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The Myth of Cross-Reactivity Between Shellfish Allergy and Iodine Contrast Media

Many people believe that you’ll be at higher risk of iodine contrast allergy if you have shellfish allergy, due to the presence of iodine in both. This is a misconception.

Iodine contrast media (ICM) are chemical agents used most commonly in radiology to increase the visibility of organs, joints, and blood vessels during medical X-ray procedures. There is some misunderstanding about the role that shellfish allergy plays to cause an intolerance to iodine-containing agents.

Even though a small amount of iodine is found in shellfish, it doesn’t cause shellfish allergy. Iodine is not regarded as an allergen. Moreover, it is essential for producing thyroid hormones that control many functions in the body such as metabolism, growth, and development. Tropomyosin, a protein from shellfish muscle tissues is primarily responsible for shellfish allergy.

A study published in the Journal of Emergency Medicine detailed that, “Allergies to shellfish, in particular, do not increase the risk of reaction to intravenous contrast any more that of other allergies.” People who have any food allergy other than shellfish have the same risk of having an adverse reaction to iodinated contrast media.

Contrast media reactions are not true allergic reactions. They don’t require the release of IgE antibodies for hypersensitivity to occur. Contrast agent molecules are far too small to trigger immunoglobulin E release. Iodine contrast hypersensitivity is a result of a direct breakdown of mast cells and basophils whereas shellfish allergy is IgE-mediated.

There are two main types of contrast media: ionic and non-ionic. Iodine molecule is found in both. Ionic agents tend to have more side effects than non-ionic. In non-ionic iodinated contrast media, the iodine is bound to an organic (non-ionic) compound and causes a low osmolality (lower concentration of charged particles) in the blood that results in lower toxicity.

Many of the side effects are due to ionic type contrast that creates more charged particles, therefore more toxic and prone to causing a potentially life-threatening reaction in high-risk patients. The non-ionic contrast media are more widely used today.

Contrast media reactions can be mild, moderate and severe. Reactions largely occur within five minutes of administering contrast.

Mild reactions include warmth, nausea, vomiting, limited urticaria, and fainting. Symptoms are self-resolving and last only for a short period.

Moderate reactions include urticaria, vomiting, sweating, rash, and drop in heart rate, and blood pressure. They often require drug treatment and a period of observation before you are cleared to go home.

Severe reactions include rash, seizures, and severe bronchospasms and are treated with emergency drugs such as adrenaline.

Risk Factors

Risk factors exist for patients with previous intravenous contrast reactions, true allergies to other medications, environmental allergens, and food. Asthma and cardiovascular diseases increase risk. Anxiety is also linked to contrast reactions. Those taking beta-blockers or metformin are also at risk. There’s a higher incidence rate of adverse reactions among females and seniors. However, there is no evidence supporting cross-reactivity between shellfish allergy and anaphylactoid reactions to contrast media.

If you suspect you could have an adverse reaction to iodine contrast media, talk to a specialist about the proper diagnosis, prevention and treatment.

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