Dermatology · Contact Dermatitis
The facts most likely to be tested
Allergic contact dermatitis is a Type IV delayed-type hypersensitivity reaction mediated by T-lymphocytes.
The clinical presentation typically features pruritic, erythematous, vesicular, or weeping lesions localized to the area of contact.
Rhus dermatitis (poison ivy, oak, or sumac) is the most common cause, characterized by linear streaks of vesicles.
The gold standard for diagnosis of chronic or recurrent cases is patch testing to identify the specific contact allergen.
First-line treatment for localized disease is high-potency topical corticosteroids.
Systemic corticosteroids, such as a tapered course of oral prednisone, are indicated for severe or widespread involvement, particularly involving the face or genitalia.
The sensitization phase requires initial exposure to the allergen, while the elicitation phase triggers the clinical rash upon subsequent re-exposure.
Vignette unlocked
A 28-year-old landscaper presents to the clinic with a 3-day history of an intensely itchy rash on his forearms and lower legs. He reports working in a wooded area last week clearing brush. Physical examination reveals erythematous, edematous plaques with multiple linear vesicles and crusting in the affected areas. The patient denies any recent changes in soaps, detergents, or medications. He has no history of atopic dermatitis.
What is the most likely diagnosis and the underlying mechanism of this condition?
Allergic contact dermatitis (Rhus dermatitis); Type IV delayed-type hypersensitivity reaction.
The vignette describes the classic linear distribution of vesicles characteristic of Rhus dermatitis, which is a Type IV hypersensitivity reaction tested in the first and third bets.
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High yield triage
Etiology / Epidemiology
Type IV hypersensitivity reaction to haptens (e.g., nickel, poison ivy). Requires prior sensitization.
Clinical Manifestations
Pruritic, erythematous, linear vesicles or bullae. Delayed-type hypersensitivity reaction.
Diagnosis
Patch testing is the gold standard. Clinical diagnosis is usually sufficient.
Treatment
Avoidance of allergen and topical corticosteroids. Avoid systemic steroids for mild cases.
Prognosis
Self-limiting if allergen is removed. Secondary infection is the primary complication.
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Epidemiology & Etiology
Common triggers include Rhus species (poison ivy/oak), nickel, fragrances, and preservatives. It is a Type IV hypersensitivity reaction occurring 24-48 hours after re-exposure. Occupational exposure is a frequent driver in adults.
Pertinent Anatomy
Involves the epidermis and superficial dermis. Lesions are strictly limited to the site of contact with the offending agent.
Pathophysiology
Sensitization phase involves Langerhans cells presenting antigen to T-cells. Upon re-exposure, memory T-cells release cytokines, causing inflammation and epidermal spongiosis. This explains the characteristic delayed onset of symptoms.
Clinical Manifestations
Patients present with intense pruritus, erythema, and vesiculation. Lesions often appear in linear streaks from plant contact. Secondary bacterial infection (impetiginization) is a red flag if crusting or purulence develops.
Diagnosis
Patch testing is the gold standard to identify specific allergens. Diagnosis is typically clinical based on history of exposure and distribution. Biopsy is rarely needed but shows spongiosis.
Treatment
First-line therapy is topical corticosteroids (e.g., triamcinolone). For severe or widespread cases, a tapering course of oral prednisone may be required. Avoid topical antihistamines due to risk of sensitization.
Prognosis
Resolution typically occurs within 2-3 weeks with allergen avoidance. Secondary infection is the most common complication requiring antibiotics. Chronic exposure leads to lichenification.
Differential Diagnosis
Irritant Contact Dermatitis: occurs immediately upon first exposure
Atopic Dermatitis: typically involves flexural surfaces
Dyshidrotic Eczema: presents with tapioca-like vesicles on palms/soles
Tinea Corporis: characterized by central clearing and scaling
Cellulitis: presents with warmth, systemic fever, and diffuse borders