Cyclic alternating pattern (CAP): The marker of sleep instability

In the box an EEG arousal during NREM sleep according to the AASM scoring rules. Notice, however, the occurrence of k-complexes (confined between the vertical black line and the box) that herald the onset of the conventional arousal.

A CAP cycle is defined as a sequence of 2 alternating stereotyped EEG patterns, each lasting more than 2 and less than 60 s, called phase A and phase B, which are the expression of a sustained fluctuation between “greater arousal” level (phase A: usually 8−12 s) and “lesser arousal” level (phase B: usually 16−25 s). At least 2 full CAP cycles in succession are needed to define a CAP sequence; thus, the minimum content of a sequence is A + B + A + B + A. Note that single K-complexes do not identify a CAP phase A.

Short CAP sequence composed by 3 A phases (A1, A2, and A3). Below each A phase, the corresponding power spectrum is shown, computed from the F4-A1 lead. Note the different reciprocal amplitude of the two main frequency components of CAP; on the left, the spectrum of the CAP A1 subtype is dominated by the slow component in the delta frequency range, in the middle, the CAP A2 subtype shows a clearly smaller delta peak with an emergency of a peak in the frequencies above 5 Hz, finally, on the right, the CAP A3 subtype shows a further reduction of the delta peak and a clear peak in the alpha frequency range.

Age-related modifications of CAP rate along the normal life span. Notice the bimodal distribution with two peaks during adolescence and senescence, respectively.

Normative age-related changes of phase A subtypes. The percentages of subtypes A1 tend to decrease along the lifespan mirrored by the reciprocal increase of subtypes A2 and A3. The opposite trends converge approximately at the age of 60 years.

Specimen of stage 2 sleep with a CAP sequence modulating a series of periodic leg jerks. Notice the concomitant occurrence between the CAP A phases and the limb movements, accompanied by a simultaneous increase of heart rate.

A paroxysmal arousal in NREM sleep (right side of the recording) preceded by a CAP sequence in a patient with nocturnal frontal lobe epilepsy.

A CAP sequence induced by repetitive obstructive sleep apneas. The respiratory events appear during the B phases of CAP, while the A phases coincide with the recovery of effective breathing, heart rate acceleration and limb jerks.

A CAP sequence associated with central sleep apneas in a patient with atrial fibrillation. Compared to obstructive events the A phases of CAP show a longer length and are not triggered by mechanoceptor stimulation. Still myoclonic manifestations and heart rate rise accompany the hyperventilation pattern driven by the CAP A phases.