SN1 SHORT NOTE 5M

Write a short note on cutaneous adverse drug reactions — patterns, causative drugs and management.

General Medicine · Final Professional

Definition

A cutaneous adverse drug reaction is any undesirable skin, mucous membrane or appendage change caused by a drug given in normal therapeutic doses. Skin is the commonest single organ affected by drug reactions, occurring in 2–3% of hospital inpatients. Most are mild, but a small proportion are life-threatening, and the whole clinical task is to separate the two quickly.

Patterns of Reaction

Pattern	Description	Typical Drugs
Exanthematous (morbilliform)	Commonest (~90%); symmetrical erythematous macules and papules on trunk spreading peripherally, 4–14 days after starting the drug; itchy, no mucosal involvement	Penicillins, cephalosporins, sulphonamides, allopurinol, anticonvulsants
Urticarial	Weals and angio-oedema within minutes to hours; IgE-mediated (type I)	Penicillins, NSAIDs, radiocontrast, opiates
Fixed drug eruption	Round, well-defined, dusky violaceous plaque recurring at the SAME site with each exposure; heals with hyperpigmentation; favours lips, genitalia, hands	Sulphonamides, tetracyclines, NSAIDs, paracetamol, dapsone
Erythema multiforme / SJS / TEN	Target lesions through to extensive epidermal detachment with mucosal erosions	Sulphonamides, anticonvulsants, allopurinol, NSAIDs
DRESS	Drug Reaction with Eosinophilia and Systemic Symptoms — rash, fever, facial oedema, lymphadenopathy, eosinophilia, hepatitis, 2–8 weeks after the drug	Anticonvulsants (phenytoin, carbamazepine, lamotrigine), allopurinol, sulphonamides, dapsone
Acute generalised exanthematous pustulosis (AGEP)	Rapid onset of sterile non-follicular pustules on erythema, with fever and neutrophilia	Aminopenicillins, macrolides, diltiazem, antimalarials
Photosensitivity	Eruption confined to sun-exposed sites	Tetracyclines (doxycycline), thiazides, amiodarone, fluoroquinolones, phenothiazines
Lichenoid	Violaceous flat-topped papules resembling lichen planus	Beta-blockers, thiazides, antimalarials, ACE inhibitors, gold
Erythroderma	Generalised erythema and scaling of > 90% body surface area	Sulphonamides, allopurinol, carbamazepine, gold
Acneiform	Monomorphic papulopustules without comedones	Corticosteroids, lithium, isoniazid, EGFR inhibitors, anabolic steroids

Red Flag Features — Severe Cutaneous Adverse Reaction

Recognising these decides whether the patient goes home or to an intensive care setting:

Mucosal involvement — oral, ocular or genital erosions
Skin pain or tenderness rather than itch
Blistering, epidermal detachment, positive Nikolsky's sign
Facial oedema and confluent erythema
Fever, lymphadenopathy, systemic upset
Laboratory: eosinophilia, atypical lymphocytes, deranged liver or renal function

DANGER / REMEMBER

Mucosal erosions plus skin tenderness and blistering means SJS/TEN until proved otherwise. Facial oedema with fever, eosinophilia and hepatitis two to six weeks after starting an anticonvulsant or allopurinol means DRESS. Both require immediate withdrawal of the drug and admission — treating either as a "simple drug rash" is a well-recognised cause of avoidable death.

Diagnosis

There is no single confirmatory test — the diagnosis is clinical and chronological.

Detailed drug chart with dates of starting and stopping every agent, including over-the-counter drugs, herbal and traditional remedies (widely used in India and frequently unreported unless asked).
Latent period helps identify the culprit: urticaria within hours, exanthem at 4–14 days, DRESS at 2–8 weeks.
Rechallenge confirms the diagnosis but is contraindicated in severe reactions.
Skin biopsy to distinguish patterns; patch testing for fixed drug eruption and lichenoid reactions; specific IgE or skin prick testing for immediate reactions.
Baseline CBC with eosinophil count, LFT, RFT and urinalysis in any widespread eruption to detect systemic involvement.

Management

Stop the suspected drug immediately — the single most important step. Where multiple drugs are implicated, stop all non-essential agents and substitute structurally unrelated alternatives for essential ones.
Mild exanthematous reaction: emollients, topical corticosteroids, oral antihistamines; the eruption fades over 1–2 weeks, often with desquamation.
Urticaria/anaphylaxis: antihistamines; adrenaline intramuscularly if there is airway compromise or hypotension.
Severe reactions (SJS/TEN, DRESS, AGEP, erythroderma): admit; fluid, electrolyte and nutritional support; skin care as for burns; ophthalmology review; systemic corticosteroids in DRESS; specialist dermatology input.
Document the reaction prominently in the case record and on a drug allergy card, counsel the patient to avoid the drug and its class lifelong, and report to the pharmacovigilance programme.

NEEDS VERIFICATION

Drug lists and the DRESS latency window are broadly agreed, but confirm the current recommended role of systemic corticosteroids and ciclosporin in SJS/TEN, which remains debated.

All questions in Dermatology

Every General Medicine chapter →