2.1 Clinical features

There are many causes of excessive sleepiness but the commonest treatable medical cause is the obstructive sleep apnoea/hypopnoea syndrome (OSAHS). This is a clinical condition, with recognisable symptoms, that occurs because the upper airway collapses intermittently and repeatedly during sleep. This collapse can be complete, with total obstruction of the airway lumen and no respiratory airflow (apnoea), or partial, with reduction in the cross-sectional area of the upper airway lumen causing hypoventilation (hypopnoea). An apnoea is arbitrarily defined in adults as a ten second breathing pause and an hypopnoea as a ten second event where there is continued breathing but ventilation is reduced by at least 50% from the previous baseline during sleep. In some centres, hypopnoeas are defined using additional criteria including associated oxygen desaturation (dips) or electroencephalogram (EEG) arousal.

As the sufferer falls asleep the muscle tone in the upper pharyngeal airway decreases leading to upper airway narrowing. This, in turn, produces an increase in inspiratory effort in an attempt to overcome this airway narrowing which then leads to a transient arousal from deep sleep to wakefulness or a lighter sleep phase which allows restoration of normal airway muscular tone and calibre. The patient then falls more deeply asleep again and the whole cycle repeats itself. This can occur many hundreds of times throughout the night leading to fragmentation of normal sleep architecture and a reduction in the quality of sleep with the generation of restless, disturbed and unsatisfying sleep. This in turn produces the symptoms of excessive daytime sleepiness, poor concentration and a reduction in alertness.

Factors predisposing to apnoeas and hypopnoeas include:

• increasing age
• male gender
• obesity
• sedative drugs
• smoking and alcohol consumption.^{4, 14}

There may also be a familial component to OSAHS possibly linked to facial or pharyngeal morphology or function but this area requires further research in order to clarify the role played by genetics.^{15, 16}

2.1.1 OSAHS AND HYPERTENSION

There is an independent association between OSAHS and hypertension. Studies have shown that patients with OSAHS have significantly higher blood pressure (BP) than matched controls.^{17, 18, 19, 20} Confounding factors such as obesity, age, gender and alcohol consumption make interpretation of many of these studies difficult.²¹ Epidemiological studies have shown that the presence of OSAHS is an independent predictor of raised blood pressure even when all known confounding variables have been allowed for.^{22, 23, 24, 25, 26} Treatment with continuous positive airway pressure (CPAP) therapy reduces BP by up to 3.3 mm Hg over 24 hours. The decrease was greatest in those with most marked nocturnal hypoxaemia (>twenty 4% desaturations/hour) in whom the mean 24 hour fall in diastolic BP was 5 mm Hg.²⁷ Reduction in BP by this magnitude may decrease cardiac risk by 20% and stroke risk by 40% over a five to ten year period.^{28, 29}

Claims of a direct association between OSAHS and myocardial infarction and stroke are as yet unproven.

2.2 Definitions

OSAHS represents one end of a spectrum with normal quiet regular breathing at one end, moving through worsening levels of snoring, to increased upper airways resistance, and to hypopnoeas and apnoeas at the other end. The frequency of apnoeas and hypopnoeas hourly is used to assess the severity of the OSAHS and is called the apnoea/hypopnoea index (AHI) or the respiratory disturbance index (RDI). It is unclear if it is the best measure of this disorder but it is the one most commonly used. Other measures including oximetry, computerised EEG analysis, autonomic arousal detection or body movement analysis, may be equally as good at characterising the severity of sleep apnoea.³⁰ As these indices are defined in different ways in different centres, comparisons may be difficult. In an attempt to overcome these difficulties recommendations aiming to standardise definitions of apnoeas/hypopnoeas and related indices have recently been published.³¹

OSAHS may be subdivided into varying degrees of breathing abnormality, for example, depending on the AHI:

• mild: AHI 5-14/hr
• moderate: AHI 15-30/hr
• severe: AHI >30/hr

Any cut off in AHI attempting to stratify the severity of OSAHS is arbitrary. Severity can vary from night to night and symptoms from day to day in any individual. Stratification is used to assign patients to an approximate level of severity when considering treatment strategies. Stratification also depends on the severity of the symptoms. In general, the more severe the breathing abnormality, the more symptomatic the patient becomes, but there may be cases where the severity of the symptoms does not correlate with the degree of breathing abnormality. AHI may rise with age in the population and require modification of any stratification system.³² Further research is required to confirm this.

The dominant symptoms of OSAHS are excessive sleepiness, impaired concentration and snoring.

Clinically significant OSAHS is likely to be present when AHI >= 15 events/hour slept, in association with unexplained daytime sleepiness or a minimum of two of the other features of the condition (see table 1). There is some evidence of benefit from the treatment of symptomatic individuals with AHI of 5-14, however further studies are required to confirm this (see section 4.3.1).^{33, 34}

It should be remembered that not every symptom is present in every case. The sufferer may also fail to recognise or indeed underplay some of these symptoms and it is often very useful to seek information from the partner regarding witnessed apnoeas, snoring, nocturnal restlessness and irritability or personality change.
Patients who present with any of the symptoms in table 1 and with specific associated symptoms may be suffering from significant underlying pathologies that merit urgent ear, nose and throat (ENT) assessment. These associated symptoms include: unilateral nasal bleeding, change in voice character, severe nasal obstruction, unexplained hoarseness, dysphagia or unusually rapid onset of symptoms in the absence of marked weight gain.

Patients suspected of serious underlying pathologies should be referred for urgent ear, nose and throat (ENT) assessment.

2.3 Excluding other causes of daytime sleepiness

OSAHS is one of the commonest medical causes of excessive daytime sleepiness, but clinicians should be aware that other conditions can produce similar symptoms, some of which are listed in table 2.