Effect of prolonged inactivity on vestibular control of balance

Postural instability and falls are a major problem in older adults, leading to injuries and increased mortality. Understanding the precise cause(s) of instability is necessary before we can implement targeted therapy programmes or lifestyle changes. One factor known to exacerbate instability and fall risk is inactivity, which tends to increase with age. This tendency towards inactivity is compounded by age-related morbidity, which often results in extended periods of profound inactivity, either in the home or in a hospital bed. Although such inactivity is known to impair postural control, we do not understand the precise mechanisms. The purpose of this project is to examine the effect of prolonged inactivity upon postural control. We will study young healthy subjects before and after a prolonged period of bed rest (60 days). By studying young adults, we will gain a pure understanding of how inactivity affects posture without the confounding co-morbidities associated with age. We will assess postural control using a technique called galvanic vestibular stimulation (GVS). GVS causes a sway response in standing subjects and can be used to test the efficacy of the balance system. We will examine GVS responses immediately before and after prolonged bed rest to determine how the response changes. The amplitude and accuracy of the sway response will be analysed separately. Any reduction in amplitude, or variability in the directional accuracy of the response, would indicate impaired postural control. Identifying such problems will provide therapeutic targets for recovery of postural control following inactivity.

This project will determine the effect of prolonged inactivity upon vestibular control of posture. Volunteers will be studied immediately before and after 60 day bed rest, plus one week following cessation of bed rest. At each time point, stochastic vestibular stimulation (SVS) will be applied to the mastoid processes during quiet stance, with the eyes closed. A force platform will be used to record the evoked postural response in terms of lateral ground reaction forces. The response will be quantified by the SVS-force cross-correlation, as well as cross-spectral gain, phase and coherence. Gain will provide the main measure of the amplitude of the vestibular-evoked postural response. We will determine if this changes following bed rest. In addition, we will assess the directional accuracy of the evoked response. In healthy subjects, the direction of the SVS-evoked sway response is fixed with respect to head coordinates, and is directed through the inter-aural axis. As the head turns, so does the evoked sway direction. This relies upon intact neck proprioception. Hence, any effects of bed rest upon proprioceptive acuity would be expected to alter the directional accuracy of the vestibular response. To determine if this is the case, subjects will adopt a set number of head positions (+/- 90 deg). The direction of the sway response will be assessed using the SVS-force cross correlation. Any changes in directional accuracy PRE and POST bed rest will be measured. In addition to the SVS response, we will also quantify any changes in baseline sway PRE and POST to determine how much postural stability is worsened by bed rest. Changes in the SVS response will be correlated with changes in stability, to determine the extent to which changes in postural control can be attributed to altered vestibular reflexes.

This project fits within a wider programme of research undertaken by Dr Reynolds to understand mechanisms of postural instability and falls in older adults. As part of this programme, Dr Reynolds has organised a series of open days and symposia in order to engage with older adults and medical professionals. He will continue to organise such events in the future, as detailed below.

The two groups who will benefit are:

- Older adults will benefit from this research by learning about factors which increase their risk of falling, and potentially by enrolling on appropriate rehabilitation programmes.

- Clinicians will benefit from being better able to identify those at high fall risk, and recommending appropriate therapies or exercise programmes. This group includes geriatricians, neurologists, ENT consultants, physiotherapists and occupational therapists. We have a good track record of engaging all these groups to inform them of our previous and planned work in the area of balance and falls.

In order to facilitate impact we will organise a series of open days and talks for the lay public, as well as a symposium aimed at professionals:

1. Winterbourne Botanical Gardens Annual Open Day: This meeting engages older adults who are interested in learning about ongoing ageing research at the university, and in volunteering to participate. We will set up a stall at this meeting where the general public can discuss the purpose of our research and volunteer if they wish.

2. Departmental Open Days: As part of our ongoing research, older adults from the general public will be invited to attend departmental open days. These days will include general talks on fall risk, as well as our findings from previous research and planned future research, presented in lay terms. It will also include a tour of our laboratories. Key stakeholders will also be invited, including local representatives of the National Osteoporosis Society and The University of the Third Age. We will separately organise a talk to be delivered to the Science Awareness Group of the local University of The Third Age.

3. Balance and Falls in Older Adults - One day professional symposium: This will include national and international invited experts who will present their research findings, in addition to us presenting our research. The invited audience will be a mix of the clinical groups mentioned above. Having recently run a similar symposium, Dr Reynolds has a considerable distribution list to target the most relevant clinicians.