Nomograms for the lower normal limits of epidermal nerve fibers number and their morphological alterations according to age and sex.

BACKGROUND: The intraepidermal nerve fiber density (IENFD) has been established as the gold standard for the evaluation of small fiber neuropathy (SFN). There is a decrease in IENFD with age, so its sensitivity depends on the accuracy of lower normal limits. IENFD lower normal limits proposed by previous multicenter studies present a wide variety of results. Additionally, their qualitative characteristics in healthy people are quantified only in one study. Therefore, the aim of this single-center study was to introduce a nomogram of IENFD regarding the age (>10 years) and quantify their morphological varieties.

METHODS: Skin biopsy 10 cm above lateral malleolus was collected from 125 healthy volunteers, 48 men, mean age 38.14 ± 17.24 years (range 10-83). After double immunofluorescence the number of intraepidermal nerve fibers (IENF) was calculated. We counted also the number of branching (BNF) and fragments fibers (FNF) as well as the number of epidermal nerve fibers with small swellings (NFS). Clinical tests including sural nerve action potential and foot warm-cold detection thresholds were used to exclude any dysfunction of the peripheral nerves.

RESULTS: The Quantile regression analysis showed that age has a statistically significant negative correlation with IENFD. Because women presented statistically higher IENFD, the lower 0.05 quantile regression regarding age computed separately for women and men. Accordingly, the proposed lower normal limits of IENFD regarding the age and sex of the subjects, are presented in one nomogram.Regarding the qualitative characteristics of the epidermal nerve fibers only the ratio IENF/NFS decreased significantly with age and lower 0.05 quantile regression was calculated to propose the age's lower normal limits. For the ratio IENF/BNF and IENF/FNF we propose the lower 5th percentile of our recordings as the lower normal limits: 2.38 and 3.05, respectively.

CONCLUSION: Using restricted methods, our single-center study results propose a nomogram for the lower normal limits of IENFD according to age and sex for first time including children > 10 years old as complement to the already published multicenter study values. Part of the above have already been applied successfully to evaluate SFN of different etiology in patients. Additionally, for the first time we propose nomogram for the epidermal nerve fibers with small swellings according to the age of healthy volunteers and lower normal limits for branching and fragments epidermal nerve fibers. Future independent studies should validate their diagnostic performance in patients with different etiologies of small fiber neuropathy.