Skin thickness in preterm neonates: relationship with skin bilirubin and predicted mortality

This study investigates the relationship between the thickness of skin in preterm infants and its link with bilirubin and mortality, as this may help understanding the potential deleterious effects of phototherapy. Observational, prospective cohort study enrolling preterm neonates needing phototherapy and admitted to a neonatal intensive care unit. Transcutaneous bilirubin (TcB) and blood bilirubin were simultaneously measured before the onset of phototherapy, if any. The skin depth was measured by high-frequency point-of-care ultrasound. Mortality risk was estimated using the critical risk index for babies-II. Correlations and multivariate regressions (adjusting for several confounders) were applied to study the relationship between skin depth, TcB, and predicted mortality. One hundred fifty-nine neonates were studied. There was a positive and steady correlation between skin depth and TcB (r = 0.402 (95%CI: 0.206; 0.568), p < 0.001) and inverse correlation between skin depth and predicted mortality (r = -0.503 (95%CI: -0.61; -0.37), p < 0.001) as well as between TcB and predicted mortality (r = -0.303 (95%CI: -0.49; -0.09), p = 0.005). Multivariate analyses showed skin depth to be the strongest risk factors associated with both increasing TcB (β = 198 (59;338), p < 0.001) and decreasing risk of death (β = -24 (-46;2), p = 0.049). Blood bilirubin and gestational age were also associated with TcB and predicted mortality, respectively.   Conclusion: In NICU-admitted preterm infants, thicker skin is associated with higher TcB levels irrespective of gestational age. Greater skin depth is also associated with lower predicted mortality irrespective of blood bilirubin. What is Known: • In preterm infants, phototherapy may improve neuro-developmental outcomes but, particularly in the smallest and sickest ones, may increase mortality. • Mechanisms behind this effect are unclear but could involve the small thickness of preterm skin. This however has never been studied in relationship with bilirubin and mortality. What is New: • In NICU-admitted preterm infants, thicker skin is associated with higher levels of transcutaneous bilirubin, irrespective of gestational age and with lower predicted mortality, irrespective of blood bilirubin. • These data suggest that a thinner skin may contain less bilirubin to be photoisomerised and protect internal tissues less efficiently.