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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Irritant contact dermatitis is direct barrier injury; allergic contact dermatitis is type IV delayed hypersensitivity
- Rhus dermatitis (poison ivy/oak/sumac) causes linear vesicular streaks after outdoor exposure
- Nickel dermatitis affects earlobes, belt buckle area, wrists; latex can cause contact urticaria or delayed dermatitis
- Treatment is allergen/irritant avoidance plus topical corticosteroids; extensive severe poison ivy may need oral prednisone taper
- Patch testing is used for persistent, recurrent, occupational, facial/hand, or unclear allergic contact dermatitis
Overview
Contact dermatitis is an eczematous eruption caused by substances contacting the skin. Irritant contact dermatitis is more common and results from toxic injury to the skin barrier, often from wet work, soaps, detergents, solvents, acids, or friction. Allergic contact dermatitis is a delayed type IV T-cell-mediated hypersensitivity reaction that requires prior sensitization. Common allergens include urushiol from poison ivy/oak/sumac, nickel, fragrances, preservatives, rubber accelerators, hair dye para-phenylenediamine, topical antibiotics such as neomycin/bacitracin, and cosmetics.
Epidemiology
Contact dermatitis is common in primary care, emergency medicine, pediatrics, and occupational health. Hand dermatitis is frequent among healthcare workers, food handlers, cleaners, hairdressers, mechanics, and patients with atopic background. Allergic contact dermatitis may appear at any age and often becomes chronic when exposure is unrecognized. Poison ivy dermatitis is one of the most common outdoor allergic contact dermatitis presentations in the United States.
Clinical Features
Symptoms
Pruritus is prominent in allergic contact dermatitis
Burning, stinging, fissuring, or pain is more prominent in irritant dermatitis
Delayed onset 24-72 hours after allergen exposure is typical for allergic contact dermatitis
Dyspnea, lip/tongue swelling, hypotension, or generalized urticaria suggests immediate hypersensitivity/anaphylaxis, not simple contact dermatitis
Signs
Erythematous papules, vesicles, weeping, crusting, or lichenification in chronic cases
Geometric or sharply demarcated distribution matching exposure
Linear vesicular streaks after poison ivy/oak/sumac exposure
Hand dermatitis with fissures in wet-work occupations
Secondary infection: warmth, purulence, honey crusting, fever, or spreading cellulitis
Investigations
First-line
Exposure historyTiming, occupation, hobbies, cosmetics, gloves, jewelry, plants, topical medications, new products, distribution, and improvement away from work
Clinical pattern recognitionLinear poison ivy, belt-buckle nickel, earlobe jewelry, eyelid cosmetic/fragrance/preservative, hand wet work, shoe dermatitis
Second-line
Patch testingGold standard for allergic contact dermatitis when recurrent, chronic, occupational, or allergen unclear. It tests delayed hypersensitivity, not IgE allergy
KOH prepIf tinea manuum/pedis/corporis is possible, especially unilateral hand or annular scaly plaques
Bacterial cultureIf purulence, recurrent infection, or treatment failure suggests secondary infection
Specialist
Skin biopsyRarely needed; consider when diagnosis remains uncertain or psoriasis, CTCL, bullous disease, or drug eruption is possible
1
Exposure control
- Identify and avoid culprit irritant or allergen; avoidance is definitive treatment
- Use gloves/barrier protection for wet work, but consider glove allergens such as rubber accelerators if dermatitis persists
- For poison ivy/oak/sumac: wash skin and contaminated clothing/tools; urushiol persists on fomites
2
Topical treatment
- Medium- to high-potency topical corticosteroids for trunk/extremity flares for limited duration
- Low-potency steroids or topical calcineurin inhibitors for face, eyelids, groin, and folds
- Cool compresses, wet dressings, emollients, and oral antihistamines for sleep if pruritus severe
3
Systemic therapy
- Extensive severe allergic contact dermatitis, especially poison ivy involving face/genitals/hands or large BSA: oral prednisone taper over 2-3 weeks to avoid rebound
- Treat secondary bacterial infection if present; do not use antibiotics for sterile vesicular poison ivy
- Emergency care with epinephrine if immediate systemic allergic reaction/anaphylaxis occurs
4
Long-term management
- Patch-test-directed allergen avoidance lists for chronic allergic contact dermatitis
- Occupational modifications for recurrent hand dermatitis
- Avoid sensitizing topical antibiotics such as neomycin/bacitracin when dermatitis worsens under treatment
Complications
- Chronic hand eczema: Fissuring, pain, occupational impairment
- Secondary infection: Impetigo or cellulitis from excoriated skin
- Post-inflammatory pigment change: Common after severe inflammation, especially in darker skin
- Rebound dermatitis: Short steroid bursts for severe poison ivy may relapse if not tapered adequately
- Allergen persistence: Ongoing exposure through cosmetics, jewelry, gloves, or contaminated clothing causes chronic disease
USMLE Step 2 CK Exam Tips
- 1Linear vesicles after hiking/gardening = poison ivy allergic contact dermatitis
- 2Allergic contact dermatitis is type IV delayed hypersensitivity; onset is delayed, not immediate
- 3Nickel allergy: itchy dermatitis under belt buckle, earrings, watch, or jean button
- 4Irritant dermatitis burns/stings and occurs with repeated wet work/soaps; allergic dermatitis is usually very itchy
- 5Patch testing identifies allergic contact dermatitis; skin prick testing identifies IgE-mediated allergy
- 6Severe poison ivy needs an adequate prednisone taper; too short a course causes rebound
- 7Topical neomycin/bacitracin can cause allergic contact dermatitis and worsen a wound rash
practicetest your knowledge on contact dermatitisApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — dermatology and beyond.
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