What are common signs of a cyclic alternating pattern (CAP) on polysomnography (PSG) and what sleep disorders are associated with it?

Cyclic Alternating Pattern on Polysomnography

Cyclic Alternating Pattern (CAP) is an EEG marker of sleep instability characterized by repetitive cycles of activation phases (phase A) followed by background activity (phase B), occurring during NREM sleep with cycles lasting 20-40 seconds, and is prominently elevated in obstructive sleep apnea and other conditions causing sleep fragmentation. 1, 2

Common PSG Signs of CAP

Basic Structure

• CAP consists of periodic EEG oscillations with two distinct phases: phase A (transient arousal/activation) that interrupts phase B (background theta/delta activity of NREM sleep) 1, 2
• Cycles typically last 20-40 seconds and recur throughout NREM sleep stages 2
• Phase A manifests as intermittent alpha rhythm, K-complex sequences, or reactive slow wave sequences depending on the sleep stage 3

Three Subtypes of Phase A

• A1 subtype: dominated by EEG synchronization with weak activation of muscle tone and autonomic variables 2
• A2 subtype: mixture of EEG synchronization and desynchronization with intermediate polygraphic activation 2
• A3 subtype: dominated by EEG desynchronization with strong activation of muscle tone and autonomic responses, representing the highest arousal power 2

Quantification

• CAP rate is calculated as (CAP time/total sleep time) or (CAP time/total NREM sleep) and provides a measure of sleep instability 3
• The F4-C4 derivation shows the best local performance for CAP detection when a minimally invasive electrode montage is preferred 4

Sleep Disorders Associated with CAP

Obstructive Sleep Apnea (Primary Association)

• OSAS shows the strongest association with elevated CAP, with 96% of respiratory events in NREM sleep and 80% in REM sleep coupled with CAP cycles 2
• In OSAS patients, CAP rate is significantly enhanced with predominance of A3 subtypes (longer, more desynchronized patterns), reflecting severe sleep fragmentation 2
• Respiratory pauses consistently begin during phase B (vulnerable period for upper airway collapse), while effective breathing recovers during phase A (especially A2 and A3 subtypes) 2
• The increase in A3 subtypes is so pronounced in OSAS that CAP can be scored even during REM sleep, which is unusual in healthy individuals 2

Other Sleep Disorders with CAP Abnormalities

• Periodic limb movement disorder and restless legs syndrome show elevated CAP due to repetitive arousals from limb movements 1
• Insomnia disorders demonstrate increased CAP reflecting hyperarousal and inability to maintain stable sleep 1
• Parasomnias may show altered CAP patterns during NREM sleep when events occur 1
• Nightmare disorder is associated with increased phasic REM sleep activity and sleep fragmentation, though CAP primarily occurs in NREM sleep 5

Clinical Significance

Physiological Interpretation

• CAP represents the brain's reorganization in response to internal or external challenges during sleep, functioning as a natural arousal rhythm 3
• Phase B creates a vulnerable background for respiratory control failure, explaining why apneas initiate during this phase 2
• Enhanced CAP (especially A3 subtypes) reflects the brain's survival mechanism in OSAS, maintaining arousal capability at the expense of sleep quality 2

Diagnostic Utility

• CAP analysis provides superior sensitivity for detecting sleep fragmentation compared to standard arousal scoring, particularly when brief, frequent arousals are prominent 2
• CAP parameters can be assessed in both adults and children, making it a lifespan tool for sleep disorder investigation 1
• Automated CAP detection algorithms now exist, though visual scoring remains the gold standard 4

Common Pitfalls

• Standard PSG arousal criteria may underestimate sleep instability that CAP analysis would reveal, particularly in conditions with subtle but frequent EEG oscillations 2
• CAP is not routinely scored in clinical practice despite its informative nature, as manual analysis is time-consuming 4
• Confusing CAP with other periodic phenomena: CAP specifically refers to the organized cycling of arousal phases during NREM sleep, not isolated arousals or respiratory-related limb movements 6, 1
• Failing to recognize that elevated CAP rate indicates pathological sleep instability requiring investigation for underlying causes like sleep-disordered breathing 1, 2