Understanding The Sleep Study


Glossary of Sleep Medicine Terms

AHI – Apnoea Hypopnoea Index = average number of apnoeas and hypopnoeas per hour.

AI – Arousal index = the total number of arousals scored per hour of sleep. These arousals are then classified as beingcaused by a respiratory event, leg movement or just spontaneously.

Apnoea –complete absence of breathing for more than 10 seconds. It can be of three types:

  • Central – where no effort is made to breathe,

  • Obstructive – where no breathing occurs despite repeated efforts due to a limitation of flow through the upper airway.

  • Mixed – where there is a combination of effort to breathe and no effort to breathe.

Arousal – The stimulation of the brain, causing the subject to go from deep to a lighter level of sleep. This can be easily detected during a sleep study as a change in sleep stage. Arousal can be due to low blood oxygen, high blood carbon dioxide, jerky leg movements, or environmental factors such as loud noises.

BiPAP – Method of ventilation with an inspiratory pressure (IPAP) and expiratory pressure (EPAP). This is usedif CPAP is not sufficient, as in patients with neuromuscular weakness or scoliosis.

Bruxism – Teeth grinding.

Cataplexy – A symptom of narcolepsy, where without warning there is sudden paralysis of the muscles (atonia) usuallycausing the person to fall to the ground. This often occurs at a time of high emotion.

Central Apnoea – This is an apnoea (cessation of airflow) in which there is no respiratory effort.

Circadian Rhythm – The normal sleep-wake cycle that allows the cyclical release of hormones and restoration of energy,and allows optimal body functions during the day.

CPAP – Continuous Positive Airway Pressure = a method of preventing throat or pharyngeal collapse. Positive pressure is delivered to the throat via a small portable electrically driven air pump, and connected by tubing to a nasal mask that is worn during sleep.

Dysomnia – Sleep disorder causing sleepiness. Such conditions are OSA and PLMS.

EEG – Electro encephalo gram, a test of electrical function of the brain. Can be used to subdivide or “stage” the level of sleep from wake, to light (stage 1), moderate (stage 2), deep (stages 3 and 4) or dreaming (REM) sleep.

EMG – Electro myelo gram, a test of electrical activity of the muscles. Sensing electrodes can be placed over any muscle, but are usually placed on the skin over the muscles under the jaw to detect upper airway muscle activity, on the ribcage to detect the contraction of the breathing muscles and on the legs to detect leg movements. They are also aid the staging of sleep, and are useful in the diagnosis of excessive leg movements at night.

ESS – Epworth Sleepiness Scale. Asks a series of 18 questions about how likely a patient is to doze off or fall asleep in a variety of situations, according to recent times, and sleepiness must not be confused with tiredness. The maximum score is 24. Each situation (sitting, reading, watching TV, driving, lying down, after lunch) is assessed on a scale of 0 (no chance of dozing) to 3 (high chance of dozing). A score of greater than 8 indicates sleepiness. Higher scores indicate greatersleepiness.

Hypopnoea – Occurs when there is a partial or incomplete absence of breathing. Like apnoeas, these may be central orobstructive. Paradoxical breathing is likely to be present if obstructive, whilst in phase respiration would be present ifcentral. Central hypopnoeas may be associated with cardiac problems, in which case they would appear at regular intervals.

Hypoventilation – Occurs when a patient breathes too slowly or too shallow to maintain normal respiration. This isaccompanied by an elevated CO2 level whilst awake, which rises further when asleep. This in turn, affects the oxygen level.
This may be seen throughout the night and not be as a result of apnoeas and hypopnoeas.

Insomnia – Inability to obtain enough sleep. This may be perceived (sleep state misperception) or real. Insomnia may be classified as either difficulty initiating sleep insomnia (DISI), difficulty maintaining sleep insomnia (DMSI) or waking too early.

Mixed Apnoea – This is an apnoea (cessation of airflow), in which there are features of both central (no effort) and obstructive (effort to breathe) apnoeas.

MSLT – Multiple Sleep Latency Test. Used to assess how long it takes to fall asleep. Usually repeated 4 or 5 times in one day. Abnormally short sleep latencies are common in instances of sleep deprivation, such as insufficient sleep period time and severe sleep apnoea. A diagnosis of narcolepsy occurs if there is a short mean sleep latency and the presence of two or more sleep-onset REMs.

MWT – Maintenance of Wakefulness test. Similar protocol to the MSLT, occurring 4 times a day at 2 hr intervals, but the patient is asked to stay awake, sitting still in a dimly lit room.

Nadir – The lowest point. SaO 2 nadir means the lowest saturation at the time specified eg. REM nadir compared to NREM nadir.

Nasal Pressure – Measures the airflow at the nares (nostrils) and monitors the change in pressure.

Narcolepsy – A condition of unknown aetiology characterised by sudden attacks of sleep or muscle weakness (cataplexy), hallucinations and sleep paralysis.

NREM Sleep – Non Rapid Eye Movement Sleep. Consists of 4 stages – NREM1 and NREM2 are light sleep, whilst NREM3 and NREM4 are deep sleep. Patients are easier to arouse from light sleep than deep sleep. NREM sleep is termed ‘quiet sleep’ in infants.

OSA/OSAS – Obstructive Sleep Apnoea / Obstructive Sleep Apnoea Syndrome. Obstructive sleep apnoea (OSA) is a serious, potentially life-threatening condition characterised by:

  1. repetitive pauses in breathing during sleep due to collapse of the upper airway,

  2. which is usually accompanied by a reduction in oxygen levels in the blood, and

  3. followed by an awakening to breathe.

Oximetry – The measurement of oxygen saturation levels in the body, expressed as a percentage. This may be measured by a finger/ear probe (SpO2) or blood gas analysis (SaO2). The baseline measurement relates to the awake resting level.

Parasomnia – Sleep disorder that generally does not cause tiredness, such as bruxism (tooth grinding), sleep walking (somnambulism), sleep talking (somniliquy) and enuiresis (bed wetting). Parasomnias tend to run in families and are more common in childhood. They usually do not require medical attention but may be a source of embarrassment for the child and family. Tiredness can occur if significant sleep fragmentation occurs as a result of the parasomnia.

PLMD – Periodic limb movement disorder. Defined as 4 or more movements (of 0.5 – 5 seconds duration) within 5-90 second period. A diagnosis of PLMD requires an index (PLMI) of at least 5.

Polysomnogram – A sleep study consisting of measurements of electrical activity in the brain, eyes, muscles and heart, in addition to measurements of breathing and oxygen.

Primary snoring – Snoring during sleep with no apnoea/hypoventilation, and no changes to blood gasses.

RDI – Respiratory Disturbance Index. The total respiratory disturbance (all scored respiratory events) per study, expressed as average number per hour. In our lab, Respiratory events are scored using the Chicago criteria. These events are greater than ten seconds in adults and more than two respiratory cycles in children (usually about 8 seconds).

REM sleep – Rapid Eye Movement sleep. REM’s are a normal characteristic of the level of sleep during which dreaming occurs., and are ssociated with extremes of muscles relaxation (atonia), which can bring on sleep apnoea if it were to occurat all. Also seen at sleep onset, and in excess quantities in narcolepsy.

RLS – Restless Leg Syndrome. Thought to affect 5% of the population whereby sufferers have uncomfortable (cramp, burning, weakness, itchy) sensations in their legs. These sensations may also be felt in the feet, thighs, trunk and arms.
Sensations can be alleviated by movement, reduction in temperature or treated with medication. More common in women(particularly during pregnancy), and can occur in children. Often runs in families. Sufferers may avoid social situations.

Sleep Architecture – Refers to the distribution of sleep stages throughout the sleep study. This changes with age and is affected by conditions disrupting sleep, such as sleep apnoea. Cycling between REM and NREM normally occurs every 90 minutes in adults and there are percentages of each sleep state that are normal for age groups.

Sleep Efficiency - Expressed as a percentage of total sleep time (TST) of time in bed (TIB). This is reduced by awakenings after sleep onset but does not consider sleep architecture.
Slow-wave Sleep – This is the deepest state of sleep, in which big, slow delta waves are present on EEG. Also known as NREM3 and NREM4 or deep sleep.

Upper Airway Resistance Syndrome (UARS) – Occurs when there are no discrete apnoeas or changes to airflow. Ventilatory responses are normal but increased work of breathing during sleep causes frequent arousals, affecting sleep quality. This can be associated with snoring and daytime symptoms such as sleepiness and reduced alertness.

UPPP – Uvulo Palato Pharyngo Plasty, surgery involving the removal of the uvula and the soft part of the palate or roof of the mouth, in addition to any redundant tissue at the back of the mouth.

Ventilation – The rhythmical entry and exit of air into the lungs it requires a trigger from the brain and muscles to keep the throat open and muscles to expand the chest, sucking in. Alternatively, it can be provided by a machine, a ventilator, used in people with Central Sleep Apnoea.