HANDOUTS : SUMMARY POINTS Steps in the EEG Analysis When discharges are found that are suspicious for interictal or ictal activity in epilepsy, there are a sequence of questions which should be asked to assist clinical interpretation. Is the discharge cerebral or artifactual? If the discharge is cerebral, is it normal or abnormal? If the discharge is abnormal, is it epileptiform? If the discharge is epileptiform, is it focal or generalized? If the discharge is focal, what is the field of the discharge?
Is it a spike or sharpwave? Milliseconds
Sharpness
Spikes
20 – 70
Apiculate
Sharp waves
70 – 200
Less sharp
Criteria to look for in interictal epileptiform discharges: 1. Clearly distinguished from the background – therefore paroxysmal. 2. Abrupt change in polarity during several milliseconds - characteristic “spikiness” evident. Other characteristics: An asymmetric appearance typically with a shorter first half and a longer and higher-voltage second half, biphasic or polyphasic morphology and after-going slow wave. 3. Duration less than 200 ms. 4. Definite Physiological field.
Ictal versus interictal epileptiform discharges Ictal discharges are usually not repetition of interictal discharges and will typically have an appearance different from multiple interictal discharges.
Criteria in Generalized epilepsy
1. One exception to this statement is generalized absence seizures, for which the distinction between interictal and ictal discharges is not always clear-cut. In generalized absence seizures it has been demonstrated that a subtle alteration of responsiveness occurs even with a single spike-and-wave discharge. On the other hand, generalized spike-and-wave discharges that are shorter than 3 seconds in duration are typically not noticed by family members, particularly in the absence of motor accompaniments. In practice, bursts of generalized spike-and-wave discharges are ictal if they last more than 3 seconds, or if they are associated with clear clinical changes. Use of the response tester during EEG will help clarify this. 2. Another pattern that can be ictal or “interictal” is paroxysmal fast activity noted in patients with symptomatic generalized epilepsy, Lennox-Gastaut syndrome. In these patients, the paroxysmal fast activity (or generalized polyspike activity) can be associated with generalized tonic seizures or could be totally asymptomatic. Occasionally, such discharges cause arousal as their only clinical manifestation.
Spectrum of focal vs. generalized epilepsy Focal versus multiple foci versus generalized spike-wave One Spike Focus : Focal epileptiform activity generally suggest focal or partial epilepsy, particularly if there is a single and consistent localization. For example, consistent right anterior temporal spikes or sharp waves suggest right anterior-mesial temporal lobe epilepsy and consistent left occipital spikes suggest left occipital lobe epilepsy. Two Spike foci : Two independent spike or sharp wave foci still suggest partial epilepsy in most instances. However, if the discharges are frontal or central they can be consistent with generalized epilepsy. In generalized epilepsy, “fragments” of generalized epileptiform discharges could be noted, particularly in sleep, in the frontal or central regions. As a rule, patients with generalized epilepsy will have generalized epileptiform discharges as well as these “fragments”. Alternatively, focal spikes and generalized spike-wave in sleep occurs in the same patient in the epileptic syndrome, Benign Epilepsy with Centrotemporal Spikes. Three or more spike foci in two hemispheres : This is termed Multiple Independent Spike Foci and the clinical outcome and prognosis is similar to secondary generalized epilepsy. When there are two or more independent foci, the localization of the epileptogenic focus becomes less certain. Many patients will still have a single ictal focus, i.e. seizures may start in a single location even though interictal epileptiform activity is bilateral or even multifocal. However, patients with independent foci are more likely to have independent seizure onsets than those with single consistent foci.
Criteria for Focal ictal discharges Before and during the discharge look for: 1. Change in rhythm: Focal voltage attenuation, followed by ictal fast rhythms. For example, beta (suggests neocortical onset). 2. Evolution of the discharge, increase followed by a decrease in voltage: 3. Change in frequency: initial fast rhythms that begin to slow; widening of the field, associated with decrease in frequency.
After the discharge: 1. Focal slowing In addition to the EEG changes, during the discharge: 1. Change in heart rate, usually tachycardia occurs. 2. Clinical changes noted: eye blinking, EMG activity, generalized or focal (if rhythmic, in an additional EMG channel monitored).
Localizing value of ictal discharges EEG finding
Localizing value
High frequency beta at onset
Neocortical involvement
Theta frequency discharge within 30 seconds of onset
Mesial temporal localization
Widespread field at onset; delta pattern at onset Lateral temporal localization No changes on surface EEG
Mesial frontal, orbitofrontal epilepsy
Secondary Bilaterally Synchrony Secondary bilateral synchrony commonly occurs in frontal epilepsy. This is to be distinguished from primary bilateral synchrony seen in generalized epilepsy. Suspect this when: 1. Bilateral discharges have a consistent asymmetry or a consistent lead on one side, 2. Some focal discharges are seen only one side 3. Consistent focal slow abnormality present.
Benign rolandic epilepsy EEG characteristics in Benign Epilepsy with Central temporal spikes: 1. Epileptiform discharges have a stereotypic appearance with broad blunt sharp waves, repetitive, increasing in drowsiness and sleep. (Rule: must record sleep before excluding this diagnosis.) 2. The classical field is negativity in the central and mid temporal regions and positivity in the bifrontal regions (look for a horizontal dipole). 3. Midtemporal focus versus anterior temporal. 4. Absence of focal slowing. Normal background! 5. Ictal discharges are rare, despite multitude of interictal discharges. 6. Generalized spike and wave discharges can coexist. 7. School going age. 8. Maybe an EEG trait.
Know the evolution of a Generalized tonic-clonic seizure Differentiate epileptic vs non epileptic seizure
How to read a page of EEG 1. 2. 3. 4. 5. 6.
First check montages, sensitivity then paper speed Identify and exclude artifacts Assess organization of the background (think in terms of a Battlefield) Assess GENERAL background Assess POSTERIOR background Look for asymmetries - can be easily explained by head position, scalp swelling, an extra-axial fluid collection, subdural 7. Next focus on the glaring abnormality - a left temporal spike, 3 Hz generalized spike-wave 8. Do not let your eye become drawn to this abnormality again and again 9. When trying to identify a waveform potential determine its field Draw iso-potential lines Check field in different montages as reference montage and may be contaminated Then determine if biologic or non-biologic field exists Classification scheme for EEGs reporting (as developed at Montefiore Medical Center by Drs. Goldenson, Moshe and fellows)
Paroxysmal activity: 1. 2. 3. 4.
Spikes Sharp waves Spike and wave, generalized- Regular, 3 Hz/regular irregular/regular, fast slow spike and wave 2-2.5 Hz
5. PLEDS 6. PEDS, periodic sharp waves. 7. Triphasic waves 8. Spindle coma 9. Alpha coma 10. Burst suppression 11. Multifocal spikes 12. Hypsarrhythmia
Slowing: 1. 2. 3. 4. 5. 6. 7.
Posterior background slowing less than 8 Hz Generalized background slowing including posterior less than 8 Hz Intermittent rhythmic slowing, focal, (for example theta) Intermittent rhythmic slowing, generalized Intermittent rhythmic slowing, focal, frontal, delta (FIRDA) Intermittent rhythmic slowing, focal, occipital, (OIRDA) Continuous polymorphic slowing, focal, (for example, temporal, delta)
Paroxysmal non-epileptiform activity: 1. 2. 3. 4. 5. 6.
Small sharp spikes (BETS) 14 and 6 positive sharp waves Phantom spike and wave SREEDA rhythm Wicket spikes POSTS
Will provide
1. Basic EEG template before normal EEG. This can be modified by the reading physician 2. Sample EEG templates of focal abnormalities, generalized abnormalities with detailed description for the reader to use when reporting EEGs 3. Method for converting the above data into a simple flat file database for records purposes, research, report retrieval etc.
Rules for identifying ictal discharges (exception is 3 Hz spike and wave)
1. Change in rhythm-voltage attenuation, then fast activity such as beta (indicates neocortical onset) 2. Evolution of the discharge would increase followed by a decrease in voltage, he increase and decrease in frequency, widening of the field of activity. 3. A change in frequency - fast rhythms that begin to slow down. Presence of a spike and wave configuration 4. Focal slowing 5. A change in the non-cephalic channels such as increase in heart rate 6. Hippocampal seizures-may have theta or alpha frequency discharge. 7. Delta frequency suggestive deeper focus of onset
Analytic Process of EEG Interpretation When viewing and discussing an EEG a working vocabulary is needed. Master the following terms:
Channel: One line of EEG recording (generated by one amplifier that amplifies the difference between the electrode(s) plugged into input 1 vs 2 of the amplifier) Derivation: a description of the electrodes placed in input 1 vs input 2 (e.g., Fp1 –
F3 is a derivation in which the electrode Fp1 has been plugged into input 1 and Fp2 into input 2 of the amplifier) difference between the electrode(s) plugged into input 1 vs 2 of the amplifier) Montage: collection of derivations Bipolar derivation or bipolar montage: adjacent electrodes on the scalp plugged into inputs 1 and 2, respectively (e.g., Fp1 – F3 is a bipolar derivation) Referential derivation or referential montage: electrodes in input 1 and 2 that are not immediately adjacent on the scalp. The electrode in input 1 is considered to be the exploring electrode and the electrode in input 2 is intended to be out of the field of the activity of interest being recorded by electrode 1 (e.g., F7 – Cz is a referential derivation where F7 is intended to be the exploring electrode and Cz is intended to be the reference).
Localization with referential and bipolar recording
Morphology
Morphology refers to the shape of the waveform. Waveform morphology is described using the following terms: » Frequency » Spike » Sharp wave » Complex » Amplitude » Polarity » Phase(s) » Symmetry » Synchrony » Rhythmicity » Reactivity
» Frequency: may be described precisely in hertz (Hz; cycles per second; 1 sec/duration of the waveform), waveform duration in milliseconds, or using the following terms: Frequency band : delta (<4.0 Hz) theta (4.0 - < 13.0 Hz) alpha (8.0-13.0 Hz) beta (>13 Hz) Spike : sharply contoured (apiculate) waveform with a duration of 20 to 70 msec Sharp wave : sharply contoured (apiculate) waveform with a duration of 70 to 200 msec complex. When the pattern consists of 2 or more waves with a distinct form, such as a spike and an aftergoing slow wave, then the waveform pattern is referred to as a complex, as in a spike and slow wave complex.
Amplitude refers to the voltage of the waveform or waveform complex. Amplitude in EEG is measured in microvolts, whereas other electrical signals commonly measured by physicians arise from muscle (EMG, ECG) are measured in millivolts. The degree to which the original signal is amplified is referred to as the gain of the amplifier. However, amplitude settings are described in terms of sensitivity in microvolts per millimeter. The sensitivity setting describes the display magnification of the waveforms. Notice that the relationship between sensitivity and gain is inverse. If the sensitivity setting is increased, the size of the waveforms on the monitor (or page) is reduced. For example, if the sensitivity setting is increased from 5 to 10 uV/mm then the size of the waveforms on the monitor will be cut in half. Polarity : The polarity of an EEG waveform is referred to as positive or negative. Polarity conventions are discussed below under, Spatial Analysis. Phase refers to that part of the waveform that begins in one direction - either up or down - and then changes to the opposite direction in a single turn and finally ends by once again turning in the opposite direction. The phase can be described by its polarity (i.e., the direction the waveform begins in), duration, and or amplitude. If the electrode detecting the waveform shows an increase in amplitude in a negative direction and then a turn in a positive direction, then that phase of the signal is said to be negative. If a waveform complex has more than one phase the phases are counted according to the number of turns (changes in polarity direction) the waveform takes. For example, a waveform that goes up, comes down and returns up again would have 2 phases and be referred to as biphasic. A waveform with 3 turns would be referred to as triphasic. Synchrony refers to waveforms that are spatially independent (usually bihemispheric) that occur simultaneously and often in phase with each other. Symmetry is a term that refers to the spatial distribution of a waveform pattern over the scalp in terms of left vs right hemisphere amplitude comparisons. Rhythmicity refers to the appearance of an uninterrupted series of monomorphic waveforms. A sine wave signal at a fixed frequency would be considered perfectly rhythmical, as would a more complex waveform that repeats in an uninterrupted series without changing its shape. Irregular patterns consist of waveforms with continuously varying shapes and durations. Patterns that combine varying degrees of rhythmical and irregular repetition intervals are referred to as semirhythmic. Reactivity refers to the degree of change that occurs in the EEG in response to exogenous or endogenous stimulation. The alpha rhythm is defined, in part, by its attenuation or complete suppression in response to sustained eye opening. Reactivity testing should be performed in every routine EEG. It should include asking the patient to open and close their eyes (to demonstrate the alpha rhythm) and guarantee an awake state recording by having the patient answer mental calculation and orientation questions asked by the technologist. If the patient is not alert when they open and close their eyes then the highest potential frequency of the alpha rhythm may be underestimated. Activation refers to the appearance or increased appearance of a particular pattern in response to an activating procedure. Routine activating procedures that should be performed in every routine EEG include hyperventilation, photic stimulation and sleep, unless medically contraindicated. Although they may activate a variety of abnormalities, they are primarily used to detect epileptiform abnormalities.
Epileptiform discharges-describe these using above terms A- spike; B- sharp wave; C- spike-and-wave complexes; D- sharp-and-slow-wave complexes; E- slow-spike-and-slow-wave complex; F- polyspike-and-wave complex; G- multiple-sharp-and-slowwave complex; H- polyspike complex; I & J- multiple sharp wave complexes. Even though spikes and sharp waves usually have after-going slow waves, the term spike-and-wave complex is usually reserved for the situation where the slow wave is very prominent, higher in voltage than the spike. The interval between vertical lines represents 200 msec.
The selection of electrodes for inputs 1 and 2 for any single amplifier channel is referred to as the derivation. The combination of multiple derivations is referred to as a montage. Montages perform the function of spatial filtering because they filter out, to varying degrees, similarly shaped waveforms that are simultaneously and widely distributed over the scalp. Indeed, montages can be compared to different lens settings in a telescope. Some montages (e.g., bipolar or Laplacian) are best suited for viewing highly localized activity (close up views) whereas others (e.g., reference) are better for viewing widespread potentials (distant views). Because spatial analysis is so important in EEG interpretation, the most important advance in routine EEG technology in the last 20 years has been the application of digital EEG montage reformatting. There are currently 5 basic kinds of montages the EEG reader should be familiar with: bipolar, common electrode reference, average reference, weighted average reference, and Laplacian (source derivation). To understand the construction and use of montages it is essential to understand differential amplification and polarity conventions in EEG. Voltage polarity in EEG (and all biological recording) is always relative because it is assigned according to the voltage difference between recording electrode inputs in each amplifier. ď‚&#x; Differential Amplification : All biological recording uses differential amplification. Differential amplifiers measure the difference (hence the term differential) in voltage between 2 electrodes placed on (or in) the body. The reason differential amplification is used is that it eliminates any electrical noise detected by both amplifier inputs (electrodes) such as 60 cycle interference. By convention, a differential amplifier subtracts the voltage in input 1 from input 2. Therefore, at any single point in time: EEG amplified signal = (input 1 voltage) – (input 2 voltage) ď‚&#x; Rule : if the voltage in input 1 is relatively more negative than in input 2, then the waveform deflection (or pen deflection) is upward. If the activity in input 1 is relatively more positive than in input 2, then the waveform deflects downward. Remember, polarity is relative. Therefore, a voltage of +30 microvolts in input one and
a voltage of +70 microvolts in input 2 would produce an upward pen deflection, even though the activity in input 1 is positive, it is relatively negative to that in input 2. According to the equation above, the amplified signal = (+30) – (+70) = - 40. Another way to express the EEG polarity convention would be to say that a relative positivity in input 2 compared to input one produces an upward deflection, whereas a relative negativity in input 2 produces a downward deflection. ď‚&#x; Montage construction and reformatting : Montage reformatting is the process of creating a new montage to view the same duration of EEG. It is surprisingly simple to understand reformatting if it is recalled the EEG signal from each channel is created by the subtraction of the activity in the electrode in input 2 from input 1. For example, when a technologist labels a channel montage as Fp1-A1, they have actually written a mathematical expression that states that the signal displayed will be Fp1 minus A1 (input 1 minus input 2). If a recording has been obtained from Fp1-A1 and F3-A1, then Fp1-F3 can be derived by subtracting F3-A1 from Fp1-A1, as follows: (Fp1 - A1) - (F3 - A1) = Fp1 - F3 + A1- A1 = Fp1-F3.
Electrode placement/Common montages
ď‚&#x; Bipolar Montages Bipolar derivations consist of an adjacent pair of electrodes of the 10-20 or international system of electrode placement inserted into inputs 1 and 2, respectively. Bipolar montages consist of a series of overlapping bipolar derivations in straight lines, either longitudinally (anterior to posterior) or transversely (left to right) across the scalp. They act as spatial filters that remove widespread potentials with similar amplitudes and phases (i.e., coherent waveforms) from the recording. Bipolar montages are therefore best for analyzing low to medium amplitude waveforms that are highly localized. Bipolar montages consist of overlapping bipolar derivations that are arranged in a chain link fashion so that an electrode at input 2 of one amplifier is in input 1 of the next amplifier as shown in the figure below. In bipolar montages the localization of a cerebral potential is made according to the direction of waveform deflections between channels. An identical potential in input 2 of one channel and input 1 of the next channel causes waveform (or pen) deflections of opposite direction between those two channels. This is referred to as an instrumental phase reversal. Recall that in EEG recording if the electrode in input 1 is more negative than the electrode in input 2, then the waveform deflection will be upward. If the electrode in
input 1 is more positive than the electrode in input 2, then the waveform deflection will be downward. Physicians who are learning to interpret EEG initially develop two fundamental misconceptions about polarity and phase reversal. The first is the belief that an upward deflection of the pen or waveform means that the scalp potential that caused the waveform was negative, and that a downward deflection was caused by a positive potential. Remember, a waveform that points up is no more positive or negative than a waveform that points down. Polarity is totally dependent on which input of the differential amplifier the electrode is in. Thus, a positive scalp potential in the electrode of input 2 makes an upgoing deflection and a negative scalp potential in the electrode of input 1 makes the same upgoing deflection. The second misconception is that a phase reversal is always an abnormal finding. Instrumental phase reversals, like polarity, are simply the byproduct of the configuration of the bipolar montage and prominent normal examples can be found on virtually every page of a bipolar recording. In contrast, true phase reversals are generated by different cortical surfaces simultaneously having different polarity voltages is far from the scalp area of activity, the following statement is true.
SUMMARY: Localization of potentials: Bipolar montage Identification by reversal of polarity 2. Identify and exclude artifacts Maximum Potential is present at a single electrode Two electrodes involved, both contained within the chain and not involving the ends of the chain Two electrodes, unequally involved, each contained within the chain, with the ends of the chain not involved: There will be reversal of polarity seen between the two channels that contain the most affected electrode. The potential will also be seen in the channel containing the less affected electrode and the unaffected electrode adjacent to it. The amplitude in the channel with the largest deflection will be equal to the sum of the amplitudes in the two channels with smaller deflections. From this, one can conclude that if there is a reversal of polarity that is not a mirror image, it indicates that there is involvement of more than a single electrode. Potential is present at the end of the chain and not involving any other electrode in the chain: In this instance there will be no reversal of polarity. A deflection will be seen in the first (or last) channel of the chain, where the potential is contained. Potential involves one end of the chain and the electrode adjacent to it, equally: There will be cancellation in the channel that contains the two affected electrodes. The channel next to it will show a deflection. There will be no deflection in subsequent channels. There will be no reversal of polarity. In a reference montage, the location of the maximal potential on the head is determined by amplitude, not by phase reversal.
Analytic Process of EEG Interpretation
Making field maps with referential and bipolar recording
Ref: Materials from Bruce Fisch, M.D. in Spelhlmann’s EEG Primer and Atlas of EEG by Drs Abou-Khalil, Misulis, and additional sources, AAN EEG course materials, Dr Goldensohn’s teaching materials at Montefiore Medical Center EEG Lab, N.Y.