To the unfamiliar, electroencephalograms appear to be complicated assemblies of disorganized, randomly superimposed, and varying activity. However, patterns exist within the collection of EEG activity and these patterns are the essential EEG results because they provide insights into cerebral function. Overall, EEG interpretation is actually identifying the patterns and determining their collective, clinical relevance to the patient. Therefore, identifying patterns is the central challenge of EEG interpretation and this can be straightforward once the patterns’ features are known.

One way to learn EEG interpretation is to first become familiar with the component features that characterize each pattern. Fortunately, the number of pattern features is manageable. With knowledge of the features, the EEG reader focuses on the features when reviewing the recording and then identifies the patterns by knowing which patterns have the features that were seen. Approaching EEG interpretation by identifying features has the advantages of building interpretation skills from a foundation and also being able to identify unfamiliar patterns based on the features. Clearly, seeking help in identifying unrecognized EEG activity is much easier when describing the unknown activity by its features. Moreover, features can be used to create a list of patterns that have the features, which is akin to creating a differential diagnosis from clinical signs and symptoms. This chapter presents a three-step approach to interpreting EEGs according to the features. First is determining which features are present in the EEG activity. Second is using the features to determine which patterns may be the unknown activity. Third is identifying the pattern by the activity’s context and nuances of the possible patterns. Overall, this approach to EEG interpretation produces both a framework for visually analyzing EEG activity and reaching an EEG interpretation.

Reviewing standard definitions is the best place to start learning this approach. The quoted definitions below are from the International Federation of Clinical Neurophysiology’s (IFCN) glossary (Chatrian et al., 1974; Noachtar et al., 1999). Each quoted definition is supplemented by clarifications that benefit its use when characterizing patterns.

Essentially, EEG patterns are the collection of named EEG findings with clinical relevance. That is, patterns are the diagnostic vocabulary of EEG and the way to communicate an EEG interpretation. According to the IFCN glossary, a pattern is “any characteristic EEG activity.” This may seem vague at first, but the critical point is that activity is considered to be one pattern when that activity has characteristic features and characteristic significance. Patterns may be normal, abnormal, artifactual, or any activity found within an EEG recording, as long as it has the characteristic features and significance. Once characterized, patterns can be named and then discussed, studied, and clinically used.

Patterns are comprised of waves, which are “any change in the potential difference between pairs of electrodes in EEG recording, may arise in the brain or outside it.” Therefore, waves are any form of EEG activity, and the term wave can be considered a segment of activity. A wave does not indicate significance, but the wave’s features are useful for identifying the pattern. In other words, patterns are identified by the waves comprising them. Implicit to this is that waves can occur in many forms, and knowing the features of waves allows for identification of patterns.

Waves vary in amplitude, duration, contour, complexity, and all other graphic features, and these features are collectively called the waveform. Straightforwardly, waveform is “the shape of an EEG wave.” Morphology is a commonly used term that can indicate waveform, but its definition encompasses a broader sense of shape. Morphology is both “(1) The study of the form of EEG waves, (2) the form of EEG waves.” Essentially, the adjectives that describe waveforms are the features that characterize the wave. As an analogy, waveforms (wave features) are the signs and symptoms of a disorder and the pattern is the actual diagnosis.

Background Activity is “any EEG activity representing the setting in which a given normal or abnormal pattern appears and from which such a pattern is distinguished.” The essential point here is that background activity is simply the activity outside of what is receiving the interpreters’ attention. As such, background activity is comprised of waves, includes patterns, and is the backdrop for the
pattern the interpreter is considering at that moment. For example, generalized slowing can be the background activity for an epileptiform discharge, but generalized slowing can be the pattern of interest (and not background activity) when it interrupts an alpha rhythm. In this example, the alpha rhythm is the background activity for the generalized slowing.

Baseline is either “(1) Strictly: Line obtained when an identical voltage is applied to the two input terminals of an EEG amplifier, or when the instrument is in the calibrate position but no calibration signal is applied, or (2) loosely: imaginary line corresponding to the approximate mean values of the EEG activity assessed visually in an EEG derivation over a period of time.” Both definitions indicate that the baseline can be considered to be a collection of zero points from which the EEG activity rises and falls. Sometimes baseline is used to indicate a portion of an EEG before something occurs, such as hyperventilation or a seizure. This usage is sufficiently common to be acceptable, and it is unlikely to lead to confusion with the IFCN definition. This usage also is distinct from background because it denotes a change related to an occurrence and not spontaneous segment amid other spontaneous activity.

Attenuation is either “(1) Reduction in amplitude of EEG activity, or (2) reduction of sensitivity of an EEG channel.” The first definition is the only one that is useful for characterizing an EEG wave because the second definition allows for attenuation to be due to changes in amplification settings or recording conditions. Attenuation does not have a specific quantitative definition. Essentially, attenuation is either a clear decrease in amplitude or persisting low amplitude.

Monophasic wave is a “wave developed on one side of the baseline.” A monophasic wave is usually defined with regard to the imaginary baseline and can be on either side of the baseline. It includes only two slopes, one up and one down, and the first slope can be either up or down.

Diphasic wave is a “wave consisting of two components developed on alternate sides of the baseline.” The two components may be either similar or different from each other in waveform, but they must occur in succession and deviate from the baseline in opposite directions. As such, a diphasic wave has four slopes. The term diphasic wave is commonly used to describe a combination of two monophasic waves that have differing waveform and opposite baseline deflection.

Triphasic wave is a “wave consisting of three components alternating about the baseline.” Analogous to the diphasic wave, the components may be either similar or different in waveform and must occur in succession. As expected, the first and third components are on the same side of the baseline. As with diphasic waves, the description usually is applied when the components differ in waveform. For example, the classic triphasic wave comprises a sharp component that is followed by a taller component and then a slower component. Triphasic wave is a waveform but it also has been used to indicate a specific pattern with particular clinical significance. This sometimes leads to confusion, so the pattern is referred to in this reference as Triphasic Pattern (see Chapter 30).

Polyphasic wave is a “wave consisting of two or more components developed on alternating sides of the baseline.” Polyphasic waves are diphasic waves, triphasic waves, and any other number of waves occurring in succession and appearing as linked to each other to form one longer wave with varying waveforms.

Complex is “a sequence of two or more waves having a characteristic form or recurring with a fairly consistent form, distinguished from background activity.” Complexes can be diphasic waves, triphasic waves and polyphasic waves, but the term often refers to a wave with at least three components that are highly different in waveform. Complexes have an overall waveform that is essentially similar across occurrences, which produces the characteristic appearance and makes it more identifiable as one type of EEG activity. Complexes are defined by features occurring in an EEG channel, so they are less specific than patterns, which are defined by additional features, such as location and distribution.

Transient is “any isolated wave or complex, distinguished from background activity.” The two key aspects of a transient are its clear disruption of background activity and its duration. As an isolated wave or complex, it implicitly has a clear beginning and end and it is present for one occurrence of the wave or complex.

Spike is “a transient, clearly distinguished from the background activity, with a pointed peak at conventional paper speeds and a duration from 20 to less than 70 milliseconds… Amplitude is variable…” Spikes are noticed because of their spikey contour at usual review settings, but they are defined by their duration. Transients that are shorter than 20 milliseconds have an even pointier contour, but they, technically, are not spikes.

Sharp wave is “a transient, clearly distinguished from the background activity, with a pointed peak at conventional paper speeds and a duration of 70 to 200 milliseconds… Amplitude is variable…” In practice, duration is the key criterion because spikes and short duration sharp waves can have highly similar contours and overall appearances.

Slow wave is a “wave with duration longer than alpha waves.” Based on this definition, waves with durations longer than 125 milliseconds are considered slow waves. Since this allows overlap with sharp waves, slow waves are usually defined as waves lasting longer than 200 milliseconds. This alternate definition is used in this reference. Slow waves can have a pointed peak if the amplitude is high, so duration is the key criterion for this wave also. Such waves are described as sharply contoured slow waves.

Rhythm is “EEG activity consisting of waves of approximately constant period.” The waves constituting rhythms are monophasic waves that each has essentially the same duration, but they may vary in amplitude.

Regular “applies to waves or complexes of approximately constant period and relative uniform appearance.” A regular pattern must be the repetition of one type of wave or complex in order for appearance to be unchanging. When a succession of monophasic waves is regular, it is rhythmic and has unchanging amplitude. As such, it appears sinusoidal. A succession of triphasic wave complexes is regular if each complex resembles the others.

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.” Both definitions indicate the timing of recurrences as the key criterion, but periodicity also depends on the waves or complexes being identifiable as repetitions of each other. As such, a characteristic waveform is another key criterion.

In addition to the standard terms above, the following terms are used in this reference to help with describing EEG activity.

Repetition is the recurrence of one or more times of a wave or complex without interruption by the background activity. In other words, it is a succession of transients. A repetition of a monophasic wave is a rhythm, but repetition can also be used to describe a continuous series of triphasic waves or spike and slow wave complexes.

Evolution applies to repetitions in which the transients within the repetition change during the repetition in their period, amplitude, contour, or distribution across the scalp. As such, evolution is the opposite of regular, which denotes a uniform appearance during the repetition.

Focal is the distribution of a wave or complex’s electrical field to one electrode and its immediate neighbors. Any type of wave or complex may be focal. This definition is more exacting than the IFCN definition, which is “a limited region of the scalp, cerebral cortex, or depth of the brain displaying given EEG activity, whether normal or abnormal.” Since “limited region” is subject to opinion, this reference has modified the definition with the intention to improve the utility while still being close to the commonly considered definition of focal.

Hemispheric is the distribution of a wave or complex’s electric field to include electrode locations that are unilateral, anterior, and posterior to the coronal midline, and have a distribution that extends across more than two interelectrode distances. All of the electrodes across one hemisphere do not need to be included. The term is intended to communicate a broad, unilateral distribution.

Bilateral is the distribution of a wave or complex’s electric field to include electrode locations that are on both sides of the sagittal midline, limited to either anterior or posterior to the coronal midline, and have a distribution that extends across more than two interelectrode distances. To avoid overlap with the term generalized, this definition is more confined than the IFCN definitions, which is “involving both sides of the head.”

Generalized is the distribution of a wave or complex’s electric field to include electrode locations that are on both sides of the sagittal and coronal midlines and extending across more than two interelectrode distances. Generalized is defined by the IFCN as “occurring over all regions of the head.” However, activity meeting this standard of all is highly rare and the use of the term generalized in practice is closer to the definition provided here.

Polarity can refer to either the polarity of electrical activity on the scalp or to the direction of a deflection on an EEG record. In this reference, EEG waves are described according to the polarity of the electrical activity on the scalp unless stated otherwise; however, this polarity is not obvious for polyphasic waves because the component waves have differing polarities. To avoid this confusion, polarity for polyphasic waves is defined here according to the component with the shortest duration. Therefore, the polarity of a triphasic complex that includes a spike and slow wave is defined here as the polarity of the spike.