Iodine, Seafood allergy and IV contrast

• Adverse reactions occur in 4% to 13% of patients who are administered radiocontrast media.
• These reactions long have been thought to be associated with allergy to seafood perhaps due to iodine.
• However, studies have documented no association between seafood allergy and reactions to radiocontrast media.
• Rather, the reaction to radiocontrast media is caused by the hypertonic nature of the media.
• Fatal reactions usually occur in patients between the ages of 20 and 50 years at a rate ranging from 1 in 100,000 to 170,000.
• These reactions are not thought to be mediated by immunoglobulin E. Instead, it is believed that the osmolality/hypertonicity of the radiocontrast media causes degranulation of the mast cells and basophils, releasing their mediators.

For most patients undergoing contrast studies, including the boy described in the vignette, no preventive therapy is needed prior to the study because the risk of anaphylaxis is low (0.04% to 0.36%). 

The administration of antihistamines has been shown to be beneficial for patients who have a history of adverse reactions to radiocontrast media and need repeat studies. The following regimen is employed:

1. prednisone 1 mg/kg up to 50 mg at 13 hours, 7 hours, and 1 hour
before the procedure;

2. diphenhydramine 1 mg/kg up to 50 mg 1 hour before the procedure;

3. in some cases, ephedrine 25 mg 1 hour before the procedure;

4. use of low-osmolality radiocontrast media.

This combination has decreased the risk of reaction to 0.7% to 3.1% and the fatality rate to 0 to 0.04%.

Pretreating a child with antihistamines and corticosteroids 1 week prior to the study is too long before the procedure. Pretreatment with only antihistamines is not adequate. There is no allergy testing for radiocontrast media because the trigger is the hypertonicity of the media. Evaluating for a shrimp allergy is not helpful because the adverse reactions are unrelated.

References:
deShazo RD, Kemp SF. Allergic reactions to drugs and biologic agents.
JAMA. 1997;278:1895-1906
Lieberman P. Anaphylaxis and anaphylactoid reactions. In: Middleton E
Jr, Reed CE, Ellis EF, Adkinson NF Jr, Yunginger JW, Busse WW, eds.
Allergy: Principles and Practice. 5th ed. St Louis, Mo: Mosby-Year
Book, Inc; 1998:1079-1092