VHL patients undergo many imaging studies requiring injected and oral contrast media, and some patients develop allergies to these contrast agents. There are many myths and misunderstandings surrounding contrast allergies leading to wide variations in policies among different imaging centers and hospitals. The goal of this short article is to dispel some of these myths.

Allergies can occur with any contrast agent, whether injected into a vein or ingested orally, whether for CT scanning or MRI. Allergies are more common with the CT contrast agents (“iodinated” contrast) than with MR contrast agents (“gadolinium” contrast) but they occur in both. There are different manufacturers of both iodinated and gadolinium contrast. Some people are allergic to one particular brand of contrast agent and not to another in the same class. Some people are born with allergies to contrast agents and some develop them over time, even after many years of reaction-free exposure to contrast media.

Question: You say that allergies to shellfish are not related, yet at every scan I am asked about allergies to shellfish. Why is this?

Answer: This is an urban legend that will not go away. Not all allergies are the same. There are five basic types of allergies and some very rare additional types that will not be discussed. Nor will I discuss medical treatment of these reactions. A reference is provided for this purpose.

The five basic types of contrast reactions are:

Skin hives: reddened splotchy areas on the skin (face, trunk, arms) that develop soon after injection. They can be very itchy and uncomfortable and take several days to go away.
Throat tightening or laryngeal edema: This is an internal hive that affects the breathing tube causing difficulty breathing. This can range from mild throat “tightening” to a severe “choking” sensation. It is often accompanied by swelling of the face.
Asthma: This reaction involves the airways in the lungs and results in wheezing. Asthmatics are more prone to this reaction.
Shock: This is a systemic reaction to contrast media where the blood pressure decreases suddenly and other serious symptoms occur. This is a medical emergency
Delayed: Some patients experience headaches, muscle pains and flu-like symptoms up to 48 hours after contrast media. This is a self limited reaction.

Skin hives and minimal throat tightening are the most common reactions and are considered mild reactions. They can usually be prevented with oral steroids and an antihistamine (benadryl) taken beforehand. Even if there is a “breakthrough” reaction it will be mild and therefore the risks associated with contrast exposure are very low. To put this in perspective approximately 2% of patients getting iodinated contrast media and fewer than 1% of patients getting gadolinium contrast media will develop this type of reaction.

The other reactions are considered more serious but fortunately are much less common (by a factor of 10). Reactions leading to death occur in about 1:250,000 patients. However, if you have had a moderate or severe reaction you obviously do not want to experience it again. So, most patients opt for another kind of study. For instance, if you are allergic to CT contrast then you would switch to MRI for your studies and vice versa.

Most adverse events occurring after contrast media are NOT allergies. Feeling warm, flushed, nauseous and even vomiting are side effects of the drug, not allergic reactions. Some people are very sensitive to contrast, but not allergic. It usually helps to slow the rate of administration of the contrast media; this will usually reduce symptoms.

Also, contrast media, especially iodinated contrast media can damage the kidneys, a process known as “contrast nephrotoxicity”. Predisposing risk factors for contrast nephrotoxicity are previous renal diseases, diabetes and hypertension. If you have had much of your kidneys removed surgically, your doctors will often change from CT to MRI to reduce the risk of contrast nephrotoxicity. Finally, allergies to “iodine” or shellfish are not related to allergy to contrast agents.

If you have developed an allergy to contrast media then there are some simple things you can do:

Always find out and write down what type of contrast and what brand name you were allergic to. It may be possible to avoid that brand in the future. I know patients who are allergic to one brand of gadolinium or iodinated contrast media but not another brand.
If the reaction was mild you can take a simple “pretreatment” or “prophylaxis” consisting of several pills of steroids and benadryl before the exam to prevent reactions.
If the reaction was moderate or severe you can change to another kind of scan using another class of contrast agents.
If the reaction was severe but your doctors still feel you need the injection of that kind of contrast then you should be pretreated but also have anesthesiologists or other emergency health personnel “standing by” to treat the reaction if it occurs. However, in reality, this is hardly ever done. In my career of 20 years I have seen it done only once. There are almost always safer alternatives.

It is still unknown why reactions occur in some people and not in others. Some doctors have devoted their whole careers to trying to understand the answer to this question. Indeed, there are some reactions that I do not discuss here that are very difficult to explain. Fortunately, modern contrast agents have very low reaction rates, and this article will not be relevant to most patients with VHL. If you do have a contrast reaction I hope these comments have shed some light on the subject.

Question: We have heard you say that oral contrast should be used for abdominal CT’s along with the intravenous dye. Yet my hospital declines to administer the oral contrast, saying they feel it is not necessary. What do you recommend and why?

Answer: Oral contrast is important for identifying the bowel and differentiating it from adjacent structures. If the bowel is not opacified by oral contrast it can be misinterpreted as a mass. One added benefit of oral contrast is that it hydrates the patient and this lessens the chance of contrast nephrotoxicity and rids the body of the contrast media faster. Some centers use water as an oral contrast agent instead of conventional oral contrast. This is an acceptable alternative to oral contrast media since it hydrates the patient and also provides a “negative” contrast. I do not feel strongly about what kind of oral contrast you take, just as long as you take something.

References:
1. Thomsen HS. “Guidelines for contrast media from the European society of urogenital radiology.” AJR 2003; 181:1463-71
2. www.chestx-ray.com/Practice/Contrast.html

As printed in the VHL Family Forum 12:4, December 2004. For permission to reprint, please contact VHL Family Alliance, editor@vhl.org. Further information is available from the VHL Family Alliance, info@vhl.org.