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CHAPTER 3
relationship in traumatized individuals and some studies reported a negative relationship consistent with our results (Bremner et al. 2003, Ehling, Nijenhuis & Krikke 2008, Stein et al. 1997), but other studies did not find any significant association (Bremner et al. 1995, Nardo et al. 2013). While it is possible that the hippocampal morphological abnormalities in our patient samples were at least, in part, involved in the dissociative symptoms, it can be speculated that the associations between morphological measures and dissociative symptoms are actually mediated by childhood traumatization. Future studies can explore this relationship by including individuals with dissociative symptoms but without childhood traumatization.
It has been reported that some psychiatric medications including typical antipsychotics (Chakos et al. 2005), anti-epileptics (Watanabe et al. 1992) and antidepressants (Vermetten et al. 2003) can change the hippocampal morphology. Therefore, it might be argued that our findings of smaller hippocampal global and subfield volumes in PTSD-DID, and to a lesser extent in PTSD-only, are due to the higher level of medications in these patients. However, when patients with a history of using typical antipsychotics (supplementary Table S3.1) or anti-epileptics were excluded (supplementary Table S3.2), the majority of our results remained preserved. Nevertheless, when patients with a history of using antidepressants (i.e., 10 PTSD-DID and 2 PTSD-only) were excluded the group differences in the hippocampal global volume remained significant but the group differences in the subfield volumes became less or non-significant (see supplementary Tables S3.3). The latter finding can be as a result of the insufficient statistical power to detect the subtle changes due to the exclusion of a large portion of the patients. Altogether, the results of these posthoc analyses may indicate that the smaller hippocampal global and subfield volume in PTSD-DID and PTSD- only as compared with HC are robust findings and are not due to the history of medication usage.
Some strengths and limitations: although our sample size of 17 PTSD-DID and 16 PTSD-only patients can be considered as modest, it is in fact the largest sample of individuals with DID in which hippocampal morphology has been studied. PTSD severity was evaluated differently across the two patient groups limiting us to investigate the morphological correlates of PTSD severity. Our findings of more pronounced morphological abnormalities in DID might be due to an interaction between (co-morbid) PTSD severity and childhood traumatization (Van Voorhees et al. 2012). Medication effects
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