Idiosyncratic Reactions

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Idiosyncratic reactions most often begin within 20 minutes of contrast media injection (2). Their occurrence is not related to the dose of contrast material administered. They can be produced by the intravascular injection of even tiny amounts (less than 5 mL) of contrast material. Although the manifestations of idiosyncratic reactions are identical to those seen in patients having true anaphylactic reactions, reactions to contrast material are not true allergic reactions in the vast majority of patients (3,4). The formal definition of an allergic reaction requires prior exposure to the offending agent to sensitize the individual and the development of antibodies against the allergen (2). Neither of these requirements is routinely met in idiosyncratic contrast reactions. Hence idiosyncratic reactions to RICM have been termed anaphylactoid or allergic-like rather than anaphylactic or allergic reactions.

Typical idiosyncratic reactions to contrast media are urticaria (hives), diffuse cutaneous and subcutaneous edema (angioedema), upper airway (laryngeal) edema, bronchospasm, and hypotension with tachycardia. However, there is some overlap with nonidiosyncratic reactions. Subclinical bronchospasm and vasodilatation (which in pronounced cases may produce hypotension) may be detectable in many patients if carefully looked for; these manifestations are probably related to the direct physiologic effects of contrast media rather than an allergic-like response.

Patients at Risk

Certain groups of patients are more likely to have idiosyncratic reactions to RICM. Patients with a history of a prior contrast reaction have about four times the risk of an adverse reaction, and patients with a history of allergies or asthma have about two to three times the risk of an adverse reaction, compared to patients who do not have these histories (1). Shellfish allergy is not a "special" allergy with respect to contrast media injection but should be managed in the same way as other non-contrast allergies (e.g., peanut, bee sting, or penicillin allergies) (5).

In our experience, some patients relate a history of allergy, often to food, that probably is not a true allergy. For example, a stated allergy to milk may, on closer questioning, reflect lactase deficiency and inability to properly digest milk. Some stated food allergies really reflect an earlier episode of food poisoning. There is no evidence that these patients are at increased risk for contrast reactions.

Reducing Risk

Choice of Contrast Medium

Several studies have shown that the frequency of reaction in the general population receiving RICM is reduced when LOCM is administered rather than HOCM (1,6,7). There are approximately one-quarter to one-fifth as many reactions when LOCM is administered intravenously compared to when HOCM is administered intravenously. This reduction applies to idiosyncratic reactions taken as a whole and to the subset of severe reactions (1). Furthermore, in addition to reducing risk in the general population, LOCM reduces risk in patients who are at higher risk for contrast reactions, such as those discussed in the previous section (1,7).

There is insufficient data to determine if iso-osmolality contrast media may reduce the rate of acute idiosyncratic reactions even further than the low rate observed with administration of LOCM. The size of the population that would have to be tested to produce a study with sufficient power to detect a small but important difference is prohibitive. However, if a patient has had a prior severe reaction to a specific contrast agent, switching to another brand of LOCM or to iso-osmolality contrast medium makes common sense even if the utility of such a change in contrast agents has not been scientifically proved.

Premedication

To reduce the risk of an idiosyncratic contrast reaction in high-risk patients, preme-dication with corticosteroids and H1 antihistamines has been recommended. Other drugs, such as H2 antihistamines and ephedrine, can be added to the regimen but are not widely used (2,8).

Premedication is generally recommended for patients who have a history of: an allergic-like reaction to contrast media; other true allergies, particularly if multiple and/or severe; and true asthma, particularly with frequent or severe attacks. It is also recommended for asthmatic patients who are currently or who have recently been symptomatic. Various premedication protocols have been proposed for high-risk patients. Ours [with the drug schedule adapted from Refs. 9 and 10] is as follows.

For oral administration prior to an elective examination: prednisone 50 mg per oral 13, 7, and 1 hour before the examination and diphenhydramine 50 mg per oral (or i.m.) 1 hour before the examination. Patients should be cautioned not to drive automobiles or perform possibly dangerous tasks after taking diphenhydramine, because it can lead to drowsiness.

For intravenous administration in urgent situations or if the patient cannot take oral medications, the following regimen has been recommended: hydrocortisone 200 mg i.v. stat and q 4 hour until the examination is complete (four total doses for an elective examination) and diphenhydramine 50 mg intravenously one hour before the procedure.

Universal steroid premedication for all patients, regardless of risk status, has been proposed for both HOCM (11) and LOCM (12) administration. When populations as a whole (regardless of risk factors) were premedicated with steroids, rates of reactions fell by approximately one-third to one-half. Although the study (12) involving LOCM administration did not have enough patients to draw statistically significant conclusions about moderate or severe reactions, it did show that the total number of reactions was reduced by steroid administration. Universal steroid preme-dication for all patients prior to RICM administration has not been widely adopted, however, probably because of logistical problems and the relatively low frequency of acute idiosyncratic reactions to LOCM (13).

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