Histopathological Differential Diagnosis of Psoriasis and Seborrheic Dermatitis of the Scalp

BACKGROUND: The differential diagnosis of psoriasis and seborrheic dermatitis can be difficult when both conditions are localized to the scalp without the involvement of other skin sites. OBJECTIVE: We aimed to evaluate the histopathological differences between psoriasis and seborrheic dermatitis on the scalp and identify favorable criteria for their differential diagnosis. METHODS: We evaluated 15 cases of psoriasis and 20 cases of seborrheic dermatitis of the scalp that had been clinicopathologically diagnosed. Skin biopsy sections stained with H&E were examined. Additional immunohistochemistry was performed, including Ki-67, keratin 10, caspase-5, and GLUT-1. RESULTS: On histopathological examination, mounds of parakeratosis with neutrophils, spongiform micropustules of Kogoj, and clubbed and evenly elongated rete ridges were significantly more frequently observed in psoriasis. Follicular plugging, shoulder parakeratosis and prominent lymphocytic exocytosis were significantly more common in seborrheic dermatitis. Moreover, significantly higher mitotic figures were observed in psoriatic lesions than in seborrheic dermatitis. Immunohistochemistry did not show any difference between psoriasis and seborrheic dermatitis. CONCLUSION: Histopathological features favoring psoriasis include mounds of parakeratosis with neutrophils, spongiform micropustules of Kogoj, clubbed and evenly elongated rete ridges, and increased mitotic figures (≥6/high-powered field). Features indicating seborrheic dermatitis are follicular plugging, shoulder parakeratosis and prominent lymphocytic exocytosis. Immunohistochemistry was not helpful in differentiating psoriasis from seborrheic dermatitis.