Sleep studies

What are sleep disorders?
Sleep disorders medicine deals with the diagnosis and treatment of patients who complain of disturbed sleep, excessive daytime sleepiness or other sleep-related problems. The spectrum of disorders is broad and may involve the primary mechanisms of sleep and arousal or may be secondary to other medical or psychiatric illnesses.

Sleep disorders are common
Insomnia affects up to one third of the population at any given time. Excessive daytime sleepiness is a common complaint in the community, and is most frequently due to sleep deprivation, but it is not normal. A survey of middle-aged adults found that sleep-disordered breathing affects 9% of women and 24% of males when defined by abnormal sleep study criteria alone and that 2% of women and 4% of men met the diagnostic criteria for obstructive sleep apnoea syndrome (abnormal sleep study and daytime hypersomnolence).¹

Sleepiness versus fatigue
Although these terms are often used interchangeably, they are not the same. Sleepiness describes the urge to sleep. The situations associated with drowsiness and falling asleep or dozing help to determine the severity of the problem.

Fatigue includes lethargy, mood disturbance, malaise and lack of motivation and does not require a sleep study as a first-line investigation.

How do you measure sleepiness?
Sleepiness can be rated using questionnaires such as the Epworth Sleepiness Scale² (Table 1). The degree of sleepiness associated with sleep disorders can be variable, or may be dismissed by the patient as part of growing older.

Questionnaires are useful, but are subjective. It is difficult to measure sleepiness objectively. The multiple sleep latency test (MSLT)³ involves measuring how long it takes an individual to fall asleep lying down with eyes closed in a quiet darkened room on 4 or 5 separate occasions over a whole day. This is of value in the diagnosis of narcolepsy where the typical features are of pathologic sleepiness (short mean sleep latency) and sleep onset REM periods. A MSLT may also be used to assess the outcome of treatment in special circumstances where documentation of abolition of sleepiness is required e.g. to allow reinstatement of driving licence.

How do you monitor sleep?
Sleep may be monitored using patient-completed sleep diaries with or without concurrent monitoring of body movement (actigraphy). Sleep measurement uses respiratory and cardiac sensors, together with continuous electroencephalogram (EEG), electro-oculogram (EOG) and electromyogram (EMG) recordings during sleep (Fig. 1). This was started at Stanford University in 1970. Such recording was finally named polysomnography in 1974.⁴ Since then, there has been rapid growth in the number of sleep centres and sleep studies performed. New terminology is also appearing (see box).

When should I order a sleep study?
Symptoms of sleep disorders are common and non-specific. Although sleep disorders are commonly associated with conditions such as obesity and psychiatric disorders, it is not cost effective to do sleep studies on all of these patients.

Sleep studies should be considered when symptoms or signs suggest that a patient is likely to have sleep-disordered

breathing, or complains of snoring, which is causing sufficient social disruption for the patient to seek treatment (Table 2). Snoring alone is insufficient justification for a sleep study.
The exception is when a patient is contemplating surgery for snoring as the presence of significant sleep-disordered
breathing will alter management.

The snoring associated with sleep apnoea is typically habitual and disruptive. The symptom that best predicts sleep-disordered breathing is a witness account of a pattern of repetitive apnoeas terminated by a choking or snorting gasp.

Symptoms of sleep apnoea in women may be less typical. They can be mistaken for depression unless the patient is specifically asked about symptoms of sleep apnoea.

Sleep studies are sometimes indicated for the investigation of other disorders. These include:

– unexplained hypertension

– polycythaemia

– pulmonary hypertension

– periodic limb movements of sleep

– precipitous arousals or violent behaviour during sleep

– persistent circadian rhythm disorders⁵

When shouldn't I order a sleep study?
Sleep studies are not indicated for the routine evaluation of transient or chronic insomnia and are of minimal assistance in diagnosing or treating insomnia due to psychiatric disorders or dementia.

Some sleep disorders may be diagnosed by a careful sleep/wake history and sleep diary (record of sleep periods, naps, intake of food, caffeine, alcohol and other drugs and activity) with or without recording of body movement (actigraphy) and may not require a sleep study e.g. circadian rhythm disorder such as advanced sleep phase syndrome.

Before the test
A full medical and psychiatric history should be obtained to determine if the test is appropriate and if there are factors which may affect the study or cause difficulties. If possible, the patient should visit the sleep laboratory before the study.

A sleep study is only a part of the total assessment of the patient. It is labour intensive and expensive (Medicare Schedule fee $443.45 as at May 1998) (Table 3). Computer-assisted sleep staging can help in the analysis, but manual verification of sleep staging and disruption is still required.

Diagnosing sleep-disordered breathing
One night is usually sufficient to diagnose sleep-disordered breathing or periodic limb movements of sleep. Two or more nights may be required if a parasomnia (e.g. sleepwalking) under investigation does not occur on the first night, or if the patient's total sleep time on the first night is inadequate for satisfactory evaluation. Provided the patient has slept adequately, a normal study rules out sleep-disordered breathing.

Sleep staging is critical
An accurate assessment of the presence/absence of sleep and the stage of sleep is critical as the frequency of events may vary with sleep stage. If the patient is unable to sleep during the study, any evaluation is useless.

The stages of sleep are identified by the combined use of EEG, EOG and submental EMG recordings. For sleep studies, two pairs of EEG electrodes are attached to the scalp in contrast to full neurological EEG studies which use 16 pairs. Routine sleep studies may therefore not detect seizure activity. Comprehensive epilepsy monitoring with overnight observation may be required to diagnose nocturnal epilepsy.

Which physiological variables are measured?
A continuous ECG recording may detect changes in association with sleep-disordered breathing. Primary cardiac abnormalities may also be detected, e.g. sinus arrest or evidence of silent myocardial ischaemia (ST segment abnormalities).

Measurements of airflow during sleep studies are generally qualitative rather than quantitative. These measurements are recorded using thermistors which respond to temperature changes in expired air. They are small probes which rest between the nose and lip rather like small oxygen cannulae, usually with a small extension which detects oral airflow.

Snoring is recorded by a small microphone.

Respiratory movement is recorded by bands around the chest and abdomen which detect movement. The presence or absence of respiratory effort/movement during an apnoea defines that apnoea as obstructive, central or mixed (Fig. 2). Paradoxical inward motion during inspiration may be seen with upper airway obstruction or profound diaphragmatic weakness.

Pulse oximetry is used continuously. Transcutaneous measurement of carbon dioxide may be used when CO₂ retention is suspected e.g. severe respiratory or neuromuscular disease or the presence of daytime respiratory failure.

Bilateral recording of EMG activity in the tibialis anterior muscles is used to diagnose periodic limb movements (PLMs) of sleep disorder. This disorder is characterised by the presence of frequent repetitive stereotyped movements (often very small) which can result in brief awakenings and may thus cause insomnia or excessive sleepiness during the day. PLMs occur in almost all patients with restless legs syndrome, but commonly occur as an isolated nocturnal syndrome.

A sensor to detect body position is usually placed on the patient's chest. The severity of sleep-disordered breathing may vary according to body position and is typically worst in the supine position.

Intervention studies
Treatment such as nasal continuous positive airway pressure (CPAP) is usually planned after a diagnostic study. Occasionally, a CPAP study will be done immediately after a 'screening study', such as overnight oximetry in a hospital inpatient, if the result suggests very severe sleep-disordered breathing. Split studies involve an initial baseline period and, once the diagnosis is confirmed, the patient is woken and CPAP applied to begin the pressure titration process. The disadvantages of split studies as a routine are that sleep-disordered breathing often becomes more severe in the latter part of the night, so the severity may be underestimated and there may be insufficient time to allow satisfactory titration of CPAP pressure. The patient, if not adequately prepared, may not accept the CPAP during the middle of the night. However, a recent study of CPAP compliance after initial full night titration compared with split studies showed that split studies did not reduce CPAP compliance.⁶

Nap studies
A nap study is generally a short daytime study of approximately 3 hours' duration. Such a study may underestimate the severity of sleep-disordered breathing and a negative nap study is indeterminate.^7,8 Therefore, nap studies are generally not recommended.

Screening studies
Monitoring systems which measure a limited number of variables can be used as an initial screen for sleep-disordered breathing to help set a priority for a full study or treatment study. Measurement of oxygen saturation alone may be used, although there are limitations. Abnormal oxygen saturation is not specific for sleep-disordered breathing and may indicate other respiratory problems. Oxygen desaturation alone lacks sensitivity due to the presence of events that cause sleep disruption without desaturation. It is thus a marker of sleep-disordered breathing, but not a direct measurement.

Interpreting sleep studies
Other factors which may affect the sleep on the night of the study should be recorded such as drug and alcohol use or unusual events or activities. An abrupt change in alcohol intake may worsen sleep quality. A less than usual intake of alcohol may lead to an underestimation of the severity of sleep-disordered breathing at home or an underestimate of the CPAP pressure required to control sleep-disordered breathing.

The sleep study report will include details of the sleep quality and structure (sleep architecture) e.g. duration of sleep stages and frequency of disturbances of sleep. It also includes the presence and description of any abnormal events and the frequency of episodes of sleep-disordered breathing, often expressed as an index of episodes per hour of sleep. This is the apnoea hypopnoea index, also known as respiratory disturbance index. A count of periodic limb movements may also be included, if clinically relevant.

Sleep studies need to be interpreted within the clinical context, and are thus not recommended as a test in isolation.

What next?
The technology of sleep studies is changing rapidly with the development of computerised data collection, portable systems for the acquisition of data outside the sleep laboratory and different types of monitoring. Further studies of newer, simplified systems are ongoing to clarify the role, specificity and sensitivity of limited variable studies.