Australian Doctor Australian Doctor 24th November 2017

Therapy Update BOX 1. THE SEVEN MAJOR CATEGORIES OF SLEEP DISORDERS • Insomnia • Sleep-related breathing disorders • Central disorders of hypersomnolence • Circadian rhythm sleep-wake disorders • Parasomnias • Sleep-related movement disorders • Other sleep disorders Source: American Academy of Sleep Medicine BOX 2. SLEEP-RELATED BREATHING DISORDERS • Central sleep apnoea syndromes • Obstructive sleep apnoea (OSA) syndromes • Sleep-related hypoventilation disorders • Sleep-related hypoxemia disorder Source: American Academy of Sleep Medicine BOX 3. CAUSES OF NOCTURNAL HYPOVENTILATION • Obesity hypoventilation syndrome • Chest wall deformities — kyphoscoliosis, fibrothorax, thoracoplasty • Neuromuscular disorders — amyotrophic lateral sclerosis, muscular dystrophies (Duchenne and Becker dystrophies), diaphragm paralysis, Guillain-Barré syndrome, myasthenia gravis • Spinal cord injury • Central respiratory drive depression • Drugs (narcotics, benzodiazepines, barbiturates), neurologic disorders (encephalitis, brainstem disease, trauma, poliomyelitis, multiple sclerosis), primary alveolar hypoventilation • COPD and other lung diseases • Myxoedema (severe hypothyroidism) 26 | Australian Doctor | 24 November 2017 from previous page Epidemiology The prevalence of hypoven- tilation syndrome varies with the underlying cause. Around 10-20% of patients with obstructive sleep apnoea (OSA) are reported to have obesity hypoventila- tion syndrome. In patients with restric- tive thoracic disorders such as chest wall deformities (such as kyphoscoliosis), the prevalence of hypoven- tilation is dependent on the degree of spinal curvature. Patients with neuromuscu- lar diseases may all eventu- ally develop hypoventilation syndrome, depending on the underlying disease, for example, if they have Duch- enne’s muscular dystrophy. In patients with underly- ing COPD, around 27% will develop nocturnal hypox- emia, but only those with associated OSA (referred to as the overlap syndrome) or with severe obstruction (FEV1 <35% predicted) may demonstrate associated hypercapnia. Brief pathophysiology Sleep is accompanied by a decrease in central respira- tory drive and reduced res- piratory muscle activity, which causes some noctur- nal hypoventilation in all individuals (see box 3). However, in healthy peo- ple, the nocturnal hypoven- tilation is minor and not clinically significant. In disorders that pro- voke hypoventilation there are additional factors that worsen the condition. More than one mecha- nism is usually responsible, such as chest wall and lung parenchymal deformities, respiratory muscle weak- ness, and increased central respiratory drive depres- sion. In many disorders, hypoventilation occurs first during sleep, when hypoxic and hypercapnic ventila- tory responses are normally decreased compared with during wakefulness, and REM inhibition of respira- tory muscles can have a major effect. The resultant carbon dioxide retention during sleep leads to the compen- satory retention of bicarbo- nate by the kidney. This further blunts central drive and promotes more carbon dioxide retention, contributing to sleep frag- mentation with arousals. In patients with obesity- related hypoventilation, a blunted central response, decreases in chest wall and lung compliance, the pres- ence of OSA, and a leptin- resistance state (a satiety protein that increases venti- lation) all contribute. www.australiandoctor.com.au In those with neuromus- cular disease, hypoventila- tion, especially during REM sleep, is the result of both the loss of accessory muscle contribution to breathing due to a weakened dia- phragm and upper airway obstruction resulting in OSA. Similar mechanisms are responsible for hypoventi- lation in patients with tho- racic cage abnormalities. Symptoms Patients with nocturnal hypoventilation may pre- sent with dyspnoea, morn- ing headaches, impaired sleep quality, sleep restless- ness and drowsiness. Clini- cal features that should raise your suspicion for nocturnal hypoventilation include:

Australian Doctor Australian Doctor 24th November 2017 | Page 26