Abstract
Background and Aim
Postural instability has been identified as one of the main causes for injurious falls in people with idiopathic Parkinson’s disease (IPD). Muscle weakness in older adults is one of the identified factors for reduced postural stability and falls in older adults and although postural instability is one of the cardinal signs of IPD, it is still not fully understood. Many contributing or associated factors for postural instability have been suggested and it is possible that the reason for postural instability may be multifactorial in nature. Muscle weakness in IPD is one of the identified factors for reduced postural stability and has been implicated as a falls risk factor in IPD. Different body composition phenotypes, determined by the proportion of fat, lean body mass, and bone density, have been implicated with low muscle strength, gait and balance deficits and falls in older adults, but this has not been investigated in people with IPD.
This thesis explored the relationship of postural instability, muscle strength and body composition phenotypes in people with IPD and an age-matched control group. The primary aim of this study was to investigate the relationships between body composition phenotypes, postural stability and lower limb muscle strength in people with IPD compared to an age and sex matched control group. The aim was also to compare the number of falls experienced in a six months self-reported falls diary, falls related injuries and physical activity participation between the two groups.
Methodology
This thesis comprised two studies. Study one was a systematic review to answer the question: What are the factors contributing to or impacting on postural instability in IPD and what is the strength of evidence for these factors? The systematic review’s findings informed the development of the second study.
The second study was a cross sectional study which investigated the relationship between postural instability, muscle strength and body composition phenotypes in people with IPD and an age and sex-matched control group. Participants for this study were recruited via the Neurology Department of the Dunedin Hospital, the Parkinson’s Society of New Zealand (Otago branch), and via public advertising. To be eligible, participants had to be diagnosed with IPD and be able to perform the required tests independently with or without assistive devices. Age and sex matched, sedentary control participants with no known neurological impairments were recruited via public advertising.
For all tests, participants with IPD were assessed during the “on” state of medical therapy, and the degree of severity and staging of their IPD was assessed with the Movement Disorders Society-Unified Parkinson’s Disease Rating Scale (MDS-UPDRS). All participants were assessed with the following tests: Postural instability was measured with the Sensory Organisation Test (SOT) and the Motor Control Test using the NeuroCom® Smart EquiTest® version 8.4.0, the basic and the high cognitive Timed Up and Go Test (TUG and TUGH, respectively) and the Step Test. The Biodex® System 3 dynamometer was used to evaluate knee joint muscle strength. All participants had a dual energy x-ray absorptiometry (DXA) scan to assess body composition phenotypes. The Activity-Specific Balance Confidence scale (ABC Scale) and the Rapid Assessment of Physical Activity (RAPA) were completed to estimate fear of falling and balance confidence, and physical activity participation, respectively. Data on the number of falls experienced were then collected prospectively for six months.
Descriptive and inferential statistical analysis were conducted to compare test results between the IPD and control groups, and a stepwise regression procedure was performed to investigate the relationships between body composition phenotypes, postural stability and strength in IPD.
Results
In the systematic review, 57 studies met the inclusion criteria, of which 42 were rated as having moderate-to-high quality. Posturography was frequently used to measure postural instability (n=36, 63%). Factors contributing to, or impacting on postural instability were broadly categorised as: (1) sensory dysfunction (n=12); (2) abnormal postural response patterns (n=11); (3) bradykinesia (n=2); (4) stiffness (n=7); (5) cognitive dysfunction (n=5); and 6) miscellaneous factors (n=5). From the studies rated as being of high quality, five factors were identified to significantly impact on postural stability: sensory dysfunction, bradykinesia, abnormal postural response patterns, L-dopa induced dyskinesia, and hypotension (the latter two factors were categorised as “miscellaneous”).
Forty-seven participants with IPD and 58 control participants were tested in the second study. The mean (SD) MDS-UPDRS of the IPD group was 46.4 (177), range 13 to 98). The majority of participants with IPD were classified as Hoehn and Yahr stage II. No significant difference was observed in the weight and BMI between the IPD and control groups. Except for the strategy and the latency composite scores, the mean differences between the IPD and control groups in the balance (equilibrium composite scores, TUG, TUGH, Step, ABC Scale) tests outcomes were significant. Both groups were mainly using the ankle strategy (79%) to maintain balance. There were significant differences (p<.01- p<.001) between groups in the knee strength variables at the speeds 60°/s and 90°/s for the peak torque/body weight, the total work, and the time to peak torque. Participants with IPD fell significantly more times than control participants during a 6 month follow up. There was no difference in the RAPA scores between groups.
There was no significant difference in the body composition phenotype variables between groups. In regards to the body composition phenotype, using the Baumgartner definition, there were no participants classified as sarcopenic obese (low lean body mass and high fat mass) in either the IPD or the control group. Thirty-two percent of the IPD group and 41% of the control group were classified as obese (high fat mass and normal lean body mass). The normal lean classification (normal lean body mass and normal fat mass) accounted for 61% and 59% in the IPD and the control groups, respectively. Of all the participants only three participants were classified as sarcopenic (low lean body mass and low fat mass), and all were males with IPD.
The stepwise regression analysis showed that sex and disease significantly predicted the equilibrium composite scores (p< .01). Sex and disease explained a significant proportion of variance in the equilibrium composite scores, (-4.1% and +6.4%, respectively p< .01). Total body lean mass and age positively predicted the latency composite score (p<.001) and explained 11.3% and 10.8% of the variance in this score p<.001. Disease, age, and leg fat significantly predicted the TUG and TUGH tests (p<.05) positively contributing 11% and 18%, 10% and 9% and 4% & 3%, respectively to the variance of the scores. Disease alone contributed 11% to the variance in the Step Test result (p<.001). There were four significant predictors (p<.01) for knee muscle strength. Leg fat mass (22%), age (14%), disease (13%), and appendicular lean mass (5%) contributed significantly (p<.01) to the variance of peak torque/body weight and the total work extension for both 60°/s and 90°/s outcomes. For total work knee extension (60°/s and 90°/s) the four significant predictors (p<.001) were appendicular lean mass, disease, age and leg fat mass, contributing 37%, 17%, 7% and 2%, respectively to the variance. Disease was the only predictor to contribute negatively to the time to peak torque extension for the speed 60°/s (p<.05, 5% contribution to the variance). Disease and age (p<.001) were the only contributors to the speed 90°/s (11% and 6% contribution to the variance, respectively).
Discussion
In this thesis, we determined postural instability and low muscle strength were existing impairments in participants with IPD, even in the early stage of the disease. Disease status, age and sex also appeared to be influential factors associated with the deficits in postural stability and muscle strength. The total body lean mass, fat mass and appendicular lean mass in the lower extremities have a significant impact on postural stability and muscle strength in people with IPD. This is the first study to explore the relationship between body composition phenotypes, postural stability and muscle strength in IPD.