How is stage II sleep defined on normal EEG?

Updated: May 15, 2018
  • Author: Selim R Benbadis, MD; Chief Editor: Helmi L Lutsep, MD  more...
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Answer

Stage II is the predominant sleep stage during a normal night's sleep. The distinct and principal EEG criterion to establish stage II sleep is the appearance of sleep spindles or K complexes. The presence of sleep spindles is necessary and sufficient to define stage II sleep. Another characteristic finding of stage II sleep is the appearance of K complexes, but since K complexes are typically associated with a spindle, spindles are the defining features of stage II sleep. Except for slow rolling eye movements, all patterns described under stage I persist in stage II sleep. Representative examples of the waveforms described here are shown in the images below.

This shows a K complex, typically a high-amplitude This shows a K complex, typically a high-amplitude long-duration biphasic waveform with overriding spindle. This is a transverse montage, which shows the typical maximum (manifested by a "phase reversal") at the midline.
Typical sleep spindles with short-lived waxing and Typical sleep spindles with short-lived waxing and waning 15-Hz activity maximum in the frontocentral regions. Note the associated slow (theta) activity that also characterizes stage II sleep.
Vertex sharp transients. This transverse montage i Vertex sharp transients. This transverse montage illustrates the maximum negativity (manifested by a negative phase reversal) at the midline. The location is similar to that of K complexes, but these are shorter (narrower) and more localized.
K complex, with its typical characteristics: high- K complex, with its typical characteristics: high-amplitude, widespread, broad, diphasic slow transient with overriding spindle. On the longitudinal montage (left), the K complex appears to be generalized. However, the transverse montage clearly shows that the maximum (phase reversal) is at the midline (Fz and Cz).
A mixture of spindles (ie, bicentral short-lived r A mixture of spindles (ie, bicentral short-lived rhythmic 14 Hz bursts) and positive occipital sharp transients of sleep (POSTS) can be seen. POSTS occur in stage I, but the presence of spindles is "diagnostic" of stage II.
A mixture of positive occipital sharp transients o A mixture of positive occipital sharp transients of sleep (POSTS) and spindles (fronto-central short-lived rhythmic 14-Hz bursts) can be seen.

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