Epidermal thickness in healthy humans: a systematic review and meta-analysis

Several skin diseases are characterized by epidermal alterations affecting epidermal thickness. Reference values of epidermal thickness in healthy humans and knowledge of possible differences regarding age, sex, skin phototype, and ethnic origin are essential in research and in clinical practice. The objectives of this systematic review were to provide epidermal thickness reference values for healthy human skin and describe possible effects of measurement methods, age, sex, ethnic origin, and skin phototype. A combined search in the databases Medline and Embase, and other sources were conducted. Searches covered a period from 1946 to 3 June 2020. Included studies were primarily observational and interventional studies providing means and spread values of epidermal thickness estimates in healthy humans, with clear reporting of skin area, age, and measurement method, and optional reporting of sex, ethnic origin, and skin phototype. Data were extracted per skin area and pooled in random-effects models. A total of 142 studies were included in the qualitative synthesis and 133 in the meta-analysis. Pooled epidermal thickness estimates were calculated for 37 skin areas. The lowest epidermal thickness of 31.2 (95% CI 27.8-34.6) μm was reported for the penis and the highest of 596.6 (95% CI 443.9-749.3) μm for the plantar aspect of the foot. Differences in epidermal thickness estimates obtained by histology, optical coherence tomography, and laser scanning microscopy were minor. High-frequency ultrasonography produces systematically higher values. The epidermis was thinner in aged skin. Differences between sexes and among ethnic origins were minor. Epidermal thickness reference values are provided for 37 skin areas. In conclusion, the epidermis tends to become thinner by ageing and does not seem to be influenced by sex. Histology, optical coherence tomography, and laser scanning microscopy might be used interchangeably to measure epidermal thickness, whereas high-frequency ultrasound should not be used.