What is sleep apnea?

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There many varying definitions for the Obstructive Sleep Apnea-Hypopnea Syndrome. The Spanish Society of Respiratory Disease (SSRD) defines OSAHS as being a disease "characterised by drowsiness, and associated with neuropsychiatric and cardiopulmonary side effects arising from anatomical and/or functional alterations to the upper airway. These alterations lead to repeated episodes of the upper airway obstruction causing a decline in the oxygen saturation levels and transient awakenings causing restless sleep".
In 2005, a national consensus on Sleep Apnea defined OSAHS as a disease characterised by "excessive sleepiness, associated with cognitive-behavioural disorders, and also associated with respiratory, cardiac, metabolic and inflammatory side-effects caused by repeated obstruction of the upper airway during sleep". These episodes are measured by what is know as the Respiratory Disturbance Index (RDI). An RDI ≥5 associated with disease related symptoms which are unexplained by other causes confirms the diagnosis.

Guilleminault et al introduced the term "Sleep Apnea" in 1973 for patients with excessive daytime sleepiness caused by OSAHS. Since then "Sleep Apnea" has been used by many authors to describe many conditions with similar symptoms leading to confusion and difficulty in diagnosing OSAHS. OSAHS has been associated with hypersomnia and periodic breathing, chronic snoring and Ondine's Curse. Currently OSAHS is classified as a Sleep Breathing Disorders (SBD) with a differential diagnosis in its own right. The abbreviations OSAHS, OSA and SAS coexist in the literature and they all refer to the same clinical entity.

A national consensus on Sleep Apnea along with the Spanish Society of Pneumology and Thoracic Surgery (SEPAR) and the Latin American Thoracic Association (ALAT) prefer the Acronym OSAHS – Sleep Apnea and Hypopnea Syndrome. The inclusion of "Hypopnea", is seen as key in the clinical development of the disease as it has the same clinical effects as apneas. It also eliminates the term "obstructive" which would exclude Central and Mixed Apneas.

Sleep Apnea consists of repeated pauses in breathing (apneas) during sleep as a result of an alteration in the anatomy or function of the upper respiratory tract which negatively affects the permeability of the airway. The upper airway has the tendency to collapse, hampering the normal respiratory cycle. Apnea is defined as a temporary cessation of breathing for more than ten seconds. These pauses lead to hypoventilation, oxygen desaturation and increased respiratory effort in response to the hypoxia and hypercapnia. These episodes of asphyxia are of variable duration, ranging from a few seconds to over a minute in some cases and may recur hundreds of time in the course of one night. The interruption of airflow is due to an obstruction of the upper airway, usually in the throat region (oropharynx) where the airway is does not have a rigid frame and is susceptible to collapse. During sleep, (mainly REM sleep), the pharyngeal muscle tone decreases (hypotonicicity) leading to loss in the co-ordination between the respiratory muscles, the diaphragm and the throat muscles.

This hypotonicity, for reasons little understood, is responsible for the collapse of the airway and the intermittent breathing associated with repeated micro-awakenings and consequent sleep disturbances, which often manifests as daytime drowsiness. SBD are also linked to hypertension, cardiovascular disease, reduced quality of life, and an increase in work and traffic accidents. A lack of deep sleep comprises the cognitive function and sometimes manifests itself as poor behaviour, personality disorders and impaired intellectual abilities. Irritability and depression are also sometimes seen. OSAHS in not considered a fatal condition in itself but it is widely accepted that life expectancy in decreased because of the syndrome.

Apnea
The term apnea was introduced by Christian Guilleminault and William C. Dement to describe the complete cessation of airflow for 10 seconds or longer. If accompanied by increased respiratory efforts it is considered obstructive and if not is considered central. Apnea is considered neurological or mixed when it is initiated by a central component and results in airway obstruction. This definition of apnea used by Guilleminault and Dement is limited in that it does not address oxyhaemoglobin desaturation, neither does it mention the presence of EEG arousals (micro-awakenings). It is accepted that a cessation of respiratory signal is considered complete or obstructive when there is a decrease in airflow by > 90%.

Hypopnea
This term was coined by Kart et al to refer to a partial reduction in airflow. Similar to apnea, hypopnea causes oxygen desaturation, sleep arousals and has clinical implications. The definition of hypopnea is difficult due to a lack of unified criteria. The American Academy of Sleep Medicine defines a hypopnea as "a discernible reduction in the respiratory signal together with decreased oxygen saturation of at least 3% and/or an electroencephalographic arousal.
The guidelines of SEPAR are generally accepted and therefore hypopnea is considered present it there is a respiratory signal reduction of between 30% and 90% in combination with transient awakenings registered on the EEG and/or oxygen desaturation >= 3%. (According to the authors these values oscillate between 2-4%.)

Unconscious micro-awakenings or arousals
Micro-awakenings, otherwise know as arousals , are a clinical feature of OSAHS in response to a respiratory pause. During sleep, the dilator muscles of the upper airway (geniohyoid, genioglossus, tensor veli palatini),loose their tone and are unable to compensate for the negative pressure produced by the intercostal muscles and diaphragm on inspiration. This partial or complete collapse of the airway disrupts the air flow causing hypoxemia and decreased oxygen saturation levels. This desaturation is detected by the Central Nervous System which initiates an immediate response via the efferent pathways provoking a micro-awakening. The transient awakening return the subject to the physiological state of wakefulness allowing the muscle tone of the upper airways to return with a rattling sound, effectively ending the apneic episode. This obviously prevents the death of the patient but it does disturb the sleep process not allowing the patient to enter into phases of deep, restful sleep. Repeated micro-awakenings throughout the night cause daytime sleepiness (somnolencia diurna) and can have a negative effect on mood, impair cognitive functions, cause a person to feel weak (asthenia), and can lead to chronic fatigue. The American Sleep Disorders Association (ASDA) defined arousal s as the sudden appearance of alpha or theta rhythms on the EEG for at least 1.5 seconds but not necessarily accompanied by increased electromyographic activity except in REM sleep.

Respiratory Effort Related Arousal (RERA)
A persons response to an apneic episode is an increase in the respiratory effort to recover the permeability of the airway and replace the airflow. There is a progressive increase in the respiratory effort for more than 10 seconds that can be detected by recording the oesophageal pressure which ends with a micro-awakening. The presence or absence of increasing thoracic efforts is the distinguishing feature between Obstructive Apnea/Hypopnea and Central Apnea/Hypopnea. Obstructive Apnea/Hypopnea is characterised by an increase in respiratory effort whereas Central Apnea/Hypopnea shows no such respiratory effort. Mixed Apneas combine aspects of both Obstructive Apnea/Hypopnea and Central Apnea/Hypopnea.

Apnea/hypopnea Index (AHI)
The Apnea/Hypopnea Index (AHI) measures the number of respiratory events (apneas plus hypopneas) per hour. An AHI of less than 5 is considered normal but does not exclude the possibility of OSAHS being present. The AHI is combined with the number of respiratory efforts which cause EEG micro-awakenings in order to make an accurate assessment and rule out false negatives.

Respiratory disturbance index (RDI)
The amount of respiratory events (apneas and hypopneas) per hour plus Respiratory Effort Related Arousals (RERA) determines the Respiratory Disturbance Index (RDI). All of these parameters are essential to determine an OSAHS diagnosis. The American Academy of Sleep Medicine has recently defined OSAHS as the presence of an abnormal RDI. (RDI = AHI + RERA). An RDI > 5 associated with relevant clinical signs and symptoms is considered to be the definitive diagnosis of OSAHS.