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Evidence already exists for the heritability of obstructive sleep apnea. The authors make reference to five published epidemiological studies, and describe one recently completed recently in which 75 out of 105 (71%) offspring of patients previously diagnosed with OSA had evidence of sleep disordered breathing themselves. Although the remaining 30 had no frank signs or symptoms of sleep disordered breathing, the authors thought that even these might show some hidden signs of being at risk for these disorders, and they explored this by comparing their responses to Inspiratory Resistive Loading (IRL) with those of healthy offspring of healthy parents.
IRL involves adding air resistance against the patient's effort to breathe. During waking, this is immediately offset by increased respiratory effort and breathing remains unchanged. During Non-REM sleep, the first breaths after application of IRL fail to increase respiratory efort, either of the diaphram or of the upper airway musculature, so that breathing volume is markedly decreased. There are large individual differences in this response during sleep, which might reflect individual differences in upper airway stability and thus the risk of developing OSA.
The authors studied 10 adult (average age 23, range 18-29) healthy offspring of fathers diagnosed with OSA (moderate to severe, AHI=21-66). These offspring were drawn from the previously mentioned group of 30 who had no evidence of sleep disordered breathing. They were compared with 14 offspring, matched for age and Body Mass Index (see Glossary), of healthy parents without OSA, at least on the basis of questionnaires and interviews of suspicious answers leading to polysomnographic testing to exclude affected individuals and their offspring.
During sleep study, the subjects breathed exclusively through a nosemask with a resistance device (a water-filled balloon centered in the inpiratory pathyway which could be filled to varying degrees to block airflow) which allowed expiration to go unimpeded while inspiration could be resisted to any desired level. Pressure and flow in this system were measured along with polysomnography during a night's sleep in which, during NREM sleep (predominantly stage 2), progressively increasing levels of inspiratory resistance were applied to find the level at which apnea or severe hypopnea would develop.
Before respiratory loading, both groups breathed in a similar manner, all free of apneas or hypopneas. All levels of IRL resulted in greater decreases in breathing in offspring of patients than in controls--39% (SD=10%) vs. 19% (SD=4%). Lower inspiratory pressure was required to elicit severe hypopnea in patient offspring than in controls (79 SD=20 vs. 153 SD=14 centimeters of water). Complete apneas without arousal occurred in three OSA offspring but no controls; three controls showed signs of arousal before the threshold for hypopnea was reached.
The authors concluded that these offspring of OSA fathers adapted less well than offspring of normals to resistive loading during sleep, with lower ventilatory flow at all levels of loading and lower levels of loading needed to elicit hypopneas. They thought the likeliest explanation lay in differences in the upper airway, perhaps more collapsible in the patient offspring. They noted that, whereas the usual risk of inheriting such "genetically based" disorders as hypertension and diabetes is les than 10-15% in first degree relatives, data so far indicating about 70% transmission in sleep apnea suggest that the number of "risk genes" for sleep apnea is rather small--and, I would add, the contribution of environmental factors rather less.
Most patients with sleep apnea who know something about it would easily accept the idea that it is heritable--after all, indications are that it is strongly based in anatomy, such as the configuration of the upper airway, and no one doubts the heritability of anatomical features. Obesity, one might think, could also mediate a family pattern of OSA; but in this study patient offspring were matched with offspring of healthy parents on body mass, yet showed a clearly distinctive pattern of respiratory impairment during sleep that could be brought out by resistive loading even though it was not apparent in any symptomatic way or during routine polysomnography.
What might well startle us all is to learn the extremely high risk of either overt OSA or its presumed preclinical risk factor, impaired response to resistive loading, apparently affecting the vast majority of children of fathers with OSA. But bear in mind that the 71% figure cited by the authors as representing the number of offspring affected by sleep disordered breathing does not represent quite the same thing as full-blown sleep apnea in all of them. However, those of us who have become informed about OSA have generally come away with a strong respect for its fundamentally physical nature, not easily modifiable by changes in sleep habits or even weight. This should arm us against people, including sometimes doctors, who tend to blame us for our symptoms--that they are due to overeating, not enough exercise, depression, laziness, whateveer. It should alert us to the high risk of our children developing, maybe much later in life, symptoms of the same disease. And perhaps we shouldn't limit our concern to our children. How many of us have parents who seem to have or have had problems with sleep apnea symptoms? What about their other children--our sisters and brothers--and theirs--our neices and nephews? It might be appropriate to educate family, as we ourselves become educated, in the signs, symptoms, complications, and treatment of sleep apnea--to hopefully enable much earlier intervention than most of us experienced, and perhaps prevention of some of the cardiovascular, respiratory, and central nervous system complications of this disease.
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