Prevention of anaphylactoid reactions in high-risk patients receiving radiographic contrast media.

OBJECTIVE: To review various pretreatment regimens for the prophylaxis of anaphylactoid reactions to radiographic contrast media (RCM) in high-risk patients. The proposed etiologies and risk factors for such reactions are also reviewed.

DATA SOURCES: A MEDLINE search of the English-language literature was used to identify pertinent human studies and reviews.

STUDY SELECTION: All studies comparing pretreatment regimens for anaphylactoid reactions to RCM were reviewed as well as studies comparing the incidence of anaphylactoid reactions between lower and higher osmolar RCM.

DATA SYNTHESIS: The two types of reactions to RCM are dose-independent, unpredictable anaphylactoid (pseudoallergic or idiosyncratic) reactions and the dose-dependent, predictable physicochemical (intrinsic, nonidiosyncratic) reactions. Prophylaxis of the former type is targeted at stemming the effects of certain chemical mediators, primarily histamine. The use of lower osmolar RCM is associated with a lower incidence of anaphylactoid reactions compared with higher osmolar RCM, but is significantly more expensive. Risk factors for such reactions are a history of previous anaphylactoid reaction to RCM, asthma, and reaction to skin allergens or penicillin. Discontinuation of any beta-blockers before the procedure is suggested. Pretesting patients with a small amount of RCM has little predictive value for an anaphylactoid reaction. Various pretreatment prophylactic regimens have been studied. Almost all included a corticosteroid to target the inflammatory response and a histamine1 (H1)-antagonist to blunt the effects of histamine. In some clinical trials, ephedrine was added for bronchodilation and cimetidine for its antagonism at the histamine2-receptor. The few controlled clinical trials that have been performed show the combination of prednisone and diphenhydramine to be most beneficial in preventing anaphylactoid reactions to RCM. The addition of ephedrine or cimetidine to a pretreatment regimen remains controversial.

CONCLUSIONS: More controlled clinical studies comparing various pretreatment regimens for high-risk patients need to be performed, especially in patients receiving lower osmolar RCM. Recommendations for high-risk patients who must receive RCM include use of a lower osmolar agent, pretreatment with a corticosteroid and an H1-antagonist, discontinuation of beta-blockers if the patient is taking any, and bedside availability of appropriate medications and equipment to treat anaphylaxis.