Rethinking post inflammatory hyperpigmentation: A prospective pilot study supporting a persistent not post-inflammatory process

Post-inflammatory hyperpigmentation (PIH) is common and challenging condition seen more frequently in skin of color. The aim of this study was to characterize PIH clinically and histopathologically using photography, colorimetry, spectrophotometry, a survey, and skin biopsy evaluation. Twenty-three subjects (18 African American, 3 Asian, and 2 White) were enrolled in this single-center, prospective pilot study. Acne was the most common cause of PIH in African Americans, followed by psoriasis and atopic dermatitis in Asians, and other causes in White participants. Colorimeter analysis revealed that deltaL* (ΔL*) between PIH and normal skin did not differ by cause of PIH but did by racial/ethnic group, with African Americans having the smallest ΔL* followed by Asian and then White participants. Compared to normal skin, PIH skin showed an increase in hyperkeratosis, acanthosis, elongation of rete ridges, mid-dermal and interstitial inflammation, plasma cells, dermal fibrosis, and dermal melanophages. Immunohistochemical staining showed PIH skin had increased staining for CD3, CD20, CD68, and melanin. For hyperpigmentation secondary to acne, atopic dermatitis, and psoriasis, persistent dermal and interstitial inflammation suggests the inflammatory process may be ongoing even after the primary condition has clinically resolved, representing persistent and inflammatory hyperpigmentation (PAIH). The proposed concept of PAIH has potential therapeutic implications, including exploration of the continued use of anti-inflammatory treatments for PAIH.