Correlations between capillary density and degree of skin pigmentation in healthy children analysed by nailfold video capillaroscopy.

Background: Nailfold video capillaroscopy (NVC) is a simple, non-invasive diagnostic tool but studies with normal values for capillary density in healthy children are rare. Ethnic background seems to play a role in capillary density; however, this is not well substantiated yet. In this work, we set out to evaluate influence of ethnic background/skin pigmentation and age on capillary density reading in healthy children. Secondary aim was to investigate whether there is a significant difference in density between different fingers within the same patient. Methods: Between 2016 and 2021, healthy children from schools around AUMC were approached, by convenience sampling. In this cross-sectional study, capillaroscopic images were obtained in a one-time videocapillaroscopy (×200 magnification) addressing the capillary density (i.e., number of capillaries per linear millimetre in the distal row). This parameter was compared to age, sex, ethnicity, skin pigment grade (I-III) and between eight different fingers, excluding the thumbs. Density differences were compared by ANOVAs. Correlations between capillary density and age were calculated with Pearson correlations. Results: We investigated 145 healthy children with mean age of 11.03 years (SD 3.51). The range of capillary density was 4-11 capillaries per millimetre. We observed a lower capillary density in the 'grade II' (6.4±0.5 cap/mm, P<0.001) and 'grade III' (5.9±0.8 cap/mm, P<0.001) pigmented-classified groups compared to the 'grade I' group (7.0±0.7 cap/mm). We did not find a significant correlation between age and density in the overall group. The fifth fingers on both sides had a significantly lower density compared to the other fingers. Conclusions: Healthy children <18 years with higher degree of skin pigmentation show a significantly lower nailfold capillary density. In subjects with an African/Afro-Caribbean and North-African/Middle-Eastern ethnicity, a significantly lower mean capillary density was observed compared to subjects with the Caucasian ethnicity (P<0.001, and P<0.05, respectively. There were no significant differences between other ethnicities. No correlation was found between age and capillary density. The fifth fingers on both hands displayed lower capillary density compared to the other fingers. This needs to be taken into account when describing lower density in paediatric patients with connective tissue diseases.