A 62-year-old man with hypertension is started on lisinopril 10 mg daily. After 3 weeks, he develops a persistent dry cough and mild angioedema of the lips. All of the following statements about this adverse reaction are CORRECT EXCEPT:

Explanation

## ACE Inhibitor–Induced Angioedema and Cough: Classification & Mechanism ### Clinical Presentation **Key Point:** ACE inhibitor–induced cough and angioedema are classic Type B (idiosyncratic) adverse reactions. - Cough: occurs in 5–10% of patients taking ACE inhibitors - Angioedema: occurs in 0.1–0.5% of patients - Both are unpredictable and dose-independent - More common in African Americans, smokers, and females ### Mechanism: NOT IgE-Mediated ```mermaid flowchart TD A[ACE Inhibitor]:::action --> B[Inhibits ACE enzyme]:::action B --> C[Reduced degradation of bradykinin]:::outcome C --> D[Bradykinin accumulation]:::outcome D --> E[Activation of B2 receptors on mast cells]:::action E --> F[Mast cell degranulation]:::action F --> G[Release of histamine, prostaglandins, leukotrienes]:::outcome G --> H[Cough + Angioedema]:::urgent ``` **High-Yield:** This is a **non-IgE-mediated** reaction. It is NOT a classical hypersensitivity reaction (Type I, II, III, or IV). Instead, it is a **pharmacological side effect** that results from the known mechanism of ACE inhibitors (bradykinin accumulation). ### Classification Paradox | Feature | Type A | Type B | ACE-I Reaction | |---|---|---|---| | Dose-dependent | Yes | No | No | | Predictable | Yes | No | No | | Mechanism known | Yes | No | Yes (bradykinin) | | IgE-mediated | No | Often | No | | Reversible | Yes | Yes | Yes | **Clinical Pearl:** Although ACE inhibitor–induced cough and angioedema are classified as Type B reactions (because they are unpredictable and occur in susceptible individuals), they are actually **pharmacologically explainable** — not truly idiosyncratic. The mechanism (bradykinin accumulation) is well-understood, which technically makes them quasi-Type A. However, because they occur unpredictably in only a subset of patients (not dose-dependent), they are conventionally classified as Type B. ### Why Option 4 is INCORRECT **Key Point:** ACE inhibitor–induced angioedema is **NOT IgE-mediated**. It is a bradykinin-mediated reaction, not a classical allergic/hypersensitivity reaction. - ~~IgE-dependent hypersensitivity~~ — WRONG. No IgE involvement. - ~~Occurs in the majority of patients~~ — WRONG. Occurs in only 0.1–0.5% of patients taking ACE inhibitors. Both claims in Option 4 are false; this is the exception. ### Management 1. **Discontinue ACE inhibitor** — first-line management 2. **Symptoms resolve** — within 1–4 weeks of stopping the drug 3. **Switch to alternative** — ARB (angiotensin II receptor blocker) is safe; bradykinin-mediated side effects do not occur with ARBs because they do not inhibit ACE **Warning:** Do NOT confuse ACE inhibitor–induced angioedema with anaphylaxis. Anaphylaxis is IgE-mediated, acute, and life-threatening; ACE inhibitor–induced angioedema is non-IgE-mediated, subacute, and self-limited. [cite:Harrison 21e Ch 297; KD Tripathi 8e Ch 10]