A 38-year-old woman with a history of asthma and multiple drug allergies requires a CT scan with intravenous contrast. Which of the following is the most common type of adverse reaction to iodinated contrast media that occurs in patients with atopy and prior contrast reactions?

Explanation

## Most Common Hypersensitivity Reaction to Iodinated Contrast Media ### Immediate Hypersensitivity (Type I) Reaction **Key Point:** Immediate hypersensitivity reactions (Type I, IgE-mediated) are the most common allergic adverse reactions to iodinated contrast media, occurring within minutes of administration. **High-Yield:** True IgE-mediated reactions to iodinated contrast occur in 1–3% of the general population but in up to 8–10% of patients with prior contrast reactions or significant atopy (asthma, eczema, allergic rhinitis). ### Classification of Contrast Reactions by Timing and Mechanism | Reaction Type | Timing | Mechanism | Frequency | Severity | |---|---|---|---|---| | **Immediate hypersensitivity (Type I)** | Seconds to minutes | IgE-mediated mast cell/basophil degranulation | 1–3% (8–10% if atopic) | Mild to severe; can be anaphylaxis | | Delayed hypersensitivity (Type IV) | Hours to days | T-cell mediated | <1% | Mild to moderate (rash, fever) | | Serum sickness-like | 1–2 weeks | Immune complex deposition | <0.1% | Mild to moderate | | Fixed drug eruption | Hours to days | T-cell mediated at specific site | <0.1% | Mild (localized lesion) | ### Clinical Presentation of Type I Reactions **Mild manifestations:** - Urticaria, pruritus, flushing - Mild angioedema (lips, face) - Nausea, vomiting **Moderate manifestations:** - Severe angioedema - Bronchospasm, wheezing - Hypotension (mild) - Tachycardia **Severe manifestations (anaphylaxis):** - Cardiovascular collapse, shock - Severe bronchospasm, laryngeal edema - Loss of consciousness - Occurs in <0.1% but is a medical emergency **Clinical Pearl:** This patient has TWO major risk factors for Type I reactions: asthma (increases risk 5–10 fold) and prior contrast reactions (increases risk 10–fold). She should receive premedication with corticosteroids and antihistamines before any contrast administration. ### Premedication Protocol for High-Risk Patients **Standard premedication regimen:** 1. Prednisone 50 mg PO at 13, 7, and 1 hour before procedure (or methylprednisolone 32 mg IV 15 min before) 2. Diphenhydramine 50 mg PO or IV 1 hour before procedure 3. Consider H₂ blocker (ranitidine 50 mg IV or famotidine 20 mg IV) 1 hour before **Efficacy:** Premedication reduces repeat reaction risk from 8–10% to <1%. **Mnemonic: ASTHMA risk** — Atopy, Serum sickness history, Thyroid disease, History of contrast reaction, Multiple allergies, Asthma ### Osmolality and Reaction Risk - **High-osmolar contrast media (HOCM):** 1500–1860 mOsm/kg → 5–8% reaction rate - **Low-osmolar contrast media (LOCM):** 600–850 mOsm/kg → 1–3% reaction rate - **Iso-osmolar contrast media (IOCM):** 290 mOsm/kg → similar to LOCM **Tip:** In this high-risk patient, use LOCM or IOCM and always have emergency medications (epinephrine, IV access, oxygen) immediately available.