pityriasis rosea

**Epidemiology:** Most common in ages 10–35, slight female predominance, more frequent in spring and fall. Not considered highly contagious.

**Etiology/Pathophysiology:** Strongly associated with systemic reactivation of human herpesvirus 6 and 7 (HHV-6/HHV-7), though the exact cause is not fully established. Often preceded by a prodromal viral-like illness (malaise, low-grade fever, sore throat, headache) in a minority of patients.

**Clinical presentation:** - **Herald patch** (~70–80% of cases): a solitary, larger (2–10 cm) oval, erythematous or salmon-colored plaque with a fine peripheral collarette of scale (scale trailing inside the edge), usually on the trunk, neck, or proximal extremities. - **Secondary eruption** 1–2 weeks later: multiple smaller oval macules/patches/plaques with collarette scale. Long axes follow Langer's lines (skin cleavage lines), producing a **"Christmas tree" pattern** on the back. - Distribution: trunk and proximal extremities. Face, palms, and soles are usually spared. - Pruritus is variable (mild to moderate in about 25–50%). - Atypical/inverse variants: involvement of the face, axillae, groin, or extremities; vesicular, urticarial, or purpuric forms. Can be more severe in pregnancy, and early pregnancy cases (especially before 15 weeks) have been associated with increased risk of miscarriage in some reports.

**Diagnosis:** Clinical. KOH prep of the herald patch to rule out tinea corporis. Consider **RPR/VDRL** to exclude secondary syphilis (palms/soles involvement, mucosal lesions, lymphadenopathy, condyloma lata) in sexually active patients, and in any atypical presentation. Biopsy is rarely needed (nonspecific: focal parakeratosis, mild spongiosis, perivascular lymphocytic infiltrate, extravasated RBCs).

**Differential diagnosis (high-yield):** - Secondary syphilis (palms/soles involved, positive serology) - Tinea corporis (positive KOH with hyphae, central clearing) - Guttate psoriasis (follows strep pharyngitis, "raindrop" lesions with thick silvery scale) - Nummular eczema - Drug eruptions (e.g., captopril, gold, metronidazole, isotretinoin, ACE inhibitors, barbiturates) - Lichen planus - Pityriasis lichenoides chronica

**Management:** - Self-limited; resolves spontaneously in ~6–8 weeks (can last up to 12+ weeks), with possible post-inflammatory dyspigmentation, especially in darker skin tones. Recurrence is uncommon. - Reassurance and supportive care: emollients, topical corticosteroids, and oral antihistamines for pruritus. - Phototherapy (UVB) may help in severe or pruritic cases. - Acyclovir has been used in some studies (high-dose, early in disease), but evidence is limited and it is not routinely recommended.

**Board pearls:** - Herald patch → Christmas tree pattern along Langer lines = pityriasis rosea. - Always rule out secondary syphilis (check palms/soles) and tinea (KOH). - Associated with HHV-6/7. Benign and self-resolving; treat symptomatically.