Update on Prevalence
Similarly to all allergic diseases, SAC and PAC have seen a worldwide trend toward increased prevalence over the past few decades. For example, increases in SAC and PAC in children have been well documented by the International Study of Asthma and Allergies in Childhood (ISAAC).[12,13] These earlier studies suggested a difference between children from western countries and those from more rural, less developed countries: although their overall health may be worse, the risk of developing allergic disease was substantially lower than that seen in Europe, North America, or Japan. A recent synthesis of the ISAAC data clarifies this: although the general trend is valid, high prevalence of allergic conjunctivitis and allergic rhinitis in Latin America and West Africa is somewhat inconsistent. Still other recent studies show that although prevalence among children in many countries may have stabilized, nations undergoing substantial economic growth report spikes in the prevalence of allergic conjunctivitis or symptoms associated with rhinitis or allergic conjunctivitis. A recent report comparing the prevalence of allergic conjunctivitis, allergic rhinitis, eczema, and asthma in Croatian children found significant increases in all between 2001 and 2010. The prevalence of allergic conjunctivitis in that time period went from 6.7 to 15.3%. In Brazil, the ISAAC Phase III data in 2006 revealed the prevalence of rhinoconjunctivitis at 12%, while a report in 2012 estimated the prevalence of allergic conjunctivitis at 20.7%. Similar values (6.6% increase in allergic conjunctivitis and 8.1% increase in allergic rhinitis) were noted in studies that repeated the ISAAC survey paradigm 10 years after Phase III in South Africa. Of equal importance are the studies establishing the impact of allergies on the quality of life and on scholastic achievement, confirming the need for effective pediatric therapies.
In most prevalence studies, there is rarely a clear distinction between allergic rhinitis and allergic conjunctivitis; in fact, many studies consider allergic conjunctivitis to be primarily a condition secondary to allergic rhinitis. A recent summary of the anatomical and functional relationship between the two disorders, however, serves to highlight the distinction between the two. In addition, most survey data suggest that allergic conjunctivitis is often underreported by patients. Despite this, patients also list 'ocular itchiness' as one of the most disruptive symptoms they associate with their allergic disease. This underreporting and lack of distinction between nasal and ocular symptoms impact our ability to interpret the results from clinical studies, as discussed in the following text.
Another factor in the rise in the prevalence of allergic disease that has received considerable attention in recent literature is the role of air pollution as an exacerbating factor in allergic signs and symptoms. Here, the focus may be on allergic asthma, but there are many new studies that have elucidated mechanistic aspects of what some have referred to as urban allergy. In preclinical studies, exposure of conjunctival epithelium to hydrocarbons and automotive exhaust particulates stimulated synthesis and secretion of cytokines including chemotactic peptides (which act to recruit inflammatory cells), IL-4, and IL-10.[22,23] In addition, several studies[24,25] have shown that airborne automotive particulates can stimulate the IL-17 inflammatory pathway that is associated with chronic allergic conjunctivitis and allergic rhinitis.
Although laboratory studies provide new clues to potential roles of pollutants in allergy, other studies provide valuable information on the real-world exposure levels. The researchers at the University of British Columbia, Vancouver, British Columbia, Canada, have used remote sensing technology to generate global estimates of fine airborne particulate matter (<2.5 μm) and ozone with high resolution. They estimate that 89% of the world's population lives in areas where the levels of particulate matter (2.5) exceed the WHO guidelines for air quality. By comparing remote sensing data from 1990 with data from 2005, they found a '6% overall increase in particulate matter (2.5) and a 1% decrease in global population-weighted ozone concentrations was apparent, highlighted by increased concentrations in East, South, and Southeast Asia and decreased concentrations in North America and Europe'. This shift follows the pattern of increases in allergic diseases in these regions.
Curr Opin Allergy Clin Immunol. 2014;14(5):451-456. © 2014 Lippincott Williams & Wilkins