Vestibular Influences on Neuropsychological Outcomes in UK Military Veterans with Mild Traumatic Brain Injury

Abstract The epidemiology and treatment of mild traumatic brain injury (mTBI), along with its co-morbid symptoms, has previously received little attention in UK military samples. In US military veterans, mTBI is among the most frequently seen and challenging of conditions to arise as a consequence of the conflicts in Iraq and Afghanistan. mTBI has been shown to represent a complicated and particularly harmful polymorbid condition when accompanied by PTSD and depression, resulting in poor life outcomes. Symptoms of dizziness are one of the most common comorbid symptoms of mTBI and PTSD. In fact, the symptomology of mTBI and PTSD share many distinct features that can also be seen in patients with vestibular disorders. This is perhaps unsurprising given the diffuse nature of the ascending vestibular pathways. Despite this anatomical feature, vestibular influences in mTBI have yet to be explored in UK military veterans. To this end, this thesis first aimed to determine if vestibular disturbance influenced the neurobehavioural and affective symptoms of mTBI. A further line of investigation examined if the vestibular pathways can be artificially modulated using galvanic vestibular stimulation (GVS) to ameliorate some of these symptoms. Chapter 1 of this thesis describes the main features of mTBI in both the UK and US military, outlining the classification/current diagnostic criteria, mechanisms of injury and co-occurring cognitive, psychiatric symptoms of mTBI. It will be argued that mTBI commonly occurs in military samples as a result of blast exposure, and is particularly difficult to diagnose and treat. Chapter 2 will illustrate that both blunt and blast mTBI frequently result's in damage to the vestibular system and thereon assesses the potential contributions of vestibular dysfunction to the chronic symptoms of mTBI. Drawing on previous intervention studies from civilian samples, it will illustrate how the vestibular system may provide a novel pathway to treat mTBI. Chapter 3 reports results from an epidemiological study of 162 UK military veterans which show that 72% of the sample reported one or more mTBI in their lifetime. Vestibular disturbance affected 69% of these individuals and was most frequently seen in those who had sustained both blunt and blast injuries. Mediation analysis indicated that when PTSD, depression and anxiety were accounted for, vestibular disturbance was directly associated with increased neurobehavioural symptoms and functional disability. These findings indicate that vestibular disturbance is common particularly after combined blunt and blast head injuries and is singularly predictive of poor long-term mental health and functional disability. In light of the newfound association between vestibular disturbance and mTBI, the remaining chapters sought to establish if and how artificial vestibular stimulation can remediate aspects of mTBI. To help determine whether to target positive or negative symptoms, in Chapter 4 I sought to determine if GVS could induce either long-term potentiation (LTP) or depression (LTD) type effects, in neurologically healthy individuals up to 24hours post stimulation. The results showed that in participants who demonstrated cortical hyperexcitability at baseline, GVS induced a significant LTD type effect at 24hours post-stimulation. This indicated that conditions such as anxiety and PTSD, which are associated with cortical hyperexcitability, should be targeted. In Chapter 5 a small proof of concept study evaluated the efficacy of GVS in treating current symptoms of anxiety in 5 UK military veterans. The results showed that state symptoms of anxiety were exacerbated at 24hours post active GVS, which although further introduces a link between the vestibular system and anxiety in mTBI, did not support therapeutic application of GVS. In Chapter 6, the general discussion concludes that, vestibular disturbance is predictive of poor long-term mental health and therefore needs to be routinely screened and treated. Further studies are also needed to establish how to yoke the novel effects of GVS on cortical excitability observed here for treatment of mTBI symptoms