What are the differential diagnoses for pityriasis rosea?

Differential Diagnoses for Pityriasis Rosea

The key differential diagnoses to consider when evaluating pityriasis rosea include secondary syphilis, guttate psoriasis, tinea corporis, nummular eczema, pityriasis lichenoides chronica, drug eruptions, viral exanthems, seborrheic dermatitis, and cutaneous T-cell lymphoma. 1, 2, 3

Primary Differential Considerations

Secondary Syphilis (Most Critical to Exclude)

• Must be ruled out in every case due to serious systemic implications and treatment requirements 1, 4
• Look for palmoplantar involvement, which is characteristic of syphilis but rare in pityriasis rosea 2
• Obtain serologic testing (RPR/VDRL and treponemal-specific tests) when clinical uncertainty exists 1
• Mucous membrane involvement and generalized lymphadenopathy favor syphilis over pityriasis rosea 3

Guttate Psoriasis

• Presents with smaller, more uniform lesions without the characteristic collarette of scale seen in pityriasis rosea 3
• Often follows streptococcal pharyngitis 2
• Lesions typically lack the salmon-pink color and peripheral scaling pattern of pityriasis rosea 1
• Does not follow Langer's lines in distribution 2

Tinea Corporis (Dermatophyte Infection)

• Presents with annular lesions with active, raised borders and central clearing 1
• Confirm diagnosis with KOH preparation showing fungal hyphae 1
• Typically more localized than the bilateral symmetric distribution of pityriasis rosea 2

Secondary Differential Considerations

Drug-Induced Eruptions

• Multiple medications can cause pityriasis rosea-like eruptions including ACE inhibitors, beta-blockers, metronidazole, and gold 1, 4
• Obtain detailed medication history including recent additions or changes 1
• Drug eruptions may lack the herald patch and typical progression pattern 2

Nummular Eczema

• Presents with coin-shaped, intensely pruritic plaques 1, 3
• Lesions are typically more chronic and lack the self-limited course of pityriasis rosea 3
• Does not follow Langer's lines and lacks herald patch 1

Viral Exanthems

• Consider in patients with systemic symptoms and acute onset 1
• Typically shorter duration than pityriasis rosea 2
• May lack the characteristic herald patch and Christmas tree pattern 1

Seborrheic Dermatitis

• Favors seborrheic areas (scalp, nasolabial folds, central chest) rather than trunk and proximal extremities 1
• Presents with greasy, yellowish scales rather than the collarette scaling of pityriasis rosea 1
• More chronic and recurrent course 4

Less Common But Important Differentials

Pityriasis Lichenoides Chronica

• Presents with polymorphic lesions in various stages of evolution 1
• More chronic course without spontaneous resolution 2
• Lesions may show central necrosis or hemorrhage 3

Cutaneous T-Cell Lymphoma (Mycosis Fungoides)

• Critical to consider in atypical or persistent cases 3
• Lesions typically more indurated and persistent beyond 8-12 weeks 3
• Skin biopsy shows atypical lymphocytes in epidermis 5
• More common in older adults rather than young patients 2

Pityriasis Versicolor

• Caused by Malassezia yeast 3
• Presents with hypo- or hyperpigmented patches with fine scale 3
• KOH preparation shows "spaghetti and meatballs" appearance 3
• Typically involves upper trunk and shoulders 1

Clinical Pitfalls to Avoid

• Do not dismiss the diagnosis in absence of herald patch, as approximately 20% of cases lack this feature 2
• Always obtain syphilis serologies when diagnosis is uncertain, particularly in sexually active patients 1, 4
• Consider skin biopsy for atypical presentations lasting beyond 12 weeks or in patients over 50 years old to exclude cutaneous T-cell lymphoma 3
• Obtain fungal culture or KOH preparation when annular lesions with active borders are present to exclude tinea corporis 1
• Review medication history carefully as drug-induced eruptions can closely mimic pityriasis rosea 1, 4