Contact Dermatitis

Contact dermatitis is an inflammatory skin reaction caused by direct contact with an external substance. It is broadly divided into Irritant Contact Dermatitis (ICD) and Allergic Contact Dermatitis (ACD).

• Symptoms: Patients report intense itching (pruritus), burning, or stinging sensations.

• Timeline (The Diagnostic Clue):

  • Irritant (ICD): Often develops rapidly (within minutes to hours) after exposure to harsh chemicals, detergents, or friction. It is a non-immune "chemical burn" effect.

  • Allergic (ACD): A delayed-type hypersensitivity reaction. Symptoms typically appear 24–72 hours after exposure. The patient may have used the product for years before suddenly developing an allergy.

• Exposure History: Ask about new jewelry (nickel), fragrances, cosmetics, "natural" plant oils (poison ivy/oak), or occupational exposures (hairdressers, construction workers, healthcare staff).

• Pattern: The patient may note the rash is perfectly shaped like a watch strap, a button, or a specific footwear pattern.

The presentation evolves through three distinct clinical phases:

1. Acute Phase:

   • Appearance: Brightly erythematous (red), edematous (swollen) skin.

   • Features: Presence of vesicles (blisters), weeping, and "oozing" crusts. It often looks "angry" and tender.

2. Subacute Phase:

   • Appearance: The redness fades to a duller hue or brownish tone.

   • Features: Dryness, scaling, and the beginning of crust formation.

3. Chronic Phase:

   • Appearance: Lichenification (skin thickening) with exaggerated skin lines.

   • Features: Fissuring (painful cracking) and persistent dryness.

4. Distribution Clues:

   • Linear: Suggests plant exposure (e.g., brushing against a leaf).

   • Localised: Under a nickel belt buckle or earring.

   • Hands/Forearms: Common in irritant dermatitis from frequent washing or industrial chemicals.

Crucial Note for Darker Skin Tones

• Erythema Masking: In darker skin, the "redness" of the acute phase may appear violaceous (purple), dark brown, or grayish-black. It may be easier to feel the heat and swelling than to see the color change.

• Post-Inflammatory Hyperpigmentation (PIH): ACD is a frequent cause of intense PIH. The dark staining can remain for months after the allergen has been removed, sometimes being more distressing than the original rash.

• Differential Diagnosis: 

  • Phytophotodermatitis: A "false" allergy caused by citrus juice (limes) + sunlight. In darker skin, this often presents as mysterious dark streaks without much preceding redness.

  • Lichen Simplex Chronicus: Chronic rubbing can look identical to chronic contact dermatitis; history of a specific trigger is key.

The most critical step in management is the identification and avoidance of the offending agent.

1. Avoidance and Protection:

   • Key Action: Patients must be educated on their specific triggers. Instruct on skin protection, such as wearing gloves for chemical exposure or avoiding nickel-containing jewelry.

   • Early Intervention (Plants): For exposure to poison ivy or similar allergens, washing with soap within two hours is essential to remove the urushiol oil before it binds to the skin.

   • Photo-protection: For photo-allergic cases, advise sun-protective clothing (sunhats, long sleeves, high collars).

2. Topical Therapy:

   • Corticosteroids: Ointments (e.g., Hydrocortisone 1% for mild cases or Clobetasol propionate 0.05% for severe cases) reduce inflammation.

   • Safety Note: Avoid high-potency steroids on "thin-skin" areas (face, genitals, skin folds) to prevent skin atrophy (thinning).

   • Calcinuerin Inhibitors: Tacrolimus or Pimecrolimus ointments are useful alternatives for Allergic Contact Dermatitis, especially on the face, as they do not cause skin thinning.

3. Symptom Management:

   • Itching: Antihistamines (e.g., Hydroxyzine or Cetirizine) can help alleviate pruritus and aid sleep.

   • Hydration: For chronic/recurrent cases, consistent use of emollients is required to maintain skin hydration and barrier integrity.

4. Systemic Therapy:

   • Severe Cases: Widespread rashes or facial swelling may require Oral Corticosteroids. These must be tapered gradually over 2–3 weeks to prevent a rebound relapse.

Darker Skin Tones – Specific Treatment Risks:

• Steroid Hypopigmentation: Long-term use of potent steroids can cause skin lightening. This must be monitored closely to ensure the "fix" doesn't cause a new cosmetic issue.

• PIH Management: Early and aggressive control of the inflammation is the best way to minimize the depth of the resulting dark spots.

• Patch Test Visibility: When performing patch tests, the "red" reaction may be subtle. Clinicians must look for skin thickening (induration) and textural changes rather than just color.

• DermNet NZ – Contact Dermatitis: https://dermnetnz.org/topics/contact-dermatitis

• StatPearls – Allergic Contact Dermatitis (2024 update): https://www.ncbi.nlm.nih.gov/books/NBK459230/

• American Academy of Dermatology – Contact Dermatitis: https://www.aad.org/public/diseases/eczema/types/contact-dermatitis