Polysomnography (PSG): Procedure and Interpretation

Definition

I. INDICATIONS

1. Diagnosis of sleep-related breathing disorders (OSA, central sleep apnea, hypoventilation syndromes)
2. CPAP/BiPAP titration and treatment efficacy assessment
3. Evaluation of violent or potentially injurious sleep behaviors (parasomnias)
4. Differentiating narcolepsy from other hypersomnolence disorders
5. Differentiating parasomnias from nocturnal seizures
6. Assessment of periodic limb movement disorder (PLMD)
7. Required the night before a Multiple Sleep Latency Test (MSLT) for narcolepsy workup
8. Neuromuscular disorders causing sleep-related hypoventilation

II. PARAMETERS MONITORED (Channels Recorded)

BOX: PSG Monitors (AASM Standard)

III. PROCEDURE — STEP BY STEP

A. Pre-Study Preparation

1. Patient instructions: Avoid caffeine, alcohol, sedatives, and naps on the day of the study. Patients should maintain normal sleep schedule.
2. History and questionnaire: Epworth Sleepiness Scale, STOP-BANG questionnaire, Berlin Sleep Questionnaire completed before arrival.
3. Arrival: Patient arrives 1–2 hours before habitual bedtime.

B. Electrode and Sensor Application

• Central leads (C3, C4) — primary for sleep staging
• Occipital leads (O1, O2) — detect alpha rhythm (wakefulness)
• Frontal leads (F3, F4) — K-complexes and slow waves

C. Recording

• Patient sleeps in a private room while a technologist monitors in an adjacent room via infrared video + audio recording.
• Digital PSG records all channels simultaneously.
• Recording speed: 10 mm/sec (standard for sleep staging).
• Epoch: Each 30-second segment of recording is called an epoch; this is the basic unit of sleep staging.
• If a technical problem (lead fall-off, artifact) occurs, the technologist re-applies the sensor without waking the patient.

D. Post-Recording

• Data is downloaded and either auto-scored or manually scored by a certified PSG technologist.
• Final interpretation is provided by a board-certified sleep medicine physician.
• A summary report includes: total sleep time (TST), sleep latency, REM latency, sleep efficiency, stage distribution, AHI, arousal index, O₂ nadir, and SpO₂ < 90% time.

IV. SLEEP STAGING — THE BASIS OF INTERPRETATION

Stage W (Wakefulness)

• EEG: Alpha rhythm (8–13 Hz) with eyes closed; low amplitude mixed-frequency with eyes open
• EOG: Voluntary/blinking eye movements
• EMG: High chin tone
• Scored when >50% of epoch contains alpha frequency

Stage N1 (Light Sleep) — 3–8% of TST

• EEG: Alpha rhythm drops to <50% of epoch; theta waves (4–7 Hz) predominate; vertex sharp waves appear
• EOG: Slow rolling eye movements
• EMG: Slightly decreased muscle tone
• Transitional stage; patient may not be aware they have fallen asleep

Stage N2 (Light Sleep) — 45–55% of TST

• EEG: Sleep spindles (12–18 Hz bursts, 0.5–2 sec duration) + K-complexes (biphasic sharp wave, ≥0.5 sec); theta activity; slow waves <20%
• EOG: Minimal or no eye movements
• EMG: Further reduced tone
• K-complex = hallmark of N2; thought to suppress arousal and consolidate memory

Stage N3 (Slow-Wave/Deep Sleep) — 15–25% of TST

• EEG: ≥20% of epoch comprised of delta waves (0.5–2 Hz, amplitude >75 μV)
• EOG: No eye movements
• EMG: Variable, often lowest tone
• Most restorative stage; growth hormone secretion; parasomnias (sleepwalking, sleep terrors) arise here
• Predominates in the first third of the night

Stage R (REM Sleep) — 20–25% of TST

• EEG: Low-amplitude, mixed-frequency activity; sawtooth waves (2–6 Hz, notched appearance)
• EOG: Rapid conjugate eye movements in all directions (phasic)
• EMG: Muscle atonia (chin EMG at lowest level — key feature)
• REM latency: normally 60–90 minutes after sleep onset
• Dreaming predominantly occurs here
• REM episodes lengthen across the night; longest REM in last third of night
• Absence of atonia = REM Sleep Behavior Disorder (RBD)

V. RESPIRATORY EVENT DEFINITIONS

VI. KEY CALCULATED INDICES

VII. INTERPRETATION OF RESULTS

A. Normal PSG

B. OSA Severity (AASM/Miller's Anesthesia Classification)

C. PSG Findings in Specific Disorders

D. Arousal Index Interpretation

• Normal: <10 arousals/hr
• Elevated arousal index without clear respiratory events → suggests UARS (Upper Airway Resistance Syndrome); requires esophageal pressure monitoring for confirmation
• Awake esophageal pressure: normally −10 to −5 cmH₂O

VIII. TYPES OF SLEEP STUDIES

IX. LIMITATIONS OF PSG

1. First-night effect: Sleep may be disturbed by the unfamiliar environment and electrode placement — sleep efficiency may be artificially reduced
2. Expensive and resource-intensive; limited laboratory availability
3. Does not replicate the patient's habitual home sleep environment
4. Attended in-lab PSG not available in many locations — rationale for home sleep testing
5. A single night may not represent every night's sleep pattern
6. HSAT (portable monitoring) cannot diagnose the full spectrum of sleep disorders

SUMMARY MNEMONIC: "PEAR-RAD"

Parameters: EEG, EOG, EMG, ECG, airflow, effort, SpO₂ Epochs: 30-second segments AHI: Classification (<5 normal; 5–14 mild; 15–30 moderate; >30 severe) REM latency: 60–90 min normally; shortened in narcolepsy and depression Restorative: N3 (slow-wave) sleep most restorative; dominates early night Atonia: REM atonia — loss → RBD Disorders: OSA, narcolepsy, PLMD, parasomnias, seizures