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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Causes of sleep apnea depend on whether the main problem is central or obstructive.

Central sleep apnea

Central sleep apnea syndromes may be divided into two groups; primary (without an underlying cause) or secondary (as a consequence of another condition). In general, central sleep apnea stems from an abnormal regulatory mechanism in the brain.

Some common causes of central sleep apnea include:

• strokes,
  
  
• heart failure,
  
  
• certain medications,
  
  
• some congenital abnormalities, or
  
  
• high altitude.

Premature infants may also be at risk for central sleep apnea.

The brain regulates breathing by monitoring levels of oxygen and carbon dioxide in the blood. If the oxygen level is low or the carbon dioxide level is high, the brain signals the breathing muscles to breathe faster in order to expire more carbon dioxide and inspire more oxygen. On the other hand, if the oxygen level is too high or the carbon dioxide is too low, then the brain slows down breathing to allow for a more normal balance.

In central sleep apnea, this regulatory mechanism is disrupted and the brain's recognition of, or response to, oxygen and carbon dioxide levels is impaired. As breathing stops or slows down, the oxygen level drops significantly lower and the carbon dioxide level increases significantly higher than the levels necessary to trigger normal breathing. This leads to a transient exaggerated over-breathing to compensate for significantly higher levels of carbon dioxide and lower oxygen levels. Subsequently, the over-breathing can result in overshooting the oxygen and carbon dioxide levels, initiating another episode of apnea.

Obstructive sleep apnea

In obstructive sleep apnea, the problem is not the regulation of breathing by the brain, but rather, it has to do with an obstruction to the flow of air into the lungs. The brain signals the muscles of breathing to take a breath. The muscles attempt to take a breath, but no air can flow due to the obstruction of air flow. Therefore, the oxygen levels fall and carbon dioxide levels rise to a level that signals the brain to wake the body up to take a breath (resulting in gasping for air).

In normal breathing:

1. the air flows through the nose and the nasal passages (or the mouth),
   
   
2. it then flows behind the soft palate and the base of the tongue,
   
   
3. through the pharynx and associated muscles, and
   
   
4. between the vocal cords before entering the lungs.

This flow of air can be compromised at any of these levels due to a variety of reasons. Some common reasons include:

• a deviated nasal septum,
  
  
• nasal congestion,
  
  
• narrow airway passages,
  
  
• enlarged tonsils,
  
  
• weak pharyngeal muscles,
  
  
• vocal cord injury,
  
  
• facial trauma leading to distorted airway passages, or
  
  
• retraction of the tongue to the back of the throat.

Some other risk factors for obstructive sleep apnea and obstructed airflow include:

• obesity and weight gain (leading to narrow air passages),
  
  
• some sedative medications and alcohol (leading to lax pharyngeal muscles, soft palate, and tongue),
  
  
• neuromuscular diseases (such as stroke, leading to weak airway muscles),
  
  
• upper respiratory infections (leading to narrow and swollen nasal passages), and
  
  
• smoking.

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