Differentiating Eczema from Psoriasis in Adults
Distinguish eczema from psoriasis by examining lesion distribution, morphology, and associated symptoms: psoriasis typically presents with well-demarcated, thick, silvery-scaled plaques on extensor surfaces (elbows, knees) and scalp, while eczema shows poorly-demarcated, erythematous patches with vesicles and weeping on flexural surfaces (antecubital/popliteal fossae) accompanied by intense pruritus.
Key Clinical Distinguishing Features
Distribution Patterns
Psoriasis:
- Extensor surfaces (elbows, knees, dorsal hands) 1
- Scalp, nails, and nail folds 2
- Both sides of wrists and dorsal hands 2
- Symmetric distribution 3
Eczema:
- Flexural areas (antecubital and popliteal fossae) 1
- Palmar surfaces, palmar side of digits, and finger pulps 2
- Poorly-demarcated borders 2
Morphologic Characteristics
Psoriasis:
- Well-demarcated, indurated plaques with thick silvery scale 1
- Hyperkeratotic plaques, particularly on palms/soles 2
- Absence of vesiculation 4
- Deeply erythematous color 1
Eczema:
- Poorly-demarcated erythematous patches 2
- Vesicles, papules, and weeping lesions 4, 2
- Fissures more common than in psoriasis 2
- Serum crusts 5
Symptomatology
Pruritus intensity: Eczema presents with significantly more intense itching compared to psoriasis (P < 0.001) 5, 2. This is one of the most reliable clinical differentiators.
Histopathologic Differentiation When Clinical Diagnosis is Unclear
When clinical features overlap, skin biopsy reveals distinct patterns:
Psoriasis hallmarks:
- Confluent parakeratosis with neutrophils in stratum corneum 4
- Loss of granular layer 4
- Suprapapillary plate thinning 4
- Regular elongation of rete ridges 4
- Minimal to absent spongiosis 4
Eczema hallmarks:
- Spongiosis and spongiotic vesicles 4, 5
- Lymphocytic exocytosis 4
- Eosinophil granulocytes in dermis 5
- Dermal edema 4
- Preserved granular layer 4
The Challenge of "Eczematized Psoriasis"
Critical caveat: Approximately 5-10% of psoriasis patients develop "eczematized psoriasis," which combines features of both conditions 5, 6. These patients present with:
- Severe itching (atypical for classic psoriasis) 5
- Histologic spongiosis within psoriatic plaques 4, 5
- Eosinophils on biopsy 5
- Often coexisting allergic contact dermatitis (35% of cases) 6
When encountering mixed features, consider patch testing to rule out allergic contact dermatitis superimposed on psoriasis 6.
Molecular Diagnostic Approach for Indeterminate Cases
For clinically and histologically indistinct cases, molecular markers provide >95% diagnostic accuracy:
- NOS2 (iNOS) elevation: Indicates psoriasis 7
- CCL27 elevation: Indicates eczema 7
- These markers work antagonistically and can be assessed via immunofluorescence on paraffin sections 7
Location-Specific Diagnostic Pearls
Hand Involvement
Psoriasis favors: Dorsal hands, wrists (both sides), nail folds, nails 2 Eczema favors: Palmar surfaces, finger pulps, palmar digits 2
Palmoplantar Involvement
Red flag for secondary syphilis: Symmetric palmoplantar red-brown macules with acute onset (weeks) require RPR/VDRL and treponemal testing to exclude secondary syphilis, which mimics both conditions 3. Psoriasis follows a chronic, stable course, not acute onset over weeks 3.
Practical Diagnostic Algorithm
- Assess distribution: Extensor (psoriasis) vs. flexural (eczema) 1, 2
- Evaluate borders: Sharp demarcation (psoriasis) vs. ill-defined (eczema) 2
- Assess scale quality: Thick silvery (psoriasis) vs. thin with vesicles (eczema) 1, 4
- Quantify pruritus: Mild (psoriasis) vs. severe (eczema) 5, 2
- Check nails: Pitting/onycholysis (psoriasis) vs. typically spared (eczema) 1, 2
- If uncertain: Perform biopsy from multiple sites for histopathology 8, 4
- If mixed features: Consider patch testing for contact dermatitis 6
Additional Systemic Considerations
Screen psoriasis patients for psoriatic arthritis: 30-33% of psoriasis patients develop inflammatory arthritis, typically 10-11 years after skin disease onset 1. Look for dactylitis ("sausage digits"), enthesitis, and inflammatory joint pain 1.