Infants and children less than 4 years old suffer chronic cognitive deficits following mild, moderate or severe diffuse traumatic brain injury (TBI). It has been suggested that the underlying neuropathologic basis for behavioral deficits following severe TBI is acute brain swelling, subarachnoid hemorrhage and axonal injury. To better understand mechanisms of cognitive dysfunction in mild-moderate TBI, a closed head injury model of midline TBI in the immature rat was developed. Following an impact over the midline suture of the intact skull, 17-day-old rats exhibited short apnea times (3-15 s), did not require ventilatory support and suffered no mortality, suggestive of mild TBI. Compared to un-injured rats, brain-injured rats exhibited significant learning deficits over the first week post-injury (p<0.0005), and, significant learning (p<0.005) and memory deficits (p<0.05) in the third post-injury week. Between 6 and 72 h, blood-brain barrier breakdown, extensive traumatic axonal injury in the subcortical white matter and thalamus, and focal areas of neurodegeneration in the cortex and hippocampus were observed in both hemispheres of the injured brain. At 8 to 18 days post-injury, reactive astrocytosis in the cortex, axonal degeneration in the subcortical white matter tracts, and degeneration of neuronal cell bodies and processes in the thalamus of both hemispheres were observed; however, cortical volumes were not different between un-injured and injured rat brains. These data suggest that diffuse TBI in the immature rat can lead to ongoing degeneration of both cell soma and axonal compartments of neurons, which may contribute, in part, to the observed sustained cognitive deficits.

Infants who experience inflicted brain injury (shaken-impact syndrome) present with subdural hematoma, brain atrophy, and ventriculomegaly, pathologic features that are suggestive of multiple incidences of brain trauma. To develop a clinically relevant model of inflicted brain injury in infants, the skulls of anesthetized 11-day-old rat pups were subjected to one, two, or three successive mild impacts. While skull fractures were not observed, a single impact to the intact skull resulted in petechial hemorrhages in the subcortical white matter, and double or triple impacts led to hemorrhagic tissue tears at 1 day postinjury. Whereas the singly impacted brain did not exhibit overt damage at 7 days, two impacts resulted in an enlarged ventricle and white matter atrophy; three impacts to the brain led to similar pathology albeit at 3 days postinjury. By 7 days, cortical atrophy was observed following three impacts. Reactive astrocytes were visible in the deep cortical layers below the impact site after two impacts, and through all cortical layers after three impacts. Swellings were observed in intact axons in multiple white matter tracts at 1 day following single impact and progressed to axonal disconnections by 3 days. In contrast, double or triple impacts resulted in axonal disconnections by 1 day postinjury; in addition, three impacts led to extensive axonal injury in the dorsolateral thalamus by 3 days. Calpain activation was observed in axons in subcortical white matter tracts in all brain-injured animals at 1 day and increased with the number of impacts. Despite these pathologic alterations, neither one nor two impacts led to acquisition deficits on the Morris water maze. While indicative of the graded nature of the pathologic response, these data suggest that repetitive mild brain injury in the immature rat results in pathologic features similar to those following inflicted brain injuries in infants.

Impact-induced head injury in infants results in acute focal contusions and traumatic axonal injury (TAI) that are associated with chronic holohemispheric cortical and white matter atrophy and may contribute to poor outcome in brain-injured children less than 4 years of age. Contusive brain trauma in postnatal day (PND) 11 or PND 17 rat pups, ages neurologically equivalent to a human infant and toddler, respectively, leads to cortical tissue loss and white matter atrophy which are associated with cognitive deficits. In adult models of brain trauma and in brain-injured humans, acute and sustained activation of the calpain family of calcium-activated neutral proteases has been implicated in neuronal death and TAI. PND 11 or PND 17 rat pups were subjected to closed head injury over the left hemisphere using the controlled cortical impact device and sacrificed at 6 h, 24 h or 3 days. Hemorrhagic contusions and tissue tears in the cortex and white matter were visible at 6 h, and neuronal loss was evident by 3 days. Calpain activation was observed in cell soma and dendrites of injured neurons at 6 h, and in degenerating dendrites and atrophic neurons at 24 h after injury at both ages. Axonal accumulation of amyloid precursor protein, indicative of TAI, was observed in the corpus callosum and lateral aspects of the white matter below the site of impact, and in the thalamus in PND 11 rats only. Intra-axonal calpain activation was observed to a limited extent in the corpus callosum and subcortical white matter tracts in both brain-injured PND 11 and PND 17 rats. Collectively, these results provide evidence that calpain activation may participate in neuronal loss in the injured cortex, but may not contribute to the pathogenesis of TAI following contusive brain trauma in the immature rat.

Although diffuse brain damage has been suggested to be the predominant predictor of neurological morbidity following closed head injury in infants and children, the presence of contusions also predicts long-term neurobehavioral dysfunction. Contusive brain trauma in the 17-day-old rat resulted in neurodegeneration and caspase activation in the cortex at 1 day, and in the thalamus at 3 days post-injury, and to a greater extent following a deeper impact. Cortical tissue loss in the 4-mm impact group was significantly greater than that in the 3-mm impact group (p < 0.05), and exhibited a time-dependent increase over the first 3 weeks post-injury. Traumatic axonal injury was observed in the white matter tracts below the site of impact at 1 day, and in the corpus callosum at 3 days, to a greater extent following 4-mm impact. In contrast, cellular caspase-3 activation in these white matter tracts was only observed at 24 h post-injury and was not affected by impact depth. Similarly, neurodegeneration and caspase activation in the hippocampus was restricted to the dentate gyrus and occurred to a similar extent in both injured groups. Only the 4-mm impact group exhibited learning deficits in the first week (p < 0.0001) that was sustained until the third week post-injury (p < 0.0001), while deficits in the 3-mm impact group were seen only at 3 weeks post-injury (p < 0.02). These observations demonstrate that increasing severity of injury in immature animals does not uniformly increase the extent of cellular damage, and that the progression of tissue damage and behavioral deficits varies as a function of injury severity.