Characteristic Features of a Psoriatic Rash
A psoriatic rash classically presents as sharply demarcated, erythematous (red) plaques covered by silvery-white scales, most commonly distributed symmetrically on the scalp, elbows, knees, and sacral region. 1
Morphology and Appearance
The hallmark lesions have several distinctive features:
- Well-demarcated borders that sharply distinguish psoriatic plaques from surrounding normal skin, creating a clearly defined edge 1, 2
- Erythematous (red to pink) base with prominent inflammation visible beneath the scale 3, 1
- Silvery-white or grey scales that are dry and easily exfoliated, reflecting the rapid keratinocyte turnover 3, 4
- Indurated (thickened) plaques that are raised above the skin surface due to epidermal hyperproliferation 4
- Symmetric distribution affecting both sides of the body in similar patterns 4, 5
Common Anatomic Distribution
Psoriatic lesions preferentially affect specific body regions:
- Extensor surfaces: elbows and knees are classic sites 3, 1
- Scalp: frequently involved, often extending beyond the hairline 3, 1
- Lumbosacral/presacral region: lower back and above the buttocks 3, 1
- Any cutaneous area can be affected, including palms, soles, nails, and genital regions (up to 60% of patients) 3, 1
Clinical Variants with Distinct Presentations
Inverse (Flexural) Psoriasis
- Appears as minimally scaly or non-scaly erythematous plaques in skin folds 1
- Affects axillae, groin, genitals, inframammary areas, and intergluteal cleft 1
- The moist environment prevents typical scale formation 6
Guttate Psoriasis
- Presents as small (1-10 mm) salmon-pink papules with fine scale 1
- Predominantly affects trunk and proximal extremities 1
- Most common in patients under 30 years, often triggered by streptococcal pharyngitis 1
Pustular Psoriasis
- Characterized by visible neutrophil collections appearing as pustules on an erythematous base 1
- Acute generalized form (von Zumbusch) presents with fever, toxicity, and widespread pustules 1
- Localized palmoplantar variant affects hands and feet 1
Erythrodermic Psoriasis
- Generalized erythema covering most body surface with variable scaling 1
- Associated with systemic symptoms including fever, chills, hypothermia, and dehydration 1
- Represents a severe, potentially life-threatening variant 6
Associated Features
Nail Changes
- Occur in approximately 50% of hand nails and 35% of foot nails 1
- Include pitting, onycholysis (nail separation), subungual hyperkeratosis, oil-drop sign, and dystrophy 1
- Present in up to 90% of patients with psoriatic arthritis 1
Symptomatology
- Lesions are often pruritic (itchy) or painful, significantly impacting quality of life 1
- The visible nature creates substantial psychosocial burden 1, 7
Key Diagnostic Pitfalls
- Do not rely solely on Auspitz sign (pinpoint bleeding when scale is removed) for diagnosis; use the constellation of clinical features described above 2
- Location matters more than extent: involvement of face, genitals, hands, feet, or scalp may warrant classification as severe disease regardless of body surface area 2
- In atypical presentations or unusual locations, consider biopsy showing parakeratosis, acanthosis, dilated vessels, and Munro microabscesses for confirmation 4