Radiopaque contrast agents are often used in radiography and fluoroscopy to help delineate borders between tissues with similar radiodensity. Most contrast agents are iodine based.

Iodinated contrast agents may be

·        Ionic

·        Nonionic

Ionic contrast agents, which are salts, are hyperosmolar to blood. These agents should not be used for myelography or in injections that may enter the spinal canal (because neurotoxicity is a risk) or the bronchial tree (because pulmonary edema is a risk).

Non-ionic contrast agents are low-osmolar (but still hyperosmolar relative to blood) or iso-osmolar (with the same osmolarity as blood). Newer nonionic contrast agents are now routinely used at nearly all institutions because they have fewer adverse effects.

The most serious contrast reactions are

·        Allergic-type reactions

·        Contrast nephropathy (renal damage after intravascular injection of a contrast agent)

Allergic-type contrast reactions

Reactions vary in severity:

·        Mild (eg, cough, itching, nasal congestion)

·        Moderate (eg, dyspnea, wheezing, slight changes in pulse or blood pressure)

·        Severe (eg, respiratory distress, arrhythmias such as bradycardia, seizures, shock, cardiopulmonary arrest)

The mechanism is anaphylactoid; risk factors include the following:

·        A previous reaction to injected contrast agents

·        Asthma/Allergies

Treatment begins by stopping contrast infusion.

1.      For mild or moderate reactions, diphenhydramine 25 to 50 mg IV is usually effective.

2.      For severe reactions, treatment depends on the type of reaction and may include oxygen, epinephrine, IV fluids, and possibly atropine (for bradycardia).

In patients at high risk of contrast reactions, imaging tests that do not require iodinated contrast should be used. If contrast is necessary, a nonionic agent should be used, and patients should be premedicated with prednisone (50 mg orally 12 hours, 7 hours, and 1 hour before injection of contrast) and diphenhydramine (50 mg IV, IM, or orally 1 hour before contrast administration).

If patients require imaging immediately, they can be given diphenhydramine 50 mg IV, IM, or orally 1 hour before injection of contrast and hydrocortisone 200 mg IV every 4 hours until the study is performed, preferably deferring imaging, if possible, until at least 2 doses of hydrocortisone have been administered.