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characterised by curvature of the spine either laterally( scoliosis) or laterally and anteroposteriorally( kyphoscoliosis).
This disorder may be idiopathic or secondary to other conditions, such as neuromuscular disorders, surgery or trauma. Spinal deformity, muscle weakness or inefficiency all contribute to inadequate nocturnal ventilation and hypoventilation in sleep.
Neuromuscular disorders Sleep disordered breathing is common in neuromuscular disorders.
The type of abnormal breathing will reflect the specific muscles involved, which can vary considerably between disorders.
For example, involvement of the diaphragm to cause hypoventilation( especially in REM sleep) usually occurs late in Duchenne’ s muscular dystrophy, while in motor neurone disease, diaphragmatic weakness may be the first presentation.
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Central alveolar hypoventilation Central alveolar hypoventilation describes patients with alveolar hypoventilation secondary to an underlying neurological disease.
Causes of central alveolar
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hypoventilation include drugs and central nervous system disease, such as cerebrovascular accidents, trauma and neoplasms.
COPD and other lung disorders Hypoventilation is not uncommon in patients with severe COPD and other chronic lung disorders. Alveolar hypoventilation in COPD usually does not occur unless the FEV1 is less than 1L or 40 % of the predicted value.
However, many patients with severe airflow obstruction do not develop hypoventilation. Therefore, other factors, such as abnormal control of ventilation, genetic predisposition, and respiratory muscle weakness, are likely to contribute.
Investigations There are daytime tests that reflect breathing and gas exchange during sleep that can be useful to detect at risk individuals who should then be further assessed with laboratory polysomnography.
Oximetry and blood gas testing Pulse oximetry is simple and readily available.
There is some relationship between daytime SpO2
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and nocturnal desaturation, allowing SpO2 to be used as a guide to identify those in need of further investigation.
In particular, nocturnal desaturation is unlikely where daytime SpO2 is more than 94 %, but likely when SpO2 is below this.
Arterial or capillary blood gas measurements provide more detailed information regarding gas exchange and hypoventilation.
Daytime hypoxemia and hypercapnia identified in this way may prompt nocturnal polysomnography to investigate nocturnal hypoventilation as the cause or an exacerbating factor to the daytime gas exchange abnormalities.
Lung function tests Spirometry and other lung function tests are helpful in assessing the severity of conditions that impact respiratory function, which can in turn cause nocturnal hypoventilation.
FEV1 is a measurement of severity in COPD and patients with severe COPD( FEV1 < 40 % predicted) in particular are at risk of significant nocturnal hypoventilation.
In patients with neuromuscular disease, nocturnal hypoventilation can be predicted on the basis of a
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vital capacity < 40 % of predicted.
In patients with thoracic wall disorders, a vital capacity < 1 – 1.5L should prompt further investigation as there is a high risk of developing respiratory failure.
A fall in vital capacity from upright to supine is a sensitive and specific indicator of diaphragmatic weakness if the change is greater than 25 %.
Patients with severe obesity may have a restrictive pattern to their spirometry and a decrease in lung volumes.
Respiratory muscle strength Respiratory muscle strength can be assessed by measuring maximal inspiratory( MIP) and expiratory( MEP) pressures generated against an occluded airway.
It is particularly useful in assessing patients with neuromuscular disorders. Nocturnal hypoventilation is likely to occur when MIP is < 40cm H 2
O. Respiratory muscle strength is also decreased in obesityhypoventilation
syndrome due to a combination of abnormal respiratory mechanics and weak respiratory muscles
X-ray The chest X-ray is normal in most cases. It is used to exclude other causes of hypoxemia. At times it can show signs of congestive heart failure.
Polysomnogram A sleep study may demonstrate hypoventilation, particularly during REM sleep. It will help determine who would benefit from nocturnal ventilation by assessing the degree of nocturnal hypoxemia, nocturnal hypercapnia and sleep fragmentation.
Additionally, a sleep study may show obstructive apnoeas in patients with
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obesity hypoventilation syndrome. In patients with COPD, it will also diagnose overlap syndrome( OSA and COPD).
Echocardiogram An echocardiogram will document the development of pulmonary hypertension( mean pulmonary artery pressure > 25mmHg) in patients with obesityhypoventilation syndrome, neuromuscular disease, and COPD.
TSH TSH testing is indicated in patients with hypercapnia who are suspected of having or who have symptoms of hypothyroidism.
Management The feature common to all the hypoventilation syndromes is the rise in carbon dioxide levels during sleep and the compensatory bicarbonate retention by the kidney. This can in turn blunt the central respiratory drive, causing more night-time carbon dioxide retention and eventually wakefulness, carbon dioxide retention and hypoventilation.
For most patients, little can be offered to alter the progression of the underlying disorder. However, the key to management is identifying and treating the sleep hypoventilation, which will improve clinical outcomes, quality of life and survival irrespective of the underlying disease process involved.
Treatment options need to be individualised and are summarised below.
Oxygen therapy This is appropriate where there is ventilation-perfusion mismatching producing hypoxemia, for example, in individuals with parenchymal lung disease in addition to hypoventilation.
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CPAP therapy This is useful where upper airway dysfunction is the underlying cause of sleep hypercapnia, for example, selected individuals with obesity hypoventilation syndrome and scoliosis.
Bi-level ventilatory support This is the most commonly used modality for managing sleep hypoventilation, producing significant objective clinical improvements and subjective symptoms.
Volume preset ventilation This is the most commonly used therapy for individuals where bi-level support has failed to control sleep hypoventilation, where tracheostomy ventilation is needed, or where ventilator support is used continuously.
Conclusion In conclusion, nocturnal hypoventilation is a common sleep breathing disorder associated with poor outcomes if not recognised and treated.
There are a number of causes, some of which can be identified early in general practice.
Understanding the causes and diagnostic approach, considering them as a differential diagnosis to OSA in some cases, and then referring these patients for specialist treatment is important for optimal care. ●
Dr Desai is senior staff specialist, department of respiratory and sleep medicine, Prince of Wales Hospital; and medical director of the Sydney Sleep Centre, Sydney, NSW.
He is also clinical senior lecturer at the University of
Sydney.
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