BackgroundMemory is one of the most impaired functions after traumatic brain injury (TBI). We used diffusion tensor imaging (DTI) to determine the structural basis of memory deficit. We correlated fractional anisotropy (FA) of the fasciculi connecting the main cerebral regions that are involved in declarative and working memory functions.MethodsFifteen patients with severe and diffuse TBI and sixteen healthy controls matched by age and years of education were scanned. The neuropsychological assessment included: Letter-number sequencing test (LNS), 2-back task, digit span (forwards and backwards) and the Rivermead profilet. DTI was analyzed by a tract-based spatial statics (TBSS) approach.ResultsWhole brain DTI analysis showed a global decrease in FA values that correlated with the 2-back d-prime index, but not with the Rivermead profile. ROI analysis revealed positive correlations between working memory performance assessed by 2-back d-prime and superior longitudinal fasciculi, corpus callosum, arcuate fasciculi and fornix. Declarative memory assessed by the Rivermead profile scores correlated with the fornix and the corpus callosum.ConclusionsDiffuse TBI is associated with a general decrease of white matter integrity. Nevertheless deficits in specific memory domains are related to different patterns of white matter damage.

Traumatic brain injury (TBI) patients have working memory deficits and altered patterns of brain activation during this function. The evolution of the impairment has not been examined to date. This study investigated longitudinal changes in brain activation during a working memory task. Twelve patients with severe and diffuse TBI and ten healthy matched controls were fMRI scanned twice at a 6-month interval during an n-back task (0-, 2- and 3-back). All the TBI patients selected presented signs of diffuse axonal injury on CT but had no evidence of focal lesions on MRI clinical examination. Significant changes in brain activation over time were observed in patients, but not in controls. During the first examination, though both groups engaged bilateral fronto-parietal regions known to be involved in working memory, activation of the right superior frontal gyrus was low in the TBI group. However, the difference between TBI and controls had decreased significantly after 6 months. A factor analysis confirmed the greater increase in activation in the right superior frontal cortex in the TBI group than in healthy controls, leading to normalization of the brain activation pattern. In conclusion, this longitudinal study provides evidence of a progressive normalization of the working memory activation pattern after diffuse axonal injury in severe TBI, coinciding with an improvement in performance on this function.

Working memory is frequently impaired after traumatic brain injury (TBI). The present study aimed to investigate working memory deficits in patients with diffuse axonal injury and to determine the contribution of cerebral activation dysfunctions to them. Eighteen patients with severe TBI and 14 healthy controls matched for age and gender were included in the study. TBI patients were selected according to signs of diffuse axonal injury on computed tomography (CT) and without any evidence of focal lesions on MRI clinical examination. Functional magnetic resonance (fMRI) was used to assess brain activation during n-back tasks (0-, 2-, and 3-back). Compared to controls, the TBI group showed significant working memory impairment on the Digits Backwards (p=0.022) and Letter-Number Sequencing subtests from the WAIS-III (p<0.001) under the 2-back (p=0.008) and 3-back (p=0.017) conditions. Both groups engaged bilateral fronto-parietal regions known to be involved in working memory, although patients showed less cerebral activation than did controls. Decreased activation in TBI patients compared to controls was observed mainly in the right superior and middle frontal cortex. The correlation patterns differed between patients and controls: while the control group showed a negative correlation between performance and activation in prefrontal cortex (PFC), TBI patients presented a positive correlation in right parietal and left parahippocampus for the low and high working memory load, respectively. In conclusion, severe TBI patients with diffuse brain damage show a pattern of cerebral hypoactivation in the right middle and superior frontal regions during working memory tasks, and also present an impaired pattern of performance correlations.

This study shows that memory problems are the most prominent and relevant impairment, although all other cognitive functions are also impaired, affecting both memory itself and general behaviour. Statistical analysis also provides preliminary evidence on the different profile of memory impairment whether cerebral anoxia had hypoxic or ischemic origin.

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