Polysomnography and Autonomic Testing
Core concepts
Know the AASM scoring framework: sleep stages W, N1, N2, N3, and R, defined by EEG, EOG, and EMG features. N1 shows low-amplitude mixed-frequency EEG with slow eye movements; N2 requires K-complexes or spindles; N3 requires slow-wave activity (≥20% of an epoch); R shows low-amplitude mixed-frequency EEG, rapid eye movements, and low chin EMG tone. Understand respiratory scoring: apnea (≥90% airflow reduction) and hypopnea (≥30% reduction with desaturation or arousal), the apnea-hypopnea index (AHI), and the role of oximetry. For autonomic testing, know heart rate variability, tilt-table testing, quantitative sudomotor axon reflex testing (QSART), and the Valsalva ratio; recognize orthostatic hypotension and postural tachycardia syndrome (POTS).
Traps
Questions often confuse central versus obstructive apnea (central = absent effort; obstructive = effort present). Another trap is misclassifying an epoch as R when chin EMG tone is not low or when slow-wave activity dominates. Remember that AHI thresholds define severity (mild 5–15, moderate 15–30, severe >30). For autonomic testing, distinguish neurogenic orthostatic hypotension (a drop with inadequate heart rate response) from POTS (heart rate rise without hypotension). QSART assesses postganglionic sudomotor function.
Worked example / rule of thumb
An epoch with a K-complex but otherwise low-amplitude mixed-frequency EEG is scored N2, not N1. Rule of thumb: stage by the dominant feature of the epoch; respiratory events require a qualifying desaturation or arousal.
Key points
- Stages W/N1/N2/N3/R defined by EEG, EOG, EMG; N3 needs ≥20% slow waves.
- N2 requires K-complexes or spindles; R requires low chin EMG and rapid eye movements.
- Central apnea = no effort; obstructive = effort present.
- AHI severity: mild 5–15, moderate 15–30, severe >30.
- QSART tests postganglionic sudomotor function.
- Distinguish neurogenic orthostatic hypotension from POTS by heart rate response.