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Normal EEG Variants

“I am not Strange.  I am just not Normal.” - Salvador Dali

There is a category of waveforms that are only partially like the above quote.

They are Strange!  But they are also Normal.


This variant of waveforms are sometimes referred to as ‘benign’ but one thing is for sure - they are not cause for concern and bonus - they often make for an interesting EEG!


Let’s review them here:


Starting with my favorite:

  • Lambda


Are you a reader?  If so, you frequently produce these.


These waveforms are positive Occipital waves.  They are bilaterally symmetric and we produce them when we are scanning something like reading a book.


Fun Fact:  Their twin waveform in the sleep state are POSTS (Positive Occipital Sharp Transients of Sleep).


And now a close 2nd favorite of mine:

  • Mu


Do you recall the Homunculus Man from your Neuroanatomy lessons?  If not, definitely look it up.  Speaking of strange!



Quick Summary of how it relates to Mu….the area under C3 and C4 controls our hand. 


Similar to when we produce alpha in the resting state with our eyes closed, the rhythm attenuates or flattens with eyes opening because light hits the back of the brain and our visional cortex reacts.


Same with Mu - except instead of reacting to eye opening and closing the alpha rhythm in the Centrals attenuates with hand movement instead.  And of course since the right side of the brain controls the left side of the body and vice versa - it’s Contralateral hand movement.


Fun Fact:  Mu can even attenuate with someone just thinking about moving their hand.


Next Up:

  • Rhythmic Mid-Temporal Theta of Drowsiness or RMTD


Just as the name suggests:  They are Rhythmic…they are in the Temporal area…they are in the Theta frequency…and they occur in Drowsiness.


They are brief - typically only lasting a couple of seconds. They can be on both sides or just one and they should not be confused with a brief ictal rhythmic discharge (BIRD).


Moving on to a few discharges that can often be mistaken for abnormal:

  • 14 & 6 Positive Spikes, Wicket Spikes and Small, Sharp Spikes (SSS)


Why are they often mistaken for abnormal?  Well for one, they have all of the right labels in the name…spikes…and for two, their appearance is out of the ordinary.  But not to be confused - they are Normal.


Starting with 14 & 6 Positive spikes:  these sharp, positive, spikey waveforms come in either a frequency of 14 Hz or 6 Hz and are typically in the posterior region and seen in the adolescent age group.  Unlike Mu, they should be symmetrical.  Similar to RMTD they often occur in Drowsiness.


Wicket Spikes:


Cricket anyone?  That’s how they get their name - they look like the 3 stumps or poles that you try to hit the ball thru when playing cricket.


They are arclike, often in the alpha frequency and they occur in the temporal regions.  Often seen in drowsiness and not to be confused for epileptiform discharges.


And then rounding up the end we have Small Sharp Spikes (SSS) or otherwise known as Benign Epileptiform Transients of Sleep (BETS)

Confused yet with the naming? It literally has the word epileptiform in the name!  Strange…yes.  But abnormal…No.

They often occur in drowsiness and they even have a field (I guess that’s why they throw in the epileptiform name) but the easiest way to recognize them as a Normal waveform instead is because they are really petite…like tiny!

Hence the name small, sharp spikes.


Hopefully this summary with visuals helps as your record your studies and prepare for your board exam.  You will likely see a question or two on the topic.


Roya Tompkins, MS, REEG/EP T, RPSGT


Resources:

Awesome pictures from LearningEEG.com



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