Classification and Diagnosis

The International Classification of Sleep Disorders (ICSD) (American Sleep Disorders Association 1997) is the classification system most widely used to diagnose OSAS (Buysse et al. 2003) and is endorsed by the American Academy of Sleep Medicine and other professional sleep societies. More recently the American Academy of Sleep Medicine (1999) produced some recommended standard definitions, criteria and severity ratings for OSAS to facilitate comparability between research and clinical practice.

The diagnostic criteria for OSAS according to the American Academy of Sleep Medicine (1999) are as follows:

An individual must fulfil criterion A or B, plus criterion C.

A. Excessive daytime sleepiness that is not better explained by other factors;

B. Two or more of the following that are not better explained by other factors:

• Choking or gasping during sleep

• Recurrent awakenings from sleep

• Unrefreshing sleep

• Daytime fatigue

• Impaired concentration;

C. Overnight monitoring demonstrates five or more obstructed events per hour during sleep. These events may include any combination of obstructive apnoeas/hypopnoeas or respiratory effort related arousals (RERAs1_).

The severity of OSAS has two components: the severity of daytime sleepiness and the frequency of respiratory events during sleep. For sleepiness, the following severity criteria are recommended:

1 _{A RERA is defined as a sequence of breaths characterized by increasing respiratory effort leading to an}

arousal from sleep, but which does not meet the criteria for an apnoea or hypopnoea. RERAs are not included in the ICSD. Note: Prevalence and disease outcome data are scarce for RERAs as it is only a recently described event.

Mild: Unwanted sleepiness or involuntary sleep episodes occur during activities that require little attention. Examples include sleepiness that is likely to occur while watching television, reading, or travelling as a passenger. Symptoms produce only minor impairment of social or occupational function.

Moderate: Unwanted sleepiness or involuntary sleep episodes occur during activities that require some attention. Examples include uncontrollable sleepiness that is likely to occur while attending activities such as concerts, meetings, or presentations. Symptoms produce moderate impairment of social or occupational functions.

Severe: Unwanted sleepiness or involuntary sleep episodes occur during activities that require more active attention. Examples include uncontrollable sleepiness while eating, during conversation, walking, or driving. Symptoms produce marked impairment in social or occupational function.

The following severity categories for sleep-related obstructive events are recommended:

Mild = 5-15 events/hour of sleep Moderate = 15-30 events/hour of sleep Severe = > 30 events/hour of sleep

In contrast to the identification of OSAS, which requires daytime sleepiness, OSA is identified on the basis of sleep-related obstructive events alone. The appropriate number of sleep-related obstructive breathing events for diagnosing a clinically significant entity has been the subject of much controversy and is still being debated in the literature. The use of five events per hour as a minimum threshold value is informed by epidemiological data (Young et al. 1993), which suggests minimal health effects, such as hypertension, sleepiness, or motor vehicle accidents. Currently there are no data to indicate an appropriate distinction between mild and moderate degrees of obstructed breathing events during sleep, therefore the recommended level of 15 events per hour is based purely on consensus opinion (American Academy of Sleep Medicine 1999)

In most sleep laboratories and research studies, sleep-related obstructive breathing events are defined by the number of obstructive apnoea and hypopnoea episodes per hour of sleep, as measured by the apnoea-hypopnoea index (AHI) or the respiratory disturbance index (RDI). The AHI is the average number of apnoeas and hypopnoeas

per hour of sleep and is most often used when referring to data from gold-standard polysomnographic monitoring. Similarly, the RDI is the average number of apnoea and hypopnoeas per hour of sleep and sometimes also includes RERAs. While the definition of apnoea is generally agreed upon, the definition of hypopnoea is not. A duration criteria of 10 seconds is generally agreed upon in adults, however, more variable definition features include the degree of airflow or respiratory effort reduction, inclusion and degree of oxygen desaturation, and inclusion of arousal from sleep. To compound the problem further, each of these is dependent on the method of detection. The implications of differing hypopnoea definitions and methods of detection have been extensively explored (Meioli et al. 2001). For example, a recent multicentered, community-based longitudinal study reported up to a 10-fold difference in the prevalence of OSA according to varying definitions of hypopnoea (Redline et al. 2000).

Across the literature, various terminologies are often used synonymously, including OSA, OSAS, sleep apnoea syndrome (SAS), and obstructive sleep apnoea hypopnoea syndrome (OSAHS). These conditions all fall within the broader category of sleep- disordered breathing (SDB). In the present study, the term OSA is used when referring to the number of sleep-related obstructive events and when combined with daytime sleepiness, the term OSAS is used to indicate a clinically relevant entity.

2.7.1 Measurement of OSA

Standard polysomnography (PSG) is the accepted gold standard for the diagnosis of sleep-disordered breathing. PSG consists of monitoring brain electrophysiological activity, eye movements, muscle tone (usually from the chin), heart rate and rhythms, respiration, blood oxygen levels, and leg movements. It has however been criticised as a method of evaluation due to its cost and inaccessibility (Pack and Gurubhagavatula 2003).

Increased awareness among the general public and health care professionals of the clinical and physiologic importance of SDB has led to an increased demand on limited clinical resources. Therefore a number of alternative strategies have been developed to decrease the number of PSG studies conducted including split-night PSG, which utilises the first half of the night to evaluate the presence of OSA and the second half to implement treatment (Rodway and Sanders 2003). Other strategies include the use of portable monitoring devices, ranging from devices that can record as many signals as

does attended PSG, to only one signal such as oximetry (Flemons et al. 2003). The different types of studies used in the evaluation of OSA are classified according to their recording ability, with Level 1 (standard PSG) considered the reference standard to which the other devices are compared (Figure 2.1).

LEVEL 1

Standard polysomnography LEVEL 2

Comprehensive portable polysomnography LEVEL 3

Modified portable sleep apnoea testing

Minimum requirements include recording of ventilation (at least two channels of respiratory movement, or respiratory movement and airflow),

ECG or heart rate and oxygen saturation

LEVEL 4

Continuous recording of one or two cardio-respiratory parameters

The signals that are most commonly used are airflow, respiratory movements, oximetry, heart rate, blood pressure and body movement. Studies based on the diagnosis of sleep apnoea using these devices vary

greatly in their degree of precision

Figure 2.1 Types of studies for OSA evaluation

(Source: American Sleep Disorders Association 1994).

One advantage of portable devices, aside from cost effectiveness, is having the patient/participant in a familiar environment, which may provide a better appraisal of night time pathology then can be obtained in the unfamiliar laboratory setting. However, one downside of portable monitoring is that it is more susceptible to artefact (Whittle et al. 1997). In general, the risk of misdiagnosis is less likely with standard PSG than with other methods. A recent review of home diagnosis of sleep apnoea highlighted the need for more rigorous research to properly evaluate these alternative diagnostic strategies. It is suggested that this research should also consider more diverse populations of patients, including primary care populations, subjects with co- morbid conditions, different ethnic populations other than White, and women (Flemons et al. 2003).

In the community sample of the present study, the MESAM4 portable device (MAP, Martinsried, Germany) was used to measure OSA. It is classified as a Level 3 device. However, because it does not measure respiratory effort, it is not a classic Level 3 device, but it is considered under this category because it does involve monitoring

several channels of breathing variables. With this level of device, and those below it, a greater degree of subjectivity is needed in the classification of respiratory events. Level 3 monitors have been shown to reduce and increase the probability that a patient may have sleep apnoea in an attended setting. However, their reliability in an unattended setting is not well established (Flemons et al. 2003).

According to the American Sleep Disorders Association (1994) practice standards, portable devices in the assessment of OSA are an acceptable alternative only in the following situations: 1) when initiation of treatment is urgent and standard PSG is not available; 2) for patients not able to be studied in the laboratory; or 3) for follow-up when a diagnosis has been established. They also recommend that only Level 2 and 3 studies are acceptable for the diagnosis and assessment of therapy of OSA, as long as they include a body position sensor, as body position often affects the severity of obstruction, thus failure to identify this positional component may lead to inappropriate treatment (Ferber et al. 1994).