Obstructive sleep apnea (OSA) is a sleep disorder whereby breathing repeatedly stops due to obstruction and collapse of the pharynx within the upper airway. The pathophysiology underlying OSA is attributable to both anatomical (structural) and neuromuscular (nonstructural) elements.
The pharynx is an important component of the upper airway that regulates respiration, swallowing, and speech. As a part of both the respiratory and digestive systems, the pharynx connects the mouth and nasal cavity above to the esophagus and larynx below.
The pharynx is made up of soft tissue and over 20 muscles, allowing it to be both flexible and collapsible. These attributes are necessary for the pharynx to carry out its role in swallowing and speech. The upper portion of the pharynx, called the nasopharynx, contains the adenoids, an area of lymphatic tissue that is naturally larger in children and can become enlarged in adults due to infection.
Enlarged adenoids can cause blockage of the upper airway and is the leading cause of OSA in children. Blockage can also occur due to mucosal buildup or chronic nasal congestion in this region. The oropharynx is the middle portion of the pharynx, located at the back of the oral cavity.
This region contains the tonsils, which can obstruct air flow during sleep if swollen. Relaxation of the tongue during sleep can also block this region. The bottom portion of the pharynx, known as the laryngopharynx, sits above the esophagus and the larynx.
Patients with OSA can have alterations in their anatomy that make them more susceptible to airway blockage.
First, genetic variation can cause some people to have a narrower upper airway than others. One study found that the calculated area of the upper airway was smaller in those who suffered from OSA as compared to healthy subjects.
Secondly, people with craniofacial abnormalities, such as those with an abnormal lower jaw position or in patients with Down syndrome, may experience higher incidences of OSA.
Patients with OSA are also more likely to have alterations in the size or location of their soft tissue that can lead to airway blockage such as enlarged tonsils or adenoids. Lastly, fatty tissue surrounding the pharynx can adversely affect airway diameter.
Obesity is strongly linked to OSA, and more than half of OSA sufferers are obese. Risk of developing OSA increases for those with a large neck, which is defined as a circumference of greater than 17 inches in men and 16 inches in women.
Neuromuscular responses stimulate the throat muscles to open and lengthen the airway in response to an obstruction. The pharynx is kept open by dilator muscles that receive signals from the central nervous system and from the pharynx itself.
The genioglossus muscle is the biggest dilator muscle in upper airway, and activity of this muscle can fluctuate from person to person. In OSA patients, the genioglossus muscle has been found to have an overall higher level of activity in order to keep the airway open during wakefulness.
This increased activity can be a compensatory mechanism that allows the airway to remain open in people with anatomical issues that make the pharynx prone to collapse. Because dilator muscles lose muscle tone as a person enters sleep, people who rely on muscle activity to prevent pharynx collapse are more likely to suffer from OSA.
The negative pressure reflex acts upon the dilator muscles to keep the airway open during inhalation. Chronic OSA can cause damage to muscles and nerves within the pharynx, resulting in a dysfunctional negative pressure reflex during sleep. In addition, variations in reflex triggering among individuals can make some people more susceptible to OSA than others.
During apneas and hypopneas, patients are aroused from sleep in order to restore dilator muscle activity and resume breathing. In some patients, the breathing reflex completely awakens the person (low arousal threshold), whereas some people can remain asleep while breathing is restored (high arousal threshold).
People with a high arousal threshold will likely experience less chronic fatigue and have improved daytime function. Restoration of airflow without complete arousal has been found to occur in healthy individuals, while people with OSA were more likely to fully awaken more often.
As blood oxygen levels fall and carbon dioxide levels rise during apnea and hypopnea episodes, ventilatory control mechanisms kick in to help regulate breathing rates. Some individuals have a more sensitive response than others, and a faulty or overly sensitive response can further exacerbate irregular breathing patterns.