The Causes of Obstructive Sleep Apnea – The How and Why

There are complex airway dynamics resulting in blockage of the upper airway producing Obstructive Sleep Apnea.  Although complex, the process can be viewed in a general non-technical manner to help one understand the causes of obstructive sleep apnea.

In such an overview, we find two components to the events producing blockage of the airway during sleep: airway muscle relaxation and airway size.  How does each of these contribute to the development of obstructive apnea?  

The basic event of sleep apnea is the relaxation of the upper airway (the throat above the Adam’s apple to the nose and mouth) narrowing it critically or totally, resulting in a blockage limiting the amount of air traveling into or out of the lungs. This causes a drop in the blood oxygen and an increase in the blood carbon dioxide.  These are changes, which the brain notices immediately.  The very next breathing effort by the diaphragm is stronger.  These efforts keep increasing until two or three breaths later, on average about 15 to 20 seconds, the effort is strong enough that the throat opens and air moves in and out again.  Commonly, but certainly not always, this is accompanied by a loud snore or snort.  These events are what produce Obstructive Sleep Apnea.

While these events alone may not be medically important, they produce problems when they occur repeatedly.  A measurement of five events per hour while sleeping is considered diagnostic of Obstructive Sleep Apnea in the United States.  Problems are thought to develop at this point.

Relaxation of the tongue and pharyngeal muscles play a role in developing these events.  As we age, muscle tone decreases.  Strength reduces.  Tissue tone becomes slack, to say the least.  We have all seen the effects of age in our family, friends and maybe even ourselves.  The same changes take place in our tongue, pharyngeal muscles and supporting tissues.  The prevalence data for obstructive sleep apnea show a steady increase with age.  Those greater than 70 years of age are reported to have an 80% frequency of occurrence.

Muscle relaxation and muscle changes with age are major components of the causation of obstructive apnea.   

What role does airway size play in causing obstructive apnea?  The size of the airway determines how much muscle relaxation is needed for the critical narrowing to occur.  Smaller airways will close more easily than larger ones

 It is best to look at the airway size as two issues – weight and genetics.  

In the 1960s, doctors discovered and described the obstructive apnea problem in obese patients.  For the first 20 years after its description, many strongly believed that obstructive apnea was just a complication of obesity, not an independent problem.  When a person gains weight, the fat tissue is added over the muscles below the skin in most areas of the body, including under the skin in the throat.  The accumulation in the throat makes the size of the airway smaller.  With a smaller airway, less relaxation is required to cause the critical narrowing resulting in an obstructive apnea event.  

It is now well known that at certain weights almost everyone will have apnea.  The medical community uses a measurement that is a calculation based on height and weight.  This is called the Body Mass Index or BMI.  A normal BMI is 20 to 25.  When a person’s BMI reaches 40, ninety percent of those individuals will have sleep apnea.

Obesity can cause sleep apnea.  What is its contribution to all those individuals with the condition?  No one agrees and there has been ongoing debate since the 1970s.  

Like most medical controversies, variation in the reported studies makes the issue more difficult to understand.  

Early attempts to understand the effects of weight loss on apnea used an improvement in symptoms as the mark of correcting the problem.  When obese patients with apnea lose a significant amount of weight, they feel and sleep better.   The authors of those reports concluded that obstructive sleep apnea was cured by weight loss.  But, did it really cure the apnea?

As more studies were done, the investigators began testing for apnea after weight loss.  To assess the effect of weight loss on correcting apnea, the reports used changes in patients’ sleep test measurements.  Typically, they would use a percentage drop in the number of apnea events or select a set level of events at 15 or 5 per hour.  The level of 5 is the diagnostic level accepted for the diagnosis of the condition.  These studies report a high level of symptomatic improvement with weight loss.  Unfortunately, only 10-15% of obese patients will drop their levels below 5 after weight loss.  Most will reduce the number of events limiting the severity, but few can be considered as cured.  For an individual, weight loss can have significant beneficial effects on treatment, need for treatment and symptoms.  However, when viewed as a cause, few can be considered to have obstructive apnea caused by obesity.

My experience reported a few years ago at a community sleep center gives an insight.  For every one hundred cases of newly diagnosed obstructive sleep apnea, 60 will have BMIs in the obese range, meaning that 60% were obese. After weight loss, we know only 10 to 15% of individuals will have their apnea event drop below 5 per hour.  In the one hundred newly diagnosed individuals with apnea, weight loss will correct the apnea in only 6-10 of the 60 who had weight issues.  The other 50- 54 previously obese now are of normal weight and still have apnea.  They will feel better after weight loss, but they will still have apnea.  There are now a total of approximately 90 who still have apnea.  Why?  

There is the problem of muscle relaxation of course, but there is the other component of airway size – Genetics.

Sleep doctors look at throats when evaluating their patients.  As a practitioner for many years, I have looked at the throats of thousands of patients with obstructive apnea.  How many abnormal throats have I observed?  Not many.   I have seen one throat malignancy in my last two thousand exams.  By including enlarged tonsils and changes due to prior surgeries, the total would still be very low.  Very few patients with apnea have abnormal throats.  However, all throats ARE DIFFERENT.  

As someone who looks at throats for a living, I can tell you that throats are like faces.  They are all similar, but they are all different. Some have large tongues, some small jaws, some low palates, some high palates, some narrow throats, some wide, but all are different.

Our throats are part of our physical anatomy given to us by our parents.  It is our genetic gift for life.   There are an infinite number of combinations that might be expected. The easiest way to describe the effects of our anatomy and genetics on our throats is simply by the throat’s size.  Some throats are large and some are small.  Smaller throats have less room for the relaxation that occurs during sleep and as a result, obstructive apnea may occur at a younger age.  A large throat may result in one never having the condition.

Over the past fifty years there are a variety of diseases and conditions that have been recognized as producing Obstructive Sleep Apnea.  Severe hypothyroidism and acromegaly are examples.  These are rare causes.  They affect muscle relaxation and airway size in a variety of ways.  Some ethnic groups have been shown to have a higher incidence of Obstructive Sleep Apnea – predominately those of Southeast Asian and black heritage.  These increases are proposed to be secondary to the bony structure of the face and cranium reducing airway size in those populations.

So these are the factors that lead to obstructive apnea: age, muscle relaxation, weight and genetics.  By the time we are in our 70s, most of us will suffer from the condition.  It could almost be considered a part of the aging condition.  Add in the genetic component and the weight factor, obstructive apnea can appear at a younger age.      


Three Predominate Components To

The Development Of Obstructive Sleep Apnea

Muscle relaxation – Increases with Age

Airway size – Genetics

Airway size – Weight

The combination of the above factors in some proportion leads to the development of obstructive sleep apnea for the majority of affected individuals.

It is best to think of Obstructive Sleep Apnea as a potential companion you will share for life.   Like grey hair and glasses for most, hearing aids for some, obstructive sleep apnea is in your future.  Once accepted and treated, it becomes just another of life’s companions and not a dreaded disease.  

RGH  June 6, 2022