A 58-year-old man with hypertension has been on enalapril 10 mg daily for 3 years with good blood pressure control. He now presents with a 2-week history of persistent dry cough, dyspnea on exertion, and wheezing. Chest X-ray shows bilateral interstitial infiltrates. Pulmonary function tests reveal a restrictive pattern. His serum creatinine has risen from 0.9 mg/dL (baseline) to 1.8 mg/dL. Echocardiography shows normal ejection fraction and no signs of pulmonary edema. Bronchoscopy with bronchoalveolar lavage (BAL) shows lymphocytic infiltration. After discontinuing enalapril, symptoms gradually resolve over 4 weeks. Which classification of adverse drug reaction best describes this presentation?

Explanation

## Classification of ACE Inhibitor–Induced Pulmonary Toxicity ### Clinical Presentation Analysis The patient develops insidious pulmonary infiltration (interstitial pneumonitis with restrictive physiology), elevated creatinine, and lymphocytic inflammation after 3 years of enalapril therapy. Symptoms resolve after drug discontinuation. ### Type B (Idiosyncratic) Reaction Characteristics **Key Point:** Type B reactions are bizarre, non-dose-dependent, unpredictable adverse effects that occur in genetically susceptible individuals and are NOT related to the drug's known pharmacological properties. ### Why This Is Type B, Not Type A or C #### Distinguishing Type B from Type A | Criterion | Type A | Type B | |-----------|--------|--------| | **Dose-dependent** | Yes | No | | **Frequency** | Common (10–20%) | Rare (0.1–1%) | | **Mechanism** | Exaggerated pharmacology | Genetic/immunological hypersensitivity | | **Predictability** | Predictable in all users | Unpredictable; only in susceptible individuals | | **Onset** | Immediate to early | Variable; can be delayed | **Why Not Type A?** - ACE inhibitors' pharmacological action is vasodilation and angiotensin II suppression → hypotension, hyperkalemia, cough (from bradykinin accumulation). - Interstitial pneumonitis with lymphocytic infiltration is NOT an exaggeration of these effects. - Not all patients on enalapril develop pulmonary toxicity—it occurs only in susceptible individuals (Type B hallmark). - The dose (10 mg) is standard; no dose escalation preceded the reaction. #### Why Not Type C (Chronic Toxicity)? **Type C reactions:** - Develop insidiously over months to years of cumulative exposure. - Result from direct tissue damage or metabolite accumulation. - Examples: methotrexate-induced cirrhosis, aminoglycoside nephrotoxicity, amiodarone-induced pulmonary fibrosis. **This case differs:** - The reaction occurred after 3 years (long latency), BUT the mechanism is immune-mediated (lymphocytic BAL infiltrate), not cumulative dose toxicity. - Enalapril does not accumulate to toxic levels; it is rapidly metabolized. - The reversibility upon discontinuation argues against cumulative tissue damage (Type C reactions are often irreversible). ### Immunological Basis of ACE Inhibitor–Induced Pneumonitis **High-Yield:** ACE inhibitor–induced pulmonary toxicity is a Type B reaction mediated by: 1. **Immune complex deposition** in the lungs (Type III hypersensitivity). 2. **Delayed-type cell-mediated immunity** (Type IV hypersensitivity) → lymphocytic infiltration (seen on BAL). 3. Genetic predisposition (HLA associations reported). **Clinical Pearl:** The lymphocytic BAL infiltrate is the smoking gun for immune-mediated (Type B) pathology, not dose-dependent toxicity. ### Type B Reactions: Classic Drugs and Manifestations | Drug | Type B Reaction | |------|----------------| | **ACE inhibitors** | Pulmonary infiltrates, drug-induced lupus | | **Sulfasalazine** | Hemolytic anemia, agranulocytosis, colitis | | **Allopurinol** | Stevens-Johnson syndrome, DRESS syndrome | | **Phenytoin** | Phenytoin hypersensitivity syndrome | | **NSAIDs** | Asthma exacerbation (in aspirin-sensitive patients) | ### Why Reversibility Matters **Key Point:** Type B reactions are often reversible upon drug discontinuation because they are not due to cumulative tissue damage but rather to an acute immune response. This patient's symptom resolution over 4 weeks after stopping enalapril is typical of Type B reactions.