How can seborrheic dermatitis be differentiated from psoriasis in a healthy adolescent or adult?

Differentiating Seborrheic Dermatitis from Psoriasis

The key clinical features that distinguish psoriasis from seborrheic dermatitis are well-demarcated, indurated plaques with thick silvery scale in psoriasis versus greasy, yellow scales with less defined borders in seborrheic dermatitis, along with personal or family history of psoriasis strongly favoring that diagnosis. 1

Clinical Examination Approach

Distribution and Morphology

• Psoriasis presents with sharply defined, thick, indurated plaques covered by silvery-white scales, typically on the scalp, elbows, knees, and trunk 1
• Seborrheic dermatitis shows poorly demarcated, thin erythematous patches with greasy, yellowish scales, predominantly affecting the scalp, central face (nasolabial folds, eyebrows), and anterior chest 1, 2
• Inspect the entire body for psoriatic plaques on extensor surfaces (elbows, knees) and examine nails for pitting, which strongly supports psoriasis 3

Scale Characteristics

• Psoriatic scales are thick, silvery-white, and adherent; removal may cause pinpoint bleeding (Auspitz sign) 1
• Seborrheic scales are greasy, yellowish, and easily removed without bleeding 1

Patient History

• A personal or family history of psoriasis strongly favors this diagnosis over seborrheic dermatitis 1, 3
• Psoriatic plaques tend to be more stable and chronic, while seborrheic dermatitis often fluctuates with stress and seasonal changes 2

Advanced Diagnostic Tools When Clinical Examination is Equivocal

Dermoscopy

When scalp lesions are isolated and clinical differentiation is challenging, dermoscopy can provide valuable additional information:

• Psoriasis shows red dots and globules, twisted red loops, and glomerular vessels on dermoscopy 4
• Seborrheic dermatitis demonstrates arborizing vessels, atypical red vessels, and featureless areas devoid of particular vascular patterns 4
• The absence of red dots and globules favors seborrheic dermatitis 4

Histopathology

Skin biopsy should be reserved for cases where clinical and dermoscopic features remain inconclusive:

Features favoring psoriasis: 5

• Mounds of parakeratosis with neutrophils
• Spongiform micropustules of Kogoj
• Clubbed and evenly elongated rete ridges
• Increased mitotic figures (≥6 per high-powered field)
• PAS-reactive serum inclusions and Munro abscesses 6

Features favoring seborrheic dermatitis: 5

• Follicular plugging
• Shoulder parakeratosis
• Prominent lymphocytic exocytosis
• Spongiosis and spongiotic vesicles
• Relatively thin horny layer 6

Common Diagnostic Pitfalls

Sebopsoriasis (Overlap Syndrome)

• Some patients present with features of both conditions, making definitive diagnosis difficult even histologically 7, 6
• In erythrodermic presentations, both conditions can appear nearly identical clinically and histologically 7
• When uncertainty persists, treat empirically based on the predominant clinical features and monitor response to therapy 1

Scalp-Only Involvement

• Differentiation is most challenging when disease is limited to the scalp without involvement of other characteristic sites 5, 4
• In these cases, dermoscopy becomes particularly valuable before proceeding to biopsy 4

Associated Conditions

• 95% of patients with seborrheic blepharitis also have seborrheic dermatitis elsewhere on the body, which can aid diagnosis 8
• Both conditions can coexist with rosacea, but seborrheic dermatitis has a stronger association (74% with seborrheic dermatitis vs. 51% with rosacea in MGD patients) 7

Practical Diagnostic Algorithm

1. Examine scale characteristics and plaque morphology - thick silvery scales with sharp borders suggest psoriasis; greasy yellow scales with ill-defined borders suggest seborrheic dermatitis 1
2. Check for psoriatic plaques on elbows, knees, and nails - presence strongly favors psoriasis 3
3. Obtain personal and family history of psoriasis - positive history is highly specific for psoriasis 1, 3
4. If scalp-only involvement with diagnostic uncertainty, perform dermoscopy - look for red dots/globules (psoriasis) vs. arborizing vessels (seborrheic dermatitis) 4
5. Reserve skin biopsy for persistent diagnostic uncertainty after clinical and dermoscopic evaluation 5