normal variants


don't overcall normal variants

Aside from the usual background findings and artifacts, EEGs will often have normal variant patterns that, to an untrained eye, may appear to be epileptiform or otherwise pathological and thus are at risk of being overcalled and leading to unnecessary tests or treatments. Technically these findings could be included in the normal awake and asleep sections, but they are not always seen and thus are not requisites for describing the background.

mu rhythm

The Mu rhythm is the idling activity of the sensorimotor cortex, similar to how the PDR is the idling rhythm of the occipital regions. However, Mu goes away with both thoughts of and actual motor activity, and so is often not present (ex. if you think of moving the right arm, the left Mu will recede). As such, its not a required part of the background to comment on, but if you see it you should mention it.

Mu appears as arch-like alpha activity (usually 7-11 Hz) over the parasagittal regions; it can be bilateral but is often predominant on one side or the other. Particularly in cases of breach rhythm, it can appear rather high amplitude and sharp, but should not be mistaken for epileptiform activity.

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Right Mu rhythm, and left hemispheric slowing with breach

This page shows prominent high amplitude, sharply contoured delta slowing throughout the left hemisphere, more prominent in the parasagittal than temporal chain. This is consistent with an underlying focal dysfunction of that region and breach artifact. There is mu rhythm seen in the right parasagittal region, marked by arch-like alpha activity that's often sharply contoured.

Wicket waves

Wicket waves are so named because they are similar in appearance to the wickets (a set of three connected poles in the ground used in cricket). Somewhat simlar to the mu waves, wickets are arch-like, alpha range (again, usually 7-11 Hz) waveforms; unlike mu, however, wickets are seen in the temporal chains.

Don't confuse wickets for epileptiform discharges (even when some wickets may be larger in amplitude than others) as wickets have no aftergoing slow wave, do not disturb the background, and are nonevolving even though they may come in runs.

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Epileptiform Discharge

This discharge is in the left temporal region, where wickets are expected to be seen. However, this particular discharge does not have the classic arch-like morphology of a wicket, instead showing a spike and slow wave appearance that is more in line with an epileptiform discharge. Furthermore, it has a field into the left parasagittal chain that would not be present with wickets.

Rhythmic mid-temporal theta of drowsiness (RMTD)

Another example of EEG having very aptly named findings, rhythmic mid-temporal theta of drowsiness (RMT) is seen as sharply contoured, rhythmic theta activity restricted to the temporal regions; it is usually quite short, only a second or so. RMTD is usually most prominent in the mid-temporal region, and can be bilateral or independently unilateral. Note that RMTD does not evolve and thus should not be confused for a temporal seizure or brief ictal rhythmic discharge (BIRD).

lambda waves

Lambda waves arise in the awake state when the patient is visually scanning something, such as with reading. Lambda waves are bilateral, symmetric sharply contoured positive occipital waves with a sail-like appearance, very similar to POSTS; unlike POSTS, however, lambdas come with other evidence of wakefulness, such as eye blinks, myogenic artifact and more, rather than early sleep.

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Bioccipital epileptiform spike and waves

The marked waveforms, while relatively symmetric as you'd expect with lambda waves, are otherwise wholly different. First off, they have a negative polarity while lambdas are positive. Second, the marked waves have the classic epileptiform morphology of a spike and slow wave, although some show this more clearly than others. Don't let them fool you just because there are a series of them; epileptiform discharges can come in nonevolving runs as you see here.

Benign epileptiform transients of sleep

Benign epileptiform transients of sleep (BETS) are also called small sharp spikes (SSS). While BETS have a slightly oxymoronic name, they are in fact a normal, benign finding that you can see in the drowsy and asleep states. In morphology they are very similar to epileptiform spikes, including having a field, but by definition BETS are low amplitude (technically less than 50µV) and short duration. They can be seen most often in the temporal chains.

Differentiating BETS from real epileptiform discharges can be hard when you're starting to learn EEG; a good rule of thumb is that if the discharge is very low amplitude and seen only once or a few times, and only during sleep, its probably a BET but you should make note of it and keep a look out for anything more suspicious.

14 and 6 positive spikes

Taking the crown for perhaps the most uninspired name of EEG findings, 14 and 6 positive spikes describe 1-2 second bursts of sharply contoured positive waveforms that come in frequencies of either 14 Hz (13-17 Hz) or 6 Hz (5-7 Hz), or an admixture of both. They should be bilateral and synchronous, and are more common in the posterior quadrants when drowsy. They are most often seen in young adults and adolescents, and are not associated with increased epilepsy risk.

In this first example below, you can see a one second run of diffuse ~14 Hz activity, consistent with 14 Hz positive spikes.

In this second example, you can see a one second run of diffuse ~5-6 Hz activity, consistent with 6 Hz positive spikes. Note that 6 Hz positive spikes have two subtypes described as WHAM (waking, high amplitude, anterior, male) or FOLD (female, occipital, low amplitude, drowsy). These subtypes describe themselves, really, and the important thing to remember is that WHAM are associated with epilepsy while FOLD are not.

Mu rhythm is the idling activity of the sensorimotor cortex, similar to the PDR for the occipital cortex. Mu is arch-like and can be unilateral or bilateral; it resolves with movement or thoughts of movement of the contralateral body

Wicket waves are sharply contoured, arch-like temporal waves that can be unilateral or bilateral

RMTD is a common finding, marked by rhythmic theta in the temporal regions when drowsy

Lambda waves look like POSTS--positive, bilateral occipital sharply contoured waves--but are seen when the patient is awake and visually scanning (ex. reading)

BETS are normal in sleep, and differ from epileptiform discharges in that they are low amplitude (<50µV) and brief

14 and 6 positive spikes are 1-2 sec runs of bisynchronous activity, seen more when drowsy in younger patients


This tracing shows multiple intervals of brief bitemporal or unitemporal sharply contoured nonevolving activity with an arch-like morphology, consistent with wickets. Note that epileptiform activity tends to disturb the backgroud more, often including a field centrally that is not present here.

Abnormal; this is left frontal paroxysmal fast activity, an epileptiform finding

Far from a normal variant, this is a highly abnormal tracing from a patient with Lennox Gastaut Syndrome.
Don't mistake the marked box for 14 and 6 positive spikes; even though they are indeed about 14 Hz, they are in the wrong area (14 & 6 spikes tend to be posterior) and come with too many other surrounding abnormalities including a highly disorganized background with multifocal phase reversing epileptiform discharges.

Lambda Waves

This tracing shows symmetric occipital, sharply contoured positive waves that look very similar to POSTS that you'd expect in sleep. However, due to the presence of an eye blink in the middle of the page, along with a lot of frontal myogenic activity, you know this patient is awake. Therefore, these POST-like waves are in fact lambda waves; this patient was reading for several hours, during which time she had nearly continuous lambda wave activity because lambdas arise from visual scanning.

Mu rhythm

This page has periods of both bilateral and independent unilateral parasagittal arch-like, sharply contoured alpha activity maximal over the centroparietal region. This is consistent with mu rhythm, the idling activity of the sensorimotor cortex. Note that while some periods of mu are better formed than others, the activity never evolves or really changes in appearance.

Abnormal; this is a portion of a right temporal seizure

While this activity might look kind of similar to mu, it is off in the wrong distribution--this is in the right temporal region--and has too broad a field, moving into the right parasagittal chain. Mu does not do either. You may also consider wickets, but this morphology does not have the arch-like activity of wickets, and is too prolonged; also, again, it has a disruptive field into the parasagittal chain that wickets typically don't. We'll look at this full seizure, focusing on its evolution in time and location, in the seizures section.

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