In an otherwise healthy adolescent, how does pityriasis versicolor differ from pityriasis rosea?

Distinguishing Pityriasis Versicolor from Pityriasis Rosea in Adolescents

Pityriasis versicolor and pityriasis rosea are fundamentally different conditions: pityriasis versicolor is a chronic superficial fungal infection caused by Malassezia yeast that produces hypo- or hyperpigmented scaly patches predominantly on the upper trunk, while pityriasis rosea is an acute self-limited viral exanthem (likely HHV-6/7) characterized by a herald patch followed by oval salmon-colored lesions along skin cleavage lines in a "Christmas tree" pattern. 1, 2, 3

Causative Agent and Pathophysiology

• Pityriasis versicolor is caused by the lipophilic yeast Malassezia (formerly Pityrosporum) converting from its normal blastospore form to a pathogenic mycelial form under predisposing conditions including high temperatures, humidity, greasy skin, hyperhidrosis, and hereditary factors 2, 3, 4

• Pityriasis rosea is likely triggered by human herpesvirus-7 and HHV-6 in some patients, presenting as an acute self-limited papulosquamous dermatosis 1

Clinical Presentation Differences

Pityriasis Versicolor

• Chronic or recurrent course with scaly hypo- or hyperpigmented macules and patches 2, 3, 4
• Predominantly affects upper trunk, neck, and upper arms 2
• No herald patch 3
• Lesions vary in color (hence "versicolor") and do not follow Langer's lines 4
• Occurs mainly at adolescence when sebaceous glands are more active 4
• No prodromal symptoms 3

Pityriasis Rosea

• Acute, self-limited course lasting 6-8 weeks 1
• Herald patch present in approximately 80% of cases—a larger, more noticeable initial lesion 1
• Generalized eruption develops 4-14 days after herald patch, continuing in crops over 12-21 days 1
• Lesions are 0.5-1 cm oval or elliptical, dull pink or salmon-colored macules with delicate collarette of scales at periphery 1
• Long axes oriented along Langer's lines of cleavage, creating "Christmas tree" pattern on back and V-shaped pattern on upper chest 1
• Mild prodrome (headaches, fever, malaise, fatigue) in approximately 5% of patients 1

Diagnostic Approach

For Pityriasis Versicolor

• Wood's light examination shows yellow-green fluorescence (unlike pityriasis rosea) 3
• KOH preparation reveals "spaghetti and meatballs" appearance of hyphae and spores 3
• Histology shows mycelial form of Malassezia in stratum corneum 3

For Pityriasis Rosea

• Diagnosis is primarily clinical based on characteristic herald patch and distribution pattern 1
• Wood's light shows no fluorescence 5
• Biopsy rarely needed but would show nonspecific findings without fungal elements 1

Treatment Differences

Pityriasis Versicolor

• Requires antifungal therapy with topical options including ketoconazole shampoo, zinc pyrithione shampoo, selenium sulfide, or topical antifungals 2, 3
• Difficult cases respond to short-term fluconazole or itraconazole 2
• Prophylactic treatment regimen is mandatory to prevent recurrence, as the condition tends to recur without ongoing management 2, 4

Pityriasis Rosea

• Reassurance and symptomatic treatment suffice in the vast majority of cases 1
• Active intervention (oral acyclovir, erythromycin, or UV phototherapy) reserved for severe cases, recurrent disease, or pregnant women 1
• Self-resolves in 6-8 weeks without antifungal therapy 1

Critical Pitfalls to Avoid

• Do not confuse the chronic, recurrent nature of pityriasis versicolor with the acute, self-limited course of pityriasis rosea 1, 2, 4
• Absence of herald patch in pityriasis rosea variants may cause diagnostic confusion, but the distribution along Langer's lines and acute onset distinguish it from pityriasis versicolor 1
• Failing to provide prophylactic antifungal therapy for pityriasis versicolor leads to rapid recurrence 2
• Pityriasis versicolor will not resolve spontaneously and requires antifungal treatment, unlike pityriasis rosea 2, 3