Table 1

Definitions of OSA/OSAS according to the International Classification of Sleep Disorders (ICSD) to date (May, 2021)

ICSD-1 (1990)
ICSD-R (1997) [6]		Terminology: obstructive sleep apnoea syndrome
Diagnostic criteria: minimal criteria A plus B plus C
No differentiation between adults and children.
No alternate names.
Duration criteria:

Acute: 2 weeks or less.
Subacute: >2 weeks but <6 months.
Chronic: ≥6 months.

  • A. The patient has a complaint of excessive sleepiness or insomnia. Occasionally, the patient may be unaware of clinical features that are observed by others.

  • B. Frequent episodes of obstructed breathing occur during sleep.

  • C. Associated features include:

    • 1. Loud snoring.

    • 2. Morning headaches.

    • 3. A dry mouth upon awakening.

    • 4. Chest retraction during sleep in young children.

  • D. Polysomnographic monitoring demonstrates:

    • 1. More than 5 obstructive apnoeas, >10 s in duration, per hour of sleep and one or more of the following:

      • a. Frequent arousals from sleep associated with apnoeas.

      • b. Bradytachycardia.

      • c. Arterial oxygen desaturation in association with apnoeic episodes.

    • 2. MSLT may or may not demonstrate a mean sleep latency of <10 min.

  • E. The symptoms can be associated with other medical disorders (e.g. tonsillar enlargement).

  • F. Other sleep disorders can be present (e.g. periodic limb movement disorder or narcolepsy).

Severity criteria
Mild: Associated with mild sleepiness or mild insomnia. Most of the habitual sleep period is free of respiratory disturbance. The apnoeic episodes are associated with mild oxygen desaturation or benign cardiac arrhythmias.
Moderate: Associated with moderate sleepiness or mild insomnia, as defined. The apnoeic episodes can be associated with moderate oxygen desaturation or mild cardiac arrhythmias.
Severe: Associated with severe sleepiness, as defined. Most of the habitual sleep period is associated with respiratory disturbance, with severe oxygen desaturation or moderate-to-severe cardiac arrhythmias. There can be evidence of associated cardiac or pulmonary failure.
ICSD-2 (2005) [7]
Separate definitions for adults and children.
Alternate names: obstructive sleep apnoea syndrome, sleep apnoea, sleep apnoea syndrome, obstructive apnoea, mixed sleep apnoea, sleep disordered breathing, sleep hypopnoea syndrome, upper airway obstruction.
Upper airway resistance syndrome subsumed under the diagnosis.
No duration criteria.
No associated cardiac or other pathophysiology is mentioned.		Terminology: obstructive sleep apnoea, adult
A, B and D, or C and D satisfy the criteria
A. At least one of the following applies:
1. The patient complains of unintentional sleep episodes during wakefulness, daytime sleepiness, unrefreshing sleep, fatigue or insomnia.
2. The patient wakes with breath holding, gasping, or choking.
3. The bed partner reports loud snoring, breathing interruptions, or both during the patient's sleep.
B. Polysomnographic recording shows the following:
1.5 or more scoreable respiratory events (i.e. apnoeas, hypopnoeas, or RERAs) per hour of sleep.
2. Evidence of respiratory effort during all or a portion of each respiratory event (in the case of a RERA, this is best seen with the use of oesophageal manometry).
or
C. Polysomnographic recording shows the following:
1.15 or more scoreable respiratory events (i.e. apnoeas, hypopnoeas, or RERAs) per hour of sleep.
2. Evidence of respiratory effort during all or a portion of each respiratory event (in the case of a RERA, this is best seen with the use of oesophageal manometry).
D. The disorder is not better explained by another current sleep disorder, medical or neurological disorder, medication use, or substance use disorder.
ICSD-3 (2014) [8]
Separate definitions for adults and children.
Alternate names: OSA syndrome, sleep apnoea, sleep apnoea syndrome, obstructive apnoea, sleep disordered breathing, obstructive sleep apnoea hypopnoea syndrome.
Upper airway resistance syndrome subsumed under the diagnosis.
No duration criteria.		Terminology: obstructive sleep apnoea, adult
A and B, or C satisfy the criteria
A. The presence of one or more of the following:
1. The patient complains of sleepiness, nonrestorative sleep, fatigue, or insomnia symptoms.
2. The patient wakes with breath holding, gasping, or choking.
3. The bed partner or other observer reports habitual snoring, breathing interruptions, or both during the patient's sleep.
4. The patient has been diagnosed with hypertension, a mood disorder, cognitive dysfunction, coronary artery disease, stroke, congestive heart failure, atrial fibrillation, or type 2 diabetes mellitus.
B. PSG or OCST# demonstrates: 5 or more predominantly obstructive respiratory events (obstructive and mixed apnoeas, hypopnoeas, or RERAs+ per hour of sleep during a PSG or per hour of monitoring (OCST#).
or
C. PSG or OCST# demonstrates: 15 or more predominantly obstructive respiratory events (apnoeas, hypopnoeas, or RERAs+) per hour of sleep during a PSG or per hour of monitoring (OCST#).

MSLT: multiple sleep latency test; RERA: respiratory effort related arousal; PSG: polysomnography; OCST: out of centre sleep testing (no electroencephalogram (EEG) is recorded). #: OCST commonly underestimates the number of obstructive respiratory events per hour as compared to PSG because actual sleep time, as determined primarily by EED, is often not recorded. The term respiratory event index (REI) may be used to denote event frequency based on monitoring time rather than total sleep time. : Respiratory events defined according to the most recent version of the American Academy of Sleep Medicine manual for the scoring of sleep and associated events. +: RERAs and hypopnoea events based on arousals from sleep cannot be scored using OCST because arousals by EEG criteria cannot be identified. Information from [6–8].