A hypopnea is a partial reduction in breathing during sleep, distinct from a full pause (apnea). A 2026 expert panel argues that current scoring practices do not consistently distinguish obstructive from central hypopneas (PMID 42658424).
Key Points
- A hypopnea is a partial reduction in breathing during sleep — less severe than a full pause (apnea), scored when accompanied by oxygen desaturation, arousal, or both, depending on which AASM criteria a laboratory applies.
- Hypopneas can be obstructive (associated with a narrowed or collapsing airway) or central (associated with a temporary reduction in the brain’s signal to breathe, though mixed obstructive-central physiology within a single event is also recognized). Current scoring criteria do not distinguish between them as clearly as apnea scoring does (PMID 42658424).
- A 2026 expert panel reports that more systematic classification identifies central hypopneas — and possible coexisting central sleep apnea — in more patients than previously recognized in the specific populations the panel discusses (PMID 42658424).
- This distinction matters because obstructive and central events are managed with different treatment approaches.
- The panel’s conclusions are based on expert consensus, not a prospective study measuring how reproducible this classification is between different sleep technologists or labs (PMID 42658424).
What Is a Hypopnea, and Why Does Its Type Matter?
A hypopnea is a partial reduction in airflow during sleep, scored under current American Academy of Sleep Medicine (AASM) criteria when accompanied by a defined drop in blood oxygen or an associated arousal. Along with apneas (complete pauses in breathing), hypopneas are counted to calculate the apnea-hypopnea index, or AHI — the number of breathing disruptions per hour of sleep.
The AHI is a primary criterion used to diagnose obstructive sleep apnea (OSA), but it is not sufficient in isolation. Per current AASM diagnostic criteria, OSA is diagnosed with an AHI ≥15 events/hour alone, or an AHI ≥5 events/hour together with associated symptoms (e.g., excessive daytime sleepiness, witnessed apneas) or relevant comorbidities (e.g., hypertension, cardiovascular disease). Severity classification is then based on the AHI value.
It is also worth noting that AASM currently maintains two parallel hypopnea definitions: a “recommended” rule (≥3% oxygen desaturation or an arousal) and an “acceptable” rule used by some payers, including CMS (≥4% oxygen desaturation only, without an arousal criterion). These two definitions can produce materially different AHI values for the same recording, and this is a pre-existing source of inter-laboratory variability independent of the obstructive-versus-central question discussed below.
What the AHI doesn’t indicate on its own is why the breathing slowed. Two mechanisms are described:
- Obstructive hypopnea: The upper airway (throat, back of tongue, soft palate) narrows, restricting airflow while the brain continues signaling the chest and diaphragm to breathe.
- Central hypopnea: The brain’s respiratory control center transiently reduces its drive to breathe, so effort itself decreases. Events are not always purely one mechanism or the other — mixed events, with both central and obstructive components, are recognized and contribute to the classification difficulty discussed below.
Apnea scoring criteria already separate these two mechanisms with relative clarity: an obstructive apnea shows ongoing breathing effort against a blocked airway, while a central apnea shows absent effort. According to the 2026 panel, hypopnea scoring has not kept pace — criteria for distinguishing obstructive and central hypopneas are less standardized, and in practice, many labs score hypopneas without routinely classifying mechanism (PMID 42658424).
How Is It Diagnosed?
Diagnosis typically begins with an in-lab polysomnogram (PSG), which records airflow, respiratory effort (chest and abdominal movement), oxygen saturation, brain activity (EEG), and other signals overnight. In select cases, a home sleep apnea test (HSAT) is used instead. HSATs generally do not record EEG and use simplified effort and airflow sensors compared to in-lab PSG; they are validated for detecting obstructive events in patients with a high pretest probability of OSA but are not well suited to reliably identifying or classifying central events. When distinguishing obstructive from central mechanisms is clinically relevant, in-lab PSG is the appropriate test.
The key diagnostic detail the panel highlights is how the technologist interprets respiratory effort during a hypopnea. If effort is preserved, the event is more likely obstructive. If effort is reduced or absent, it points toward a central mechanism. Because current guidelines don’t require this distinction to be scored as rigorously as it is for apneas, some labs may report an AHI without specifying how many of those events are central versus obstructive — meaning a patient could have central events that are not called out in the report (PMID 42658424).
Why the Distinction Matters for Treatment
This is not solely a coding technicality — it affects which therapies are appropriate.
| Hypopnea type | Underlying mechanism | Typical first-line approach |
|---|---|---|
| Obstructive | Airway narrows or collapses despite breathing effort | CPAP, oral appliance therapy, upper-airway surgery, hypoglossal nerve stimulation |
| Central | Brain reduces respiratory drive; effort decreases | Identify and address underlying cause (e.g., heart failure, opioid use, high-altitude physiology); central-apnea-specific therapies may be considered in select cases under specialist management |
Standard obstructive-sleep-apnea treatments — CPAP, oral appliances, airway surgery — target the airway itself and are not designed to correct a central reduction in respiratory drive. Adaptive servo-ventilation (ASV) is one such central-apnea-specific therapy, but it is contraindicated in patients with symptomatic heart failure and reduced ejection fraction (LVEF ≤45%), based on findings of increased cardiovascular mortality in this population (SERVE-HF trial) and reflected in current FDA labeling. Central hypopnea/apnea management is individualized by a sleep physician and requires cardiac evaluation before ASV or similar therapies are considered.
The panel notes that emerging therapies, including certain neurostimulation devices and pharmacologic agents under investigation, are being developed with specific mechanisms in mind, which makes accurate classification relevant to selecting appropriate candidates (PMID 42658424).
Which Treatment Is Right for Me?
The appropriate treatment depends on which mechanism — or combination of mechanisms — is driving the events, not solely the total AHI. This is determined by a sleep physician based on:
- The proportion of central versus obstructive events on the sleep study.
- Underlying medical contributors, such as heart failure, opioid medication, or altitude, which are associated with central events.
- Response to initial therapy — central events can sometimes appear or increase after CPAP therapy is initiated, a pattern sometimes called treatment-emergent central sleep apnea.
- Candidacy for newer therapies, several of which are designed for specific mechanisms and may require confirmed classification before use (PMID 42658424).
Because current hypopnea scoring criteria are, per the panel, less standardized than apnea criteria — and because two different desaturation thresholds are already in parallel clinical use — patients with a mixed or ambiguous picture may benefit from having their study reviewed with this distinction specifically in mind, or from a repeat/expanded evaluation if the original report does not specify hypopnea type.
When to See a Specialist
Consider a referral to a sleep medicine specialist or a dentist trained in dental sleep medicine if:
- An AHI has been reported without specification of whether events are obstructive, central, or mixed.
- CPAP therapy has not resolved symptoms, or central events appeared after starting CPAP.
- A condition associated with central sleep apnea is present (heart failure, opioid use, stroke history).
- Neurostimulation, a newer pharmacologic therapy, or oral appliance therapy is being considered and confirmation of the underlying mechanism is needed first.
For Referring Providers
The panel’s central argument is methodological: current AASM hypopnea scoring rules do not require the same rigor in obstructive-versus-central classification that apnea scoring rules do, and the panel suggests this gap may under-identify coexisting central sleep apnea, particularly among patients being evaluated for neurostimulation or pharmacologic therapies (PMID 42658424). Whether this gap extends broadly across the general OSA population was not established by the panel and should not be assumed. This has potential relevance when triaging patients for mechanism-specific therapies — for example, hypoglossal nerve stimulation candidacy assessment includes exclusion of predominant central sleep apnea per existing device labeling, independent of the panel’s discussion — where central disease may be a relative contraindication or require a different device configuration. The authors are explicit that this is a consensus/expert panel discussion, not a prospective reliability study — inter-scorer reproducibility of proposed classification criteria has not yet been established, and the panel calls for standardized, validated scoring approaches before broad clinical implementation (PMID 42658424). Until such standards exist, requesting an explicit central-versus-obstructive breakdown on PSG reports for patients being considered for newer OSA therapies is a reasonable interim practice.
References
- The importance of classifying central hypopneas: a call to action. Journal of Clinical Sleep Medicine (JCSM), 2026. PMID 42658424.
