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This is basically a review article leading to a theory which I will focus on, instead of detailing the individual studies reviewed.
Upper airway muscles--the pharyngeal dilator muscles--help keep the airway open despite the negative pressures created in the airway by the act of breathing in. These muscles relax to some degree in everyone at sleep onset. If the airway is smaller, as in sleep apnea, this may lead to blockage. The patient's body responds by increasing activity in these muscles to reopen the airway. In fact, in sleep apneics the activity of these muscles is usually higher than normal, whether in waking or sleep, so that further increases of muscle tone require extreme activity. Increasing muscle tone and activity in any part of the bodycan have the helpful effect of increasing muscle strength, but it can also have the harmful effect of causing muscular abnormalities, such as inflammation. Even the normal process of enlargement of muscles that accompanies increases in their strength can have the harmful effect of adding to the blockage of the airway in sleep apneics. Eventually, the entire structure of the muscle can be disrupted by excessive activity--especially when the muscle is contracting but at the same time being stretched by overwhelming opposite forces. This may in turn lead to a deterioration in muscle function and worsening of the apnea.
Muscle injury caused by excessive activity is well-known, in marathon runners as well as sleep apneics. This has been shown to exist in microscopic examination of upper airway muscles in sleep apnea. This has been done with biopsies and with post-mortem examinations of human tissue, but the English Bulldog as a model of human sleep apnea lends itself especially well to experimental studies. Some of the changes in muscles observed have included muscle overgrowth, fibrosis, pathological changes in the muscle fibers, change in type of muscle fibers, and increases in enzymes characteristic of injury.
So, some of the muscular changes can be considered adaptive and others maladaptive in terms of increasing muscle strength, but both kinds of change could contribute to eventual worsening of sleep apnea. There is not enough data on long-term course of sleep apnea yet to say whether this long-term process of worsening is typical, but at least there is some to suggest that over time patients progress from simple snoring to mild obstructive sleep apnea to more severe forms. This evaluation of muscle changes in the upper airway may offer one explanation for this course of events.
It isn't hard to understand that, as more work is demanded of muscles anywhere in the body, they become larger and stronger. So it may be with the upper airway dilator muscles (like the genioglossus and myohyoid muscles) that must become more active to keep the airway open in sleep apnea or open it up again after an apeic event. It is less obvious, but equally believable, that extreme demands made on muscles may also cause inflammation and other damage to them.What may be harder to follow is the idea that in sleep apnea, the muscles that keep the airway open may be both larger and more active than in normals, yet by their own bulk obstruct the airway.Also, it may be less than obvous that long-term overexertion may eventually lead to a decline in muscular effectiveness, again worsening the airway obstruction. The authors have drawn evidence from various studies to support each part of the picture they have drawn, one which would predict a long-term worsening of obstruction in sleep apneics. In some cases, this may be true, but we have also seen evidence for a "burn-out" of apnea later in life. Furthermore, it may be hoped that relief of excessive demands on upper airway muscles as a result of CPAP and other treatment measures may allow the muscles time to recover--if not overly damaged before the treatment began. This consideration would lead us to be especially concerned about the lengthy delays of many years between initial symptoms and eventual diagnosis and treatment in many patients currently showing up at sleep clinics, and to hope that the future will more of their children detected and showing up for apnea treatment earlier in the course of the disease.
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