EEG Analysis of Waveforms
EEG Analysis of Waveforms[edit | edit source]
When approaching the evaluation of the EEG you will see continuous lines that represent fluctuating cerebral potentials and are composed of several types of waveforms. To describe a waveform in an organized and reproducible way, we break it down into its component dimensions of voltage, frequency and morphology.
Voltage: At the scalp surface, the amplitude of the cerebral activity is measured on the order of microvolts. This is in contrast to EKG potentials which are recorded in millivolts. The EEG waveform represents the summed electrical potential of a population of cortical neurons just below the electrode. This signal recorded at the scalp is attenuated by the cerebral tissue itself, the CSF, dura, skull and scalp and the EEG equipment before being displayed on the oscilloscope, paper or computer screen. The height of the waveform on the tracing or display depends of the sensitivity of the EEG amplifier, and is described in microvolts/mm.
Frequency: Many EEG patterns are rhythmic or semirhythmic in nature. Frequency is a measure of the number of repetitive waveforms with respect to time and is described in waves per second or Hertz. Frequencies are grouped into bands that are relevant to patterns of normal or abnormal brain activity. Physiological brain rhythms are rarely pure, and are usually composed of a mixture of frequency bands, but they can be described and assigned a frequency based on the relative dominance of one particular frequency band.
Morphology: The morphology is the shape of a waveform. Descriptive adjectives used to characterize waveform morphology include monomorphic, sinusoidal, polymorphic, arciform, biphasic, triphasic, spiky, or sharp with or without slow wave components. These patterns may be incorporated into ongoing EEG activity, occur as bursts or as individual paroxysmal discharges that stand out from the background activity. Whether the morphology of a particular waveform on the EEG represents benign, normal activity or implies pathology depends on where and when it occurs in the context of the overall EEG.
Electrical events on the EEG are described in terms of timing, rhythmicity, quantity and location
Timing: It is important to describe the number of occurrences of a waveform that occurs paroxysmally during the EEG or the duration of a pattern. When the events occur is equally important. Some EEG events may occur more often or exclusively in association with other physiological events, changes in clinical state or induction procedures during the EEG such as eye closure, drowsiness, sleep, photic stimulation or hyperventilation.
Rhythmicity: Waveforms on the EEG can occur as isolated sporadic events but may also occur repetitively. When events are repetitious, they can be described in terms of their periodicity or regularity. Rhythmic events have a relatively consistent frequency. Some waveforms are less regular, but still have a periodicity. Certain periodic waveforms with particular frequencies are associated with specific disorders. For example, rhythmic 3 Hz spike and wave discharges are associated with Genetic/Idiopathic/Primary Generalized Absence Epilepsy. Slow Periodic Lateralized Discharges (LPDs) are associated with Sub-Sclerosing Panencephalitis (SSPE).
Quantity: An EEG pattern can be described as continuous or intermittent. If it is paroxysmal a qualitative or quantitative description of how often it occurs allows the next EEG reader to assess the degree of change without pulling up the original study. For example, if there slowing, what is the prevalence of the slowing Continuous: ≥ 90% of record/epoch, Abundant: 50% to 89% of record/epoch, Frequent: 10% to 49% of record/epoch, Occasional: 1% to 9% of record/epoch or Rare: <1% of record/epoch? If there are discharges, are they Abundant: ≥ 1 per 10 seconds, but not periodic ( estimated average and maximum number of spikes per 10-second epoch), Frequent: ≥ 1/minute but less than 1 per 10 seconds, Occasional: ≥ 1/hour but less than 1/minute, or Rare: <1/hour?
Location: The purpose of a montage is to help localize the region of the brain from which a particular waveform arises. The rules of localization are based on interpretation of bipolar and referential montages. EEG events are called generalized when they affect leads over both hemispheres of the EEG simultaneously. Lateralized features affect only one hemisphere. Regional events are seen in one region of the brain, typically affecting a group of contiguous electrodes. Focal events may be seen in only one or two electrodes at a time.
Normalcy of an EEG is dependent on patient characteristics and clinical state.
Before interpreting an EEG, it is important to appreciate what is considered normal in reference to the patient’s clinical state and age. Identifying the state of the patient during the EEG is essential to the interpretation. Findings that might be considered normal during one state may be abnormal in another state. The age of the patient is also an important identifier. For instance, more slowing is allowed in as part of a normal pediatric EEG than is allowed in an adult EEG.