Thank you for your inquiry.
Cold urticaria can result in systemic reactions including anaphylaxis (1, 2). In a series of 30 children with this condition, a third had experienced episodes of anaphylaxis (3). To my knowledge, there is no known "cutoff" temperature at which the threshold for an anaphylactic event occurs. In my personal experience, this may vary from patient to patient. We do know that the factors that determine whether or not anaphylaxis will occur are the degree of temperature, the duration of exposure, and the percent of the body exposed. Swimming is perhaps the most common trigger of anaphylactic events in patients with cold urticaria. Importantly, fatalities have been recorded (1).
For these reasons it is normally recommended that patients with cold urticaria never enter a body of water without first testing the temperature, and if there is any question that the temperature is cool enough to precipitate a reaction, they avoid exposure. As noted, there is no clear-cut established threshold temperature, but it is often recommended that patients avoid temperatures lower than 25 degrees Centigrade (2). In addition, in situations where there may be generalized cold exposure, patients should be prescribed an automatic epinephrine injector. Unfortunately, there is no strong evidence that prophylactic antihistamines will prevent episodes of anaphylaxis due to systemic cold exposure.
I am not aware of any study that has looked at the efficacy of a wetsuit in preventing cold urticaria, and could not find any reports of this strategy in the literature. Therefore I would be reluctant to be more permissive about systemic exposure to cold based upon the use of a wetsuit.
Finally, antihistamines, as you are using, are the cornerstone of treatment for cold urticaria patients, and sometimes higher than "normal" doses are required. Potential side effects such as drowsiness of course must be weighed against the benefit of adding additional antihistamine treatment. However, you could certainly increase the dose of fexofenadine to an adult dosage with little risk. If symptoms were still bothersome, you might try a low dose of doxepin (10 mg qd to b.i.d.) if it did not make her too drowsy.
In summary, I would discourage swimming in any circumstance in which the temperature of the lake or pool was lower than 25 degrees Centigrade, or any activity which might cause systemic exposure (such as boating). I would also supply your patient with an automatic epinephrine injector. The present antihistamine therapy that you are using is certainly appropriate, but you might increase the dose as mentioned above if symptoms were not adequately controlled.
Thank you again for your inquiry and we hope this response is helpful to you.
1. Wanderer AA, et al. Clinical characteristics of cold-induced systemic reactions in acquired cold urticaria syndromes: Recommendations for prevention of this complication and a proposal for a diagnostic classification of cold urticaria. J Allergy Clin Immunol 1986; 78:417-423.
2. Isk S, et al. Idiopathic cold urticaria and anaphylaxis. Pediatr Emerg Care 2014; 30(1):38-39.
3. Alangari AA, et al. Clinical Features and Anaphylaxis in Children with Cold Urticaria. Pediatrics 2014; 113:313-317.
Phil Lieberman, M.D.