What are the criteria for diagnosing the severity of Obstructive Sleep Apnea (OSA)?

Criteria for Diagnosing the Severity of Obstructive Sleep Apnea (OSA)

OSA severity is defined as mild for Respiratory Disturbance Index (RDI) ≥ 5 and < 15, moderate for RDI ≥ 15 and ≤ 30, and severe for RDI > 30/hr. 1

Diagnostic Parameters

Primary Diagnostic Criteria

• OSA is diagnosed when the Apnea-Hypopnea Index (AHI) is ≥ 5 events per hour with associated symptoms or when AHI ≥ 15 regardless of symptoms 1, 2
• AHI is calculated as the sum of apneas and hypopneas per hour of sleep 1
• Respiratory Disturbance Index (RDI) may include respiratory effort-related arousals (RERAs) in addition to apneas and hypopneas 1

Severity Classification

• Mild OSA: AHI/RDI ≥ 5 and < 15 events per hour 1
• Moderate OSA: AHI/RDI ≥ 15 and ≤ 30 events per hour 1
• Severe OSA: AHI/RDI > 30 events per hour 1

Definition of Respiratory Events

• Apnea: Complete cessation of airflow for ≥ 10 seconds 3
• Hypopnea: Commonly defined as ≥ 30% decrease in nasal flow with a ≥ 4% oxygen desaturation, or alternatively, a ≥ 50% decrease with a ≥ 3% desaturation or arousal 1, 3
• When using portable monitors, RDI is calculated using total recording time rather than total sleep time, which may underestimate severity compared to polysomnography (PSG) 1

Beyond AHI: Additional Severity Indicators

Recent evidence suggests that AHI alone may not fully capture OSA severity or predict clinical outcomes 4, 5. Additional parameters to consider include:

• Oxygen desaturation metrics: 5

  • Minimum oxygen saturation level
  • Time spent with oxygen saturation below 90%
  • Oxygen desaturation index (ODI)

• Obstruction event characteristics: 4

  • Duration of apneas
  • Severity of individual obstruction events
  • Adjusted AHI (incorporating severity of events) may better identify patients at highest risk for adverse outcomes

• Symptom severity: 1, 3

  • Excessive daytime sleepiness (measured by Epworth Sleepiness Scale)
  • Quality of life impairment
  • Presence of comorbidities

Diagnostic Testing

• Polysomnography (PSG): Gold standard test for OSA diagnosis 2, 6

  • Records airflow, respiratory effort, oxygen saturation, sleep stages, and body position
  • Provides the most comprehensive assessment of sleep architecture and breathing disturbances

• Portable monitoring (PM): Alternative for patients with high pre-test probability 1

  • Should include at minimum: airflow, respiratory effort, and blood oxygenation
  • Should be performed under the auspices of an accredited sleep medicine program
  • May underestimate AHI compared to PSG due to calculation based on recording time rather than sleep time

Common Pitfalls in OSA Severity Assessment

• Relying solely on AHI: AHI correlates poorly with daytime symptoms in many patients 5, 3
• Ignoring oxygen desaturation patterns: The depth and duration of desaturations may be more important than frequency in some patients 5
• Not accounting for positional or REM-related OSA: Some patients have significant OSA only in certain sleep positions or during REM sleep 6
• Overlooking comorbidities: Presence of cardiovascular disease, metabolic disorders, or other comorbidities may influence clinical significance of a given AHI 1, 2

Clinical Implications of Severity Classification

• Severity classification guides treatment decisions and predicts health outcomes 1, 6
• Moderate to severe OSA (AHI ≥ 15) is associated with increased cardiovascular morbidity and mortality 4
• Even mild OSA may be clinically significant in patients with prominent symptoms or comorbidities 2, 3

Remember that while AHI/RDI thresholds provide standardized criteria for OSA severity, the clinical significance should be interpreted in the context of symptoms, comorbidities, and other physiological parameters for optimal patient management 1, 4, 5.