Sleep apnea (or sleep apnoea in British English) is a sleep disorder characterized by pauses in breathing during sleep. Each episode, called an apnea (Greek: ἄπνοια (ápnoia), from α- (a-), privative, πνέειν (pnéein), to breathe), lasts long enough so that one or more breaths are missed, and such episodes occur repeatedly throughout sleep. The standard definition of any apneic event 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), a blood oxygen desaturation of 3-4% or greater, or both arousal and desaturation. Sleep apnea is diagnosed with an overnight sleep test called a polysomnogram, or a "Sleep Study".
Clinically significant levels of sleep apnea are defined as five or more episodes per hour of any type of apnea (from the polysomnogram). There are three distinct forms of sleep apnea: central, obstructive, and complex (i.e., a combination of central and obstructive) constituting 0.4%, 84% and 15% of cases respectively. Breathing is interrupted by the lack of respiratory effort in central sleep apnea; in obstructive sleep apnea, breathing is interrupted by a physical block to airflow despite respiratory effort. In complex (or "mixed") sleep apnea, there is a transition from central to obstructive features during the events themselves.
Regardless of type, the 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 first reports in the medical literature of what is now called obstructive sleep apnea date only from 1965, when it was independently described by French and German investigators. However, the clinical picture of this condition has long been recognized as a character trait, without an understanding of the disease process. The term “Pickwickian syndrome” that is sometimes used for the syndrome was coined by the famous early 20th century physician, William Osler, who must have been a reader of Charles Dickens. The description of Joe, "the fat boy" in Dickens's novel, The Pickwick Papers, is an accurate clinical picture of adult obstructive sleep apnea syndrome.
The early reports of obstructive sleep apnea in the medical literature described individuals who were very severely affected, often presenting with severe hypoxemia, hypercapnia and congestive heart failure. Tracheostomy was the recommended treatment and, though it could be life-saving, post-operative complications in the stoma were frequent in these very obese and short-necked individuals.
The management of obstructive sleep apnea was revolutionized with the introduction of continuous positive airway pressure (CPAP), first described in 1981 by Colin Sullivan and associates in Sydney, Australia. The first models were bulky and noisy but the design was rapidly improved and by the late 1980s CPAP was widely adopted. The availability of an effective treatment stimulated an aggressive search for affected individuals and led to the establishment of hundreds of specialized clinics dedicated to the diagnosis and treatment of sleep disorders. Though many types of sleep problems are recognized, the vast majority of patients attending these centers have sleep disordered breathing.
Obstructive Sleep Apnea
Obstructive sleep apnea (OSA) is the most common category of sleep-disordered breathing. The muscle tone of the body ordinarily relaxes during sleep and at the level of the throat the human airway is composed of collapsible walls of soft tissue which can obstruct breathing during sleep. Mild, occasional sleep apnea, such as many people experience during an upper respiratory infection may not be important, but chronic, severe obstructive sleep apnea requires treatment to prevent low blood oxygen (hypoxemia), sleep deprivation, and other complications. The most serious complication is a severe form of congestive heart failure called cor pulmonale.
Individuals with low muscle tone and soft tissue around the airway (e.g., due to obesity), and structural features that give rise to a narrowed airway are at high risk for obstructive sleep apnea. The elderly are more likely to have OSA than young people. Men are more typical sleep apnea sufferers than women and children, although it is not uncommon in the latter two.
Common symptoms include loud snoring, restless sleep, and sleepiness during the daytime. Diagnostic tests include home oximetry or polysomnography in a sleep clinic.
Some treatments involve lifestyle changes, such as avoiding alcohol or muscle relaxants, losing weight, and quitting smoking. Many people benefit from sleeping at a 30 degree elevation of the upper body or higher, as if in a recliner. Doing so helps prevent the gravitational collapse of the airway. Lateral positions (sleeping on a side), as opposed to supine positions (sleeping on the back), are also recommended as a treatment for sleep apnea, largely because the gravitational component is smaller in the lateral position. Some people benefit from various kinds of oral appliances to keep the airway open during sleep. "Breathing machines" like the continuous positive airway pressure (CPAP) may help. There are also surgical procedures to remove and tighten tissue and widen the airway.
OSA symptoms, signs and sequelae
As already mentioned, snoring is almost a uniform finding in an individual with this syndrome. Snoring is the turbulent sound of air moving through the back of the mouth, nose and throat. Although not everyone who snores is experiencing difficulty breathing, "snoring" in combination with other conditions such as overweight and obesity has been found to be highly predictive of OSA risk. The loudness of the snoring is not indicative of the severity of obstruction, however. If the upper airways are tremendously obstructed, there may not be enough air movement to make much sound. Even the loudest snoring does not mean that an individual has sleep apnea syndrome. The sign that is most suggestive of sleep apneas occurs if snoring stops. If it does, along with breath, while the persons' chest and body tries to breathe - that is literally a description of an event in obstructive sleep apnea syndrome. When breathing starts again, there is typically a deep gasp, and then the resumption of snoring.
The term "sleep disordered breathing" is commonly used in the US to describe the full range of breathing problems during sleep in which not enough of air reaches the lungs (hypopnea and apnea). Sleep disordered breathing is associated with an increased risk of cardiovascular disease, stroke, high blood pressure, arrhythmias, diabetes, and accidents. When high blood pressure is caused by OSA, it is distinctive in that, unlike most cases of high blood pressure (so-called essential hypertension), the readings do not drop significantly when the individual is sleeping. Stroke is associated with obstructive sleep apnea. Sleep apnea sufferers also have a 30% higher risk of heart attack or premature death than those unaffected.
In the June 27, 2008, edition of the journal Neuroscience Letters, researchers revealed that people with OSA show tissue loss in brain regions that help store memory, thus linking OSA with memory loss. Using magnetic resonance imaging (MRI), the scientists discovered that sleep apnea patients' mammillary bodies were nearly 20 percent smaller, particularly on the left side. One of the key investigators hypothesized that repeated drops in oxygen lead to the brain injury.
Central Sleep Apnea
In pure central sleep apnea or Cheyne-Stokes respiration, the brain's respiratory control centers are imbalanced during sleep. Blood levels of carbon dioxide, and the neurological feedback mechanism that monitors it does not react quickly enough to maintain an even respiratory rate, with the entire system cycling between apnea and hyperpnea, even during wakefulness. The sleeper stops breathing, and then starts again. There is no effort made to breathe during the pause in breathing: there are no chest movements and no struggling. After the episode of apnea, breathing may be faster (hyperpnea) for a period of time, a compensatory mechanism to blow off retained waste gases and absorb more oxygen.
While sleeping, a normal individual is "at rest", as far as cardiovascular workload is concerned. Breathing is regular in a healthy person during sleep, and oxygen levels and carbon dioxide levels in the bloodstream stay fairly constant. The respiratory drive is so strong that even conscious efforts to hold one's breath do not overcome it. Any sudden drop in oxygen or excess of carbon dioxide (even if tiny) strongly stimulates the brain's respiratory centers to breathe.
In central sleep apnea, the basic neurological controls for breathing rate malfunction and fail to give the signal to inhale, causing the individual to miss one or more cycles of breathing. If the pause in breathing is long enough, the percentage of oxygen in the circulation will drop to a lower than normal level (hypoxia) and the concentration of carbon dioxide will build to a higher than normal level (hypercapnia). In turn, these conditions of hypoxia and hypercapnia will trigger additional effects on the body. Brain cells need constant oxygen to live, and, if the level of blood oxygen goes low enough for long enough, the consequences of brain damage and even death will occur. Fortunately, central sleep apnea is more often a chronic condition that causes much milder effects than sudden death. The exact effects of the condition will depend on how severe the apnea is, and the individual characteristics of the person having the apnea. Several examples are discussed below, and more about the nature of the condition is presented in the section on Clinical Details.
In any person, hypoxia and hypercapnia have certain common effects on the body. The heart rate will increase, unless there are such severe co-existing problems with the heart muscle itself or the autonomic nervous system that makes this compensatory increase impossible. The more translucent areas of the body will show a bluish or dusky cast from cyanosis, which is the change in hue that occurs due to lack of oxygen in the blood ("turning blue"). Overdoses of drugs that are respiratory depressants (such as heroin, and other opiates) kill by damping the activity of the brain's respiratory control centers. In central sleep apnea, the effects of sleep alone can remove the brain's mandate for the body to breathe. Even in severe cases of central sleep apnea, the effects almost always result in pauses that make breathing irregular, rather than cause the total cessation of breathing.
Normal Respiratory Drive: After exhalation, the blood level of oxygen decreases and that of carbon dioxide increases. Exchange of gases with a lungful of fresh air is necessary to replenish oxygen and rid the bloodstream of built-up carbon dioxide. How do the changing blood levels of oxygen and carbon dioxide result in a breath? In a healthy animal, including humans, oxygen and carbon dioxide receptors in the blood stream (called chemoreceptors) send nerve impulses to the brain, which then signals reflex opening of the larynx (so that the opening between the vocal cords enlarges) and movements of the rib cage muscles and diaphragm. These muscles expand the thorax (chest cavity) so that a partial vacuum is made within the lungs and air rushes in to fill it. The body inhales.
Physiologic effects of central apnea: During central apneas, the central respiratory drive is absent, and the brain does not respond to changing blood levels of the respiratory gases. No breath is taken despite the normal signals to inhale. The immediate effects of central sleep apnea on the body depend on how long the failure to breathe endures. At worst, central sleep apnea may cause sudden death. Short of death, drops in blood oxygen may trigger seizures, even in the absence of epilepsy. In people with epilepsy, the hypoxia caused by apnea may trigger seizures that had previously been well controlled by medications. In other words, a seizure disorder may become unstable in the presence of sleep apnea. In adults with coronary artery disease, a severe drop in blood oxygen level can cause angina, arrhythmias, or heart attacks (myocardial infarction). Longstanding recurrent episodes of apnea, over months and years, may cause an increase in carbon dioxide levels that can change the pH of the blood enough to cause a metabolic acidosis.
Polysomnography of sleep apnea shows pauses in breathing that are followed by drops in blood oxygen and increases in blood carbon dioxide. In adults, a pause must last 10 seconds to be scored as an apnea. However in young children, who normally breathe at a much faster rate than adults, the pause may be many seconds shorter and still be considered apnea. The cessation of airflow in central sleep apnea has an association with no physical attempts to breathe. On polysomnograms, there is an absence of rib cage and abdominal movements while airflow ceases at the nose and lips. Obstructive sleep apnea shows pauses in breathing for at least 10 seconds causing a decrease in blood oxygen and associates with physical attempts to breathe.
Hypopneas in adults are defined as a 50% reduction in air flow for more than ten seconds, followed by a 4% desaturation, and/or arousal. The Apnea-Hypopnea Index (AHI) is expressed as the number of apneas and hypopneas per hour of sleep.
Any individual, no matter how healthy, who is given enough of a central respiratory depressant drug will develop apnea on a central basis. Generally, drugs that are central respiratory depressants also have sedative effects, and so the individual taking a toxic dose of such a drug is likely to be asleep, or at least in an altered state of consciousness, when breathing becomes irregular. Alcohol is such a central respiratory depressant in large doses, so are opiates, barbiturates, benzodiazepines, and many other tranquilizers. Some individuals have abnormalities that predispose them to central sleep apnea. The treatment for the condition depends on its specific cause.
Similarly, in any person who has some form of sleep apnea (including obstructive sleep apnea), breathing irregularities during sleep can be dangerously aggravated by taking one of these drugs. Quantities that are normally considered safe may cause the person with chronic sleep apnea to stop breathing altogether. Should these individuals have general anesthesia, for example, they require prolonged monitoring after initial recovery, as compared to a person with no history of sleep apnea, because apnea is likely to occur with even low levels of the drugs in their system.
Premature infants with immature brains and reflex systems are at high risk for central sleep apnea syndrome, even if these babies are otherwise healthy. Fortunately, those premature babies who have the syndrome will generally outgrow it as they mature, providing they receive careful enough monitoring and supportive care during infancy to survive. Because of the propensity toward apnea, medications that can cause respiratory drive depression are either not given to premature infants, or given under careful monitoring, with equipment for resuscitation immediately available. Such precautions are routinely taken for premature infants after general anesthesia. Caffeine has been found to help reduce apnea in preterm infants and to aid in care after general anesthesia. Sudden infant death syndrome is sometimes theorized to be attributable to sleep apnea.
Congenital Central Hypoventilation Syndrome: This rare, inborn condition involves a specific gene, PHOX2B. This homeobox gene guides maturation of the autonomic nervous system, and loss-of-function mutations lead to the failure of the brain to effectively control breathing during sleep in patients with the syndrome. There may be a pattern of recognizable facial features among individuals affected with this syndrome.
Once almost uniformly fatal, congenital hypoventilation ("abnormally low ventilation") syndrome is now treatable. The children who have it must have tracheotomies and access to mechanical ventilation on respirators while sleeping, but most do not need to use a respirator while awake. The use of a diaphragmatic pacemaker may offer an alternative for some patients. When pacemakers have enabled some children to sleep without the use of a mechanical respirator, reported cases still required the tracheotomy to remain in place, because the vocal cords did not move apart with inhalation. This form of central sleep apnea has been called Ondine's curse. Now that some children with the syndrome have grown up, there is particular need for their avoidance of adolescent behaviors, such as alcohol use, which can easily be lethal.
Adults suffering from congestive heart failure are at risk for a form of central sleep apnea called Cheyne-Stokes respiration. This is periodic breathing with recurrent episodes of apnea alternating with episodes of rapid breathing. In those who have it, Cheyne-Stokes respirations occur while both awake and asleep. There is good evidence that replacement of the failed heart (heart transplant) cures central apnea in these patients. The use of some medications that are respiratory stimulants decrease the severity of apnea in some patients.
Mixed Apnea and Complex Sleep Apnea
Some people with sleep apnea have a combination of both types. When obstructive sleep apnea syndrome is severe and longstanding, episodes of central apnea sometimes develop. The exact mechanism of the loss of central respiratory drive during sleep in OSA is unknown, but is most commonly related to acid-base and CO2 feedback malfunctions stemming from heart failure. There is a constellation of diseases and symptoms relating to body mass, cardiovascular, respiratory, and occasionally, neurological dysfunction that have a synergistic effect in sleep-disordered breathing. The presence of central sleep apnea without an obstructive component is a common result of chronic opiate use (or abuse), due to the characteristic respiratory depression caused by large doses of narcotics.
Complex sleep apnea has recently been described by researchers as a novel presentation of sleep apnea. Patients with complex sleep apnea exhibit OSA, but upon application of positive airway pressure, the patient exhibits persistent central sleep apnea. This central apnea is most commonly noted while on CPAP therapy, after the obstructive component has been eliminated. This has long been seen in sleep laboratories, and has historically been managed either by CPAP or BiLevel therapy. Adaptive servo-ventilation modes of therapy have been introduced to attempt to manage this complex sleep apnea. Studies have demonstrated marginally superior performance of the adaptive servo ventilators in treating Cheyne-Stokes breathing, however, no longitudinal studies have yet been published, nor have any results been generated which suggest any differential outcomes versus standard CPAP therapy. At the AARC 2006 in Las Vegas, NV, researchers reported successful treatment of hundreds of patients on Adapt SV therapy, however these results have not been reported in peer reviewed publications as of July, 2007.
An important finding by Dernaika, et al., (Chest 2007, 132) suggests that transient central apnea produced during CPAP titration (the so called "complex sleep apnea") is "... transient and self-limited." The central apneas may in fact be secondary to sleep fragmentation during the titration process. As of July 2007, there has been no alternate convincing evidence produced that these central sleep apnea events associated with CPAP therapy for obstructive sleep apnea are of any significant pathophysiologic import.
Research is ongoing, however, at the Harvard Medical School, including adding dead space to positive airway pressure for treatment of complex sleep-disordered breathing.