Shellfish Allergy Doesn't Predict Reaction to Imaging Agents

Pam Harrison

March 19, 2013

Patients who are allergic to shellfish are no more likely to react to radiocontrast materials used in imaging than those with other allergies or asthma, according to a new study.

An estimated 10 million radiocontrast-material-facilitated procedures are performed each year in the United States, and only about 0.01% to 0.40% result in severe immediate reactions. The vast majority of reactions are mild to moderate in nature.

Patients with allergies are 2 to 3 times more likely to have a reaction to imaging agents than those with without allergy, said Joelle Borhart, MD, from Georgetown University in Washington, DC. She presented the study results at the 19th Annual American Academy of Emergency Medicine Scientific Assembly.

Dr. Joelle Borhart

Dr. Borhart pointed out that studies from the 1970s show that up to 15% of patients who had an acute reaction to radiocontrast materials had a history of seafood allergy.

But so did virtually identical numbers of patients who reported allergies to foods other than shellfish and those who reported having asthma.

"For some reason, those with shellfish allergy were separated out as special and unique; that's probably how the myth got started," Dr. Borhart told Medscape Medical News. "We're partially to blame for perpetuating this myth because we keep asking patients about it, even though there is no evidence to support us doing that."

We keep asking patients about [shellfish allergy], even though there is no evidence to support us doing that.

In fact, a survey of 113 radiologists and cardiologists found that 65% of radiologists and 89% of cardiologists asked patients if they had a history of shellfish allergy prior to giving them radiocontrast materials (Am J Med. 2008;121:158.e1-4). In addition, more than one third of radiologists and half of cardiologists indicated that they would withhold treatment if patients were allergic to shellfish.

"When I was doing my residency, I had a conversation with my attending physician who told me he found it strange that people had asked him if he was allergic to seafood before he underwent cardiac catheterization," explained Andrew Beaty, MD, from the Allergy and Asthma Centers of the Metroplex in Keller, Texas, who was involved with that survey.

Because of that, "we sent a questionnaire to academic medical centers where we thought people would be on top of what's current in medicine and who were using radiocontrast materials regularly." It turned out that they "were still asking patients about seafood allergies," he told Medscape Medical News.

Dr. Beaty noted that "the younger generation of physicians may be turning around on this, [but] once something gets ingrained in medicine, it takes a while to ungrain it — even when it's outdated and archaic."

The other half of the myth is that most patients with shellfish allergy think they are allergic to the iodine in shellfish, Dr. Borhart said.

"Shellfish are high in iodine, but that's not what's triggering the reaction," she noted. "People are reacting to tropomyosins, which are muscle proteins in shellfish. You can't be allergic to iodine," she explained.

It is not known whether reactions to radiocontrast materials are mediated by the classic immunoglobulin E or by nonallergic mechanisms. "They are clinically similar," Dr. Borhart pointed out, "and treatment is the same — epinephrine (1:1000) 0.3 mg IM, plus antihistamines and steroids."

Prevention is ideal, Dr. Borhart noted. However, there is no clear evidence that premedication with either corticosteroids or antihistamines prevents severe reactions to radiocontrast materials.

Only 12-hour protocols have been shown to reduce the incidence of allergic reactions, but even these only helped in mild to moderate cases; they did not help in the most severe cases.

The only prevention strategy that does reduce allergic reactions to radiocontrast materials is the use of nonionic low-osmolar agents. Dr. Borhart explained that for patients with any kind of food allergy or asthma, such agents should be used.

Another common myth in the emergency department pertains to the risk for necrosis if epinephrine plus lidocaine are used for a digital block.

Purported Necrosis Risk

This myth comes from a small number of case studies in which anesthetics other than lidocaine were used. No additional cases of finger necrosis have been reported in association with epinephrine and lidocaine use, Dr. Borhart reported.

"You do not want to use epinephrine with lidocaine in patients with Raynaud's disease or in those with otherwise compromised blood flow in their fingers," Saeed Chowdhry, MD, from the University of Louisville in Kentucky, told Medscape Medical News. However, "in the appropriate setting and in the appropriate patient, I think it's a safe technique."

He was involved in a literature review of the technique (Plast Reconstr Surg. 2010;126:2031-2034). His team looked at 1111 patients who received 1% plain lidocaine (2 to 10 cc) injections for surgery on the hands and fingers, and 611 patients who received injections of 1% lidocaine plus epinephrine (0.5 to 10 cc).

No patients in the epinephrine group developed gangrene, and there were no complications associated with the use of epinephrine in digital blocks.

In addition, a randomized controlled study of 60 digital blocks, 31 of which used epinephrine plus lidocaine, reported no complications from the combination (Plast Reconstr Surg. 2001;107:393-397).

In another study of 50 digital bocks, 21 with epinephrine and lidocaine, there was a decreased need for redosing and tourniquet use and the onset of anesthesia was faster (Plast Reconstr Surg. 2003;111:1769-1770).

Dr. Borhart pointed out that the combination should be used with caution in patients with poor perfusion. However, "evidence supports the safety of using lidocaine with epinephrine in the fingers, toes, and nose," she concluded.

The investigators, Dr. Beaty, and Dr. Chowdhry have disclosed no relevant financial relationships.

The 19th Annual American Academy of Emergency Medicine Scientific Assembly. Presented February 11, 2013.