Pityriasis rosea

Pityriasis rosea is a common, acute, self-limiting inflammatory skin eruption. While the exact cause is unknown, it is thought to be a reactive response to a viral infection (likely Human Herpesvirus 6 or 7).

• Prodrome: Some patients recall "flu-like" symptoms roughly a week before the rash appears, including sore throat, mild fever, headache, or fatigue.

• The "Herald Patch": Most patients (50–90%) describe a single, large, scaly "mother patch" that appeared 1–2 weeks before the rest of the rash.

• Evolution: Following the initial patch, a "crop" of smaller, similar-looking spots appears across the trunk and limbs.

• Symptoms: Itching (pruritus) is the most common complaint, ranging from mild to severe. In some cases, it can be exacerbated by hot showers or exercise.

Diagnosis is based on the characteristic morphology and the unique "Christmas tree" distribution.

1. The Herald Patch:

   • Appearance: A single, oval, 2–10 cm plaque with a fine, scaly surface.

   • The Collarette Scale: A pathognomonic sign where the scale is attached at the periphery but free in the center, creating a "ring" or "collarette" effect.

2. The Secondary Eruption:

   • Morphology: Smaller, oval macules and papules that mimic the appearance of the herald patch.

   • Distribution: Primarily on the trunk and upper limbs. The long axes of the ovals follow the skin's cleavage lines (Langer's lines), creating a "Christmas tree" or "fir tree" pattern on the back.

3. Healing: The rash typically resolves spontaneously over 6–12 weeks.

Crucial Note for Darker Skin Tones

• Color Variation: In lighter skin, the patches are "salmon-pink." In darker skin, they frequently appear deep brown, violaceous (purple), or hyperpigmented.

• Inverse Pityriasis Rosea: This variant is notably more common in Black patients, particularly children. The rash may be "atypical," appearing in the armpits, groin, and face rather than the classic trunk distribution.

• Post-Inflammatory Hyperpigmentation (PIH): Pityriasis rosea often leaves behind significant dark spots in patients with darker skin. Patients should be warned that these marks may last for several months after the scaling and redness have gone.

• Differential Diagnosis: 

  • Secondary Syphilis: A "great imitator." It also presents with a scaly trunk rash but crucially involves the palms and soles and lacks a herald patch. A plasma reagin (RPR) test should be considered if there is any doubt.

  • Tinea Corporis (Ringworm): A herald patch is often misdiagnosed as ringworm. Ringworm typically has a more aggressive, pustular border and is not followed by the "Christmas tree" eruption.

General Principles:

• Reassurance: This is the most important treatment. Explain that the rash is not contagious, is not related to poor hygiene, and will go away on its own without leaving permanent scars (though PIH may occur).

• Avoid Irritants: Advise lukewarm showers and mild soaps, as heat and harsh chemicals can worsen the itch.

Medication

• Topical Corticosteroids: For mild-to-moderate itching, low- to medium-potency corticosteroids like hydrocortisone (1%) or triamcinolone acetonide can be applied twice daily to affected areas.

• Oral Antihistamines: For persistent itch, oral antihistamines such as diphenhydramine (25–50 mg at night) or cetirizine (10 mg daily) can be prescribed.

• Phototherapy: UVB phototherapy may help in severe or persistent cases, shortening the disease duration, although it is usually reserved for severe pruritus or when cosmetic appearance is a concern.

• Antiviral Therapy: In certain severe or widespread cases, acyclovir (800 mg five times daily for a week) may be considered as an off-label option.

Darker Skin Tones – Specific Treatment Risks:

• PIH Concerns: Because the risk of hyperpigmentation is higher, aggressive management of the "itch" is vital to prevent scratching, which worsens pigment deposition.

• UVB Caution: While phototherapy can help, it must be used cautiously in darker skin to avoid causing further hyperpigmentation.

• DermNet NZ – Pityriasis Rosea: https://dermnetnz.org/topics/pityriasis-rosea

• StatPearls – Pityriasis Rosea (2024 update): https://www.ncbi.nlm.nih.gov/books/NBK532283/

• The Pharmaceutical Journal – Pityriasis rosea in skin of colour: https://pharmaceutical-journal.com/article/ld/pityriasis-rosea-in-skin-of-colour