Obstructive Sleep Apnea

01Pathophysiology

The upper airway is a collapsible muscular tube that lacks rigid cartilaginous or bony support in the pharyngeal segment. During wakefulness, the pharyngeal dilator muscles (most importantly the genioglossus) actively maintain airway patency through tonic and phasic contraction. During sleep, physiological muscle relaxation reduces this neuromuscular drive, narrowing the airway lumen.

In patients with OSA, this normal relaxation becomes pathological because the airway is already anatomically compromised. Factors such as excess adipose tissue surrounding the pharynx, enlarged tonsils or adenoids, retrognathia, or macroglossia reduce the baseline airway caliber. When muscle tone drops during sleep, the narrowed airway reaches a critical closing pressure (Pcrit), and the transmural pressure becomes insufficient to keep it open. The result is either partial collapse (hypopnea) or complete collapse (apnea).

Each obstructive event causes a cascade of consequences. Airflow cessation leads to progressive hypoxemia and hypercapnia, which stimulate peripheral and central chemoreceptors. This triggers a cortical arousal, a brief awakening that restores pharyngeal muscle tone and reopens the airway. The patient resumes breathing, often with a loud snort or gasp, and then falls back to sleep, only for the cycle to repeat. This can occur dozens to hundreds of times per night.

The clinical consequences stem directly from two processes. Sleep fragmentation (repeated arousals preventing deep restorative sleep) causes the hallmark excessive daytime sleepiness, impaired cognition, and morning headaches. Chronic intermittent hypoxia activates the sympathetic nervous system, promotes oxidative stress, systemic inflammation, and endothelial dysfunction. This is why OSA is an independent risk factor for systemic hypertension (particularly resistant hypertension), atrial fibrillation, heart failure, stroke, pulmonary hypertension, and type 2 diabetes. The large negative intrathoracic pressure generated by inspiratory effort against a closed airway (Mueller maneuver) also increases cardiac afterload and promotes atrial stretch, contributing to arrhythmias and cardiac remodeling.

In children, the pathophysiology is dominated by adenotonsillar hypertrophy rather than obesity. The consequences include failure to thrive, behavioral problems (often misdiagnosed as ADHD), enuresis, and poor school performance.

02Classification and Clinical Manifestation

Severity Classification by AHI

Key Definitions

Symptom Domains

03Diagnostic Workup

Summary of Diagnostic Tests

The workup for OSA begins with clinical suspicion. A vignette that describes a middle-aged, obese man brought in by his wife because she noticed he stops breathing at night, combined with excessive daytime sleepiness, is the classic setup. The STOP-BANG questionnaire (Snoring, Tiredness, Observed apneas, high blood Pressure, BMI > 35, Age > 50, Neck circumference > 40 cm, male Gender) is a validated screening tool used in primary care and preoperative settings to stratify risk.

The best initial diagnostic test for a patient with a high pretest probability and no comorbidities (no heart failure, no chronic lung disease, no central sleep apnea suspicion) is a Home Sleep Apnea Test (HSAT). This portable device measures airflow, respiratory effort, and oxygen saturation. It is convenient and cost-effective, but it tends to underestimate severity because it cannot measure total sleep time (it uses recording time instead). Therefore, a negative or inconclusive HSAT in a clinically suspicious patient should be followed by a full in-laboratory PSG.

The gold standard and most accurate test is in-laboratory polysomnography (PSG). This is mandatory when HSAT is inconclusive, when comorbid sleep disorders are suspected (e.g., central sleep apnea, periodic limb movements, narcolepsy), or in patients with significant cardiopulmonary disease. PSG simultaneously records brain activity (EEG), eye movements (EOG), muscle activity (EMG), heart rhythm (ECG), airflow (nasal pressure transducer and thermistor), respiratory effort (thoracic and abdominal belts), and oxygen saturation. The AHI derived from PSG is the cornerstone metric for diagnosis and severity grading.

A split-night study is an efficient variant: the first half of the night is diagnostic, and if the AHI exceeds 40 events/hour (or exceeds 20 with clear clinical indication), the second half is used for CPAP titration in the same session.

Always consider checking thyroid function in patients presenting with new fatigue and sleepiness, as hypothyroidism can both mimic and exacerbate OSA. In obese patients with daytime hypercapnia, an arterial blood gas helps distinguish pure OSA from overlap with obesity hypoventilation syndrome (Pickwickian syndrome).

04Management and Treatment

The cornerstone of OSA management is CPAP therapy. CPAP delivers a constant stream of positive pressure through a nasal or oronasal mask, functioning as a pneumatic splint that prevents airway collapse during sleep. The pressure is typically titrated during a PSG or through auto-titrating CPAP (APAP) devices that adjust pressure in real time. CPAP is indicated as first-line therapy for all patients with an AHI $\ge \backslash geq$ 15, and for those with an AHI of 5 to 14 who have symptomatic daytime sleepiness or cardiovascular comorbidities.

The principal challenge with CPAP is adherence. Adequate use is defined as $\ge \backslash geq$ 4 hours per night for $\ge \backslash geq$ 70% of nights. Common barriers include mask discomfort, nasal congestion, claustrophobia, and aerophagia. Interventions to improve compliance include heated humidification, mask fitting optimization, ramp features, and behavioral coaching.

Weight loss is strongly recommended for all overweight patients and should be pursued alongside CPAP. Bariatric surgery may be considered for patients with severe obesity (BMI $\ge \backslash geq$ 40, or $\ge \backslash geq$ 35 with comorbidities) who have failed conventional weight loss. Significant weight reduction can sometimes cure mild OSA entirely and reduce severity in moderate-to-severe cases, though it rarely normalizes AHI in severe disease alone.

For patients who cannot tolerate CPAP, mandibular advancement devices (MADs) are the primary alternative. Custom-fitted by a dentist trained in sleep medicine, these devices pull the mandible and tongue base forward, expanding the retrolingual airway. They are most effective in mild-to-moderate OSA and in patients with retrognathia.

In the pediatric population, adenotonsillectomy is the undisputed first-line treatment. This is a high-yield exam point: when a child presents with snoring, mouth breathing, and behavioral problems, and the exam reveals tonsillar hypertrophy, the next best step is referral for adenotonsillectomy, not CPAP.

Hypoglossal nerve stimulation (upper airway stimulation) is a newer option for adults with moderate-to-severe OSA who are intolerant of CPAP, have a BMI under 35, and do not have concentric palatal collapse on drug-induced sleep endoscopy. The implanted device stimulates the hypoglossal nerve to protrude the tongue during inspiration, preventing retrolingual collapse.

Tracheostomy is the ultimate rescue therapy for life-threatening cases refractory to all other interventions, particularly in patients with severe desaturations and cor pulmonale. It is rarely needed but is essentially 100% effective by completely bypassing the pharyngeal airway.

All patients should be counseled to avoid alcohol and sedative-hypnotics before sleep, as these substances reduce pharyngeal muscle tone and lower the arousal threshold, worsening obstructive events.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap on exam questions about OSA involves confusing it with central sleep apnea. Both can present with excessive daytime sleepiness and witnessed apneas, but the critical distinction lies in the presence or absence of respiratory effort. In OSA, the patient is actively trying to breathe against a collapsed airway (paradoxical chest and abdominal movement may be seen). In central sleep apnea, there is no respiratory effort at all. If the vignette mentions heart failure, opioid use, or Cheyne-Stokes breathing, think central. If it mentions obesity, loud snoring, and a large neck, think obstructive.

Another frequent pitfall is choosing the wrong initial management in pediatric OSA. Students often default to CPAP because it is first-line in adults. In children, the answer is almost always adenotonsillectomy when tonsillar hypertrophy is present. CPAP in children is reserved for cases where surgery is not indicated or has failed to resolve the condition.

A third testable trap involves patients with resistant hypertension. When a vignette describes a patient on three or more antihypertensives (including a diuretic) whose blood pressure remains uncontrolled, the question is testing whether you recognize OSA as the most common secondary cause of resistant hypertension. The next best step is screening for OSA, not adding another antihypertensive.

Finally, be aware that HSAT can underestimate severity. If a patient has a clinical picture that strongly suggests moderate-to-severe OSA but the HSAT shows a low or borderline AHI, the correct next step is to proceed with in-laboratory PSG, not to rule out the diagnosis. The exam tests whether you understand the limitations of the screening test and know when to escalate.

The core competency being tested across OSA questions is threefold: (1) pattern recognition of the classic presentation (obese, snoring, witnessed apneas, daytime sleepiness), (2) understanding the hierarchy of diagnostic tests (clinical screening to HSAT to PSG), and (3) selecting the correct first-line treatment based on the patient population (CPAP for adults, adenotonsillectomy for children).