Acetylcholine-induced whealing in cholinergic urticaria - What does it tell us?

BACKGROUND: Cholinergic urticaria (CholU) is characterized by the occurrence of itchy wheals induced by sweating. Intradermal injections of acetylcholine (ACh) have been proposed to help with diagnosing CholU and subgrouping of patients, but controlled studies are largely missing.

OBJECTIVE: To compare the rates of positive ACh test results in well characterized CholU patients and controls and to identify clinical features of CholU linked to ACh reactivity.

METHODS: Acetylcholine was injected intradermally into 38 CholU patients and 73 matched healthy controls. Wheal and flare skin responses were assessed after 15 and 30 min and correlated with clinical features of CholU.

RESULTS: At 15 min after intradermal injections of ACh, wheal and flare responses were significantly more frequent in CholU patients than healthy controls, wheals: 34 % vs.15% (P = 0.028); flares: 50 % vs.18 % (P <0.001). Also, wheals were 37 % and flares 172 % larger and of longer duration in CholU patients than in healthy controls (both P < 0.01). CholU patients with ACh-induced wheals (ACh+ ) had larger flare but not wheal responses in response to histamine than those without (ACh- ; P = 0.011). Also, ACh-induced wheal responses were significantly correlated with sweating (r = 0.54, P = 0.046) in CholU patients. Finally, wheal responses lasted longer in ACh+ than in ACh- patients (P = 0.03).

CONCLUSION: Intradermal ACh testing does not allow for the identification of CholU patients due to its low sensitivity. ACh-induced wheals, in patients with CholU, is linked to sweating and longer lasting symptoms. Intradermal ACh testing is an interesting tool for mechanistic studies in CholU.