The initial step in evaluating an electroencephalogram (EEG) is to identify the recording montage. The typical initial montage is bipolar—each electrode is compared to a single adjacent electrode rather than to a common reference electrode—and longitudinal, indicating that comparisons are made sequentially along a chain progressing from anterior to posterior. There will usually be longitudinal chains covering both the lateral (temporal) and medial (parasagittal) regions over both the left and right hemispheres. Alternative montages may also be used. For example, using transverse rather than longitudinal bipolar may help to demonstrate sleep patterns, while referential montages may help to evaluate and localize possibly epileptic activity.

Next, identify the state of the patient. The waking state is most easily identified due to the presence of a posterior dominant rhythm (PDR), an 8–12 Hz “alpha” rhythm seen prominently in the posterior electrodes. Faster activity (beta) may be seen anteriorly. The PDR usually appears with eyes closed and attenuates or disappears with eyes open. Artifacts from muscle movement and eye blinks are often prominent. Sleep may be identified by several different features. Most notably, the PDR is absent, theta (4–8 Hz) or delta (< 4 Hz) rhythms predominate, and muscle and eye blink artifacts disappear. Depending on the stage of sleep, vertex waves, sleep spindles, K complexes, positive occipital sharp transients (POSTS), slow waves, or artifacts from rapid eye movements may be seen.

The background should be classified as normal or abnormal; abnormal background patterns indicating mild-to-moderate diffuse cerebral dysfunction—for example, as seen in intoxication or mild renal dysfunction—may include slowing of the PDR to frequencies < 8 Hz, disappearance of the anterior-to-posterior gradient of amplitude and frequency (known as “disorganization”), or absent PDR with diffuse theta or delta slowing. With obtundation or coma, the background may be interrupted intermittently by brief periods of much lower-amplitude signal (“discontinuity”). Finally, with severe global brain dysfunction, including after anoxic injury or with high-dose barbiturates, the background may be burst-suppressed, in which a low-voltage or flat pattern is only occasionally interrupted by a “burst” of activity, or is completely suppressed.

Focal slowing should be identified. This may include theta or delta frequencies seen asymmetrically, for example, over a single temporal lobe. Focal slowing indicates dysfunction in a specific region of the brain, although spatial resolution is limited. This may correlate to a structural injury such as infarction or tumor. It may also be seen in focal epilepsy, although it is not specific. One exception is unilateral, rhythmic delta slowing, for example, temporal intermittent rhythmic delta activity (TIRDA), which is frequently associated with focal epilepsy.

Epileptiform discharges may be either focal or generalized. The appearance of these may vary, but at a minimum they should be clearly distinct from the background and should have a physiologically plausible distribution (“field”). The morphology typically includes a negative (pointed up ) “spike” or “sharp wave,” or at times a polyspike, followed by a negative “slow wave.” Generalized epileptiform discharges usually indicate generalized epilepsy. Discharges repeating at ~ 4–6 Hz are commonly seen in juvenile myoclonic epilepsy (JME), at ~ 3 Hz in childhood absence epilepsy (CAE), and at < 3 Hz in Lennox-Gastaut syndrome (LGS). The location of a focal discharge helps to indicate the source of seizures, but is relatively imprecise. Epileptiform discharges must be distinguished from sharply contoured features without the typical epileptiform morphology. These “sharp transients,” which often are less distinct from the background, often represent normal variants, and it is critical to avoid misidentifying them as epileptic in origin. In clinical practice, misidentification is common and may lead to an incorrect diagnosis of epilepsy.

Seizures are typically distinguished by a rhythmic pattern with a clearly identifiable start and end (i.e., standing out from background activity) and with evolution over time in frequency, spatial distribution, or morphology. Seizures may be clinical or subclinical. A seizure on an outpatient or routine EEG is effectively pathognomonic for epilepsy. A seizure on an inpatient EEG also usually suggests a diagnosis of epilepsy, but may also be seen with acute brain injury, and in the setting of persistently altered mental status should raise suspicion for nonconvulsive status epilepticus.

Rhythmic patterns and periodic discharges can vary widely in their appearance and significance. Generalized periodic discharges (GPDs) can be seen after anoxic brain injury or with nonspecific global dysfunction, and depending on morphology may be epileptic—“GPEDs”—or may indicate a metabolic abnormality, such as with so-called “triphasic waves.” Lateralized periodic discharges (LPDs, often known as “PLEDs”) usually indicate a high risk for seizures, but can also be seen in the setting of focal structural injury such as stroke. Terminology and understanding of these patterns both remain in flux, and they should typically be interpreted only in light of the complete clinical scenario.