve matter of judgement and experience which render clinical EEG as art as much as a science