The itchy, watery eyes that plague allergy sufferers may be, at least in part, a reflex response triggered by the nose, a new study shows.

Researchers have previously demonstrated that when an allergen is introduced into one nostril, the other nostril will respond, demonstrating what’s known as a “nasonasal reflex,” Dr. Fuad M. Baroody and colleagues from the University of Chicago explain in their report in the Annals of Allergy, Asthma & Immunology.

Given that the eyes and nasal cavity share a nerve supply, they add, it’s possible that ocular hay fever symptoms aren’t only related to direct exposure to the allergy-producing substance, but could also be a result of a similar reflex response.

To investigate, the researchers studied the reactions of 20 people, who were allergic to grass or ragweed, under experimental conditions.

Each participant underwent a “challenge” with an extract of grass or ragweed placed in the nose. In a subsequent experiment, the researchers first sprayed a placebo or antihistamine into the nose, and then challenged them again with the amount of allergen that had produced a response in the previous experiment. The subjects then switched from placebo to antihistamine and vice versa, and were tested again.

When they had been given antihistamine before allergen exposure, symptoms were reduced in both nostrils, and eye symptoms also were reduced - suggesting that histamine release in the nose helps trigger the eye reflex.

While there is plenty of evidence that putting allergen into a person’s eye will produce symptoms, the researchers say, the amount of allergen that eyes are typically exposed to in real life is probably much smaller than the amount they inhale into the nasal passages.

Furthermore, the fact that corticosteroids administered via the nose can also reduce eye symptoms provides additional evidence that eye symptoms are reflex-related, they add.

“The eye symptoms associated with allergic rhinitis probably arise, in part, from a naso-ocular reflex,” the researchers conclude.

SOURCE: Annals of Allergy, Asthma & Immunology, March 2008.