The Role of Comorbidities on Outcomes After Traumatic Brain Injury
Traumatic brain injury (TBI) is a leading cause of disability and mortality worldwide resulting in a myriad of difficulties including cognitive impairment, functional disability, postconcussion syndrome (PCS) symptoms and diminished quality of life, that have been known to last many years. The epidemiological transition coinciding with rising trends in both TBIs and chronic health conditions, requires greater attention to be placed on understanding the outcomes of those with TBI who are increasingly also likely to have a health condition or comorbidity. In doing so, it furthers our current limited understanding about the additional mediating and/or moderating role that comorbidities are likely to play on delayed recovery and difficulties experienced after TBI. In the measurement of such outcomes, detailed examination of the psychometric properties of scales using modern methods such as Rasch analysis plays a central role. Psychometric analysis not only ensures that outcome measures uphold validity and reliability for use in the intended populations, but also contributes to precision medicine by increasing measurement accuracy of scales. The aims of this thesis were twofold. The first objective was to apply Rasch analysis to evaluate the performance of the Cumulative Illness Rating Scale (CIRS), Rivermead Postconcussion Questionnaire (RPQ), as well as the WHO Quality of Life Questionnaire-BREF (WHOQoL-BREF) and its shorter derivatives in TBI and orthopaedic populations. The second objective was to develop and test a conceptual and empirically-derived model that illustrates the structural relationships between the presence of comorbidities and injury characteristics, and their impact on PCS symptoms and quality of life in TBI and orthopaedic samples. These study aims were achieved using an age-sex matched case-control sample consisting of n=109 TBI and n=114 orthopaedic patients recruited from the Midland Trauma Registry in the Waikato district of New Zealand. Participants were assessed via telephone interviews regarding their health history, ongoing symptoms and current quality of life at six months to six years post-injury. Results from the first study using Rasch analysis demonstrated that the RPQ and WHOQoL-BREF scales are reliable and unidimensional measures of persistent postconcussion symptoms and quality of life, respectively, that can be used for assessment in individuals with either TBI or orthopaedic injuries. Shorter versions such as the EUROHIS-QOL-8 and the development of a new 12-item WHOQoL version were also shown to be reliable measures especially useful in time-restrained settings, or where minimising respondent burden is a priority. In the second study, multivariate linear regression identified that individuals with TBI or orthopaedic injuries, and with a history of neurological or psychiatric problems were at increased risk of reporting prolonged PCS symptoms and diminished quality of life. Findings from structural equation modelling also revealed that the persistence of post-injury psychological, neurological and musculoskeletal difficulties influenced long-term symptoms and quality of life in both groups. Previous research has misattributed PCS symptoms to the presence of a TBI, whereas a notable finding arising from these findings is that these symptoms are a significant predictor of post-injury quality of life in TBI and orthopaedic patients alike. This thesis has made practical contributions to psychometric research, adhering to best practice guidelines with the provision of ordinal-to-interval conversion tables for improving clinical assessment. The development of a conceptual model that highlights the impact of comorbidities on injury outcomes serves as an important prognostic tool that can be useful for clinicians to identify high-risk individuals who are likely to have persisting difficulties. Prognostic modelling can therefore enable rehabilitation practitioners to modify treatment and tailor rehabilitation strategies from an early stage according to a patient’s individual needs.