Quantification of Palmar and Digital Epidermal Thickness and Sweat Duct Density.

Palmar digital skin, with its high density of epidermal sweat glands and unique stratum corneum, derives its autonomic innervation from peripheral nerves. Understanding the changes to these structures could serve as objective proxy markers following peripheral nerve injury and interventions to promote regeneration. There are limited data in humans for these measures, and normative reference ranges are yet to be defined. Light-based imaging techniques, such as optical coherence tomography (OCT), can detect quantifiable changes in these epidermal parameters. This study aimed to define the normal epidermal thickness and sweat duct density (SDD) of volar hand skin. A total of 53 healthy, non-injured participants, 21 males and 32 females, were enrolled. Two- and three-dimensional images of defined 1 × 1 mm areas of volar fingertip skin were obtained using OCT. The mean epidermal thickness and SDD were manually calculated. Results were compared across all digits and by age and gender. The 95% reference intervals for both parameters were also calculated. There was a significant (p < 0.0001) progressive reduction in epidermal thickness across the digits, from the thumb towards the little finger. Females had a significantly thinner (p = 0.011) epidermal layer in comparison to males. Sweat duct density was significantly lower (p < 0.030) in the thumb compared to the middle, ring, or little fingers. There was a general downward trend in the number of sweat glands per mm2 with age (p < 0.0001). The reference interval for epidermal thickness was 0.37-0.79 mm, while for SDD the interval was 30-86 ducts/mm2. This study has outlined normative differences in SDD and epidermal thickness in palmar digital skin. Our results can be used as control measures for prospective studies looking into the regenerative process following peripheral nerve injuries or identifying potential trauma to digital nerves.