Abstract
It has been 20 years since functional near-infrared spectroscopy (fNIRS) was first used to investigate the evoked hemodynamic response to a stimulus in newborns. The hemodynamic response to functional activation is well-established in adults, with an observed increase in concentration change of oxygenated hemoglobin (Δ[HbO2]) and decrease in deoxygenated hemoglobin (Δ[HHb]). However, functional studies in newborns have revealed a mixed response, particularly with Δ[HHb] where an inconsistent change in direction is observed. The reason for this heterogeneity is unknown, with potential explanations arising from differing physiology in the developing brain, or differences in instrumentation or methodology. The aim of this review is to collate the findings from studies that have employed fNIRS to monitor cerebral hemodynamics in term newborn infants aged 1 day−1 month. A total of 46 eligible studies were identified; some studies investigated more than one stimulus type, resulting in a total of 51 reported results. The NIRS parameters reported varied across studies with 50/51 cases reporting Δ[HbO2], 39/51 reporting Δ[HHb], and 13/51 reporting total hemoglobin concentration Δ[HbT] (Δ[HbO2] + Δ[HHb]). However, of the 39 cases reporting Δ[HHb] in graphs or tables, only 24 studies explicitly discussed the response (i.e., direction of change) of this variable. In the studies where the fNIRS responses were discussed, 46/51 cases observed an increase in Δ[HbO2], 7/51 observed an increase or varied Δ[HHb], and 2/51 reported a varied or negative Δ[HbT]. An increase in Δ[HbO2] and decrease or no change in Δ[HHb] was observed in 15 studies. By reviewing this body of literature, we have identified that the majority of research articles reported an increase in Δ[HbO2] across various functional tasks and did not report the response of Δ[HHb]. Confirming the normal, healthy hemodynamic response in newborns will allow identification of unhealthy patterns and their association to normal neurodevelopment.
Highlights
Monitoring brain activity in newborn populations up to 1 month old is of increasing interest for neuroscientists and psychologists who want to develop a deeper understanding of the brain and its development, and for clinicians to derive prognostic markers of neurodevelopment following perinatal brain injury [such as hypoxic-ischaemic encephalopathy (HIE)]
A total of 46 studies using functional near-infrared spectroscopy (fNIRS) in neonates were identified, with a total of 51 sets of results arising from some studies investigating more than one stimulus type
Some studies looked at both term and preterm newborns (Isobe et al, 2001; Ozawa et al, 2011a; Naoi et al, 2013; Carlier-Torres et al, 2014; Frie et al, 2017; Arimitsu et al, 2018) or included subjects older than 30 days (Meek et al, 1998); the results presented here only include the responses from the term infants
Summary
Monitoring brain activity in newborn populations up to 1 month old is of increasing interest for neuroscientists and psychologists who want to develop a deeper understanding of the brain and its development, and for clinicians to derive prognostic markers of neurodevelopment following perinatal brain injury [such as hypoxic-ischaemic encephalopathy (HIE)]. Since the first functional near-infrared spectroscopy (fNIRS). Study in newborns in 1998 by Meek et al (1998) there have been a number of studies using this technique to investigate brain function and development in newborns, as well as older infants and children (Lloyd-Fox et al, 2010; McDonald and Perdue, 2018). It relies on the fact that tissue is relatively transparent to light in the near-infrared region (650–1,000 nm), and oxygenated- (HbO2) and deoxygenated- (HHb) hemoglobin are strong absorbers in this region. Similar to functional magnetic resonance imaging (fMRI), fNIRS is able to detect functional activity indirectly via detection of hemodynamic changes. Whilst fMRI is able to detect changes in HHb, fNIRS has the ability to differentiate between HbO2 and HHb, providing additional hemodynamic and oxygenation information. It is an appropriate tool to study the newborn brain
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