Pickwickian Syndrome Definition
Pickwickian syndrome, now formally termed Obesity Hypoventilation Syndrome (OHS), is defined by the triad of obesity (BMI > 30 kg/m²), sleep-disordered breathing, and awake daytime hypercapnia (PaCO₂ > 45 mm Hg at sea level), after excluding other causes of hypoventilation. 1, 2
Core Diagnostic Components
All three of the following criteria must be present simultaneously: 2
- Obesity: Body mass index exceeding 30 kg/m² 1, 2
- Daytime hypercapnia: Arterial PaCO₂ greater than 45 mm Hg measured during wakefulness at sea level 1, 2
- Sleep-disordered breathing: Documented on polysomnography or sleep respiratory polygraphy 2
- Exclusion requirement: Other causes of hypoventilation (neuromuscular disease, severe COPD, restrictive lung disease, hypothyroidism) must be ruled out 1, 2
Historical Context
The term "Pickwickian syndrome" originates from Charles Dickens' character Joe in "The Pickwick Papers," who was obese and excessively sleepy. 3, 4 While this historical name persists in clinical parlance, the modern medical terminology is Obesity Hypoventilation Syndrome. 3, 4, 5
Pathophysiologic Mechanisms
OHS develops from multiple simultaneous failing mechanisms that distinguish it from simple obesity or obstructive sleep apnea alone: 1
- Mechanical respiratory dysfunction: Chest wall weight and abdominal fat restrict diaphragmatic excursion and lung expansion, increasing work of breathing 6
- Impaired central respiratory drive: Decreased ventilatory responsiveness to rising CO₂ prevents appropriate compensatory hyperventilation 6
- Chronic nocturnal hypoventilation: Inadequate respiratory muscle strength during sleep when respiratory drive is naturally reduced 6
- Leptin resistance: Blunts ventilatory drive and chemoreceptor response to hypercarbia and hypoxemia 7
Relationship to Obstructive Sleep Apnea
Approximately 90% of OHS patients have coexistent obstructive sleep apnea (AHI > 5 events/h), with nearly 70% demonstrating severe OSA (AHI > 30 events/h). 1, 2 However, OHS and OSA are distinct entities—OHS can occur without OSA, and OSA commonly occurs without the chronic daytime hypercapnia that defines OHS. 5
Clinical Severity and Prognosis
OHS represents the most severe form of obesity-induced respiratory compromise: 1
- Significantly higher mortality compared to eucapnic obese patients with OSA alone 1, 2
- Pulmonary hypertension develops in 30-88% of patients 1, 2
- Chronic heart failure and cor pulmonale from chronic hypoxemia and hypercapnia 1, 6
- High risk of acute-on-chronic hypercapnic respiratory failure requiring hospitalization 6
Epidemiology
The prevalence is estimated at 8-20% among obese patients referred to sleep centers for sleep-disordered breathing evaluation, and 0.15-0.4% in the general adult population. 2 The condition often remains undiagnosed until late in disease progression, making early recognition critical. 4
Common Clinical Pitfall
Do not confuse Pickwickian syndrome with simple obstructive sleep apnea. The defining feature is chronic daytime hypercapnia (PaCO₂ > 45 mm Hg while awake), not just nocturnal breathing disturbances. 1, 5 Many obese patients with OSA maintain normal daytime CO₂ levels and therefore do not have OHS. 5