K Complexes

Description

The K complex is a transient with a vertex localization that occurs in stages 2 and 3 of non–rapid eye movement (NREM) sleep. It almost always is polyphasic and usually has two or three phases. Most commonly, it immediately precedes a sleep spindle, and some K complex definitions include a sleep spindle after a polyphasic discharge as the overall K complex transient (Erwin et al., 1984). Another definition specifies that a sleep spindle must follow the discharge, so the polyphasic discharge alone is the K complex. However, two standard definitions allow for it to occur in the absence of sleep spindles, and it is considered according to these definitions in this reference (Chatrian et al., 1974; Noachtar et al., 1999; Rechtshaffen and Kales, 1968). Among its two or more phases, K complexes include a couplet of waves with opposite polarity that are greater in amplitude than the surrounding background. The first wave of the pair is negative and usually has a shorter duration than the second with each wave having a duration typically between 0.2 and 0.4 seconds. The duration for the K complex as a whole often is greater than 0.5 seconds and this duration is a required attribute when identifying K complexes for sleep staging using standard polysomnography. The duration rarely is more than 1 second and averages 0.6 seconds when defined with a minimum duration of 0.5 seconds (Silber et al., 2007). The distributions of the K complex and the vertex sharp transient (VST) are similar with a frontal–central or central maximum near the vertex and extension of the field as far as the neighboring frontal, parietal, and temporal regions. The first occurrence of K complexes is at around the age of 5 to 6 months, and their amplitude in infants is greater than at any other age with at least one phase commonly greater than 200 μV. Peak-to-peak amplitude in adults ranges from 100 to 400 μV. The wave elements during infancy are not as sharply contoured as they become in childhood and remain through adulthood. Similar to sleep spindles, K complexes occur less often and have a lower amplitude in late adulthood, even among neurologically healthy individuals (Crowley et al., 2002).

Distinguishing Features

• Compared to Vertex Sharp Transients

The waveform of K complexes is more complicated than the waveform of VSTs, and this feature is the most differentiating. Although K complexes also have a negative phase reversal at the vertex, the waveform has a longer duration, a less sharp contour, and more even amplitude across the phases than the VST waveform. Indeed, the polyphasic pattern of K complexes is more evident than the triphasic pattern of a VST because more than one of its phases has an amplitude greater than the background. Similar to VSTs, K complexes occur individually; however, unlike VSTs, they do not occur in trains of successive waves and often are immediately followed by a sleep spindle.

Co-occurring Patterns

K complexes occur only in stages 2 and 3 NREM sleep; thus, other features of these states always co-occur. These features include sleep spindles and background activity with principal frequencies that are in the theta or delta frequency ranges. Positive occipital sharp transients of sleep (POSTS) and VSTs, which first occur during stage 1 NREM sleep, also may be present.

Clinical Significance

Similar to VSTs, K complexes are normal transients that are provoked by various external stimuli, but they also can occur in the absence of any external stimulation and may be due to internal stimuli (Bastien et al., 2002; Fushimi et al., 1998;
Gora et al., 2001

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