Our findings in 2-3-year-olds in this high-risk South African birth cohort, support the emerging literature reporting putative widespread effects of PDE on white matter microstructural integrity in young children. Although we did not detect group differences at neonatal age, we describe changes at 2–3 years associated with PDE in major association and projection tracts connecting cortical and subcortical regions including the sagittal stratum, superior fronto-occipital fasciculus, and corona radiata, and the posterior region of the cingulum which extends to the hippocampus of the limbic system. We further describe association of these white matter alterations with cognitive development at this age.
Our findings suggest that the sagittal stratum may play a key role with regard to the impact of maternal depression on early child neurodevelopment. Group effects in 2-3-year-olds were large compared to group differences in other tracts, and controlling for substance use did not alter effects. The sagittal stratum is a large intricate white matter bundle that relays major tracts such as the inferior frontal-occipital fasciculus, and superior and inferior longitudinal fasciculi, between cortical and subcortical regions (Jellison et al. 2004). We observed differences in FA, MD and RD in PDE compared to controls in this region, a difference in RD suggesting altered myelination. Given the importance of the sagittal stratum in terms of connections, alteration of white matter integrity in this tract bundle may have widespread implications for effective integration of numerous functions including emotional processing, regulation of mental distress, sensory and motor function (Hinton et al. 2019; Juttukonda et al. 2019; Patel et al. 2018; Poletti et al. 2020). One prior study in neonates found sex-by-symptom interaction effects for FA in the sagittal stratum for a composite of prenatal depression and anxiety (Dean et al. 2018). The involvement of the sagittal stratum in adolescents and adults with major depression has been previously described (Henderson et al. 2013; Hermesdorf et al. 2017; Kieseppä et al. 2010; Ota et al. 2015). Further, studies in depressed middle-aged adults have also described altered FA, MD, RD and AD in the sagittal stratum (Kieseppä et al. 2010; Korgaonkar et al. 2011; Ota et al. 2015).
We also noted altered FA and RD in the superior fronto-occipital fasciculus, the latter parameter having lost significance when controlling for substance use. There is some support for altered MD and RD in tracts originating from frontal regions including the inferior fronto-occipital fasciculus in 2-5-year-old children with a history of PDE (Lebel et al. 2016). Since the superior frontal-occipital fasciculus had altered white matter integrity in adult mega studies of major depressive disorder (van Velzen et al. 2020), this tract may well be of interest across the lifespan following PDE. The superior fronto-occipital fasciculus extends to frontal and parietal lobes, and medially to the thalamus (Wakana et al. 2004). The study by Dean and colleagues (Dean et al. 2018) in neonates also reported altered microstructure in the posterior thalamic radiation, and in white matter adjacent to the hippocampus; consistent with our findings at 2–3 years of alterations in the posterior thalamic radiation and hippocampal portion of the cingulum.
Further, the posterior and superior corona radiata had altered white matter integrity in our cohort at 2–3 years. This is partly consistent with previous studies. Lebel and colleagues found in 2-5-year olds with PDE, associations of postpartum depression with diffusion in tracts of the superior frontal cortex including the anterior portion of the corona radiata (Lebel et al. 2016); while studies in neonates with prenatal exposure to depression and anxiety found altered MD, RD and AD across the corona radiata (Dean et al. 2018). These tract regions also extend to the posterior thalamic radiation, while the larger corona radiata connect most cortical regions via a mixture of projection, association and callosal tracts (Jellison et al. 2004; Wakana et al. 2004).
It is unclear why we did not observe associations of PDE at neonatal age in our cohort as inconsistent with some previous findings. We had some overlap in the children scanned as neonates and at 2–3 years. This may be explained by challenges common to neuroimaging of young children including different neurodevelopmental stages, practical difficulties, movement and other technical artefacts resulting in smaller sample size (Barkovich et al. 2019; Raschle et al. 2012; Thieba et al. 2018).
Regarding associations with development in our group of 2-3-year-olds with PDE, this is the first time to our knowledge, that altered white matter integrity in PDE has been associated with worse cognitive function at this early age. It is notable that we observed associations in the corona radiata, considering evidence of it being affected following PDE in this as well as other paediatric cohorts, and in adults with major depression. In our cohort, lower RD, suggesting altered myelination, and lower MD, both associated with poorer cognitive and motor function in 2-3-year-olds following PDE. In a separate analysis in the larger DCHS cohort, antenatal maternal depression associated with poorer cognitive function at 2–3 years (Donald et al. 2019). This is consistent with a recent longitudinal study of 2679 children exposed to pre- and postpartum maternal depression that described cognitive and motor delays at age 2 years (Chorbadjian et al. 2020). Another study of 223 infants aged 18 months found poorer cognition after PDE independent of postnatal depression (Koutra et al. 2013). Further, white matter integrity of the corona radiata, described as having a central role in emotion regulation, was particularly affected in adults with major depression (van Velzen et al. 2020).
We did not find white matter alterations to associate with internalizing and externalizing symptoms at 24 months in this study. This is inconsistent with a small literature addressing this issue. One study in neonates with PDE, using clustering coefficients of individual structures derived from DTI data, found that this coefficient of the right amygdala predicted later internalizing and externalizing behavior at 24 months (Wee et al. 2017). Other researchers have found, associations of frontal functional connectivity with internalizing and externalizing behavior at 24 months following PDE (Soe et al. 2016). In turn, altered structural connectivity in prefrontal-limbic pathways mediated externalizing behavior following PDE in 2-6-year-old children (Hay et al. 2020). The CBCL may not be a reliable measure in our setting or at this very young age when parent reports of behavior challenges tend to be less stable.
The pattern in this cohort of widespread associations of PDE with decreased white matter integrity is notable. Though studying the effects of maternal depression on subsequent neurodevelopment of offspring, our findings are consistent with an adult literature on depression, and widespread alterations in white matter integrity (Davis et al. 2019; van Velzen et al. 2020). These continuities across the life span are consistent with the Developmental Origins of Health and Disease (DOHaD) model of intergenerational transfer of biological alterations through epigenetic mechanisms, including for maternal distress especially in LMIC settings where the risk for development of major depression is high (Monk et al. 2019). One study of neonates exposed to major depression prenatally, found associations of depression with modified control of synaptic plasticity in cortical-limbic regions of neonates (Qiu et al. 2017). These and other complex neurobiological changes have been associated with cognitive, emotional and behavioral outcomes in children following PDE (Gelaye et al. 2016; Gentile 2017; Herba et al. 2016). In support as described above, we found altered white matter integrity in the corona radiata, in this cohort with poorer cognitive and motor performance at age 2–3 years.
A number of limitations deserve emphasis. First, we did not determine whether postnatal depression further contributed to the association of PDE with white matter integrity, as few mothers (n = 7) in our sample had persistent depression. Second, although we controlled for key confounders known to affect white matter microstructure, we cannot rule out the potential for residual confounding from unmeasured factors.