Diagnostic dilemma: Teenager's hives turned out to be caused by rare water allergy

The patient: A teenage girl in Canada

The symptoms: The teen was referred to a clinic for recurring outbreaks of hives, which appeared whenever her skin came into contact with water. She had hives at the time of her clinic visit; the inflamed bumps were surrounded by patches of red skin measuring about 0.4 to 1.2 inches (1 to 3 centimeters) wide.

What happened next: The patient told physicians that her first hive outbreak in response to water happened about two years earlier, soon after she began menstruating. Following that, whenever her skin got wet — regardless of the water's temperature or the type of water — welts appeared.

She developed hives within 20 minutes of bathing or showering; swimming in pools or in the ocean; or after getting soaked by rain. The hives and redness would then disappear without any treatment after about 30 to 60 minutes. Sweat or tears on her skin did not trigger her symptoms, she told the doctors.

Hives, also known as urticaria, are typically triggered by an allergic reaction to something that the affected person has touched or eaten. When the allergen enters the body, immune cells release a chemical called histamine that launches an immune response that can include itchy hives.

There was no history of unexplained hives in the patient's family, nor did she report other symptoms that sometimes accompany allergic reactions, such as dizziness, wheezing or shortness of breath, her doctors wrote in a report of the case. Laboratory tests of the patient's blood and urine showed that all of her vital functions were within normal ranges.

She was allergic to dust, rabbits and cats, but these known allergens did not overlap with her bouts of hives after water exposure.

The diagnosis: The doctors prescribed a week-long course of antihistamines to the patient. Then, they discontinued the medication and performed a provocation test, which involved deliberately exposing her skin to the suspected allergen by placing a cloth saturated with room-temperature water onto her abdomen. Welts became visible within 20 minutes.

Based on this result and the patient's history, the physicians diagnosed her with aquagenic urticaria — a rare allergy to water.

The treatment: The girl's mother had previously tried to manage her daughter's hives with montelukast, which treats and prevents asthma symptoms, such as swelling and inflammation of the airways. The medication moderately improved the teen's symptoms, but it did not entirely resolve them.

Doctors instead prescribed her a daily dose of the antihistamine cetirizine, which targets the cause of hives and a range of other allergic symptoms, such as sneezing and itchiness in the eyes, nose and throat. During a follow-up visit eight months later, the patient reported that her symptoms returned only if she missed a dose of the antihistamine.

Her condition continued to improve over time. When she returned for her 14-month follow-up appointment, she said she was able to take part in normal daily activities "without any restrictions" and that her quality of life was unaffected by the allergy as long as she continued to take the cetirizine, according to the report.

What makes the case unique: Researchers have described only 100 cases of aquagenic urticaria to date. Its cause is unknown, although the condition generally appears at the onset of puberty and is thought to affect women more frequently than men.

This allergic response is difficult to diagnose because it can be challenging to isolate water as the cause of the hives, given that hives can also be triggered by pressure, extreme temperatures and exercise, as well as a range of allergens.

In this case, the patient's detailed medical history, along with the results of the provocation test, enabled doctors to successfully pinpoint the cause of her unusual condition.

For more intriguing medical cases, check out our Diagnostic Dilemma archives.