3.1 Subjective assessment of sleepiness

Many patients may initially present with non-specific symptoms such as irritability, personality change, work or family problems or poor concentration. This may result from poor sleep quality and a high index of suspicion of OSAHS is necessary to allow the diagnosis to be made. The following questions should be asked whenever a diagnosis of OSAHS is under consideration:

• Is this patient falling asleep regularly against their will?
• Is this patient often sleepy whilst driving?
• Is this patient experiencing difficulties at work because of excessive sleepiness?
• Is surgery for snoring being contemplated?

Various equations have been developed in an attempt to try to predict the likelihood of a patient having OSAHS from the history and examination findings but none have been successful or helpful in clinical practice.^{35, 36}

Patients with significant sleep apnoea may not realise that they have a problem as many of the features may be reported by a spouse or partner. Subjective assessment of sleepiness (by both patient and partner) is important as it is unlikely that patients will accept treatment unless they can perceive benefit with a reduction in subjective sleepiness or improvement in work performance. This benefit is related to the severity of pre-existing impairment.³⁷ Evidence level 2+

The Epworth Sleepiness Scale (ESS; see appendix 1) is a validated method of assessing the likelihood of falling asleep in a variety of situations. The maximum score is 24.³⁸ The score can be used to clinically subdivide the patients into either the normal range (ESS <11), mild subjective daytime sleepiness (ESS=11–14), moderate subjective daytime sleepiness (ESS=15 –18) or severe subjective daytime sleepiness (ESS>18).³⁹ The Scale should be completed independently by both the patient and their partner as the patient may underestimate the severity of their sleepiness due to its insidious onset, or in order to hide concerns over driving ability. Although the correlation between ESS and OSAHS severity is relatively weak, the ESS is the best available tool to guide the clinician as to the patient’s perception of his/her sleepiness.^{40, 41, 42} Evidence level 2+,3

ESS can also be used to predict the likelihood of long term compliance with nasal CPAP (see section 4.3.1).³⁷ Evidence level 2+

All patients who have suspected sleep apnoea and their partners should complete an Epworth questionnaire to subjectively assess the degree of pretreatment sleepiness.

3.2 Referral

A subjective measure of daytime sleepiness (ESS >10) or sleepiness in dangerous situations, even with a normal ESS, in combination with symptoms associated with OSAHS (see table 1) should prompt referral to a sleep service. The main category of patient requiring urgent assessment to establish the presence of underlying sleep apnoea is one who has excessive daytime sleepiness, despite a normal time in bed at night, which may interfere with his/her driving ability or occupation.

Patients with severe obstructive sleep apnoea may decompensate with cor pulmonale or hypercapnic respiratory failure especially if there is coexisting chronic obstructive pulmonary disease (COPD).

The combination of severe OSAHS and COPD is potentially dangerous. In such cases clinicians should consider urgent referral to a sleep centre.

Patients with symptoms suggestive of OSAHS, who are sleepy whilst driving or working with machinery, or are employed in hazardous occupations should be considered for urgent referral to a sleep centre, as should those with ventilatory failure.

OSAHS should be excluded in patients before they are considered for surgery for snoring.

Patients and their partners should routinely complete questionnaires about their sleep habits and key symptoms before the initial consultation with a specialist to aid overall assessment. A sample sleep habit questionnaire is available on the SIGN website.

Hypertension is associated with sleep apnoea and successful treatment of the apnoea may lead to a mean fall in diastolic blood pressure of 5 mm Hg in those with over twenty 4% desaturations/hr.^{27, 29} Evidence level 1+

This may translate to a reduction in risk of death from both coronary artery disease and cerebrovascular disease. Hypertensive patients may not require urgent referral, as obstructive sleep apnoea may cause cardiovascular morbidity to develop over many years and not acutely.

3.3 Objective assessment of sleepiness

The Multiple Sleep Latency Test (MSLT) measures the time to fall asleep (using EEG criteria) in a darkened room on at least four separate occasions across the day following an instruction to fall asleep.^{43, 44, 45} This period of time is known as sleep latency. Each challenge is terminated at 20 minutes or upon sleep onset, and an average time of seven minutes or less is regarded as evidence of pathological sleepiness. An alternative to the MSLT is the maintenance of wakefulness test (MWT) where the subject is instructed to stay awake, rather than to fall asleep.^{46, 47} This test is terminated at 40 minutes and a result of <20 minutes is regarded as abnormal. A variant of the MWT (the OSLER test), using a behavioural assessment of sleep onset rather than one based on EEG, gives similar results but requires less technical input.⁴⁸ Although these tests measure sleepiness and its resolution with treatment, there is still a relatively poor correlation with other indices of OSAHS severity. In addition, it is unclear whether these objective tests predict functional impairment in real life, such as driving. At present they should therefore not generally be relied on to make individual clinical decisions as to fitness to drive, for example. These objective tests are not available outside large sleep centres.

3.4 Physical examination

Examination by itself cannot allow an accurate diagnosis of OSAHS but it does help to exclude other causes for the patient’s symptoms.

• weight and height should be documented at the first clinic visit. Continue to documentweight at all subsequent visits. Approximately 50% of patients with OSAHS are obese (BMI >30 kg/m2)
• measure neck circumference. Patients with OSAHS often have a neck circumference >17” (43 cm)
• visually inspect for abnormally small mandible size
• assess nasal patency visually
• assess upper airway for obvious obstruction using indirect laryngoscopy if available
• inspect the tongue for macroglossia, assess dentition and presence or absence of teeth
• assess pharyngeal appearance (tonsillar size, uvular appearance, lumen size)
• measure BP (with an appropriately sized cuff)
• perform routine respiratory, cardiovascular and neurological examination to detect any coexisting disease (eg cor pulmonale, chest wall deformity, myopathies) as clinically indicated
• measure forced expiratory volume (FEV1) and forced vital capacity (FVC) to detect any significant spirometric abnormalities
• the possibility of hypothyroidism, acromegaly and Marfan’s syndrome as underlying causes for OSAHS should always be considered and thyroid function tests are often indicated

3.5 Diagnostic tools

3.5.1 SLEEP STUDIES

The main purposes of a sleep study are to confirm the clinical suspicion of OSAHS and to assess its severity in order to guide the therapeutic choices to offer patients. To do this, sleep studies measure some aspect of ventilation and assess its possible compromise by upper airway obstruction, and any consequences on sleep quality. There are many ways to assess these aspects of OSAHS and the choice of sleep study equipment depends on many factors.

Studies of patterns of sleep, breathing and/or movements during sleep can be performed with decreasing degrees of complexity, varying from full polysomnography (PSG) eg 12-30 channels of various electrophysiological, breathing and movement signals, to one channel of information, eg oximetry. These may be recorded during part or all of a night’s sleep. Limited sleep studies give less information than a full PSG and may only give indirect information about breathing patterns and often none about sleep duration or quality. Studies using oximetry alone are even more limited in the information they provide.

Simple or complex sleep studies can be performed either in hospital or at home depending on local and personal circumstances but the relative merits of these are unclear at present.

3.5.2 POLYSOMNOGRAPHY

Polysomnography records sleep and breathing patterns simultaneously. It is conventionally performed in a sleep centre with the aid of a technician, but portable home based versions are available.

PSG is carried out overnight at a sleep centre and is a relatively intrusive and costly study whose interpretation can be complex. A standard PSG typically consists of EEG, segmental (+/-) tibialis electromyogram, electro-oculogram, respiratory airflow (usually measured by oronasal flow monitors), thoraco-abdominal movement and oxygen saturation tracings (oximetry). Electrocardiogram (ECG) and body position are also frequently monitored, as is snoring.

Polysomnography usually requires about 30-60 minutes set up time before sleep and about 30 minutes detachment time in the morning. Staff must be available for at least ten hours overnight to perform and monitor this test. The study can then take up to four hours to analyse. The cost of PSG depends on the staffing levels employed, the number and complexity of studies performed and the cost of the equipment and premises.⁴⁹

Although PSG is accepted in North America as the gold standard test for the diagnosis of sleep apnoea it has never been independently validated. Observational studies indicate that PSG may be useful in the diagnosis of sleep apnoea although there is night-to-night variation in PSG reproducibility. Different centres also use different thresholds in the diagnosis of sleep apnoea.^{50, 51, 52} Evidence level 2++,4

The clinical value of performing PSGs on all patients with daytime sleepiness has been questioned. In a prospective study of 200 patients with possible OSAHS, overnight PSG records were analysed to determine which signals contributed to diagnosis. Respiratory variables (thoraco-abdominal movement and oximetry) and the leg movement sensors were found to be helpful but neurophysiological signals did not contribute significantly to the diagnosis. ⁵³ Evidence level 3

3.5.3 LIMITED SLEEP STUDIES

Theoretically, limited sleep studies may use any reduced combination of the full range of variables present with a full PSG. In practice, they usually incorporate some measurement of respiratory signals often with an indirect measure of arousal. Common combinations are airflow, thoraco-abdominal movement, oximetry and heart rate measurement with some adding snoring and indirect evidence of episodes of airflow obstruction. Many new machines are emerging and this field is changing rapidly.^{49, 50, 54}

Whether PSG should be the gold standard with which to compare these simpler techniques is unclear. One recent trial has measured the improvement in symptoms following CPAP and correlated these with sleep study signals to identify the best predictors of this improvement.

In this study PSG was not the best predictor.³⁰ In this, and other studies, PSG derivatives (such as AHI, or EEG arousals) were no better at predicting improvement than the simpler indices (such as the number of oxygen desaturation dips or numbers of body movements).^{30, 55}

Three studies of hospital-based partial channel PSGs involving 213 patients with AHI >10 revealed a sensitivity range from 82-94% and a specificity ranging from 82-100% compared to full channel PSGs.^{56, 57, 58}

Limited sleep studies can often be performed at home by the patient him/herself after adequate instruction from sleep technicians. A written instruction sheet should be provided. Studies performed in this way can save both the costs of the accommodation in the sleep centre and the attendant staff costs.⁴⁹ The cost of a typical home based limited study may be only approximately 20% of the cost of a hospital based PSG. If a technician has to go to the patient’s home to attach the equipment this may be as expensive as a sleep centre PSG once travel time and costs are included. A disadvantage of this approach is the possible decrease in diagnostic certainty as limited studies do not allow assessment of sleep presence, quality or duration. Some respiratory events occurring during wakefulness will be scored and patients who fail to sleep will have negative results (although this is rare in OSAHS as these patients usually fall asleep very easily whatever the circumstances). There is also an inherent inability for home studies to diagnose conditions other than OSAHS with the technology currently available. Evidence level 2+

There may be significant problems with equipment failure rates, night-to-night reproducibility, cost, compliance and reliability when used in the unattended home environment. Sensitivity ranges from 32-100% and specificity from 33-100% compared to full PSG in a sleep centre.⁵⁹ These figures are highly dependent on the equipment and definitions of events used. Evidence level 1++

It has been suggested that patients diagnosed with OSAHS in limited sleep studies have poorer CPAP use thereafter⁶⁰ but this was not found in another study when the CPAP education process was carried out carefully.⁴⁹

Provided these concerns are appreciated, limited studies may be useful, cost-effective and convenient for patients and can significantly speed up the investigation pathway. Hospital-based PSGs are the overnight investigation of choice for a minority of patients who cannot be investigated adequately at home or whose home study result does not fit with the clinical suspicion of the investigating doctor.

3.5.4 OXIMETRY

Oximetry alone is often used as the first screening tool for OSAHS due to the universal availability of cheap recording pulse oximeters. They are spectrophometric devices that detect and calculate the differential absorption of light by oxygenated and deoxygenated haemoglobin in blood to produce a measurement called the SpO2. This is an assessment of the oxygen saturation of the arterial blood arriving at the fingertip or earlobe with each pulse beat.

Oximeters, however, have significant limitations which must be fully appreciated before they can be used alone to diagnose clinical problems or to influence patient management. They have an accuracy of ± 3% between individuals and all become less reliable if tissue perfusion is poor or if coloured nail varnish is used. They can give false negative results if used on young, thin patients who generally fail to desaturate during short apnoeic or hypopnoeic episodes as they maintain their lung volumes when lying flat and their baseline oxygen saturation tends to be on the plateau part of the oxygen dissociation curve as opposed to obese individuals who desaturate easily in a similar situation.

Oximeters are easy to use but they differ in probe design, sensitivity, sampling frequency, artefact rejection computation and averaging time. There can also be differences present in the techniques used to analyse the oximetry signals produced. Commonly employed methods include counting the number of oxygen desaturations (dips) per hour greater than an agreed value (often a 4% SpO2 dip rate of more than 10 per hour) or alternatively the time spent during the study at less than an agreed SpO2 level (often 90%). Thus it can be difficult to compare results from different centres using different machines or modes of analysis.

Oximeters also register oscillations in SpO2 when the baseline SpO2 is low, such as in COPD which may be confused with dips due to OSAHS. This is because at SpO2 values below 93% the slope of the haemoglobin saturation curve is steep and normal physiological variations in ventilation and PaO2 produce large changes in SpO2. In the Cheyne-Stokes breathing of heart failure, oscillations in SpO2 can be indistinguishable from those due to OSAHS.

Oximeters also measure heart rate and brief increases are an indirect marker of transient arousal from sleep. With each arousal the heart rate rises by about 6-10 beats per minute. Reviewing oximeter tracings with the accompanying pulse rate can provide information about sleep fragmentation.

Studies using oximetry alone to diagnose OSAHS have reached widely diverging conclusions. Some found oximetry useful^{61, 62, 63} while others did not.^{53, 64} These differences may be explained by differences in the oximeters themselves, the analysis algorithm, and the diagnostic criteria used. Studies which required desaturation to occur before a hypopnoea could be diagnosed, found better correlations between AHI and desaturation frequency.³¹ When compared with full PSG, oximetry alone showed a mean sensitivity of 87% (SE 4%, CI 36-100%) and mean specificity of 65% (SE 7%, CI 23-99%), which suggests that oximetry may be useful in selected patients with significant symptoms (AHI range 24-47).⁵⁹ A trained observer can diagnose OSAHS from positive oximetry traces, but false negatives occur in up to a third of patients with OSAHS.⁵³ The limitations of oximetry need to be clearly understood, namely, oximetry can positively diagnose OSAHS but cannot be used to exclude OSAHS. Evidence level 2+,4

A normal oximetry tracing does not exclude OSAHS.

The characteristic oximetric tracing in OSAHS is a sawtooth pattern, however care should be taken interpreting this pattern as it may also appear in patients with COPD or congestive heart failure.

Automated analysis is available for many sleep systems but the validity of this should be checked by an experienced operator.

3.5.5 FLOW VOLUME LOOPS

Inspiratory flow volume loops have not proved to be useful in diagnosing OSAHS.^{65, 66}

3.5.6 RADIOLOGICAL IMAGING

Studying upper airway size or shape by computed tomography (CT), magnetic resonance imaging (MRI), or cephalometric radiology does not accurately differentiate patients with OSAHS from normal subjects and cannot be recommended in the routine assessment of patients with possible OSAHS.^{67, 68, 69, 70}

3.5.7 QUESTIONNAIRES

Three studies were identified comparing questionnaire sampling in OSAHS patients to full PSG. These reported a mean sensitivity and specificity of only 42% and 68% respectively. Questionnaires are useful in the initial assessment of the potential OSAHS patient but cannot, by themselves, make the diagnosis.^{38, 70, 71}

3.5.8 NASENDOSCOPY UNDER SEDATION

Direct visualisation of the site of airway obstruction during sedation has been widely used by ENT surgeons to predict the occurrence and site of airway obstruction during sleep. There are no good prospective studies showing that these measures reflect what actually happens during spontaneous sleep and nasendoscopy cannot be recommended.

3.5.9 TRAINING

The appropriate training of personnel involved in the assessment of patients and their sleep studies is paramount. Although there is no direct evidence about who should perform such assessments, the research evidence upon which treatment effectiveness is based uses well-trained individuals in well-equipped centres.

Training in sleep studies is essential in the specialist registrar (SpR) programs for respiratory medicine, anaesthesia, and ENT surgery. Those intending to run a specific sleep service require a minimum of 12 months, and all respiratory trainees should spend three months, within a specialist referral centre.⁷² A specialist centre would need to treat at least 100 new patients with CPAP a year to allow an SpR to see at least 25 cases during a three month attachment.

Training for the support staff, such as specialist technicians and nurses is equally important. At present, training is best acquired through an apprenticeship, although there are an increasing number of specialist courses designed for support staff.

3.6 Summary of diagnostic strategies

Numerous diagnostic strategies incorporating the tests discussed in section 3.5 have been reported, although the published evidence for the individual tests is limited.

A combination of a positive limited sleep study in the context of clinical suspicion of sleep apnoea (excessive daytime sleepiness, Epworth subjective sleepiness score of more than 10, witnessed apnoeic episodes and loud snoring) allows a diagnosis of significant sleep apnoea with banding into mild, moderate or severe subdivisions. Such a limited sleep study should include one or more of the following: oximetry; thoraco-abdominal respiratory movement and airflow; recordings of snoring, heart rate or general video.⁵⁹ Evidence level 2++

Limited sleep studies to assess respiratory events are an adequate first-line method of diagnostic assessment for OSAHS.

Individual sleep centres should examine the balance of benefits associated with using a specific sleep study against their resources, their geographical catchment area, the equipment available and the diagnostic algorithm used.

Full PSG with EEG-based sleep staging is not necessary to diagnose sleep apnoea in most patients. It should be available in regional sleep centres for patients who have typical symptoms of excessive daytime somnolence but no objective evidence of obstructive sleep apnoea on limited testing.

Oximetry studies cannot exclude OSAHS. Studies using oximetry alone may have a role in the initial assessment of OSAHS, however their significant limitations must be fully appreciated before using them to make diagnostic and therapeutic decisions.

The specific technology used to make the diagnosis is less important than the level of experience and training available to interpret the results.

Other types of assessments using anthropometric measurements, ENT and dental assessments, radiological measurements and questionnaires do not offer sufficient sensitivity or specificity to be of value.