BACKGROUND Capillary rarefaction (CR) is an established hallmark of essential hypertension (EH). The aim of this study was to examine early changes in capillary density (CD) and blood pressure (BP) in low birth weight (LBW) infants who are at risk of developing EH in later life. METHODS We studied 77 LBW infants and 284 normal birth weight (NBW) infants, all born to mothers with normotension, in a longitudinal multicenter study. Intravital capillaroscopy was used to measure functional basal capillary density (BCD) and maximal capillary density (MCD) at birth, 3, 6, and 12 months. RESULTS We found that LBW infants, born preterm and at term, had a significantly higher CD at birth, then underwent significant CR in the 1st 3 months culminating in a CD similar to that seen in NBW infants. NBW infants showed a gradual reduction in CD between birth and 12 months. Non-Caucasian ethnicity and preterm birth were significant predictors of a higher CD at birth. Systolic BP in NBW infants increased significantly from birth to 3 months, and we identified a significant negative correlation between systolic BP and MCD. CONCLUSIONS This study has identified a process of early “accelerated capillary remodeling” in LBW infants, which corrects their higher CD at birth. This remodeling is unlikely to explain the CR seen in adult individuals with, or at risk of developing EH. Further follow-up studies are required to determine the timing and mechanisms involved in CR, which is likely to occur after the 1st year of life but before early adulthood.

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Introduction: Capillary rarefaction, defined as a reduction in capillary density, is an established hallmark of essential hypertension. Low birth weight (LBW) infants, known to have an increased risk of developing hypertension as adults, were unexpectedly found to have a significantly higher capillary density at birth when compared to normal birth weight (NBW) infants. We therefore hypothesised that there is a microcirculatory window in the 1 st year of life of LBW infants, during which a process of extensive capillary loss, known as “hyperpruning”, occurs. Methods: The George’s Capillary Rarefaction Offspring Study (G-CROS) is a longitudinal, multi-centre study of which 284 infants were NBW, born at term, and 77 were LBW. Intravital microscopy was used to measure the functional (also known as basal) capillary density (BCD), and the structural (also known as maximal) capillary density (MCD) at birth, 3 months, 6 months and 12 months. Results: LBW infants had a significantly higher capillary density at birth when compared to NBW infants ( p < 0.0001). NBW infants showed a gradual reduction in capillary density between birth and 12 months, with their greatest reduction occurring between birth and 3 months (BCD mean difference = -27.62 cap/field, p <0.0001 and MCD mean difference = -31.49 cap/field, p <0.0001). Similarly, the most significant reduction in BCD and MCD, in the LBW cohort, was between birth and 3 months (BCD mean difference = -47.01 cap/field, p < 0.0001 and MCD mean difference = -48.01 cap/field, p < 0.0001). However, within this same 3-month interval, LBW infants demonstrated a significantly higher percentage reduction in BCD (mean difference = 7.81%, p = 0.0194) and MCD (mean difference = 8.29%, p = 0.0361) when compared to NBW infants. Conclusions: Once again, LBW infants were shown to have a higher capillary density than NBW infants at birth. However, this study has shown for the first time that there appears to be a microcirculatory window in the first 3 months of life during which LBW infants undergo capillary “hyperpruning”. Further follow-up studies are required to investigate the role of early capillary rarefaction in the pathogenesis of hypertension, as well as the mechanisms orchestrating these early microcirculatory changes.