contact dermatitis

**Types**

1. **Irritant contact dermatitis (ICD)** – about 80% of cases. - Mechanism: direct chemical or physical damage to keratinocytes and disruption of the skin barrier. It is non-immunologic, needs no prior sensitization, and can occur on first exposure. - Onset: minutes to hours after exposure. It is dose-dependent. - Causes: soaps and detergents, solvents, acids and alkalis, and repeated wet work ("housewife's/hand dermatitis," healthcare workers). - Symptoms: burning and stinging more than itching. Findings include erythema, dryness, fissuring, and sharply demarcated borders.

2. **Allergic contact dermatitis (ACD)** – type IV (delayed-type) hypersensitivity. - **Sensitization phase:** a hapten (a small molecule) penetrates the skin and binds self-proteins. Langerhans cells and dendritic cells present it to naïve T cells in the lymph nodes. This generates memory CD4⁺ (Th1) and CD8⁺ T cells and takes about 10–14 days. - **Elicitation phase:** re-exposure leads to memory T-cell activation. These cells release IFN-γ and TNF-α and recruit macrophages and cytotoxic T cells. The reaction appears 12–72 hours after exposure (peak at 48–96 hours). - Findings: pruritic erythematous papules, vesicles, and bullae, often in a linear or geometric pattern that matches the exposure. Chronic disease shows lichenification and scaling. - Histology: spongiosis (intercellular edema) with a perivascular lymphocytic infiltrate. Eosinophils may be present.

**Classic allergens (high-yield)** - **Urushiol** (poison ivy, oak, sumac, and mango rind): linear vesicles and streaks. Fluid from blisters does NOT spread the rash. The spread is from residual oil on skin, clothing, or pets. - **Nickel:** jewelry, belt buckles, watch backs, and jean snaps. Causes periumbilical, earlobe, and wrist dermatitis. It is the most common contact allergen. - **Neomycin and bacitracin:** topical antibiotics. - **Latex (rubber) gloves:** latex can cause either type IV (contact dermatitis from accelerators/thiurams) or type I (IgE-mediated urticaria and anaphylaxis) reactions. - **Fragrances and preservatives:** Balsam of Peru, formaldehyde, and parabens. - **Others:** potassium dichromate (leather, cement), p-phenylenediamine (hair dye, henna tattoos), and topical anesthetics such as benzocaine. - **Cosmetic and hair products:** eyelid dermatitis from nail polish, since the hands transfer the allergen.

**Diagnosis** - Clinical, based on history and distribution. - **Patch testing** is the gold standard for ACD. Allergens are applied under occlusion for 48 hours and read at 48 and 72–96 hours. Do not confuse it with prick/IgE testing, which is for type I reactions. - Biopsy is rarely needed.

**Treatment** - Identify and avoid the trigger. This is the most important step. - Emollients and barrier protection. - Topical corticosteroids for localized disease. Use a short course of systemic steroids (taper over 2–3 weeks to avoid rebound) for extensive disease or involvement of the face or genitals. - Cool compresses and oral antihistamines for symptomatic relief only. Antihistamines do not treat the underlying type IV reaction. - Refractory or chronic cases: topical calcineurin inhibitors, phototherapy, or steroid-sparing immunosuppressants.

**Key comparisons for boards**

| Feature | Irritant | Allergic | |---|---|---| | Mechanism | Direct toxicity | Type IV hypersensitivity | | Prior sensitization | No | Yes | | Onset | Minutes–hours | 12–72 h after re-exposure | | Symptom | Burning, pain | Intense itching | | Distribution | Confined to contact area | May extend beyond contact site | | Diagnosis | Clinical | Patch test |

**Related concepts** - Other type IV hypersensitivity reactions: TB skin test (PPD), granuloma formation, and graft rejection. The mnemonic is "ACID" (Allergic contact dermatitis, TB skin test/Type IV, Immune graft rejection, Delayed). - Photocontact dermatitis: the reaction requires UV light exposure (for example, with sunscreens, fragrances, and phytophotodermatitis from lime juice or furocoumarins). - Atopic dermatitis is a distinct entity (type I with Th2 skew, filaggrin mutations). Nickel and other allergens can cause hand eczema in atopic patients.