ETIOLOGY OF CONTRAST REACTIONS
There are 2 basic types of contrast reactions; the first is the anaphylactoid or idiosyncratic, and the second is the nonanaphylactoid. Contrast reactions may occur from either one or a combination of both of these effects.
Anaphylactoid/Idiosyncratic Reactions
As the name suggests, the exact etiology for these reactions is less well understood, and they tend to mimic an anaphylactic (allergic) reaction. The proposed mechanism of these reactions includes enzyme induction, causing the release of vasoactive substances such as histamine and serotonin and the activation of a physiologic cascade and eventually the complement system.
These are the most frequent type of adverse reactions and may have serious, occasionally fatal, complications. These reactions are more frequent in patients with asthma (5 times), patients with previous reactions (4–6 times), patients with cardiovascular and renal disease, and individuals on β-blockers.
Anxiety, apprehension, and fear may play a part in this type of reaction. Such reactions usually begin within 20 min of injection and are independent of the dose administered.
Symptoms associated with anaphylactoid reactions are classified as mild (skin rash, itching, nasal discharge, nausea, and vomiting), moderate (persistence of mild symptoms, facial or laryngeal edema, bronchospasm, dyspnea, tachycardia, or bradycardia), and severe (life-threatening arrhythmias, hypotension, overt bronchospasm, laryngeal edema, pulmonary edema, seizure, syncope, and death).
Nonanaphylactoid Reactions
Nonanaphylactoid reactions are also called physiochemotoxic or nonidiosyncratic reactions. These reactions are believed to result from the ability of the contrast media to upset the homeostasis of the body, especially the blood circulation.
These reactions are dependent on the physical properties of the contrast medium such as ionicity (which causes free ions in the circulation, which in turn may disrupt electrical charges associated with nervous or cardiac activity) and osmolality (which causes large shifts in fluid volumes).
Increasing iodine concentration also increases the risk of these reactions. Finally, the volume and route of administration of contrast also increase the likelihood of such reactions (larger volume or intraarterial administration are more likely to produce a reaction).
The cardiovascular, respiratory, urinary, gastrointestinal, and nervous systems are most commonly affected by physiologic changes produced by contrast media. The symptoms of nonanaphylactoid reactions are warmth, metallic taste, nausea, vomiting, bradycardia, hypotension, vasovagal reactions, neuropathy, and delayed reactions.