Features of aquagenic urticaria:
(1) pruritic, perifollicular hives (pin-head wheals with surrounding erythematous flare)
(2) onset a few minutes after contact with water and lasting about 1 hour
(3) associated with a local release of acetylcholine in the skin (inhibited by hyoscine) with mast cell degranulation and increase in blood histamine concentration
(4) suppressed by pretreatment with scopolamine
(5) challenge testing involves placement of water compresses at body temperature (37°C)
Features of aquagenic pruritus:
(1) intense skin itching
(2) no visible skin changes
(3) brief contact with water at any temperature
(4) associated with a local release of acetylcholine in the skin (inhibited by hyoscine) with mast cell degranulation and increase in blood histamine concentration
(5) responsive to anti-histamines
Differential diagnosis:
(1) thermal (cold or heat related) urticaria
(2) solar urticaria
(3) chemical-associated urticaria
(4) dermatographia (with wheals caused by towelling or showering)
A patient with aquagenic urticaria may have other physical urticaria.
