Obstructive Sleep Apnoea–Hypopnoea Syndrome (OSAHS)

Pathophysiology

•    Recurrent upper airway collapse during sleep → apnoeas (>10 sec) or hypopnoeas (≥30% airflow reduction with desaturation)

•    Causes repeated arousals → sleep fragmentation

Clinical Features

•    Loud snoring

•    Witnessed apnoeas

•    Excessive daytime sleepiness (Epworth Sleepiness Scale)

•    Morning headaches

•    Poor concentration

Risk Factors

•    Obesity (most important)

•    Male sex

•    Craniofacial abnormalities, large neck circumference (>17 inches)

Investigations

•    Polysomnography (gold standard)

•    Home oximetry (simpler screening)

Management

•    Weight loss (first-line for all)

•    CPAP (continuous positive airway pressure) → mainstay for moderate-severe cases

•    Mandibular advancement devices if mild or CPAP intolerant

Obesity Hypoventilation Syndrome (Pickwickian syndrome)

Definition

•    BMI >30 kg/m²

•    Daytime hypercapnia (PaCO₂ >6.0 kPa)

•    Sleep disordered breathing (often coexistent OSA)

Pathophysiology

•    Chronic hypoventilation due to obesity → blunted respiratory drive → chronic hypercapnia and hypoxaemia

Clinical Features

•    Dyspnoea

•    Morning headaches

•    Daytime somnolence

•    Signs of right heart failure (cor pulmonale)

Investigations

•    ABG: hypercapnia, hypoxaemia

•    Sleep studies

•    Exclude other causes of hypoventilation

Management

•    Weight loss (definitive)

•    Non-invasive ventilation (BiPAP) (especially at night)

•    Avoid excessive oxygen therapy (can worsen hypercapnia)

Neuromuscular Weakness and Respiratory Failure

Causes

•    Motor Neuron Disease (ALS)

o    Progressive weakness; early diaphragm involvement → orthopnoea, nocturnal hypoventilation

•    Myasthenia Gravis

o    Fatigable weakness; possible myasthenic crisis → respiratory failure

•    Guillain–Barré Syndrome

o    Rapid ascending paralysis; monitor vital capacity closely

•    Myopathies (e.g., muscular dystrophies)

Clinical Clues

•    Orthopnoea, morning headaches (hypercapnia)

•    Reduced vital capacity, especially when lying flat (>25% reduction from upright suggests diaphragm weakness)

•    Paradoxical abdominal movement (diaphragmatic weakness)

Investigations

•    Vital capacity monitoring

•    ABG: nocturnal or daytime hypercapnia

•    Sleep studies (if nocturnal hypoventilation suspected)

Management

•    Early consideration of non-invasive ventilation (NIV)

•    Respiratory physiotherapy

•    Specialist referral for underlying disease management

Extra Revision Pearls

•    Epworth Sleepiness Scale ≥10 → suggests pathological sleepiness

•    OSA → strong association with hypertension and metabolic syndrome

•    BiPAP (bilevel) preferred in hypercapnic respiratory failure; CPAP for upper airway obstruction

•    Obesity hypoventilation → often under-recognised; always measure daytime ABG if OSA suspected and obesity present

•    MND clue → paradoxical breathing when lying flat; vital capacity used to decide NIV timing

•    Avoid over-oxygenation in hypercapnic patients (risk of CO₂ narcosis)