The Mayo Clinic Proceedings recently had a review of adverse reactions to the various types and routes of injection of the most common iodinated contrast agents used in obtaining X-rays. Contrast media are usually safe, but severe reactions can occur.
The injection sites – in order of increasing risk for reaction – include into tissue spaces or cavities, into an artery and into a vein.
The two main causes for acute reactions are allergic and anaphylactoid, with the latter accounting for the vast majority of events.
The allergic mechanism involves the allergic antibody IgE, and requires that a person have some previous exposure to contrast media that can lead to the development of the allergy. The anaphylactoid mechanism involves activation of defensive complement proteins by the immune system, and requires no previous exposure to cause an adverse event.
Regardless of the mechanism, the symptoms are the same: rash, hives, angioedema, wheezing and possible shock.
Risk factors identify people with the greatest risk of adverse reactions: a history of asthma, a history of a prior reaction to contrast media and a history of being an allergic individual. Persons who are at higher risk should be pre-medicated. A pre-treatment regimen of medications that has had the best success in preventing reactions includes giving corticosteroids in several doses, starting the day before the X-ray test. Next, two types of antihistamines are given shortly before the test – an H-1 blocker such as Benadryl and an H-2 blocker such as Zantac.
Use of this three-pronged prevention eliminates most reactions or lessens their severity.