Dentate gyrus abnormalities in sudden unexplained death in infants: morphological marker of underlying brain vulnerability

Abstract

Sudden unexplained death in infants, including the sudden infant death syndrome, is likely due to heterogeneous causes that involve different intrinsic vulnerabilities and/or environmental factors. Neuropathologic research focuses upon the role of brain regions, particularly the brainstem, that regulate or modulate autonomic and respiratory control during sleep or transitions to waking. The hippocampus is a key component of the forebrain-limbic network that modulates autonomic/respiratory control via brainstem connections, but its role in sudden infant death has received little attention. We tested the hypothesis that a well-established marker of hippocampal pathology in temporal lobe epilepsy-focal granule cell bilamination in the dentate, a variant of granule cell dispersion-is associated with sudden unexplained death in infants. In a blinded study of hippocampal morphology in 153 infants with sudden and unexpected death autopsied in the San Diego County medical examiner's office, deaths were classified as unexplained or explained based upon autopsy and scene investigation. Focal granule cell bilamination was present in 41.2% (47/114) of the unexplained group compared to 7.7% (3/39) of the explained (control) group (p < 0.001). It was associated with a cluster of other dentate developmental abnormalities that reflect defective neuronal proliferation, migration, and/or survival. Dentate lesions in a large subset of infants with sudden unexplained death may represent a developmental vulnerability that leads to autonomic/respiratory instability or autonomic seizures, and sleep-related death when the infants are challenged with homeostatic stressors. Importantly, these lesions can be recognized in microscopic sections prepared in current forensic practice. Future research is needed to determine the relationship between hippocampal and previously reported brainstem pathology in sudden infant death.

Fig. 1

Examples of focal granule cell dispersion in the dentate gyrus (DG) and associated abnormalities in infants with sudden unexplained in death. a Normal infant hippocampus with landmarks for reference at its mid-body [level of the lateral geniculate nucleus (not shown)]. The DG forms the shape of a “C” at this level. Hematoxylin and eosin (H&E), ×4. b Control DG in a 4-month-old infant with explained cause of death. The DG consists of densely packed granule cells in a linear structure in its straight limbs. H&E, ×20. c Hippocampus of an infant with sudden unexplained death with focal granule cell bilamination (arrow) in the DG. H&E, ×4. d Hippocampus of a second infant with sudden unexplained death with focal granule cell bilamination (arrow) in the DG. H&E, ×10. e Hippocampus of a third infant with sudden unexplained death with focal granule cell bilamination (arrow) in the DG. H&E, ×10. f Hippocampus of a fourth infant with sudden unexplained death with focal granule cell bilamination (arrow) in the DG. H&E, ×10. g Granule cell duplication at the bend (hook) of the DG (arrow), associated with FGCB along the straight limb, in an infant with sudden unexplained death. H&E, ×20. h There are single, ectopic granule cells in the molecular layer (arrowheads). In the DG, there are immature cells, suggestive of immature neurons (Fig. 2), present in the subgranular layer. H&E, ×20. i There are immature cells in the DG (arrowheads), as well as clusters of granule cells in the molecular layer (arrow). H&E, ×20. DG dentate gyrus, ML molecular layer

Fig. 2

Clusters of immature cells in the DG of a human infant with sudden unexplained death. a Histology of immature cells in the deep subgranular layer of the DG (long arrow) with small, dark nuclei and negligible cytoplasm compared to mature cells in the upper layers with large nuclei and cytoplasmic differentiation (arrowhead). H&E, ×40. b The immature cells express cytoplasmic Tuj1, a marker of immature cells of the neuronal cell lineage. The insert demonstrates the high nuclear to cytoplasmic ratio of these immature cells, and positive Tuj1 immunostaining within the scant cytoplasm. These cells are not reactive inflammatory cells, as demonstrated by negative immunostaining for GFAP (reactive astrocytes) (c), or for CD68 (activated microglia) (d). ×40. DG dentate gyrus, GFAP glial fibrillary acidic protein

Fig. 3

Features of acquired injury, consistent with hypoxia–ischemia. a Focal granule cell bilamination associated with hypoxic–ischemic changes (FGCB-HI) in the DG, with a separated line of granule cells (arrows) adjacent to the molecular layer, associated with pyknotic neurons and vacuolation (arrowheads). H&E, x10. b Neuronal necrosis (pyknotic nuclei, shrunken cytoplasm) with vacuolation (edema), consistent with acute hypoxic ischemic injury, in the DG of an infant with explained death. H&E, x20. c Acutely necrotic pyramidal neurons in CA1 with hyper-eosinophilic cytoplasm (arrows), associated with vacuolation of the neuropil. H&E, x20. DG dentate gyrus, ML molecular layer

Fig. 4

Frequency of the following features in the unexplained (black) and explained (white) groups and their subcategories. a Focal granule cell bilamination (FGCB). b Immature clusters of neurons in the subgranular layer of the dentate gyrus. c Hypoxic changes in the dentate gyrus and/or Ammon’s horn of the hippocampus, with FGCB. d FGCB associated with hypoxic–ischemic changes. P value is from a Fisher exact test across the five subcategories

Fig. 5

Examples of microdysgenetic features of the hippocampus and temporal lobe assessed in the present study. a Vertical cortex with linear columns of neurons (>8 row) (arrow) in the temporal cortex. H&E, ×4. b Single interstitial neurons (arrows) in the white matter of the temporal lobe. H&E, ×20. c Heterotopia (arrow), comprised of a collection of misplaced granule cells, in the molecular layer. H&E, ×4

Fig. 6

Examples of thick walled blood vessels compared to thin walled blood vessels in the hippocampus proper. a Medium-sized artery with thick walls (arrowhead) compared to thin walled capillary (arrow) in the molecular layer. H&E, ×10. b Thick walled vessel (arrow) compared to thin walled vessel in the dentate gyrus. H&E, ×20. DG dentate gyrus, ML molecular layer