What are the EEG waveform features of drowsiness in stage I sleep?

Updated: May 15, 2018
  • Author: Selim R Benbadis, MD; Chief Editor: Helmi L Lutsep, MD  more...
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The features of drowsiness are as follows:

  • Slow rolling eye movements (SREMs): SREMs are usually the first evidence of drowsiness seen on the EEG. SREMs of drowsiness are most often horizontal but can be vertical or oblique, and their distribution is similar to eye movements in general (see EEG Artifacts). However, they are slow (ie, typically 0.25-0.5 Hz). SREMs disappear in stage II and deeper sleep stages.

  • Attenuation (drop out) of the alpha rhythm: Drop out of alpha activity typically occurs together with or nearby SREM. The alpha rhythm gradually becomes slower, less prominent, and fragmented.

  • Central or frontocentral theta activity

  • Enhanced beta activity

  • Positive occipital sharp transients of sleep (POSTS): POSTS start to occur in healthy people at age 4 years, become fairly common by age 15 years, remain common through age 35 years, and start to disappear by age 50 years. POSTS are seen very commonly on EEG and have been said to be more common during daytime naps than during nocturnal sleep. Most characteristics of POSTS are contained in their name. They have a positive maximum at the occiput, are contoured sharply, and occur in early sleep (stages I and II). Their morphology is classically described as "reverse check mark," and their amplitude is 50-100 µV. They typically occur in runs of 4-5 Hz and are bisynchronous, although they may be asymmetric. They persist in stage II sleep but usually disappear in subsequent stages.

  • Vertex sharp transients: Also called vertex waves or V waves, these transients are almost universal. Although they are often grouped together with K complexes, strictly speaking, vertex sharp transients are distinct from K complexes. Like K complexes, vertex waves are maximum at the vertex (central midline placement of electrodes [Cz]), so that, depending on the montage, they may be seen on both sides, usually symmetrically. Their amplitude is 50-150 µV. They can be contoured sharply and occur in repetitive runs, especially in children. They persist in stage II sleep but usually disappear in subsequent stages. Unlike K complexes, vertex waves are narrower and more focal and by themselves do not define stage II.

  • Hypnagogic hypersynchrony: Hypnagogic hypersynchrony (first described by Gibbs and Gibbs, 1950 [3] ) is a well-recognized normal variant of drowsiness in children aged 3 months to 13 years. This is described as paroxysmal bursts (3-5 Hz) of high-voltage (as high as 350 µV) sinusoidal waves, maximally expressed in the prefrontal-central areas, that brake after the cerebral activity amplitude drops during drowsiness.

Clinical correlation

The importance of normal sleep patterns is that they should not be mistaken for pathologic sharp waves. Several normal stage I patterns easily can be mistaken for epileptic sharp waves or spikes, including vertex sharp transients, POSTS, and even fragments of alpha rhythm as it drops out.

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