Pathophysiology of Neonatal White Matter Injury

White Matter Injury in the Preterm Neonate

Preterm birth is a major public health issue affecting an estimated 13 million babies worldwide; one in eight deliveries in the United States are now preterm. Over the past two decades, improved neonatal intensive care unit (NICU) therapies have reduced the mortality and increased the survival of preterm newborns. Despite these advances in neonatal intensive care, preterm birth remains a leading cause of childhood and lifelong disability. These disabilities place enormous burdens on the children and their families. Even with modern intensive care, more than half of very preterm infants will have serious neurodevelopmental challenges that often follow severe intraventricular hemorrhage (IVH) and white matter injury (WMI). Here we focus on the pathophysiology and consequence of WMI, the most significant brain injury in contemporary cohorts of children born preterm.

Injury to the preterm developing brain leads to functional consequences in several domains: motor, cognition, language, behavior, mental health, vision, and hearing. Impairments in these domains often co-occur. For example, the consequence of severe WMI, cystic periventricular leukomalacia (PVL), includes spastic diplegic cerebral palsy often with associated impairments in vision, cognition, and learning. Importantly, the cognitive and behavior issues that are prevalent in children born preterm most often occur in the absence of PVL and remain evident through young adulthood. These prevalent neurocognitive impairments point to widely distributed brain abnormalities or problems with brain connectivity. Increasing evidence suggests that the normal trajectory of brain maturation is disrupted by common systemic illnesses such as recurrent infections and exacerbated further by pain and stress. The diverse spectrum of neurocognitive and motor outcomes following preterm birth points to widespread cellular maturational disturbances that target cerebral gray and white matter.

Historically, the prevalent pattern of WMI in the preterm neonate was destructive white matter lesions that resulted in cystic PVL. PVL was accompanied by secondary cortical and subcortical gray matter degeneration. Fortunately, contemporary cohorts of preterm neonates with WMI typically have less severe injuries that do not appear to involve gross glial or neuronal loss. However, even this milder form of WMI can be accompanied by diffuse gliosis and reduced cerebral and cerebellar growth. In this chapter, we will review human and experimental studies that indicate how impaired brain growth is related to distinct responses in gray and white matter. More specifically, we cover cerebral white matter myelination abnormalities related to aberrant regeneration and repair responses of the oligodendrocyte (OL) lineage.

We will review studies of the response to premyelinating oligodendrocyte (preOL) death, whereby early OL progenitors rapidly proliferate and differentiate, as well as evidence for OL precursor maturation arrest in the setting of hypoxia and human WMI. Despite these responses to injury, potentially regenerative preOLs fail to mature along normal pathways to become the myelinating cells critical to normal white matter maturation ( Fig. 132.1 ). We will also address the response of immature neurons in the preterm brain to hypoxia and hypoxia-ischemia, which includes widespread disturbances in maturation of their dendritic arbors. Together, the maladaptive responses of immature oligodendroglia and neurons to injury involves a widespread failure of normal maturation during a critical window in the development of brain circuitry ( Fig. 132.2 ). This contemporary spectrum of WMI, with abnormal maturation of cerebral gray and white matter, raises new diagnostic challenges for clinicians and opens new potential avenues for therapies to promote optimal brain health and improve neurodevelopmental outcomes.

Neonatal Brain Health Predicts Functional Outcomes

The functional consequences of preterm brain injury affect several domains that are essential to quality of life: motor, cognition, behavior, mental health, vision, and hearing. Cognitive and behavior problems in preterm-born children persist to young adulthood and impact function in family, school, employment, and society. Impairments in cognition and motor skills (e.g., cerebral palsy) have been described extensively. More recently, it is recognized that preterm children with broadly normal IQ nonetheless have processing deficits including problems in attention and executive functions (e.g., cognitive flexibility, working memory), visually based information processing and language. ^{, ,}

Considerable progress has been made to define the structural and functional consequences of preterm birth with magnetic resonance imaging (MRI). MRI is currently the “gold-standard” diagnostic test for the safe and reliable clinical identification of injury in the developing brain. Brain injuries (e.g., WMI) are indicated by discrete (focal) areas of MRI signal abnormalities. WMI occurs in a characteristic topology, with most lesions occurring in the periventricular central region, followed by posterior and frontal regions. WMI is most readily evident on MRI early in life rather than at term-equivalent age. Quantitative mapping of MRI-defined punctate WMI demonstrates that lesion volume and location are important contributors to the prognostic significance of WMI, with frontal lesions being of particular concern. The predictive value of frontal WMI volume highlights the importance of lesion location when considering the neurodevelopmental significance of WMI. This imaging classification tool that considers location is useful to predict preschool age outcomes in children born preterm.

When considering the functional consequences of preterm brain injury, there is increasing recognition of the complex interplay of neonatal brain injury and the socioeconomic circumstances of the child. In normative populations, among children from lower-income families, small differences in income predict relatively large differences in brain surface area. The impact of socioeconomic status is most prominent in regions supporting language and executive functions. In children born preterm, speech, language, and communication concerns are common and independently associated with increasing levels of neighborhood deprivation and gestational age. This is especially relevant given the suggestion that epigenetic dysregulation mediates the effects of preterm birth on neurodevelopmental outcome. Among extremely preterm neonates, moderate to severe neonatal brain injury predicts impaired cognitive functions even when accounting for the level of maternal education. At preschool age, in a cohort of 170 very and extremely preterm children, cognitive outcome was comparably associated with maternal education and neonatal brain injury. The association of brain injury with poorer cognition was attenuated in children born to mothers of higher education level, indicating the need to consider the implications of neonatal brain health in the context of the child’s environment. The neurobiology of these relationships, and the long-term impact of both higher and lower socioeconomic status, need to be determined.

As early as the newborn period, sophisticated imaging methods can also detect and quantify developmental abnormalities of brain structure or function as they evolve from the acute to longer-term recovery phases. Observations with advanced MRI techniques indicate that WMI and some common clinical conditions impede brain maturation in areas that appear normal on conventional (T1 and T2) MRI. ^,

Several brain MRI measures can now be applied in the research and clinical setting to measure white matter dysmaturation in the preterm neonate: volumetrics, diffusion tensor imaging (DTI), magnetic resonance spectroscopic imaging (MRSI), functional connectivity MRI (fcMRI), and magnetoencephalography (MEG). Structure : High-resolution MRI analyzed with deformation-based morphometry can quantify volumetric growth of cortical and subcortical brain structures. Microstructure : DTI measures regional brain microstructure reflected in parameters such as fractional anisotropy (FA), a measure of the directionality of water diffusion in each pixel of the DTI image. Using these MRI tools, brain dysmaturation is increasingly recognized as the component of WMI critical to brain health for preterm neonates.

Despite the widespread use of diagnostic MRI in clinical practice, it is nonetheless important to recognize the limitations of MRI at regularly used clinical field strengths (e.g., 1.5 or 3 T). This is particularly relevant for detection of the full spectrum of WMI because MRI currently has limited sensitivity for imaging microscopic foci of necrosis, and conventional imaging may not fully define early diffuse WMI. The definition of diffuse WMI by MRI is of particular interest, given that these lesions correspond to foci of dysmaturation with preOL maturation arrest and abnormal myelination.

White Matter Injury and Brain Dysmaturation: An Overview

Patterns of injury in the neonatal brain result from “selective vulnerability” of certain cell populations during distinct times in development. In the preterm neonate’s brain, some types of OL progenitors (i.e., late progenitors; preOLs) are vulnerable to insults that do not affect mature myelin-forming OLs. This selective preOL vulnerability in the developing white matter is a central feature of the propensity for WMI in preterm neonates. WMI in preterm neonates is linked to ischemia (e.g., hypotension), infections, and inflammation. ^{, ,} After preOL death has occurred, the primary mechanism of myelination failure in the human preterm neonate involves a dysmaturation process where preOLs fail to differentiate in diffuse chronic lesions enriched in reactive astrocytes (i.e., preOL maturation arrest).

As noted earlier, multifocal WMI predominates in preterm neonates and predicts later visual, motor, and cognitive dysfunction. ^{, ,} Yet, WMI is the tip of the iceberg in regards to brain abnormalities. ^{, ,} Punctate white matter lesions are associated with widespread neuroanatomic abnormalities as revealed by multimodal MRI. The changes in white matter FA, a metric derived from DTI, are associated with brain maturation and correspond with maturation of the OL lineage. WMI is followed by diffusely abnormal microstructural (e.g., FA) and metabolic brain maturation, as preterm neonates grow to term. ^{, , ,} Abnormalities in brain maturation persist through childhood and are associated with adverse neuro-developmental outcomes. ^{, ,} The adverse outcomes do not appear to be due to a loss of the preOL pool. In fact, glial progenitors respond to WMI by partially, but incompletely, mounting a repair process that regenerates and expands the preOL pool, which is then blocked from maturation and myelination. ^{, ,} Ultimately, this brain dysmaturation disrupts the brain’s capacity for regeneration and repair following injury.

Abnormalities in gray matter structures such as the thalamus, cortex, hippocampus, and cerebellum are also increasingly recognized in association with WMI by MRI. ^, For example, delayed maturation of the cerebral cortex is detected with DTI in preterm neonates with impaired postnatal growth. Importantly, in a preterm sheep model, cortical growth impairments measured with DTI following moderate hypoxia-ischemia were associated with diffuse disturbances in the dendritic arbor and synapse formation of cortical neurons. This simplification of the dendritic arbor of immature neurons contrasts sharply with mature neurons in the full-term neonate that degenerate from activation of excitotoxic and apoptotic pathways.

Subplate neurons (SPNs) are a transient cell population important for developing thalamocortical connections. Similar to other neuronal populations, SPNs degenerate in more severe lesions arising from perinatal hypoxia-ischemia in rodents ^, and human PVL. Notably, in association with less severe multifocal WMI, fetal ovine SPNs did not degenerate, but rather displayed functional maturation disturbances. Given the central role of SPNs in establishing thalamocortical connectivity, this dysmaturation response may contribute to aberrant thalamocortical connections in the visual cortex of preterm neonates with WMI, for whom visual dysfunction is common.

Preterm neonates at term age also exhibit reduced functional thalamocortical connectivity on fcMRI, particularly with WMI. MRI studies of preterm neonates from 25 to 45 postmenstrual weeks suggest more effective use of impaired white matter reserve, as short-range corticocortical connections and connectivity involving thalamus, cerebellum, superior frontal lobe, and cingulate gyrus and were related to the severity of prematurity. Furthermore, cognitive scores at 2 years are correlated with structural connectivity between the thalamus and extensive cortical regions at term. Altered functional connectivity in children and adolescents born preterm is a critical risk factor for adverse cognitive outcomes. ^, There is altered cortical activation and functional connectivity during language and visual processing in preterm-born children and adults who have normal cognitive function, highlighting the important role of brain imaging, neurophysiology, and neuroscience to fully understand the spectrum of injury in this population. ^, Studies have shown late postnatal migration of cortical interneurons in human brain, ^, suggesting another vulnerable population, critical to neural circuit formation, in preterm brain injury. Indeed, because the majority of such interneurons traffic through the subventricular zone, a prediction is that even focal IVH would prevent them from reaching their intended targets.

Taken together, recent data support that brain dysmaturation (1) involves white matter and gray matter and (2) disrupts an essential developmental window characterized by rapid brain growth and the enhancement of neuronal connectivity through myelination, elaboration of the dendritic arbor, and synaptogenesis. Furthermore, disturbances in brain maturation appear to be multifactorial and related to clinical factors such as malnutrition, infections, and lung disease.

Spectrum of Injury and Dysmaturation in Preterm White Matter Injury

The spectrum of WMI seen in human preterm neonates is very similar to that generated by cerebral ischemia in a preterm fetal sheep model. This model has provided access to ex vivo ultra-high field (12 T) MRI images that were aligned at high resolution with WMI visualized in the same brains by histopathology. At this ultra-high field strength, MRI defined three types of subacute lesions that correspond to distinct forms of human necrotic or diffuse WMI:

• 1.

  Cystic necrotic WMI : Cystic PVL describes foci of necrosis that are typically larger than one millimeter in diameter with degeneration of all cell types including glia and axons on pathologic examination. Pronounced necrotic WMI is visualized on MRI as hyperintense signal abnormalities on T2-weighted (T ₂ W) images or as volume loss of major white mater tracts such as the corpus callosum or optic radiations. These large necrotic lesions are highly enriched in macrophages.

• 2.

  Microscopic necrotic WMI : Despite the pronounced reduction in incidence of cystic PVL on MRI, these small foci of necrosis (less than a millimeter in diameter) are evident on neuropathologic examination (i.e., microcysts). Similar to cystic PVL, microcysts are also enriched in cellular debris, degenerating axons, and phagocytic macrophages. The extent to which microcysts contribute to functional impairments or are clinically silent is unclear, because they are poorly visualized by MRI at clinical field strengths. In preterm fetal sheep and human autopsy cases, microcysts were observed in at least one third of cases imaged at 12 T. Yet, because of their size and extent, microcysts comprise only a small proportion, less than 5%, of lesion burden, highlighting the importance of diffuse WMI.

• 3.

  Diffuse WMI: The imaging characteristics of diffuse WMI differ substantially at clinical field strengths compared with those at higher field strengths. Diffuse WMI is visualized by ultra-high field MRI as diffuse hypointense signal abnormalities on T ₂ W images. Early subacute signal changes correspond to reactive astrogliosis and myelination disturbances arising from maturation arrest of preOLs. In diffuse WMI, there is selective degeneration of preOLs, while axonal degeneration is initiated at foci of microscopic necrosis. ^, Diffuse WMI is not as directly evident on diagnostic MRI. At clinical MRI field strengths, WMI is indicated by discrete focal or multifocal areas of MR signal abnormalities. Structural or metabolic abnormalities related to diffuse WMI may also be detected by advanced MRI techniques such as DTI and spectroscopic imaging. Disturbances in DTI parameters such as FA correspond in part to maturation of the OL lineage consistent with the role of preOL arrest in the myelination disturbances in diffuse WMI. The importance of diffuse WMI must be stressed because this is the most common form of WMI in autopsy studies of contemporary human cohorts. In fetal sheep, diffuse WMI comprised nearly 90% of the total volume of WMI.

Although the extent of diffuse WMI is difficult to define, these lesions display a robust inflammatory reaction that extends considerably beyond the apparent lesion boundaries defined by conventional neuropathology. Although the chronic inflammatory reaction is incompletely understood, it occurs in response to early WMI, which targets the human OL lineage and axons via oxidative damage of a severity consistent with hypoxia-ischemia. ^,

Advances in imaging science are providing a new window on diffuse WMI and white matter maturation. For example, pixel-based analysis is a novel imaging framework that reveals alterations in microstructural and macrostructural changes in axonal fiber populations within regions of crossing fibers. It is sensitive to the effect of preterm birth and aspects of neonatal intensive care such as days on ventilation and total parenteral nutrition. It will be interesting and important to further evaluate such MRI biomarkers against pathologic analysis in suitable cases using the latest tools, such as immunohistochemical and mRNA markers of neural precursors, mature cell types, and signaling pathways in situ.

Clinical Risk Factors for the Spectrum of White Matter Injury in the Preterm Neonate

The conditions that contribute to both focal and diffuse WMI are multifactorial. There is greater recognition of the importance of clinical factors as predictors of brain dysmaturation, the critical lesion in diffuse WMI. Postnatal illness severity is a better predictor of brain health than gestational age at birth. ^, Postnatal illness severity is also a stronger predictor of focal and diffuse WMI than are several prenatal factors. ^{, , , ,} Such conditions include infection, bronchopulmonary dysplasia (BPD), and chronic hypoxemia, as well as repetitive exposure to painful procedures.

• 1.

  Infection. In preterm neonates, complex systemic illness contributes to the risks of brain injury and adverse outcomes. For example, recurrent postnatal infections in preterm neonates predict a significantly increased risk of WMI. ^{, ,} These neonates are also at risk for “progressive WMI,” where the punctate lesions of WMI are more readily evident on MRI scans at term equivalent age rather than on earlier scans. In addition, preterm neonates with postnatal infections show evidence of altered white matter pathway development and even more widespread impairments in brain development. Postnatal infections, even without positive cultures, predict impaired neurodevelopmental outcome consistent with the neonatal brain imaging findings of widespread delayed brain maturation. ^,

• 2.

  BPD, requiring prolonged treatment with mechanical ventilation and supplemental oxygen, is strongly associated with adverse cognitive outcome, even after accounting for other morbidities. In cohorts of preterm neonates studied with MRI, BPD, and days of ventilation are associated with impaired white matter and cortical development. BPD in preterm neonates is associated with hypoxic episodes. This is especially concerning given observations that brief hypoxic episodes induce robust structural and functional disturbances in the dendritic maturation of CA1 neurons in an ovine experimental model. BPD treatment with corticosteroids is also associated with impaired growth of the cerebellum. Notably, extremely preterm neonates born at U.S. academic centers over the past 20 years display increased rates of BPD despite modest improvements in other neonatal morbidities.

• 3.

  Exposure to Pain and Analgesics. During a period of rapid brain development, preterm neonates are exposed to multiple painful and stressful procedures as part of their life-saving NICU care (see Chapter 128 ). Pain is a central factor that predicts dysmaturation ; neonates with infections are exposed to more painful procedures, and these painful procedures predict poor somatic growth and altered brain maturation from preterm early life to term age. ^, The amount of procedural pain and stress to which a preterm neonate is exposed is particularly linked to altered maturation of sensory regions of the thalamus. Early pain exposure, especially in the youngest preterm neonates, has the most robust relationship with dysmaturation of the thalamus. Analgesic and sedative practices vary considerably among hospitals, even for infants with similar characteristics and illness severity. Medications commonly used to provide analgesia or sedation include opiates, benzodiazepines, and sucrose, but their use varies considerably across sites. ^, At least 70% of preterm neonates receive narcotics or benzodiazepines. ^, Reports suggest that some medications regularly used for analgesia and sedation in the NICU have unanticipated harmful effects with regional specificity. ^, Midazolam is associated with slower growth of the hippocampus, a finding congruent with experimental rodent models in which Midazolam results in widespread neurodegeneration, impairments in synapse formation, and long-term memory deficits. ^, In addition, morphine is associated with slower growth of the cerebellum, and glucose for analgesia with slower growth of the basal ganglia. ^, Future work is needed in relevant experimental models and clinical trials to determine the analgesia strategies that best promote brain maturation and neurodevelopmental outcome in this population.

Consistent with neonatal brain imaging observations, the increasing amount of pain to which a preterm neonate is exposed during NICU care predicts poorer neurodevelopment, a relationship moderated by parent-child interaction. ^, The influence of parent-child interaction on outcomes indicates how factors that follow the NICU period, including parental stress, may also contribute to cognitive and behavioral outcomes of preterm survivors. Preterm-born infants exposed to a parental intervention for sensitivity training demonstrated enhanced maturation and connectivity on MRI at term equivalent age. A longer time-horizon for potential interventions to improve brain development and outcomes is suggested by experimental and clinical evidence that parent-infant interactions may modify early brain maturation. ^, Future work is needed in relevant experimental models and clinical cohorts to define the evolution of cerebral white matter lesions over years to define the period over which WMI may be repaired and optimal brain development promoted.