Date Published: January 23, 2017
Publisher: Public Library of Science
Author(s): Dana Waltzman, Salil Soman, Nathan C. Hantke, J. Kaci Fairchild, Lisa M. Kinoshita, Max Wintermark, J. Wesson Ashford, Jerome Yesavage, Leanne Williams, Maheen M. Adamson, Ansgar J. Furst, Christian Schmahl.
Given the high prevalence and comorbidity of combat-related PTSD and TBI in Veterans, it is often difficult to disentangle the contributions of each disorder. Examining these pathologies separately may help to understand the neurobiological basis of memory impairment in PTSD and TBI independently of each other. Thus, we investigated whether a) PTSD and TBI are characterized by subcortical structural abnormalities by examining diffusion tensor imaging (DTI) metrics and volume and b) if these abnormalities were specific to PTSD versus TBI.
We investigated whether individuals with PTSD or TBI display subcortical structural abnormalities in memory regions by examining DTI metrics and volume of the hippocampus and caudate in three groups of Veterans: Veterans with PTSD, Veterans with TBI, and Veterans with neither PTSD nor TBI (Veteran controls).
While our results demonstrated no macrostructural differences among the groups in these regions, there were significant alterations in microstructural DTI indices in the caudate for the PTSD group but not the TBI group compared to Veteran controls.
The result of increased mean, radial, and axial diffusivity, and decreased fractional anisotropy in the caudate in absence of significant volume atrophy in the PTSD group suggests the presence of subtle abnormalities evident only at a microstructural level. The caudate is thought to play a role in the physiopathology of PTSD, and the habit-like behavioral features of the disorder could be due to striatal-dependent habit learning mechanisms. Thus, DTI appears to be a vital tool to investigate subcortical pathology, greatly enhancing the ability to detect subtle brain changes in complex disorders.
Posttraumatic stress disorder (PTSD) and traumatic brain injury (TBI) are a common occurrence in Veterans. The prevalence of PTSD in Veterans ranges from 10–30% [1, 2] and 15–30% for TBI [3, 4]. In addition, a common sequelae of both PTSD and TBI are impairments in memory. The memory systems in the medial temporal lobe and basal ganglia have different properties. The medial temporal lobe supports declarative memory, which is the conscious recollection of facts and events, while the basal ganglia support nondeclarative memory, which is nonconscious and is implicitly learned through performance rather than recollection . Furthermore, it is not well understood how the different memory systems (declarative vs non-declarative memory) interact in the brain with each other [6, 7], but it is possible that both systems are involved in the pathology of PTSD and TBI, as summarized below.
This study sought to determine whether the structural integrity among the hippocampus and caudate differed between Veterans with PTSD, TBI, and neither PTSD nor TBI. Our results demonstrated no macrostructural differences between the groups in these neuroanatomical structures. By contrast, we did observe significant alterations in microstructural DTI indices in the caudate and hippocampus. We found that the PTSD group showed worse integrity across all DTI metrics than the VC group. This finding is consistent with accumulating evidence that DTI is a useful tool to detect subtle microscopic brain tissue alterations before neuronal degradation and atrophy are detectable on a macroscopic level [37, 38].