Clinical meaning
Cutaneous adverse drug reactions (CADRs) affect 2-3% of hospitalized patients and represent a spectrum from benign morbilliform eruptions to life-threatening conditions (SJS/TEN, DRESS). The NP must differentiate these patterns, identify the causative agent, and determine which reactions are self-limited versus which require emergent intervention.
Morbilliform (exanthematous) drug eruption: The MOST COMMON drug eruption pattern (~95% of all drug rashes). Pathophysiology involves a delayed-type (Type IVb) T-cell-mediated hypersensitivity reaction. Drug molecules or their metabolites are presented by antigen-presenting cells (Langerhans cells) to CD4+ T-helper cells via MHC class II molecules, triggering release of IL-5 and other Th2 cytokines that recruit eosinophils to the dermis. The reaction typically develops 7-14 days after FIRST exposure to the drug (or 1-3 days upon RE-EXPOSURE). Clinical presentation: symmetric, erythematous macules and papules starting on the trunk and spreading centrifugally to the extremities, often becoming confluent; pruritus is common; mucous membranes are SPARED (important distinction from SJS/TEN). The eruption is typically self-limited, resolving within 1-2 weeks of drug discontinuation, often with desquamation. Most common causative drugs: aminopenicillins (amoxicillin — especially during EBV infection/mononucleosis, where the rash rate is 70-100%), sulfonamides (TMP-SMX), cephalosporins, allopurinol, anticonvulsants (phenytoin, carbamazepine, lamotrigine), and NSAIDs.
Fixed drug eruption (FDE): A distinctive reaction that recurs at EXACTLY THE SAME anatomic site each time the causative drug is administered. Pathophysiology involves tissue-resident memory CD8+ T cells that remain in the skin at the site of a previous reaction. These intraepidermal CD8+ T cells express CD69 and retain drug-specific memory; upon re-exposure, they rapidly release interferon-gamma and cytotoxic mediators (perforin, granzyme B), causing localized keratinocyte necrosis. The lesion appears within 30 minutes to 8 hours of re-exposure (much faster than morbilliform eruptions). Clinical presentation: one or a few well-demarcated, round to oval, dusky red to violaceous plaques, often with central blistering or erosion; the most common locations are the lips, genitalia, hands, and feet (particularly the glans penis — a classic exam pearl). After resolution, a characteristic post-inflammatory HYPERPIGMENTATION remains at the site, which is pathognomonic for FDE. Common causative drugs: NSAIDs (particularly naproxen and ibuprofen), TMP-SMX, tetracyclines, barbiturates, phenolphthalein (laxatives), pseudoephedrine, and acetaminophen.