The items that the younger participants find harder than those who are older are the items relating to orientation in poor light and peripheral obstacles, which had the highest item difficulties in the overview scale. The items that are specifically harder for the older participants were easier tasks within the overview scale and relate to finding products and use of public transport.
Although older participants tended to have had visual loss for a longer duration (duration of visual impairment = (0.48 × age) − 2.51, R2 0.24, P = .000), there are differences in the items with differential item functioning when considering duration of visual loss. Those who had been visually impaired for longer found some items relating to finding products and public transport more difficult, similar to the older participants. However, those who had been visually impaired for less time found the items on fatigue specifically more difficult. These were the “easiest” items overall in the overview scale.
In terms of gender, male participants expressed more difficulty with furniture shopping and reading traffic signs. Female participants expressed more difficulty with fatigue, as had those who had been visually impaired for less time. Females in the sample did have a significantly (t(163) = 2.85, P = .005) shorter duration of visual impairment (19 ± 14 years) than males (25 ± 17 years).
Mobility aid users found the most difficult outdoor tasks of orientating in poor light and avoiding inferior obstacles significantly easier than those not using mobility aids. Those using mobility aids found visual search tasks around finding products more difficult, indicating that mobility aids are not helping with such central vision tasks. There were no other items with significant differences in difficulty reported by mobility aid users and non-users. Therefore, the use of mobility aids seems to equate the difficulty of mobility in general and makes specific mobility tasks easier than for those not using aids.
The items that are specifically difficult to those with less severe impairment resulting from retinitis pigmentosa, orientating in poor light and peripheral obstacles, reflect the difficulties that people with retinitis pigmentosa may initially present with in terms of night blindness and restricted peripheral field. The items of greater specific difficulty for those registered severely sight impaired, around utilizing visual information and finding products, reflect how further visual field restriction and central vision reductions lead to particular difficulties with visual search and central reading tasks that are not experienced in the earlier stages. Several items that are more difficult for those registered severely sight impaired are consistent with those also seen to be more difficult for those of greater age, duration of visual impairment, and who use mobility aids. As might be expected, those registered severely sight impaired are more likely to be older (t(163) = −4.22, P = .000), to have been impaired for longer (t(163) = −4.72, P = .000), and to be more likely to use mobility aids (Mann–Whitney U 1903, Z = −5.74, P = .000).
The overview scale produced here by assessing the tasks underpinning the most difficult goals of the Dutch Activity Inventory for people with retinitis pigmentosa allow consideration of the most difficult tasks that need to be incorporated into rehabilitation programs. The most challenging tasks overall (Table 5) relate to mobility in poor and bright light both outdoors and indoors, and to avoiding peripheral obstacles outdoors. These are not unexpected activity limitations, given that the effect of retinitis pigmentosa on photoreceptors is such that the presenting visual impairments are usually poor scotopic vision and reduced peripheral field. The high prevalence of posterior subcapsular cataract in retinitis pigmentosa24 is also likely to impact on difficulty in bright light conditions when the pupil constricts. Greater difficulty with mobility in reduced illumination by people with retinitis pigmentosa has previously been reported in questionnaire studies,25 and observed objectively in terms of slower walking speed and increased mobility incidents when negotiating a mobility course.5,26 The remainder of the “top 10” most difficult tasks (finding products in shops only visited occasionally, orientating in a store/hospital, avoiding inferior obstacles indoors, and noticing road users) also relate to the utilization of peripheral vision. These are tasks that may be more difficult because they are less frequently done and/or involve changes that cannot easily be predicted.
To take an alternative perspective, what are the least challenging of the tasks assessed and can these suggest appropriate rehabilitation strategies? Fatigue, asking for help, and traveling in familiar environments are the least challenging of these tasks underpinning difficult goals (Table 5). Given these findings are for people with largely established visual impairment (median duration of loss 16 years), it suggests that key aspects to effective rehabilitation for those in the initial stages of the condition could include learning skills by making tasks and travel routes more familiar and to provide confidence in asking for help. Tiredness was found more difficult by those who had been impaired for less time and also by women. Those who have been visually impaired for less time are likely to have less severe visual loss, but may not yet have developed techniques or compensatory strategies for achieving tasks that those with more longstanding impairment have, or gained familiarity with undertaking these tasks as a visually impaired person, and it therefore takes more effort to undertake and achieve these tasks.
The use of Rasch analysis also allowed the derivation of subscales, identifying unidimensional themes that might be usefully considered as independent aspects of particular difficulty for those with retinitis pigmentosa to be considered in the rehabilitation process. This complements the overview analysis that addresses relative item difficulty but in a slightly less unidimensional way. The analogy is that the overview scale is like an assessment of “maths” and the subscales identify the relative components of this overarching theme, such as “addition,” “algebra,” or “calculus.” However, the novelty of the subscales as derived here is that the tasks are reorganized from those that are grouped together by goal (researcher driven) to those perceived as requiring similar skills by people with retinitis pigmentosa (patient driven).
The complexity and diversity of needs makes it necessary to adapt any rehabilitation program, such as orientation and mobility training, to an individual client’s needs.27 The most effective methods to teach the use of mobility aids, or orientation and mobility in general, are unclear with no good quality evidence currently available.27,28 There has been interest in determining whether a standardized orientation and mobility teaching protocol would have better outcomes than usual care,29 but the results of the trial suggested little difference between techniques30 although the trial was hampered by trainers deviating from the standardized protocols, potentially to deliver more client-centered training. The subscales derived in the present study may be useful in informing the development of training programs, suggesting tasks that can be considered together in training as forming a unidimensional construct from the client’s perspective. The subscales do seem to group together skills that are considered similar by rehabilitation specialists: for example, the subscales of “going up and down” and “using public transport” represent higher risk activities that would be taught after more basic skills had been acquired.
There is also little current evidence of the value of training provided by rehabilitation specialists.31,32 The subscales outlined here could provide a potentially valuable specific tool to provide evidence of success in client-centered rehabilitation training. Clients undertaking initial training could have changes in perceived ability assessed by administration of relevant subscales pre- and post-training. After successful initial training and review of appropriate further goals, more advanced skills could be taught and efficacy assessed using further relevant subscales.
The use of differential item functioning also allows consideration of which tasks are more specifically difficult for particular groups, which can again inform the rehabilitation needs of those with retinitis pigmentosa. The most difficult items overall, those around poor light and obstacle avoidance (subscale 4), are even more difficult for those in the earlier stages of the condition, namely those who are younger, who do not use mobility aids, and who are either not registered or registered only as sight impaired. Because mobility difficulties tend to increase as the extent of visual field loss increases,7,26,33 it might have been expected that mobility tasks such as avoiding peripheral obstacles would have been more difficult for those in the later stages of the condition, who are more likely to be registered as severely sight impaired. However, it could also be argued that as the disease progresses, adjustments to approaches to mobility are made from an early stage, such that these tasks seem less of an issue than they do earlier in the disease process, and newer difficulties, such as those with visual search as the field contracts to very small levels, are relatively more difficult as they need to become adapted to.
Items around visual search (subscale 1) are found more difficult by those with more profound visual loss as indicated by their registration as severely sight impaired, use of mobility aids, older age, and longer duration of visual impairment. Items around utilizing visual information (subscale 3) are also found more difficult by those registered severely sight impaired. These specific activity limitations are likely to relate to visual impairments that become more of an issue later in the disease process, with very limited visual fields affecting visual search, and additional progressive loss of central visual acuity and contrast sensitivity. Use of public transport (subscale 2) is found more difficult by those who are older or have been impaired for longer. Thus, the need for rehabilitation training in the use of public transport and the use of visual search strategies may need to be reviewed over time, as these areas become more difficult over time. It is a limitation of the study that by using an online questionnaire, the only indication of severity of visual loss is registration status. It is acknowledged that not every participant will be in the most appropriate registration category for their visual function because registration is voluntary. It would be useful in future research to be able to determine visual acuity and visual field extent to determine at what level of visual function these tasks become problematic.
Of particular note are the items for which differential item functioning is seen between people who do and do not use mobility aids, and also the items for which differential item functioning is not seen. People using mobility aids find many tasks no more difficult than those who do not use aids (such as those included in public transport (subscales 2 and 6), going up and down (subscale 5), and street safety (subscale 7)) and express significantly less difficulty with the most difficult tasks overall of orientation in poor light and bumping into peripheral obstacles compared to people not using mobility aids. These findings provide evidence that mobility aids do make a difference to performance. Although it is anecdotally clear that mobility aids such as canes enhance mobility function, because users continue to find value in using them, evidence in previous literature for the benefits of mobility aids and training is not clear cut, and the present finding is therefore relatively novel. A Cochrane systematic review27 found no high-quality evidence of the effects of orientation and mobility training for adults with low vision, and very little evidence has been found in the literature28 on methodology or effectiveness of symbol cane training. Guide dog users with retinitis pigmentosa have, however, been shown to walk faster and with greater ease with their dog than without.34 Thus, further research to demonstrate whether and how mobility aids enhance mobility function would be beneficial. A limitation of the present study with regard to mobility aid use is that we did not distinguish between symbol cane and long cane users in our “cane users” category. The use of canes and guide dogs were asked about separately, and of the 82 participants who used a mobility aid, 64 used a cane only, 4 used a dog only, and 14 used both a cane and a dog. Given the low number of people in the study using guide dogs as mobility aids, the data has considered all mobility aid users together.
Overall, the most difficult tasks underpinning the most difficult activity and participation goals for people with retinitis pigmentosa are orienting in poor light and avoiding peripheral obstacles. However, we also show that these specific tasks are relatively more difficult for those who are not registered and registered as sight impaired. They are also less difficult for those people who use mobility aids, demonstrating the benefits of such aids. Those who are registered severely sight impaired find visual search tasks relatively more difficult. Derivation of unidimensional subscales identifies groups of tasks considered similarly by people with retinitis pigmentosa, which can be used as an evidence base for developing and assessing orientation and mobility training protocols.
Department of Vision and Hearing Sciences
Anglia Ruskin University
Cambridge, CB1 1PT
The authors thank the members of RP Fighting Blindness for their enthusiasm in taking part in this project. Presented in part at the 10th Annual Rasch User Conference, Durham, United Kingdom, March 2016.
Received June 3, 2016; accepted October 31, 2016.
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Keywords:© 2017 American Academy of Optometry
retinitis pigmentosa; activities of daily living; rehabilitation; visual impairment; Rasch analysis; orientation and mobility; mobility aids