Radiocontrast Media Hypersensitivity

Radiocontrast media hypersensitivity is an immediate or delayed reaction to iodinated (or, rarely, gadolinium based) contrast agents. Most reactions are mild, and severe reactions are uncommon with modern low osmolar agents.

Pathophysiology. Immediate reactions (within 1 hour) are mostly non-IgE mast cell activation, with some true IgE-mediated allergy. Delayed reactions (1 hour to 7 days) are T cell mediated, appearing as a benign rash, and rarely as severe cutaneous reactions.

Immediate reactions occur in about 0.5 to 3% of administrations with low osmolar contrast, and fatal reactions are very rare. A previous reaction raises the chance of recurrence about fivefold.

Allergy to shellfish or iodine is not a risk factor specific to contrast. Asthma, beta blocker use and mast cell disease raise the severity of reactions.

• Immediate: flushing, urticaria, itching, angio oedema, nausea
• Bronchospasm, laryngeal oedema, hypotension, collapse
• Delayed (1 to 3 days): maculopapular rash, itching, delayed urticaria
• Severe delayed reactions: DRESS, Stevens-Johnson syndrome, toxic epidermal necrolysis
• Contrast induced kidney injury and thyroid effects (separate, non-allergic)
• Reaction on first exposure, since a prior exposure is not needed

• History: agent, route, time to onset, features, treatment needed, and prior contrast exposures.
• Serum tryptase within 1 to 2 hours of a reaction, with a baseline later.
• Skin prick and intradermal tests with the culprit and alternative contrast agents (undiluted for prick, diluted 1 in 10 for intradermal) 4 to 6 weeks after the reaction: a negative test with an alternative agent supports its use. Delayed reactions need delayed reading intradermal and patch tests.
• Specific IgE is not available for iodinated contrast.
• Do not test or challenge after a severe cutaneous reaction.

Differential diagnosis.

• Vasovagal reaction and anxiety
• Contrast extravasation or toxicity
• Cardiac event, pulmonary embolism, or arrhythmia
• Mast cell disorder
• Drug rash from co-administered drugs
• Contrast induced kidney injury

• Acute reaction: stop the injection, give oxygen, and for anaphylaxis adrenaline 0.5 mg intramuscularly (children 0.01 mg/kg, maximum 0.5 mg) into the anterolateral thigh, repeated after 5 minutes if no improvement, plus intravenous fluids. Observe for at least 6 hours. Mild urticaria: chlorphenamine 10 mg intravenously or orally.
• After a prior moderate or severe immediate reaction: if the study is needed, switch to a different non-ionic contrast agent with a different structure (preferably with a negative skin test), and give premedication. An example adult regimen is prednisolone 50 mg orally at 13, 7 and 1 hours before, plus chlorphenamine 10 mg orally or intravenously 1 hour before.
• After a prior mild reaction: use a different agent, and premedication is optional.
• Prevention in unselected patients: premedication is not routine, since it does not prevent most reactions.
• Delayed benign rash: avoid the culprit and use a different agent, with antihistamine and topical corticosteroid for the rash.
• Severe cutaneous reaction: avoid all iodinated contrast permanently unless essential, and then only after specialist testing.
• Gadolinium agents: hypersensitivity is rare, and an agent with a different structure is chosen after a reaction.
• Medical alert with the culprit agent named.

Special groups.

• Patients on beta blockers: severe reactions are harder to treat, so glucagon 1 to 2 mg intravenously may be needed for adrenaline resistant shock.
• Patients with shellfish allergy: no special precaution is needed.

Monitoring. Observe for at least 30 minutes after contrast in patients with prior reactions, and review 24 to 72 hours later for delayed rash.

Referral and escalation. Refer to allergy for any moderate to severe reaction, for testing and planning of future contrast studies. Emergency for airway swelling or hypotension.