16th February 2025

Obstructive sleep apnoea and obesity hypoventilation

Obstructive sleep apnoea syndrome (OSA)

OSA is the third most common respiratory problem in the UK after asthma and COPD, with 2.5% of women and 5% of men in Western populations having severe enough disease to warrant treatment.

Pathophysiology and risk factors

Loss of pharyngeal dilator tone during REM (rapid eye movement) sleep results in occlusion of the upper airway, leading to apnoea lasting at least 10 seconds, with transient hypoxia and hypercapnia. The patient partially wakes, leading to an increase in airway tone to allow ventilation. The cycle repeats hundreds of times per night, leading to fragmented sleep.

Sympathetic stimulation during apnoeas with resultant hypertension is thought to explain the increased risk of poor cardiovascular outcomes, which is independent of obesity.

Risk factors include obesity, large neck circumference, large tonsils, hypothyroidism, acromegaly, increasing age, smoking, and use of sedatives or alcohol. Although obesity is a risk factor, OSA is also common in those without a raised BMI.

Presentation and diagnosis

Presentation may include loud snoring, daytime sleepiness, morning headaches, mood change, and poor memory and concentration. Their partner may have observed apnoeic episodes.

The STOP-BANG score and Epworth Sleepiness Score are frequently used screening tools.

Polysomnography (sleep study) is the diagnostic investigation of choice; it is comprised of six continuous elements: oximetry, EEG, EMG, ECG, airflow monitoring and electrooculogram.

The apnoea hypopnoea index (AHI) is the number of apnoeas or hypopnoeas per hour. An AHI > 14 in the presence of symptoms indicates moderate disease.

Polysomnography will differentiate OSA from other causes of daytime sleepiness, such as: central sleep apnoea (for example due to heart failure or stroke), nocturnal seizures, gastro-oesophageal reflux disease (GORD), restless legs syndrome, and periodic limb movement disorder.

Management of OSA

Management options include:

• Conservative – weight loss, smoking cessation, avoidance of alcohol/sedatives, mandibular advancement devices
• CPAP (continuous positive airway pressure) – typically nasal CPAP is used with pressures of 5-15 cmH2O. It works by splinting open the upper airway and has been shown to be highly effective in improving quality of life
• Surgical – pharyngeal surgery, bariatric surgery

Obesity hypoventilation syndrome (OHS)

Diagnosis of OHS

Obesity hypoventilation syndrome is a diagnosis of exclusion, defined as where a patient with BMI > 30 has an awake pCO₂ > 6 kPa, with no other explanation.

Patients present non-specifically with features similar to OSA and, in severe cases, may additionally show features of right-heart failure. A minority of patients present acutely with severe type II respiratory failure requiring hospital admission.

Management of OHS

Non-invasive ventilation (NIV) is typically started in the presence of acidosis, somnolence or cor pulmonale.

• OSA is co-morbid in 90% of patients, and for these patients typically nighttime CPAP is the initial mode, with escalation to BiPAP (bilevel positive airway pressure) for those who fail to respond adequately
• For the remaining 10% of patients who have few obstructive events during sleep and for patients who present acutely, nighttime BiPAP is the initial treatment

BiPAP may be started with an inspiratory positive airway pressure (IPAP) of 8 cmH2O and an expiratory positive airway pressure (EPAP) of 4 cmH2O. These pressures can then be titrated up. The EPAP will improve airway opening, and the driving pressure (IPAP – EPAP) will improve tidal volumes.

As per management of OSA, conservative or surgical measures to encourage weight loss also form part of OHS management.

Patients should be assessed and treated for complications of OHS, which include:

• Pulmonary hypertension (present in 60%) – all patients should have ECG and echocardiography
• Hypertension, congestive cardiac failure, insulin resistance

References

• Chapman SJ, Robinson GV, Shrimanker R, Turnbull CD, Wrightson JM. Oxford handbook of respiratory medicine. 4th ed. London, England: Oxford University Press; 2021
• Bourke SJ. Respiratory medicine: Lecture notes, 10th edition: Lecture Notes. 10th ed. Hoboken, NJ: Wiley-Blackwell; 2022
• Kline LR. Clinical presentation and diagnosis of obstructive sleep apnea in adults [Internet]. Collop N, editor. UpToDate. 2022 [cited 2022 Dec 9]. Available from: https://www.uptodate.com/contents/clinical-presentation-and-diagnosis-of-obstructive-sleep-apnea-in-adults
• Piper A, Yee B. Clinical manifestations and diagnosis of obesity hypoventilation syndrome [Internet]. Safwan Badr M, editor. UpToDate. 2022 [cited 2022 Dec 9]. Available from: https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-obesity-hypoventilation-syndrome
• Martin T. Treatment and prognosis of the obesity hypoventilation syndrome [Internet]. Safwan Badr M, editor. UpToDate. 2022 [cited 2022 Dec 9]. Available from: https://www.uptodate.com/contents/treatment-and-prognosis-of-the-obesity-hypoventilation-syndrome
• Safwan Badr M. Central sleep apnea: Risk factors, clinical presentation, and diagnosis [Internet]. Chervin R, editor. UpToDate. 2022 [cited 2022 Dec 9]. Available from: https://www.uptodate.com/contents/central-sleep-apnea-risk-factors-clinical-presentation-and-diagnosis