Sleep Apnea - symptoms, causes, and treatment of central and obstructive sleep apnea on MedicineNet.com

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Sleep Apnea (cont.)

What is obstructive sleep apnea and what causes it?

In obstructive sleep apnea (OSA), apneas have four components.

First, the airway collapses or becomes obstructed.
Second, an effort is made to take a breath, but it is unsuccessful.
Third, the oxygen level in the blood drops as a result of unsuccessful breathing.
Finally, when the amount of oxygen reaching the brain decreases, the brain signals the body to wake up and take a breath. (This is what the bed partner hears as a silence followed by a gasp for air.)

First, it is necessary to describe a "normal breath." A normal breath of air passes through the nasal passages, behind the soft palate and uvula (part of the soft palate), then past the tongue base, through the throat muscles, and between the vocal cords into the lungs. An obstruction to the flow of air at any of these levels may lead to apnea. The following are some examples:

airflow can become diminished if a person has a deviated septum (the middle wall of the nose that separates the two nostrils). A septum can be deviated to one or both sides narrowing the air passages;
there are filters in the nose called turbinates that can obstruct airflow when they become swollen;
if the palate and uvula (the part of soft palate that hangs down in the back of the throat) are long or floppy, they can fall backwards and close the area through which air flows;
the back of the tongue can also fall backwards and obstruct breathing especially when individuals lay flat on their backs; or
the side walls of the throat can fall together to narrow or close the airway.

To break it down even further:

the muscles of breathing work to expand the chest and lower the diaphragm to degenerate a negative pressure between the airways of the lungs and outside;
this negative pressure literally sucks air into the lungs;
the nasal passages, palate, tongue, and pharyngeal tissues can all contribute to narrowing of the airway;
if during an attempt to breathe the airway collapses or is obstructed the tissues of the airway are sucked together by the negative pressure;
the harder the chest tries to pull air in the greater the negative pressure and the more the tissues of the airway are sealed together; and
finally, when the oxygen in the blood stream decreases the person wakes up or the level of sleep becomes more shallow in order to more consciously take a breath.

People with obstructive sleep apnea have an airway that is more narrow than normal, usually at the base of the tongue and palate. When lying flat, the palate is above the air passage. When the pharyngeal muscles (muscles of the pharynx or throat ) relax the palate can fall backwards and this can obstruct the airway.

The genioglossus muscle is located where the base of the tongue attaches to the jawbone in front. Most people have enough space behind the tongue to take a breath without needing to pull the tongue forward. However, when obstructive sleep apnea patients are awake, this muscle needs to be active to pull the base of the tongue forward to open the airway. During sleep, most muscles including the genioglossus relax. During the stage of rapid eye movement (REM), the muscles completely relax. Relaxation of the genioglossus muscle during sleep allows the base of the tongue to fall backwards and the airway closes.

Patients with obstructive sleep apnea often don't report waking up during the night with each episode of apnea. Frequently, during the apnea the brain only awakens from a deep sleep (stages 3, 4, or REM) to a shallow level of sleep. The genioglossus muscle then contracts and pulls the tongue forward so that a breath can be taken. The patient may remain asleep, but the deep sleep that is important to be fully rested the following day is disrupted.