Gender differences in asthma

There are clear-cut gender differences in the prevalence and natural history of childhood asthma. These differences have been well described over the past 20 years, although the reasons for them are less well investigated. Boys have a greater prevalence of doctor diagnosed asthma than girls, and this relative male predominance is maximal before puberty. ' If incidence rates are examined, the equalization of incidence by gender is related to puberty.2 After the age of 20 years, asthma prevalence remains approximately equal between the two genders with, if anything, a slight female pred~minance. ~ These trends in the prevalence and incidence of asthma have been confirmed in the recent prevalence survey conducted in Switzerland and reported in this issue of Pediatric Pulrnonol~gy. ~ What is of even greater interest is that the authors also reported on the prevalence of various asthma related symptoms and their prevalence by gender for children ages 7, 12, and 15. The symptoms examined included wheeze, exercise-induced symptoms, allergeninduced symptoms, morning chest tightness, and chronic night cough. What was most interesting was that the reported prevalence of these symptoms revealed a much less striking male predominance that was also age related. In fact, certain symptoms (such as night cough) had a male predominance at age 7, an equal male/female prevalence at age 12, and a female predominance at age 15 years.4 In a second and related paper,5 these same investigators examined the prevalence of bronchodilator treatment in the same group of children; they found that although the presence of any asthma symptom was roughly equivalent in males and females at the three ages, bronchodilator use was less frequent in females than males at all three ages and was significantly below the male rate of bronchodilator use for the 7 and 12 year olds. ' This was largely related to the difference in diagnostic labeling. Children with a diagnostic label of asthma were more likely to receive treatment than children without such a label. When this was examined by symptom, the greatest difference was for night cough, which was the most common symptom in girls. Only bronchodilator prescribing was assessed, not anti-inflammatory prescriptions. The lack of complete characterization of the prescribing patterns of the physicians relative to the symptoms reported precludes an assessment of the appropriateness of therapy in these subjects. Nevertheless, the reduced diagnostic labeling of girls with night cough raises the possibility of reduced recognition of asthma symptoms in girls. What could be going on to explain both the gender differences in symptoms and treatment for asthma? First, the patterns of symptom reporting could be influenced by true biological differences between boys and girls. There are two intermediate phenotypes that are relevant to asthma: increased airway responsiveness and atopy . Atopy is clearly more common in boys than in girls, although this difference equalizes after puberty.6 Increased airway responsiveness is less clearly male predominant and, when normalized for baseline lung function and lung size, excessive airway responsiveness appears similar in male and female children. Boys have smaller airway diameters relative to their lung volume (dysanapsis) which may predispose them from a mechanical point of view to more airway obstruction than girls. As a result, boys may be more likely to manifest the classic symptom of asthma, namely wheeze. * It remains unclear whether relative or absolute airway size is the primary determinant of symptoms. Absolute FEV, is equal in boys and girls by age 6 , yet wheeze is still much more common in boys than girls at that age. Since relative airway size is still reduced in boys relative to girls at this age, it suggests that, at least before puberty, relative airway size is the predominant correlate of the classic symptom of asthma, namely wheeze. It is worth noting that environmental exposures can influence the gender differences in relative airway and lung size. Maternal cigarette smoke exposure in utero can lead to smaller airways relative to lung volume in girls but not in boys, which may potentially confound the normal pattern of gender differences in nonexposed children . 9 Mechanical factors are not the only ones which explain the gender differences in asthmdsymptodtreatment prevalences by gender. Although the prevalence of atopy is greater in boys than in girls before puberty, the degree of airway inflammation in girls for a given allergen exposure could be greater, leading to a classical clinical presentation in boys but ultimately milder disease, as more male symptomatology could be related to anatomic factors rather than inflammation. With increasing age allergy, rather than mechanical factors, becomes the predominant factor influencing symptoms. Girls may also