Cognitive factors mediate placebo responses in patients with house dust mite allergy.

BACKGROUND: Placebo effects have been reported in type I allergic reactions. However the neuropsychological mechanisms steering placebo responses in allergies are largely unknown. The study analyzed whether and to what extend a conditioned placebo response is affecting type I allergic reactions and whether this response can be reproduced at multiple occasions. METHODS: 62 patients with house dust mite allergy were randomly allocated to either a conditioned (n = 25), sham-conditioned (n = 25) or natural history (n = 12) group. During the learning phase (acquisition), patients in the conditioned group received the H1-receptor antagonist desloratadine (5mg) (unconditioned stimulus/US) together with a novel tasting gustatory stimulus (conditioned stimulus/CS). Patients in the sham-conditioned control group received the CS together with a placebo pill. After a wash out time of 9 days patients in the conditioned and sham-conditioned group received placebo pills together with the CS during evocation. Allergic responses documented by wheal size after skin prick test and symptom scores after nasal provocation were analyzed at baseline, after last desloratadine treatment and after the 1(st) and 5(th) CS re-exposure. RESULTS: Both conditioned and sham-conditioned patients showed significantly decreased wheal sizes after the 1(st) CS-evocation and significantly decreased symptom scores after the 1(st) as well as after the 5(th) evocation compared to the natural history control group. CONCLUSIONS: These results indicate that placebo responses in type I allergy are not primarily mediated by learning processes, but seemed to be induced by cognitive factors such as patients' expectation, with these effects not restricted to a single evocation

Experimental design.

<p>During acquisition, patients in the conditioned group received desloratadine (US) in combination with the CS. During evocation, the drug was replaced by placebo pills. Sham-conditioned patients received the CS together with placebo pills throughout the study. During 3 subsequent days during acquisition (2–4) and evocation (17–19) patients were instructed to intake the pills together with the drink (CS) at home.</p

Sociodemographic and psychological characteristics.

<p>BDI = Becks Depression Inventory; STAI X2 = Trait anxiety form of the State-trait anxiety inventory.</p>*<p>Results of Chi² Test or univariate ANOVA.</p

Conditioned and sham-conditioned patients show reduced allergic reactions during evocation.

<p><b>A</b> After skin prick test wheal size (mm) was analyzed before and after acquisition as well as before and after the 1^st and 5^th evocation to the CS in patients in the conditioned, sham-conditioned as well as patients in the natural history group. <b>B</b> Symptom scores after nasal provocation were analyzed before and after acquisition as well as before and after the 1^st and 5^th evocation to the CS in patients in the conditioned, sham-conditioned as well as patients in natural history group. Data are presented as mean ±SEM. *p<0.05 **p<0.01 ***p<0.001, comparison against natural history group.</p