Drug Eruptions: Morbilliform, Urticarial, and Fixed Drug Reactions
Drug eruptions are adverse cutaneous reactions to medications that can present in various forms, most commonly as morbilliform, urticarial, or fixed drug reactions. Understanding these reactions is crucial for the diagnosis and management of patients experiencing drug-induced skin conditions.
Topics: drug reaction, adverse effects, exanthem
Overview / Definition Drug eruptions are skin reactions caused by the administration of medications. They can manifest in several forms, with morbilliform, urticarial, and fixed drug eruptions being among the most common types. These reactions may vary in severity and can affect patient management and medication adherence. Epidemiology Drug eruptions are a significant concern in dermatology, accounting for approximately 2-3% of all dermatologic consultations. The incidence can vary based on the population studied and the medications used. Certain classes of drugs, such as antibiotics and anticonvulsants, are more commonly associated with cutaneous reactions. Common drugs implicated include: Penicillins Cephalosporins Sulfonamides Anticonvulsants (e.g., phenytoin, carbamazepine) Pathophysiology / Mechanism The pathophysiology of drug eruptions involves a complex interaction between the drug, the immune system, and the skin. These reactions can be classified based on the mechanism of action: Type I hypersensitivity: Immediate reactions mediated by IgE, resulting in urticarial eruptions. Type II hypersensitivity: Cytotoxic reactions leading to cell destruction. Type IV hypersensitivity: Delayed reactions mediated by T cells, commonly seen in morbilliform eruptions. Clinical Presentation Drug eruptions can present with a variety of clinical features, depending on the type of reaction: Morbilliform reactions: Characterized by a diffuse rash resembling measles. Typically appears 1-2 weeks after drug exposure. Pruritus may or may not be present. Urticarial reactions: Appear as raised, itchy wheals. Can occur within hours of drug administration. May be associated with angioedema. Fixed drug eruptions: Localized lesions that recur at the same site with re-exposure to the drug. May present as erythematous patches or plaques. Typically resolve with post-inflammatory hyperpigmentation. Diagnosis / Workup The diagnosis of drug eruptions is primarily clinical, based on the histor