The effect of ageing on vestibular control of balance

Falls are a major problem for the elderly. Approximately 30% of people aged over 65 experience one or more falls per year, rising to 50% after 80. 10-15% of falls result in serious injury, costing the NHS and social services up to £1billion annually. This problem can only be exacerbated by an ageing population. Various factors such as muscle weakness and sensory loss have been associated with increased risk of falling, but despite the magnitude of the problem, we know little about mechanisms underlying falls. A structure which is known to play an important role in maintaining balance is the vestibular system, which resides in the inner ear. Vestibular feedback tells us how the head is moving in space and this information is used to reduce postural sway. Vestibular reflexes are therefore important in arresting falls. However, for them to work properly requires the information to be transformed from a head-centred to a body-centred frame of reference. This requires a precise sense of head orientation, and also central neural processing. These factors are known to degrade with age and this may cause inaccurate or absent responses, leading to postural instability and falls. This will be tested by assessing the ability of different age groups to use vestibular information for controlling balance. Electrical stimulation of the vestibular nerve will be used to induce a sensation of head movement. This is a painless technique which produces reliable sway responses in standing subjects. The precision of the sway response will be assessed in young (18-40), middle aged (40-65) and elderly (>65) subjects. This will determine the effect of age upon the ability to use vestibular information for balance. Volunteers will also be assessed for fall risk, using a series of sensory-motor tests, and a questionnaire. This will reveal any link between vestibular control of balance and the risk of falling. Reasons underlying altered vestibular responses will also be investigated. Specifically, the ability to sense head orientation, and the integrity of the peripheral vestibular apparatus, will both be tested. This multi-factorial approach will provide an overall picture, from the reasons underlying altered vestibular control, to the consequences for postural stability and falls. This research has implications for identifying individuals at high risk of falling, and for designing exercise and therapy programmes for reducing risk. This is becoming increasingly important in our ageing population.

The primary objective of this proposal is to determine the effect of normal ageing upon vestibular control of balance. This will be achieved using techniques recently developed by the applicant. Stochastic vestibular stimulation (SVS) will be applied to standing subjects and the resulting sway response will be quantified by cross-correlating the SVS signal with ground reaction forces. In young subjects this response is systematically modulated by head orientation, reflecting transformation of vestibular signals from head to body coordinates. Loss of this modulation pattern would reflect an inability to use vestibular information to restore balance. Responses will be assessed in young (18-40), middle-aged (40-65) and elderly (>65) participants. Subjects will adopt various head orientations, ranging between +45 and -45 degrees in pitch and yaw. The precision with the vestibular-evoked balance response is modulated by head orientation will be compared across age groups. The second objective is to investigate the relationship between vestibular control of balance, postural instability and fall risk. Postural stability will be assessed by measuring the excursion of the centre of pressure during quiet stance. Fall risk will be assessed with validated questionnaire and tests of sensory-motor function, both predictive of falls. The third objective is to reveal the sensory factors affecting the precision of the vestibular sway response. Head proprioception will be measured by asking subjects to align their head with their feet in darkness. Peripheral vestibular function will be tested by measuring eye movement responses to low frequency chair rotation. The relationship between age, vestibular control of balance, sensory factors and fall risk will be assessed statistically using a structural equation model. This will determine how the known effect of age upon fall risk is mediated by these other factors, addressing all three objectives.

Other than academics interested in postural control mechanisms, the main beneficiaries of this research will be clinicians who treat the elderly, and the elderly themselves. Overall, the impact of this research will be to improve quality of life for the elderly, and to reduce the economic impact of fall-related injuries. Identifying people at high risk of falling is important, because patients can be advised on strategies to avoid falls, and be prescribed targeted exercise programmes. One corollary of this research will be better identification of those at high risk of falling. This will be of benefit to clinicians working with the elderly. The findings will be communicated to clinicians via links with existing collaborators. In terms of reducing fall risk, this research is relevant to physiotherapists and occupational therapists. There is evidence that exercise programmes can reduce fall risk. However, these programmes differ greatly and it is not clear what type of exercise works best. In helping to identify the mechanisms underlying falls, this research will identify weak points in sensorimotor function. This might include neck proprioception, for example, in which case physiotherapy could be targeted at improving neck sensation. Existing collaborations and teaching links with physiotherapists will be used to publicise this work to those who deal with the elderly on a daily basis. I have recently become a member of the KT-EQUAL group. This is a collection of researchers and health professionals interested in ageing, funded by the EPSRC. I have attended their falls prevention workshop in Bath, where I met a variety of professionals interested in ageing research, including academics, physicians, therapists and local authority staff. This group offers an excellent opportunity to make my work known to people who would be otherwise unaware of it. This will accelerate the transfer of research findings into clinical practice. I will attend future workshops to disseminate the results of my research. The ultimate aim of this research is to benefit the elderly themselves, and it is therefore important to engage directly with this group. To this end, the RA and I will get involved with community outreach. We will send copies of published papers to elderly participants (along with lay summaries), and give presentations in day care centres where our research will be described in a format suitable for non-specialists. In summary, I am in a good position to be able to inform those people who will benefit most from this research, leading to the greatest impact.