Quantitative Histopathologic Analysis of Pigmentation and Melanophage Distribution in Dermal Postinflammatory Hyperpigmentation.

BACKGROUND: Dermal postinflammatory hyperpigmentation (PIH) is frequently refractory to treatment, and therapeutic response is thought to depend on the depth and distribution of dermal pigment. Key quantitative histopathologic features, however, remain poorly defined. METHODS: Ninety-five biopsy-proven cases of dermal PIH were analyzed. Fontana-Masson-stained sections underwent image analysis to quantify epidermal and dermal pigmentation intensity, melanophage density, cross-sectional area, and depth. Associations with inflammatory etiology, Fitzpatrick skin type (FST), and sun-exposure status of the biopsy site were assessed. RESULTS: Epidermal pigmentation intensity was significantly increased in spongiotic dermatitis, higher FST, and sun-exposed sites. Dermal pigmentation intensity was associated with sun exposure. Melanophage density increased with higher FST but was lower in spongiotic dermatitis compared with lichenoid and vacuolar interface dermatitis. In contrast, melanophage cross-sectional area and depth showed no significant associations with etiology, FST, or anatomic site. Median melanophage depth was 295.0 μm (IQR, 220.0-407.0 μm) from stratum corneum and 163.0 μm (IQR, 111.2-235.5 μm) from basement membrane, with consistent measurements across subgroups. CONCLUSION: Despite variability in epidermal pigmentation and melanophage density, melanophage depth is uniform across dermal PIH. These findings refine the conventional epidermal-dermal PIH classification and support a quantitative, anatomy-based framework to inform depth-targeted treatment strategies.