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Dr. Collop is Professor of Medicine, Division of Pulmonary and Critical Care Medicine, University of Mississippi Medical Center.
Correspondence to: Nancy A. Collop, MD, FCCP, Professor of Medicine, Division of Pulmonary and Critical Care Medicine, University of Mississippi Medical Center, 2500 N. State St, Jackson, MS 39216; e-mail: ncollop{at}aol.com
John Gray’s famous book, Men Are From Mars, Women Are From Venus, focuses on the differences between men and women.1 This book deals with relationships between men and women using a metaphor that Martians (men) and Venusians (women) met, fell in love, and developed loving relationships; on moving to Earth, they forgot they were from different planets. Subsequently, their relationships deteriorated. Maybe the sexes are more different than we think, and maybe our understanding of disease states will benefit from closer examinations of the variations of specific diseases between the sexes.
In the context of Gray’s book, obstructive sleep apnea (OSA) is a Martian’s disease. The reason for this is not completely understood. The study in this issue of CHEST (see page 1442) by Mohensin compares upper-airway size utilizing the acoustic reflectance technique in a group of men and women referred to a university sleep center. Similar studies2 3 utilizing other imaging techniques in normal subjects have shown that although women have smaller pharynges when seated, on recumbency the differences between men and women disappear. The current study population, because they were a group of patients at risk for OSA, were more obese than patients in prior studies (mean body mass index [BMI] > 33 kg/m2). The authors showed that although the women had higher BMIs and smaller pharynges than the men, they had less severe OSA. Additionally, the size of the pharynx (< 3.2 cm2) correlated with the severity of OSA only in men. This article suggests there is something inherently different about the properties of the upper airway in men compared to women.
Collapsibility of the upper airway depends on its size, the surrounding muscle tone, and the characteristics of the tissue. All of these are interrelated: if the muscles that surround and dilate the airway tone are lax, the airway will narrow; if there is an increased amount of fat in surrounding tissues, the airway may be compressed. Other studies have investigated the gender differences related to muscle tone in the upper airway.4 5 Upper-airway resistance increases with sleep onset, and this results in a reduction in minute ventilation. Awake genioglossus electromyelogram (EMG) activity in women may be related to hormonal status (luteal vs follicular phase of the menstrual cycle vs postmenopausal state), difference does not seem to affect upper-airway resistance.4 There appears to be no significant difference in muscle tone in upper-airway dilators (genioglossus and tensor veli palatini) between men and women during sleep, although men have a higher upper-airway resistance.5 Unfortunately, these studies were done in normal subjects, so how this extrapolates to OSA patients is unclear.
Sex hormones have also been thought to influence the development of OSA. A recent study by Bixler et al6 showed that OSA was much more prevalent in postmenopausal women who were not receiving hormone replacement therapy (HRT), compared to postmenopausal women receiving HRT or premenopausal women. HRT has also been shown to decrease sleep-disordered breathing indexes (apnea/hypopnea index [AHI]) in postmenopausal female patients.7 Other reports8 9 have shown that OSA is more prevalent in women with androgen excess, for instance, polycystic ovary syndrome. New onset of OSA developed in a woman who received exogenous testosterone.9 This suggests testosterone may also play an important role.
Other investigators have examined the surrounding tissue in the upper airway. MRIs of the upper airway have shown differences between the sexes; nonobese male subjects were shown to have more fat in their necks compared to the rest of their bodies.10 Another study,11 looking specifically at physical examination findings, showed that male patients with a narrowing of the lateral pharyngeal walls and/or tonsil enlargement were at greater risk for OSA, but no specific physical examination finding was predictive of OSA in female subjects.
Polysomnography findings have also been shown to differ between men and women with OSA. Not only do women have lower AHI results than men, most of the difference in AHI between men and women occurs during nonrapid eye movement (NREM) sleep. The AHI in patients during rapid eye movement (REM) sleep is equal in men compared to women, suggesting that whatever protects women from upper airway collapse in NREM sleep disappears on entering REM sleep.12 13
Is there a common theme to explain the gender difference in OSA? The size of the airway seems to make a difference in men, and this may be an effect of a difference in muscle tone (lower in men) and tissue characteristics (floppier in men). The difference in tone may be abolished when REM sleep is entered, which would explain why the AHI difference between men and women disappears in REM sleep. Differences in tissue characteristics could not explain that change. Clearly, further studies are needed to elaborate on the differences between men and women. Identification of what protects women from more severe OSA could potentially lead to new treatments. As stated in an excerpt from Gray’s book1 : "When you remember that men are from Mars and women are from Venus, everything can be explained."
References
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