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Reactions to iodinated contrast
Distinguish an allergic-like reaction (urticaria, bronchospasm, laryngeal oedema, hypotension with a fast pulse) from a physiologic one (nausea, warmth, a vasovagal reaction with a slow pulse) — the first is treated with intramuscular adrenaline, the second with leg elevation, fluids and atropine.
Why it matters in the exam
The exam almost always presents a scenario: a man who received contrast for CT develops hoarseness and difficulty breathing within minutes, or facial swelling, or a fall in blood pressure — and the question is the first treatment. Hoarseness and stridor mean laryngeal oedema: oxygen and intramuscular adrenaline (1 mg/mL, 0.5 mg in an adult). Moderate bronchospasm: oxygen, an inhaled bronchodilator, and adrenaline if there is no improvement or the reaction is severe. Hypotension with a fast pulse is anaphylaxis: leg elevation, large-volume fluids and intramuscular adrenaline. Hypotension with a slow pulse is a vasovagal reaction: leg elevation, fluids, and atropine 0.6–1 mg IV up to 3 mg.
The second theme is who is at risk. The strongest risk factor is a previous allergic-like reaction to contrast of the same class; next come asthma and multiple allergies. "Shellfish allergy" or "iodine allergy" is not a specific risk factor and does not require premedication. Beta-blockers do not increase the frequency of reactions but make them harder to treat. With low-osmolality agents an allergic-like reaction occurs in about 0.6% and a severe reaction in about 0.04%.
ACR premedication for an elective patient with a previous reaction: prednisone 50 mg by mouth 13 hours, 7 hours and 1 hour ahead of the study, and diphenhydramine 50 mg an hour before. In an urgent setting: methylprednisolone 40 mg or hydrocortisone 200 mg IV every 4 hours until the examination, and diphenhydramine 50 mg IV an hour before — preferably with at least 4–5 hours of steroids. Premedication does not reliably prevent a reaction ("breakthrough" reaction) and is not intended for physiologic reactions. Delayed reactions (an hour to a week) are mostly a transient maculopapular rash. Contrast extravasation is managed in most cases with limb elevation, compresses and observation; refer to a plastic surgeon when there is worsening pain, blistering, impaired perfusion or altered sensation.
Key points
- The strongest risk factor — a previous reaction to the same class of agent.
- Laryngeal oedema and severe bronchospasm — intramuscular adrenaline.
- Low pressure and fast pulse — anaphylaxis; slow pulse — vasovagal.
- Elective premedication: prednisone 50 mg 13, 7 and 1 hours before.
- Extravasation — conservative management in most cases.
Summary table
| Reaction | Signs | First treatment | Note |
|---|---|---|---|
| Mild urticaria | Limited itchy rash | Observation; diphenhydramine as needed | No adrenaline needed |
| Bronchospasm | Wheeze, falling saturation | Oxygen, bronchodilator; intramuscular adrenaline if severe | Monitor saturation |
| Laryngeal oedema | Hoarseness, stridor, choking sensation | Oxygen and adrenaline 0.5 mg intramuscularly | Do not wait for an antihistamine |
| Anaphylaxis | Hypotension with a fast pulse | Leg elevation, fluids, intramuscular adrenaline | Call the resuscitation team |
| Vasovagal reaction | Hypotension with a slow pulse | Leg elevation, fluids, atropine 0.6–1 mg IV | Up to 3 mg atropine |
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Sources
- ACR Manual on Contrast Media, 2024 — Allergic-like and physiologic reactions to intravascular iodinated contrast media; Treatment of acute reactions
- ESUR Guidelines on Contrast Agents, version 10.0 (2018) — Acute adverse reactions; Late adverse reactions; Extravasation
- World Allergy Organization Anaphylaxis Guidance, 2020 — Initial treatment with intramuscular epinephrine
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.