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Non-renal adverse reactions to contrast media: separating fact from myth

Based on my personal experience, there are few drugs which are associated with as many myths and misunderstandings as contrast media. During more than 30 years working with contrast media, I have collected a number of myths and misunderstandings about non-renal adverse reactions. I have selected eight of the most frequent myths of recent years to discuss here, because this type of misinformation has the potential to lead to significant harm to patients. It is essential that radiologists have a correct knowledge of the classification of adverse reactions and a clear understanding of their associated risk factors, pathophysiology, prevention, and treatment so that they can manage patients undergoing contrast-enhanced magnetic resonance (MR) and computed tomography (CT) imaging properly. 1. Myth: Gadolinium-based contrast media never cause acute adverse reactions and therefore it is unnecessary to be prepared to recognize or treat such reactions. Fact: Clinical trials and experience have shown that, in general, gadolinium-based contrast agents produce relatively few acute reactions (1). However, although allergylike reactions occur after ,1% of gadolinium-based contrast agent administrations, acute reactions do occur. These reactions are the same as those seen after iodine-based contrast agents. In most cases they are mild, but a few severe anaphylactoid reactions have been reported. Allergy-like reactions to gadolinium-based contrast media may occur despite pretreatment with corticosteroid and antihistamine. 2. Myth: If the patient has had previous contrast agent administration without incident, they will not have an adverse reaction in the future. Fact: A previous reaction to an iodine-based contrast medium is the most important patient factor predisposing to an acute idiosyncratic reaction (2). With ionic agents, the risk of a reaction in a patient who reacted previously has been reported to be 16–35% and to be 11 times greater than the risk in a non-reactor. When a patient who previously reacted to an ionic agent is given a non-ionic agent, the risk of a repeat reaction is reduced to approximately 5%. However, the fact that a patient previously tolerated contrast medium administration does not mean that they will not have an acute adverse reaction when they receive contrast medium on another occasion. 3. Myth: Certain contrast agents are associated with lower risks of severe hypersensitivity reactions than others, i.e. the severity of a hypersensitivity reaction, and its outcome, depend on which contrast agent is given. Fact: No significant difference in the incidence of acute non-renal adverse reactions between the various nonionic iodine-based contrast media has ever been reported (3–5). Nor have differences in reaction rates been found between the different extracellular gadolinium-based contrast media despite the considerable differences in osmolality between the different agents. The rate is lower after gadolinium-based contrast media than after iodinebased contrast agents (5). 4. Myth: Reducing the dose of a contrast agent will reduce the risk of a hypersensitivity reaction. Fact: Acute idiosyncratic systemic reactions (also described as allergy-like or anaphylactoid) are defined as unpredictable reactions which occur within 1 h of contrast medium administration, and which are unrelated to the amount of contrast medium above a certain level (2). This definition aims to distinguish them from chemotoxic reactions, which are dose-related and dependent on the physicochemical properties of the contrast medium. 5. Myth: People with an allergy to seafood are at higher risk of an allergic reaction to iodine-based contrast agents because seafood contains iodine. Fact: Allergy to foodstuffs which contain iodine, such as seafood, often causes particular anxiety (6–8). However, the available data suggest that allergy to seafood is not associated with a greater risk of acute adverse reactions than allergy to any other foodstuff. 6. Myth: It is not important to distinguish between a serious allergic/anaphylactoid reaction (endogenous histamine release, edema, hypotension, and secondary tachycardia), a vasovagal reaction (endogenous vagal nerve stimulation, bradycardia, and secondary hypotension), and a panic attack (endogenous adrenaline release, tachycardia, and hypertension) since they are treated the same – just give oxygen, fluids, antihistamines, and steroids – and have the same consequences. Fact: The vast majority of patients with severe anaphylactoid-type reactions recover if they are treated quickly and appropriately (9). However a correct diagnosis of the type of reaction is essential if appropriate treatment is to be given. Talking to the patient while checking their pulse rate provides useful initial information: breathing can be assessed, bradycardia suggesting a vagal reaction can be detected and an estimate of systolic blood pressure can be made. Important first-line management includes establishing an adequate airway, giving supplementary oxygen and intravascular physiological fluids and measuring the blood pressure and heart rate. A diagnosis of an anaphylactoid reaction will be made if there is, for example, severe vomiting, marked urticaria, bronchospasm, facial or laryngeal edema, and/or hypotension and tachycardia, and intramuscular adrenaline should be administered. A patient who has hypotension and bradycardia and is having a vasovagal attack should be given atropine, not adrenaline. Endogenous adrenaline release is already occurring in a patient having a panic attack, so they should not be