Differentiating solar lentigines from post-inflammatory hyperpigmentation with hyperspectral imaging: A pilot study

Solar lentigines and post-inflammatory hyperpigmentation (PIH) are common pigmentation disorders, and they are usually differentiated by a trained dermatologist. Accurate identification of the spot type is essential for providing patients with the most appropriate skin treatment. The aim of this study was to evaluate the capacity of hyperspectral imaging (HSI) in characterizing and discriminating solar lentigines from PIH, that is, to identify the colorimetric features and chromophore concentrations associated with each pigmentation disorder.

Acquisition of HSI was performed using a liquid-crystal tuneable filter spectral scanning spectrometer. The recruited panel consisted of female Japanese subjects (n = 11) with one solar lentigo on both the left and right cheeks. Five of these subjects also had one PIH spot on both the left and right cheeks.

Significant differences were found in the mean difference relative to the surrounding skin between PIH spots and solar lentigines for the following features (all p < 0.05): ΔOxyhaemoglobin (PIH spots (2.3 ± 1.1) × 10^-2 g·cm·L^-1 vs. solar lentigines (-0.1 ± 0.6) × 10^-2 g·cm·L^-1), ΔDeoxyhaemoglobin (PIH spots (6.3 ± 2.6) × 10^-2 g·cm·L^-1 vs. solar lentigines (0.9 ± 1.7) × 10^-2 g·cm·L^-1), Δa* values (PIH spots 5.3 ± 2.0 vs. solar lentigines 1.6 ± 1.3) and Δb* values (PIH spots -1.2 ± 0.7 vs. solar lentigines -0.1 ± 0.4). No significant difference was observed in ΔL* values and ΔMelanin content between the two hyperpigmentation types (all p > 0.05).

Compared with solar lentigines, PIH spots displayed higher haemoglobin content and Δa* values and lower Δb* values. To strengthen these preliminary findings, further clinical studies need to be conducted on a larger cohort, aiming at confirming the benefit of HSI in accurately differentiating these two skin pigmentation disorders to support clinical assessment.