What is atypical psoriasis?

What is Atypical Psoriasis?

Atypical psoriasis refers to presentations of psoriasis that deviate from the classic plaque-type morphology, including inverse, erythrodermic, pustular, and guttate variants, as well as cases of plaque psoriasis that lack typical clinical or pathological features and may mimic other dermatoses. 1

Two Distinct Meanings of "Atypical Psoriasis"

1. Non-Plaque Psoriasis Variants (Classical Atypical Forms)

These are well-recognized subtypes that differ morphologically from typical plaque psoriasis:

Inverse (Flexural) Psoriasis

• Characterized by erythematous plaques with minimal scale due to moisture in affected areas 1
• Locations include axillary, genital, perineal, intergluteal, inframammary areas, and flexural surfaces like antecubital fossae 1
• The lack of typical silvery scale makes diagnosis challenging 2

Erythrodermic Psoriasis

• Generalized erythema covering nearly the entire body surface area with varying degrees of scaling 1
• Can develop gradually from chronic plaque disease or acutely with little preceding psoriasis 1
• Life-threatening complications include altered thermoregulation leading to hypothermia, fluid loss causing dehydration, and systemic symptoms like fever and malaise 1

Pustular Psoriasis

• Clinically apparent collections of neutrophils forming pustules 1
• Generalized form (von Zumbusch variant): uncommon, severe, with widespread pustules on erythematous background, accompanied by fever and toxicity 1
• Localized form: involves palms and soles, with or without classic plaque-type disease elsewhere 1

Guttate Psoriasis

• Dew-drop-like, 1-10mm salmon-pink papules with fine scale 1, 2
• Common in individuals younger than 30 years 1
• Primarily affects trunk and proximal extremities, occurring in less than 2% of psoriasis patients 1

2. Atypical Plaque Psoriasis (Diagnostically Challenging Cases)

This refers to plaque psoriasis cases that lack typical clinical features or pathological hallmarks (Munro microabscesses and spongiform pustules of Kogoj), creating diagnostic confusion with psoriasis mimics. 3

Key Distinguishing Features from Mimics:

Pathological clues that favor atypical plaque psoriasis over mimics: 3

• Type of parakeratosis pattern
• Epidermal capture of extravasated erythrocytes
• Absence of focal basal liquefied degeneration (more common in mimics)
• Specific inflammatory cell types and distribution
• Depth of dermal inflammation

Clinical clues: 3

• Presence and character of scales and crusts differ significantly from conditions like pityriasis rosea, pityriasis lichenoides chronica, and subacute dermatitis

Clinical Context and Importance

The term "atypical" is clinically significant because these presentations can significantly impact morbidity and quality of life, yet may be misdiagnosed or have delayed diagnosis. 4, 5

• Classic psoriasis presents with well-demarcated, erythematous plaques with silvery scale on elbows, knees, scalp, and presacral region 6
• Atypical forms require different treatment approaches than plaque psoriasis, particularly inverse and guttate variants which respond better to topical therapies and phototherapy, while pustular and erythrodermic forms may require biologics like ustekinumab 2

Common Diagnostic Pitfall

Do not rely solely on Auspitz sign for diagnosis—clinicians should evaluate the constellation of clinical features, distribution, and overall presentation rather than depending on any single diagnostic sign. 6 When diagnostic uncertainty exists in atypical cases, biopsy is indicated to distinguish psoriasis from its mimics. 3