Diagnostic Testing and Study Interpretation Concepts
What the study measures
Polysomnography records EEG (typically F4-M1, C4-M1, O2-M1), EOG, chin EMG, limb EMG, ECG, airflow (thermistor and nasal pressure), respiratory effort by belts, pulse oximetry, snoring, body position, and video. Scoring follows AASM rules: an apnea is a ≥90% airflow reduction for ≥10 seconds; a hypopnea under the recommended rule is a ≥30% drop in nasal pressure lasting ≥10 seconds with a ≥3% desaturation or an arousal. Central events lack respiratory effort; obstructive events show continued or paradoxical effort.
The apnea-hypopnea index divides events by hours of sleep. Severity bands are 5–15 mild, 15–30 moderate, and above 30 severe, with the 5–15 range generally requiring symptoms or comorbidity to be clinically meaningful. Also report nadir oxygen saturation, time below 90%, arousal index, and supine versus non-supine AHI.
Home sleep apnea testing lacks EEG, so events are indexed to recording time and results may underestimate severity; it is inappropriate when central apnea, significant comorbidity, or another suspected sleep disorder is present. The MSLT follows a PSG documenting at least 6 hours of sleep, offers four to five naps two hours apart, and is abnormal with a mean sleep latency of 8 minutes or less; two or more sleep-onset REM periods support narcolepsy. The MWT assesses the ability to stay awake.
Traps
Watch the denominator (sleep time versus recording time), the three-second EEG shift needed to score an arousal, and pediatric rules that differ from adult criteria.
Rule of thumb: treat an AHI as meaningless without its denominator and its clinical context - the same number can mean different things in sleep time versus recording time.