Differentiating Seborrheic Dermatitis from Psoriasis
The most reliable distinguishing features are scale character (greasy yellowish in seborrheic dermatitis vs. silvery-white in psoriasis), lesion borders (poorly demarcated in seborrheic dermatitis vs. well-demarcated in psoriasis), and distribution pattern (sebaceous areas including groin/axillae in seborrheic dermatitis vs. extensor surfaces with groin/axillae sparing in psoriasis). 1, 2, 3
Key Distinguishing Clinical Features
Scale Characteristics
- Seborrheic dermatitis presents with greasy, yellowish scales that are oily in texture 2, 3, 4
- Psoriasis displays silvery-white, dry scales that are thicker and more adherent 1, 5
- The greasy quality of seborrheic dermatitis scales is one of the most specific differentiating features 2, 3
Lesion Borders and Morphology
- Psoriasis shows well-demarcated, sharply defined erythematous plaques with clear borders 1, 5
- Seborrheic dermatitis has poorly defined borders with patches that blend into surrounding skin 2, 3
- Psoriatic plaques are typically thicker and more raised than seborrheic dermatitis lesions 1, 5
Distribution Pattern (Most Diagnostically Useful)
Seborrheic dermatitis:
- Affects sebaceous-rich areas: scalp, central face (nasolabial folds, eyebrows, glabella), ears, chest, and characteristically involves groin and axillary regions 6, 2, 3, 4
- Commonly presents on the anterior chest in a "V" distribution 3, 4
Psoriasis:
- Classic involvement of extensor surfaces: elbows, knees, scalp, and presacral region 1, 5
- Typically spares groin and axillary regions - this is a critical negative finding 6
- May affect flexural areas in inverse psoriasis, but this represents a distinct variant 5
Pruritus
- Seborrheic dermatitis is frequently pruritic, with itching being a prominent complaint 2, 3
- Psoriasis may be pruritic but is often less intensely itchy than seborrheic dermatitis 6
Dermoscopic Findings (When Available)
If dermoscopy is accessible, vascular patterns provide objective differentiation:
Psoriasis shows:
- Red dots and globules
- Twisted red loops
- Glomerular vessels 7
Seborrheic dermatitis shows:
- Arborizing vessels
- Atypical red vessels
- Absence of red dots and globules
- Featureless areas without specific vascular patterns 7
Clinical Pitfalls to Avoid
Sebopsoriasis Overlap
- A hybrid condition called "sebopsoriasis" exists where features of both diseases coexist, particularly on the scalp 5, 4
- In these cases, look for lesions elsewhere on the body to establish the primary diagnosis 5
- Greasy scales may predominate, but silvery scales can appear in some areas of sebopsoriasis 5
Infancy Considerations
- In infants, seborrheic dermatitis and atopic dermatitis are particularly difficult to separate and may overlap 6
- Seborrheic dermatitis in infants typically affects groin/axillae and is not pruritic, whereas atopic dermatitis spares these areas and causes significant scratching 6
Scalp-Only Presentations
- When lesions are isolated to the scalp, differentiation is most challenging 7
- Examine the entire body surface for additional lesions to clarify the diagnosis 7, 5
- Dermoscopy of scalp lesions can be particularly valuable in this scenario 7
Diagnostic Algorithm
Assess scale quality: Greasy/yellowish → seborrheic dermatitis; Silvery/dry → psoriasis 2, 3, 1
Examine lesion borders: Well-demarcated → psoriasis; Poorly defined → seborrheic dermatitis 1, 2
Check groin and axillae: Involved → seborrheic dermatitis; Spared → psoriasis 6, 3
Identify primary distribution: Extensor surfaces (elbows/knees) → psoriasis; Central face/chest → seborrheic dermatitis 1, 3
If diagnosis remains unclear: Consider dermoscopy for vascular pattern assessment or skin biopsy, though biopsy may not always be definitive 7, 6