Sleep apnea (or sleep apnoea in British English) is a sleep disorder characterized by abnormal pauses in breathing or instances of abnormally low breathing, during sleep. Each pause in breathing, called an apnea, can last from a few seconds to minutes, and may occur 5 to 30 times or more an hour. Similarly, each abnormally low breathing event is called a hypopnea. Sleep apnea is diagnosed with an overnight sleep test called a polysomnogram, or "sleep study".
There are three forms of sleep apnea: central (CSA), obstructive (OSA), and complex or mixed sleep apnea (i.e., a combination of central and obstructive) constituting 0.4%, 84% and 15% of cases respectively. In CSA, breathing is interrupted by a lack of respiratory effort; in OSA, breathing is interrupted by a physical block to airflow despite respiratory effort and snoring is common.
Regardless of type, an individual with sleep apnea is rarely aware of having difficulty breathing, even upon awakening. Sleep apnea is recognized as a problem by others witnessing the individual during episodes or is suspected because of its effects on the body (sequelae). Symptoms may be present for years (or even decades) without identification, during which time the sufferer may become conditioned to the daytime sleepiness and fatigue associated with significant levels of sleep disturbance.
The diagnosis of Sleep Apnea is based on the conjoint evaluation of clinical symptoms (e.g. excessive daytime sleepiness and fatigue) and of the results of a formal sleep study (polysomnography, or reduced channels home based test). The later aims at establishing an "objective" diagnosis indicator linked to the quantity of apneic events per hour of sleep (Apnea Hypnea Index(AHI), or Respiratory Disturbance Index (RDI)), associated to a formal threshold, above which a patient is considered as suffering from Sleep Apnea, and the severity of his sleep apnea can be then quantified. Nevertheless, due to the number and variability in the actual symptoms and nature of apneic events (hypopnea vs apnea, central vs. obstructive...), the variability of patients physiology, the intrinsic imperfections of the experimental setups and methods, this field is opened to debate . Within this context, the definition of an apneic event depends of several factors (e.g. patient's age) and account for this variability through a multi-criteria decision rule described in several, sometimes conflicting, guidelines . One example of a commonly adopted definition of an apnea (for an adult) includes a minimum 10 second interval between breaths, with either a neurological arousal (a 3-second or greater shift in EEG frequency, measured at C3, C4, O1, or O2) or a blood oxygen desaturation of 3–4% or greater, or both arousal and desaturation.
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