What is the differential diagnosis of pityriasis rubra pilaris?

Differential Diagnosis of Pityriasis Rubra Pilaris

The differential diagnosis of pityriasis rubra pilaris (PRP) primarily includes erythrodermic psoriasis, which is often the most challenging distinction, followed by atopic dermatitis, seborrheic dermatitis, cutaneous T-cell lymphoma, and other erythrodermic conditions. 1

Primary Differential Diagnoses

Erythrodermic Psoriasis

This is the most difficult differential diagnosis for PRP, particularly when PRP presents as diffuse erythroderma with psoriatic-like scale in younger patients 1. Key distinguishing features include:

Clinical features favoring psoriasis over PRP:

• Deeply erythematous color of skin eruption 1
• Absence of "skip" areas (islands of normal skin) 1
• Lack of significant palmoplantar keratoderma 1
• Presence of indurated plaques with silvery scale 1
• Personal or family history of psoriasis 1

Clinical features favoring PRP over psoriasis:

• Presence of characteristic "skip" areas (islands of spared normal skin) 1
• Prominent palmoplantar keratoderma 1
• Yellow-orange erythroderma rather than deep red 2
• Follicular papules as initial presentation 2

Histologic differentiation:

• Psoriasis shows neutrophils in mounds of parakeratosis, thin rete ridges, thin suprapapillary plates, and broad dermal papillae 3
• PRP demonstrates alternating orthokeratosis and parakeratosis (both vertical and horizontal), focal or confluent hypergranulosis, thick suprapapillary plates, and broad rete ridges 3

Atopic Dermatitis

PRP may resemble atopic dermatitis, particularly in erythrodermic presentations 1. Distinguishing features include:

• Atopic dermatitis typically has a history of childhood onset, personal or family history of atopy, and elevated IgE levels 1
• PRP lacks the typical distribution pattern of atopic dermatitis (flexural involvement in children) 1
• Atopic dermatitis does not typically present with follicular papules or palmoplantar keratoderma 1

Seborrheic Dermatitis

Seborrheic dermatitis enters the differential, particularly in early or limited disease 1:

• Seborrheic dermatitis affects groin and axillary regions, which PRP typically spares 1
• Seborrheic dermatitis is less pruritic than PRP 1
• Distribution favors seborrheic areas (scalp, nasolabial folds, central chest) 1

Cutaneous T-Cell Lymphoma (Mycosis Fungoides/Sézary Syndrome)

This is a critical diagnosis not to miss 1:

Key diagnostic steps:

• Skin biopsy looking for atypical lymphocytes 1
• Flow cytometry and Sézary cell count to assess blood involvement 1
• T-cell receptor gene rearrangement for clonality 1
• HIV screening, as HIV can present as erythroderma 1

Contact Dermatitis

Contact dermatitis may present similarly, particularly in widespread reactions 1:

• History of exposure to allergens or irritants is key 1
• Patch testing may be indicated in unclear cases 1

Secondary Differential Diagnoses

Drug-Induced Erythroderma

Medication history is essential, as numerous drugs can cause erythrodermic reactions that mimic PRP 1

Pityriasis Alba and Pityriasis Versicolor

These conditions are included in broader differential diagnoses of papulosquamous disorders 1, 4:

• Pityriasis alba presents as hypopigmented patches with fine scale, primarily in children 4
• Pityriasis versicolor shows hyperpigmented or hypopigmented macules with fine scale caused by Malassezia species 4
• Wood's lamp examination can aid diagnosis 4

Immunotherapy-Related Eruptions

In patients receiving immune checkpoint inhibitors, PRP-like eruptions have been reported 1:

• Erythrodermic papulosquamous eruptions including psoriasis or PRP-like presentations occur as immune-related adverse events 1
• These typically appear as well-delimited erythematous scaly plaques 1

Sarcoidosis

Sarcoidosis can present with skin manifestations that may mimic PRP 1:

• Nodular or papular lesions may be present 1
• Systemic features (pulmonary involvement, lymphadenopathy) help distinguish sarcoidosis 1
• Biopsy showing non-caseating granulomas is diagnostic 1

Diagnostic Approach

When PRP is suspected, the following systematic evaluation is recommended:

1. Detailed clinical examination looking for:

   • Follicular papules progressing to yellow-orange erythroderma 2
   • Characteristic "skip" areas of normal skin 1
   • Well-demarcated palmoplantar keratoderma 2
   • Distribution pattern and color of erythema 1

2. Skin biopsy to evaluate:

   • Alternating orthokeratosis and parakeratosis in vertical and horizontal directions 3
   • Focal or confluent hypergranulosis 3
   • Thick suprapapillary plates and broad rete ridges 3
   • Follicular plugging (when present in follicular lesions) 3

3. Additional testing when diagnosis is uncertain:

   • Flow cytometry and Sézary cell count if cutaneous T-cell lymphoma is suspected 1
   • HIV screening in appropriate clinical contexts 1
   • Fungal culture or KOH preparation if pityriasis versicolor is considered 4

Clinical Pitfalls

Common diagnostic errors to avoid:

• Assuming all erythroderma with scale is psoriasis without looking for PRP's characteristic skip areas and palmoplantar keratoderma 1
• Overlooking cutaneous T-cell lymphoma in patients with persistent erythroderma; always consider biopsy with immunohistochemistry in refractory cases 1
• Missing drug-induced causes by failing to obtain thorough medication history 1
• Relying solely on histology when clinical features are more distinctive; PRP and psoriasis share many histologic features 3, 5
• Failing to recognize that narrowband UVB may be contraindicated in adult PRP and should be used with caution 1