Chorioamnionitis, a common etiology of preterm birth, has often been cited as a risk factor for the development of cerebral palsy and brain injury. Additionally, cystic periventricular leukomalacia (PVL), damage to deep white matter, is thought to be a precursor to cerebral palsy in that 60-100% of infants with cystic PVL go on to develop cerebral palsy suggesting that white matter injury may be associated with intrauterine infection. A metaanalysis documented that chorioamnionitis is a risk factor for cerebral palsy in both term and preterm delivered infants. Although cerebral palsy is a severe manifestation of brain injury, there are likely other subclinical effects associated with exposure to chorioamnionitis. Subclinical or less severe manifestations of abnormal brain development (including both gray and white matter damage) may ultimately play a role in cognitive development or other pathologic processes.

The association between intrauterine infection and brain injury is strongly supported in experimental studies utilizing animal models. In addition to white matter damage observed following experimentally induced intrauterine infection, Gavilanes et al demonstrated global and diffuse effects on the central nervous system, including decreased number of neurons in the cortex, hippocampus, and substantia nigra. Together, these findings, as well as others, suggest the potential for global and diffuse neurologic injury as a direct result of intrauterine infection and that this damage extends beyond white matter injury alone.

There is a paucity of data describing the consequences of intrauterine infection for brain development in human beings. Preterm birth is associated with compromised brain development. Given the evidence from animal models that intrauterine infection leads to widespread neurological deficits and the prevalence of infection among women delivering preterm, it is plausible that exposure to chorioamnionitis may contribute to additional neurologic impairments observed in children born preterm beyond that of prematurity alone. To date, there are only 2 studies attempting to evaluate the effects of chorioamnionitis on brain structure and neither used a whole-brain approach; one assessed the effectiveness of a specific technique in the evaluation of isolated white matter injury in the neonate and the other performed a randomly chosen region-specific analysis at term gestational age. Thus far, there have been no long-term follow-up studies attempting to identify persisting differences in brain development in children exposed to chorioamnionitis prenatally using whole-brain magnetic resonance imaging (MRI) techniques. Our hypothesis, largely driven by the experimental literature, is that chorioamnionitis would lead to widespread and diffuse alterations in child brain morphology and as such consider the whole-brain approach preferable. Therefore, the objective of the current investigation was to determine if chorioamnionitis is associated with neurologic differences in children born preterm as measured by cortical thickness and volume of subcortical structures.

Materials and Methods

The institutional review board for protection of human subjects at the University of California, Irvine, approved the study protocol. Written and informed consent was obtained from mothers prior to study enrollment. Children aged 6-10 years were recruited into a study of prenatal influences on child brain development, and a subset of these were assessed with structural MRI. Children were born at either the University of California, Irvine, Medical Center or Long Beach Memorial Medical Center/Miller Children’s Hospital, a community hospital affiliate of the university.

Subject selection

Participants included 11 children who were born preterm with documented exposure to chorioamnionitis and 16 children born preterm who were not exposed to chorioamnionitis, but with similar neonatal characteristics. Subjects participated in a larger study of prenatal influences on child development at the University of California, Irvine, and were eligible for participation in this substudy based on the following criteria: subjects were included if they were singletons and >28 and <37 gestational weeks at birth (based on American College of Obstetricians and Gynecologists dating criteria). Exclusion criteria for participation included congenital or genetic anomaly, maternal preeclampsia or HELLP syndrome, maternal drug use, or postnatal steroid administration ( Figure 1 ) . Subjects who met inclusion criteria were recruited in 2 groups: those with and without antenatal exposure to chorioamnionitis. Exposure to chorioamnionitis was based on clinical and/or histopathological assessment. Clinical criteria for chorioamnionitis was defined as fever (≥100.4°F) in the presence of preterm labor (PTL), preterm premature rupture of membranes (PPROM), maternal or fetal tachycardia, or foul-smelling discharge or amniotic fluid. Subjects in the comparison group were born concurrently with the target group and had placental histopathology reports documenting the absence of chorioamnionitis.

Flow diagram of subject selection

MRI , magnetic resonance imaging.

Hatfield. Chorioamnionitis exposure and MRI findings. Am J Obstet Gynecol 2011.

Maternal and neonatal characteristics

Background characteristics including demographic variables were determined at the time of study entry by standardized maternal interview. Maternal intellectual performance was determined by the Perceptual Reasoning Scale of the Wechsler Adult Intelligence Scale. Neonatal and maternal medical characteristics including birth outcome data were determined by chart abstraction. Hypotension was defined by either the use of pharmacologic pressor support or documentation of hypotension in the medical records. The presence of intraventricular hemorrhage was evaluated by cranial ultrasounds that were obtained as part of standard convention. Sepsis was defined by positive blood cultures and diagnosis documented in the medical record.

MRI protocol

T1-weighted scans were acquired in a 3T Philips Achieva system (Philips Healthcare, Andover, MA) using a 3-dimensional magnetization prepared rapid gradient echo pulse sequence that covered the whole brain. The images were acquired in the sagittal orientation with field of view = 240 × 240 mm ² , 1 mm ³ isotropic voxel dimensions, 150 slices, time to repetition = 11 milliseconds, echo time = 3.3 milliseconds, inversion pulse delay = 1100 milliseconds, flip angle = 18 degrees. No signal averaging and no sensitivity encoding acceleration were used.

Image processing

Cortical surface reconstruction and volumetric segmentation was performed with the FreeSurfer image analysis software suite, which is available for download online ( http://surfer.nmr.mgh.harvard.edu/ ). It should be emphasized that the calculations of cortical thickness are generated directly by the software and independent of the operator. Further, the operator was not aware of the study hypothesis. Streamlined image processing procedures are provided in this software package, which first begins by applying intensity normalization prior to segmentation to minimize errors in identifying the boundaries. This is followed by removal of nonbrain tissues. Then, the images are transformed into the Talairach space and subcortical white matter and subcortical gray matter structures are segmented. Pial and white matter surfaces were located by finding the highest intensity gradient, which defines the transition from one tissue class to the other. Once the preprocessing steps are completed, surface inflation is applied to each individual brain and the inflated brains are registered to a spherical atlas. This procedure utilizes individual cortical folding patterns to achieve accurate registration of cortical geometry across subjects. Cortical thickness is calculated as the closest distance from the gray matter/white matter surface to the pial surface at each vertex on the tessellated surface. Procedures for the measurement of cortical thickness have been validated against histologic analysis and manual measurements. The cortical surface images generated by the FreeSurfer software were visually inspected for errors in segmentation and corrections were made as needed.

Analysis of group differences in cortical thickness

Differences between groups in cortical thickness were analyzed at each and every node on the cortical surface using a standard statistical approach for whole-brain analyses. In this method, the software represents the brain surface with a dense array of vertices. At each vertex, the brain thickness is calculated for each subject. Then the software applies a general linear model to test the significance of independent variables (ie, chorioamnionitis) on the variations of cortical thickness across subjects at each and every vertex location. If an effect has strong correlation with the variations in cortical thickness across subjects at that vertex, one gets a strong effect size. This effect size is tested against the null hypothesis using a t test and the resultant t scores are reported. Spatially normalized cortical thickness maps of each subject were entered into an analysis of variance model using the false discovery rate (FDR) correction for multiple comparisons ( P < .05) as recommended by Genovese et al and is the standard approach to avoid a type 1 error with this whole-brain analysis.

Analysis of volumetric differences in subcortical gray matter

The volume of each structure was calculated by the FreeSurfer program. Group differences in volume of subcortical structures were analyzed using analysis of covariance with intracranial volume as a covariate.