Contact Dermatitis: Allergic

ACD Basics - Skin's Delayed Fury

• Definition: Allergic Contact Dermatitis (ACD) is an immune-mediated, delayed (Type IV) hypersensitivity reaction to an external allergen. Unlike Irritant Contact Dermatitis (ICD), which is direct non-immune tissue damage. 📌 ACD = Allergen Causes Delayed reaction.
• Pathophysiology (Type IV Hypersensitivity):

⭐ ACD is a classic example of a Type IV hypersensitivity reaction mediated by T-lymphocytes.

Allergen Parade - The Usual Suspects

📌 For common metals: No Chrome Contact (Nickel, Cobalt, Chromium).

⭐ Parthenium dermatitis is a significant cause of airborne contact dermatitis in India.

Rash Report - Itch & Tell

• Onset: 24-72 hours post-exposure in sensitized individuals.
• Morphology:
  • Acute: Erythema, edema, vesicles/bullae (may be tense), oozing, intense pruritus.
  • Subacute: Scaling, crusting, less vesicular, resolving erythema.
  • Chronic: Lichenification (thickened skin), fissuring, hyper/hypopigmentation, dryness.
• Distribution:
  • Often sharply demarcated, localized to site of direct contact.
  • Linear streaks: Plant contact (e.g., Poison Ivy, Parthenium - 📌 think "streaky rash from a leafy dash").
  • Specific Patterns: Consider: Earlobes (nickel), eyelids (cosmetics, nail polish transfer), feet (footwear components like chromates, rubber).

⭐ The presence of well-demarcated, eczematous lesions in an area of allergen exposure is highly suggestive of ACD.

Patch Test Puzzles - Finding Culprits

• History is Key: Occupation, hobbies, cosmetics, topicals, clothing, jewelry.
• Patch Testing: Gold Standard
  • Identifies specific allergen.
  • Technique: Allergens applied, readings at 48 & 72/96 hrs.
  • Interpretation: + (weak), ++ (strong), +++ (extreme), IR (irritant).
  • Series: Indian Standard Series.
• Differential Diagnosis:
  • Irritant Contact Dermatitis (ICD)
  • Atopic Dermatitis
  • Seborrheic Dermatitis
  • Psoriasis
  • Fungal infections

⭐ A positive patch test must be clinically relevant to the patient's dermatitis to confirm ACD.

Soothe & Shield - Calming Chaos

• Cornerstone: Allergen identification & strict avoidance.
• Patient Education: Allergen sources, cross-reactants, alternative products.
• Topical Therapy:
  • Corticosteroids: Potency based on site/severity.
  • Topical Calcineurin Inhibitors (TCIs): e.g., Tacrolimus, Pimecrolimus for sensitive areas, long-term use.
• Systemic Therapy (Severe/Widespread ACD):
  • Oral Corticosteroids: e.g., Prednisolone $0.5-1 \text{ mg/kg/day}$ tapered over 2-3 weeks.
  • Antihistamines: For pruritus.
• Supportive Care: Emollients, wet compresses.

⭐ Short courses of potent topical steroids are preferred over long courses of weaker steroids to minimize side effects.

High‑Yield Points - ⚡ Biggest Takeaways

• Type IV hypersensitivity reaction, delayed onset (24-72 hours post-exposure).
• Common allergens: nickel (jewelry), poison ivy (urushiol), fragrances, preservatives, PPD (hair dye).
• Clinical: intensely pruritic, erythematous papules/vesicles, well-demarcated at contact site.
• Patch testing is gold standard for diagnosis, identifies specific allergen.
• Management: strict allergen avoidance, topical corticosteroids (mainstay), emollients; systemic steroids for severe/widespread.
• Chronic ACD: lichenification, scaling, fissuring; may become generalized or id reaction (autosensitization).