Recent epidemiologic studies demonstrate that the prevalences of allergic diseases are increasing significantly. The exact reasons are not known, although a number of possible risk factors have been suggested as explanations. Importantly, there is a strong association between rhinitis and asthma, and evidence that nasal dysfunction may have significant effects on the lower airway. This article reviews the epidemiology of allergic rhinitis and asthma, focusing on the relationship between these two conditions.
EPIDEMIOLOGY OF ALLERGIC RHINITIS AND ASTHMA
The prevalences of asthma and rhinitis have increased worldwide during the past 20 to 30 years, and the increase has been greatest in children, teenagers, and young adults. In the United States, the prevalence of pediatric asthma changed from 3% to 4% between 1981 and 1988.1 A large study of military conscripts in Sweden demonstrated that the prevalence of asthma increased from 2% to 5% between 1971 and 1981, whereas the prevalence of rhinitis increased from 6% to 12%.2 Even greater increases were observed in Finnish adolescents, as asthma prevalence rose from 1% to 3% and rhinitis prevalence increased from 5% to 15% between 1977 and 1991. 3 In England and Scotland, the prevalence of asthma in children increased from approximately 3% in 1982 to 6% in 1985 and was nearly 9% in 1992.4
RISK FACTORS FOR ALLERGIC AIRWAY DISEASE
A number of risk factors play important roles in atopic conditions. These include a family history of atopy, exposure to tobacco smoke, allergen sensitization, air quality, and viral infections.
A positive family history confers up to a fourfold increased risk of having asthma and a sixfold increased risk of having allergic rhinitis.5 Importantly, a family history of a particular allergic disease does not limit the risk to that condition only.
Exposure to indoor tobacco smoke during infancy has been an important risk factor for both asthma and allergic sensitization.6 Although self smoking is a significant risk factor for asthma in adulthood, maternal smoking is most critical for the presentation of asthma in young children.
Sensitization to airborne allergens appears to play a key role in the development of allergic diseases in children. Allergen exposure may be most important early in life, when the immune system is most vulnerable to the environment. Allergens that seem to be most strongly related to subsequent asthma include house dust mites, cockroach antigens, and cat dander/ whereas seasonal allergens are less important. Conversely, the risk of allergic rhinitis appears to be most highly associated with early life exposure to seasonal pollens.8
Indoor air quality, especially high indoor humidity, has been shown to be an important risk factor for asthma.9 Similarly, outdoor air pollution may play a role in both asthma and rhinitis. For example, a study of adolescents in Bochum, Germany, found traffic density correlated significantly with the prevalences of wheezing and allergic rhinitis.10 Despite this suggestive evidence, it has been difficult to definitively link discrete levels of atmospheric pollutants (eg, particulates) to the prevalences of allergic diseases.
A clear relationship between viral infections and the development of asthma and atopy has not been established. There is evidence that infections during infancy may play roles in the sensitization process or, conversely, may protect against the development of atopy.11,12
EPIDEMIOLOGIC ASSOCIATION BETWEEN ALLERGIC RHINITIS AND ASTHMA
A large number of cross-sectional studies have demonstrated that allergic rhinitis and asthma commonly occur together. Many report that nasal symptoms occur in up to 80% of children and adolescents with asthma,13"15 as compared with a 5% to 20% incidence in the general population.16 Generally, these studies did not use standardized interview questionnaires or sensitive instruments for detecting rhinitis. A more recent study used a standardized questionnaire for 478 patients and demonstrated that rhinitis is ubiquitous in patients with asthma, occurring in 93% of such adolescents.17 Conversely, asthma has been shown to afflict up to 38% of patients with allergic rhinitis.13,15 This is much higher than the 3% to 5% prevalence in the general population.18
The onsets of rhinitis and asthma also appear to be temporally related. A number of studies in children and adolescents have shown that upper airway symptoms either preceded or started at the same time as asthma in most patients with both conditions.15'17,19'20 Although these studies suggest that rhinitis often begins before asthma, prospective studies are required to accurately assess the risk of asthma in patients who have rhinitis alone. Settipane et al. prospectively observed 694 college freshmen who had no evidence of bronchial asthma at the onset of the study.21 Students who reported nasal symptoms in 1961 had asthma three times more often (10.5%) than did individuals without rhinitis (3.6%) during the subsequent 23 years.
Although patients with rhinitis appear to be more likely to have asthma on average, it has not been possible to predict which patients in this group are at greatest risk for lower airway symptoms. Patients with allergic rhinitis and no clinical evidence of asthma frequently exhibit bronchial hyperresponsiveness to bronchoconstrictor agents such as methacholine or histamine.22'23 This high incidence of hyperresponsiveness has caused some investigators to postulate that bronchial hyperreactivity may represent an intermediate phase between nasal allergy and symptomatic asthma. Although at least two small investigations suggested that lower airway hyperresponsiveness conferred a higher risk for symptoms and signs of asthma,24,25 future prospective studies will be needed to confirm these findings.
ALLERGIC RHINITIS IS RELATED TO ASTHMA SEVERITY
Recent pharmacoeconomic studies have attempted to correlate rhinitis with both asthma severity and health care costs attributable to asthma. Huse et al. compared patients who had significant nasal allergy with those who had mild or no symptoms of nasal disease among 1,261 patients with asthma.26 Patients with more severe rhinitis were much more likely to have nocturnal awakening due to asthma (19.6% vs 11.8%), "moderate to severe asthma" as defined by the National Asthma Education Program (60.2% vs 51.2%), or work loss related to asthma (24.1% vs 21.1%). Similarly, Halpern et al. observed that patients with asthma who had symptomatic rhinitis used more asthma medications, particularly more inhaled corticosteroids and supplemental oral corticosteroids, compared with those who did not have asthma.27
Based on these observations, significant allergic rhinitis seems to be related to asthma severity and the requirement for more potent anti-asthma medications. Although these data suggest that rhinitis itself may contribute to asthma severity, an alternative explanation may be that nasal inflammation is associated with worse asthma because it is only a marker for increased dysfunction of the entire respiratory tract.
Allergic respiratory diseases are increasing throughout the world, and are related to a number of important risk factors. Rhinitis and asthma coexist to a great extent, and nasal disease may serve as a risk factor for the later development of asthma. Future studies will determine whether early interventions for allergic rhinitis will affect the development and expression of bronchial asthma.
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