Orientation and mobility training for adults with low vision (2024)

Monitoring Editor: Gianni Virgili, Gary Rubin, and Cochrane Eyes and Vision Group

University of Florence, Department of Specialised Surgical Sciences, Via le Morgagni 85, FlorenceItaly, 50134

Institute of Ophthalmology, Bath Street, LondonUK, EC1V 9EL

Orientation and mobility training for people with low vision

Progressive visual impairment often affects people as they age. Training is used to help people with low vision maintain travel independence, with new orientation and mobility skills to compensate for reduced visual information. Orientation is the ability to recognise one's position in relation to the environment, whereas mobility is the ability to move around safely and efficiently. Orientation and mobility (O&M) training teaches people to use their remaining vision and other senses to get around. Canes and optical aids may also be used.

We found two small studies with a total of 63 people comparing O&M training delivered by a trained volunteer to physical exercise. These studies did not show a difference between the two interventions, but they had little power to do so because of the small sample size and poor methodological quality. There were no adverse effects of O&M training in these studies.

There is little evidence from randomised controlled trials on which type of O&M training is better for people with low vision who have specific characteristics and needs.

Description of the condition

The World Health Organization (WHO) has established criteria for low vision which are used in the International Classification of Diseases (WHO 1992). Visual impairment is defined as a best‐corrected visual acuity of less than 0.5 logMAR (Snellen 6/18 or 20/60) but equal to or better than 1.3 logMAR (3/60 or 20/400) in the better eye. Blindness is defined as a best‐corrected visual acuity of less than 1.3 logMAR or a remaining central field of 20 degrees diameter. In the United States, legal blindness is defined as a visual acuity of 1.0 logMAR (6/60 or 20/200) or less in the better eye.

Blindness is one of the most common disabilities (Congdon 2003). An estimated 40 million people were blind a decade ago, the time of the last accurate assessment. Among persons older than 40 years in 2002 in the USA, 937,000 were blind. Figures for both the developing world, where 90% of world blindness exists, and the developed world are expected to increase significantly during the next decades as the world's population ages.

Causes of blindness in the USA depend on race and ethnicity (EDPRG 2004). Age‐related macular degeneration is the most common cause in white people whereas cataract, glaucoma and diabetic retinopathy are the leading causes in Hispanic and black people. Treatable or preventable conditions are the most frequent causes of blindness in developing countries: infectious disease, nutritional deficiency, cataract and refractive error (Congdon 2003).

One of the most significant handicaps produced by visual impairment is the limit it imposes on the ability to travel independently (Golledge 1991). During travel we receive the majority of the information about our environment through our visual system (Soong 2001b). Many aspects of routine life may be affected by difficulty or inability to travel, such as opportunities to participate in social activities and to find or retain employment (Long 1996). Loss of vision may, therefore, have a major effect on the individual's overall quality of life, either directly or indirectly (Soong 2001b).

The SEE Project (West 2002b) showed that disability, defined as deficit in performance relative to a population, cannot be characterised on the basis of a single cut‐off value of visual acuity. In fact, for most daily living tasks performance gradually declines with progressive visual loss. Moreover, given tasks require specific levels of visual function. In the SEE Project reading and face recognition were very demanding in terms of visual function as they were found to be impaired in most people with even mild visual loss (0.5 or less, Snellen 6/12 or 20/40). On the other hand, the mobility tasks considered in the study were affected in almost half of people when visual acuity approached 0.1 (Snellen 6/60 or 20/200) (West 2002b).

Vision functions other than standard visual acuity may affect day‐to‐day functioning of older adults. In most eye diseases loss of contrast sensitivity and the extent of visual field damage are correlated with mobility impairment at least as much as is visual acuity (West 2002a; West 2002b). Loss of peripheral visual field is particularly prominent in people with retinitis pigmentosa, 80% of whom experienced mobility difficulty based on a questionnaire in one study (Geruschat 1998). Finally, environmental factors are critical to mobility tasks (Kuyk 1996). The ability of visually impaired persons to avoid obstacles is significantly impeded under mesopic (reduced) illumination. Object contrast and location are also factors determining success in avoiding obstacles on the travel path (Kuyk 1996).

Description of the intervention

Travel in the environment involves skills of orientation and mobility (O&M). Orientation is the ability to recognise and establish position in relation to environment. Mobility is the physical ability to move in an orderly, efficient and safe manner through the environment (Novi 1998). To maintain travel independence it is essential for a visually impaired adult to learn new (O&M) skills to be able to compensate for reduced visual information (Jacobson 1993; Seybold 1990).

How the intervention might work

Enhancing the ability of low vision people to safely navigate their home and the external environment is the aim of O&M services. People are taught how to cope with vision loss to maximise the ability to live independently. Given the fact that O&M training is an established practice, the most interesting question to people with low vision and O&M therapists is which techniques are more effective.

Orientation and mobility training may be a component of multidisciplinary rehabilitation delivered by some low vision services. An ongoing Cochrane review focusses on such complex interventions (Langelaan 2007), and therefore these interventions will not be the purpose of our review. Another Cochrane review underway is currently investigating the effect of environmental and behavioural interventions for reducing activity limitation and improving quality of life in community dwelling visually impaired older people. Our review focusses on objective performance whilst the other two Cochrane reviews adopt quality of life as the primary outcome measure.

Why it is important to do this review

The Academy for Certification of Vision Rehabilitation and Education Professionals performed a survey among O&M training specialists in order to evaluate the knowledge, skill and ability required in their profession (Wiener 2000). This survey established a ranking of importance of professional competencies according to the opinion of 200 O&M experts that responded to the questionnaire. However, current assessment and rehabilitation techniques are not standardised and to date there has not been a systematic review of the evidence for the effectiveness of the various interventions used by O&M training specialists.

Criteria for considering studies for this review

Search methods for identification of studies

Data collection and analysis

Description of studies

Risk of bias in included studies

The results of the assessment of methodological quality are shown in the 'Characteristics of included studies' table and summarised in Figure 1.

Effects of interventions

See: Table 1

The authors of the two included studies reported analyses of the total score (Analysis 1.1) as well as analyses of three subscales (Analysis 1.2, Analysis 1.3, Analysis 1.4). Since this was a training and assessment instrument, grouping of similar skills in the subscales should have been pre‐planned which would avoid the risk of "post hoc" analyses but as yet we have not had this confirmed by the authors. Since discussing statistical heterogeneity of two small studies is questionable, we provide a meta‐analysis for descriptive purposes and discuss clinical heterogeneity issues.

An issue in Straw 1991a was the good functioning of most individuals, either because they already had some independent mobility in their living environments, or because of the characteristics of the measurement tool (too easy for the people included). In fact, baseline scores were in general high, from 73 up to 95 in all scales, which may have limited the ability to detect an improvement (ceiling effect). The score was a percentage of tasks done correctly; high scores were markedly skewed when plotted as a continuous outcome variable. Taking into account these limitations, there was no statistically significant difference between groups in any score at the final examination (Analysis 1.1; Analysis 1.2; Analysis 1.3; Analysis 1.4).

All baseline scores were much lower in Straw 1991b (range: 47 to 70), suggestive of less O&M skills in these people who had more room for improvement. At the final assessment, all the scores in the treated group increased, but the difference between comparison groups was close to statistical significance only for the Independent subscale (Analysis 1.4). The authors observed anecdotally that blind persons with no O&M experience benefit more, while those living in their homes for years already had skills. Participants with less severe visual impairment did not have O&M needs.

Apart from the inclusion of people with more severe disability in Straw 1991b compared to Straw 1991a, a further difference was the use of trained professional outcome assessors in the former, which the authors believed may have improved measurement sensitivity.

Summary of main results

The two small quasi‐randomised controlled studies included in this review (Straw 1991a; Straw 1991b) could not find a significant difference between O&M training, delivered by a volunteer and physical exercise. Straw 1991a and Straw 1991b evaluated the effects of orientation and mobility (O&M) training in adults with low vision, comparing it to physical exercise. The studies were consecutive phases of development and implementation of the same training curriculum and assessment instrument. No difference between O&M training and physical exercise was found in Straw 1991a. In Straw 1991b O&M training tended to be better than physical exercise, but only the difference for the independent subscale of the training/ assessment instrument was close to statistical significance. This scale assessed the ability to travel indoors independently.
The differences between the two studies may have been due to the improvement of the intervention and assessment instrument, which was developed to be tailored to a patient's needs and characteristics in the second study. An important limitation of Straw 1991a was the high baseline scores obtained by the participants, close to the maximum achievable with that instrument (ceiling effect). Another difference between these studies could be greater O&M abilities of participants before training. Very low vision was slightly less common among participants in Straw 1991a as compared to Straw 1991b (77% versus 84%), and the authors observed that there is little need for O&M training among those using vision for travel. Even differences among blind people may exist, since those who are very familiar with their home environment already have such skills, but no details are available in the articles.

Overall completeness and applicability of evidence

Given the complexity of O&M interventions and the small size of the two studies in this review, as well as the fact that they were conducted by the same study group, we cannot assume that our findings are applicable to other settings. The precision obtained in each study is low since analysing data as standardised mean difference (i.e. the effect size) could not exclude a large effect size, i.e. approaching 1, based on 95% confidence interval limits.

From the researchers' point of view, these two studies suggest that it is possible to develop an O&M training curriculum coupled with an assessment tool that is able to capture change with time for at least some O&M skills. Nonetheless, this message can be less useful today, given the shift towards the use of quality of life questionnaires as an outcome measure and towards complex rehabilitation or support models as an intervention.

Quality of the evidence

The size of the studies (which are small) and the inadequate method of randomisation place these studies at risk of selection bias and confounding. Masking of recipients of care and care providers could not be achieved and indeed this may be difficult or impossible when delivering an O&M intervention. In at least one study the outcome assessors were not masked. On the other hand, the choice of outcome measures that allow masking of at least outcome assessors is important when scoring O&M performance. The shift from objective to subjective outcomes, i.e. from scoring physical performance to self‐perceived performance or quality of life measured with validated questionnaires, would change the perspective from that of the trainer to that of the patient, thus allowing masking of both assessors and a patient‐centred outcome measure. Such a choice has been made in the ongoing study Zijlstra 2009. This study is validating a standardised O&M training model in The Netherlands while assessing its acceptability and efficacy compared to standard care.

Agreements and disagreements with other studies or reviews

In this update of the review we have pooled the results of the two studies for descriptive purposes. However, the limitations in the analyses remain, particularly the fact that data were markedly skewed in Straw 1991a, making the use of parametric tests improper. The I² value was more than 60% for two subscales, but, as reported above, we decided not to make formal analyses of statistical heterogeneity in the update of this review because it cannot be reliably calculated with few studies in the meta‐analyses (Higgins 2009b; Ioannidis 2007).

Since we found little literature that met the more stringent criteria for this review, we provided a description of three non‐randomised controlled studies found by the electronic searches that have evaluated the effects of this intervention, to delineate the most recently used investigation methods.

Szlyk 1998 compared the performance of eight people who received both bioptic amorphic lenses and O&M training with that of seven people who received neither, during an observation period of three months. All participants were affected by peripheral visual field loss and the aim of the lenses was to expand their visual field. No randomisation procedure was reported but the authors state that the participants were matched as much as possible for ocular disease, level of visual impairment and age. The group that served as a control in the first three months received the same intervention during the second trimester. All participants were evaluated with clinical, psychophysical, laboratory and O&M assessment, which included driving skills, at baseline and at three and six months. A merit of this study is that the authors explored the feasibility of using a number of laboratory and real‐world tests grouped in six fundamental visual skills that are relevant to O&M (recognition, peripheral detection, scanning, tracking, visual memory and mobility). The authors provided examples of such tasks. Examples of recognition tasks were counting the number of people in a passing car; locating building addresses; reading posted prices of items within a cafeteria. As an index of mobility, participants were graded on their ability to cross intersections, walk stairs, and negotiate crowded public places. Scanning tasks included finding particular items on a store shelf, finding a particular entrance in the student union building, and locating the "deli station" in a cafeteria. Tracking tasks included watching moving vehicles as they pass down the street and following particular individuals in public places. The certified O&M specialist coded performance on the mobility tests using a scale from 1 (not able to perform) to 5 (no difficulty). On each task, either performance or speed or both were coded. To define a trained person as improved in a particular task they had to obtain an increase in score of at least the average change recorded in the observation group after three months. With this definition, there was a reported 25% to almost 50% improvement of performance, depending on the type of task, after both immediate and delayed training. The investigators found that the amount of improvement was larger in those with narrower visual field. Data on reliability of the measures are not provided.

Szlyk 2000 used a similar battery of assessment methods and study design to evaluate the effectiveness of bioptic lenses with or without O&M training in people with central vision loss. Participants were assigned to receive bioptic telescopes with training (n = 9), telescopes alone (n = 8) or no intervention (n = 8). This last group was offered delayed use of lenses and training from three to six months. No method of randomisation was used but the authors state that groups were matched and statistically equivalent in age, sex, central scotoma size, visual acuity and letter contrast sensitivity. As in the former study, the authors defined an improvement in a task as an increase in score by more than the average of the test‐retest difference (baseline versus three months) for the untreated group. Since they found no difference between the improvement obtained with immediate versus delayed training, they combined data of those given lenses and training initially with those who received the intervention in the second trimester only; these individuals had served as controls in the first trimester. They found a statistically significant effect of training plus lenses compared to lenses alone for some visual tasks, such as recognition (P = 0.05, t‐test), peripheral identification (P = 0.02), scanning (P = 0.03), but not for mobility (P = 0.06), tracking (P = 0.15) or visual memory (P = 0.07), although all comparisons favoured the intervention. When driving related tasks were evaluated separately from other O&M tasks a statistically significant effect of training was observed for this task (P = 0.02). This work shows that differences between treatment groups can be elicited based on numerous and complex psychophysical, laboratory and real‐world tests but statistical methods used to analyse them are not reported in detail in the paper. One statistical issue is the failure to take into account multiple testing, which increases the likelihood of finding a statistical significance by chance. As an example, performing 20 comparisons adopting the conventional level of statistical significance (P=0.05), leads to a probability of (1 to 0.95²⁰) or 64% to obtain one or more significant findings only by chance.

Soong 2001b investigated the effect of O&M training on mobility performance of 19 visually impaired individuals compared to 18 people who were matched as closely as possible for ocular disease, level of visual impairment and age. No randomisation procedure was adopted. All the participants were physically active and went out of their homes with or without other people. Sixteen people in the treated group were prescribed long canes or support canes or identity canes. All participants were tested at two visits four weeks apart. At each visit, the person's mobility performance was assessed twice as walking efficiency (Percentage Preferred Walking Speed (PPWS)) and error score on an indoor obstacle course. The error score improved at the second visit only in the treated group but not to a statistically significant extent, while PPWS improved only in the untrained group. The authors comment that the mental effort spent by trained participants trying to use the new techniques may have compromised their walking efficiency. Also the design of the walking path may be critical and future studies should consider other outcome measures such as self reported mobility performance and mental effort needed.
The studies presented above as well as the literature obtained in our search point out that trials considering O&M training may use very different tests to measure participants' performance. The objective of our review was not to find and discuss all the studies that have used or evaluated O&M assessment instruments. Nonetheless, we recognise that the discussion of this issue is critical for those who plan to conduct RCTs in this field of research. Therefore, we provide some examples that are useful for this purpose. Soong 2001b used an indoor laboratory course with obstacles set up along the course and measured time and errors made while completing the task. Szlyk 1998 used 41 indoor and outdoor tests exploring six visual skills categories (recognition, peripheral detection, scanning, tracking, visual memory and mobility). Geruschat 1989 used total time and mobility incidents on real‐world routes. Straw 1991a and Straw 1991b developed an assessment instrument for assessing O&M in older visually impaired adults, which has been described above and which they claimed to have acceptable inter‐observer reliability.

The complexity of an O&M intervention and its evaluation is such that assessment instruments will usually focus on several aspects of the performance rather than on a single objective measure. While objective measurements of performance have theoretically better properties, being easier to standardise, their use and interpretation may not be straightforward. As an example, Soong 2001b used PPWS and error score to evaluate mobility. They could not demonstrate a benefit from training with respect to the control group, although there was a trend in favour of treated people for the error score outcome.

The authors state that it may be necessary to assess mobility performance after a longer period, such as at three and six months after training, to allow visually impaired adults to have sufficient practice in adopting new travel skills. It must also be considered that O&M instructors specifically teach clients to vary walking speed and to increase the duration of preview while walking through an unfamiliar area so that faster may not always be better for some real‐world tasks. In addition, many of the participants in the training group used a long cane on the post‐test, but not the pre‐test. This may have further slowed their progress as they would have been relatively inexperienced with the cane and would have had to use it to manoeuvre round the confined and cluttered obstacle course that was being used to assess performance. This could explain the opposite direction of training effect on PPWS and error score in treated and control participants in the study by Soong 2001b. Even if such effects were demonstrated, how they relate to a person's quality of life is not clear. Indeed, if laboratory mobility tests are used they should both be sensitive to meaningful changes in performance and correlate with real‐world tasks. An advantage of using laboratory tests is that they can be described in detail, as in Soong 2001b, and possibly replicated by others. Finally, tests arranged in a laboratory may not be comparable across studies because of differences in their construction, such as number and positioning of obstacles along the path, their characteristics or positioning of light and glare sources.

The training and assessment methods of the studies presented in this discussion indicate that, despite the apparent lack of standard measures of the effect of O&M training, several travel‐related activities which are relevant to a person's life should be considered as primary outcome measures and the training should also be personalised according to patients' needs and characteristics. It may also be expected that enhanced mobility will have an impact on a person's wellbeing and self‐esteem and on their relationship with other people. Results from laboratory‐based mobility tests can be considered as surrogate outcome measures as they are expected to relate to performance in daily life. Specifically designed quality of life questionnaires may also be suitable to measure the effect of O&M intervention. The advantages of these tools are the ability to capture not only physical, but also mental and social dimensions of health using measures of both functionality and well‐being based on self‐evaluation. One example of such instruments is the Independent Mobility Questionnaire (IMQ), developed by Turano et al. (Turano 1999) for people with retinitis pigmentosa. An overview on quality of life and other types of visual function assessment questionnaires is offered in Massof 2001.

Implications for practice

The clinical practice of O&M instructors suggests that the need for mobility training of people with low vision is self evident. Nonetheless, the possibility of quantitative assessment of the effect of training is needed to study which techniques are more useful. Such results would help O&M instructors choose the most appropriate techniques, as well as guiding social health programmes in deciding to support this intervention in an era of cost‐containment.
Two small quasi‐randomised controlled studies could not find a significant difference between O&M training, delivered by a volunteer, and physical exercise.

Implications for research

Comparison 1

Characteristics of included studies [ordered by study ID]

Characteristics of excluded studies [ordered by study ID]

Characteristics of ongoing studies [ordered by study ID]

Straw 1991a {published data only}

• Straw LB, Harley RK, Zimmermann GJ. A program in orientation and mobility for visually impaired persons over age 60. Journal of Visual Impairment and Blindness 1991;85(3):108‐13. [Google Scholar]

Straw 1991b {published data only}

• Straw LB, Harley RK. Assessment and training in orientation and mobility for older persons: program developing and testing. Journal of Visual Impairment and Blindness 1991;85(3):291‐6. [Google Scholar]

Soong 2001 {published data only}

• Soong GP, Lovie‐Kitchin JE, Brown B. Does mobility performance of visually impaired adults improve immediately after orientation and mobility training?. Optometry and Vision Science 2001;78(9):657‐66. [PubMed] [Google Scholar]

Szlyk 1998 {published data only}

• Szlyk JP, Seiple W, Laderman DJ, Kelsch R, Ho K, McMahon T. Use of bioptic amorphic lenses to expand the visual field in patients with peripheral loss. Optometry and Vision Science 1998;75(7):518‐24. [PubMed] [Google Scholar]

Szlyk 2000 {published data only}

• Szlyk JP, Seiple W, Laderman DJ, Kelsch R, Stelmack J, McMahon T. Measuring the effectiveness of bioptic telescopes for persons with central vision loss. Journal of Rehabilitation Research and Development 2000;37(1):101‐8. [PubMed] [Google Scholar]

Zijlstra 2009 {published data only}

• Zijlstra GA, Rens GH, Scherder EJ, Brouwer DM, derVelde J, Verstraten PF, et al. Effects and feasibility of a standardised orientation and mobility training in using an identification cane for older adults with low vision: design of a randomised controlled trial. BMC Health Services Research 2009;9:153. [PMC free article] [PubMed] [Google Scholar]

Banja 1994

• Banja JD. The determination of risks in orientation and mobility services: ethical and professional issues. Journal of Visual Impairment and Blindness 1994;88(5):401‐9. [Google Scholar]

Blasch 1995

• Blasch BB, Stuckey KA. Accessibility and mobility of persons who are visually impaired: a historical analysis. Journal of Visual Impairment and Blindness 1995;89(5):417‐22. [Google Scholar]

Blasch 1997

• Blasch BB, Wiener WR, Welsh RL. Foundations of Orientation and Mobility. 2nd Edition. New York: American Foundation for the Blind, 1997. [Google Scholar]

Campbell 2005

• Campbell AJ, Robertson MC, Grow SJ, Kerse NM, Sanderson GF, Jacobs RJ, et al. Randomised controlled trial of prevention of falls in people aged > or =75 with severe visual impairment: the VIP trial. BMJ 2005;331(7520):817. [PMC free article] [PubMed] [Google Scholar]

Congdon 2003

• Congdon NG, Friedman DS, Lietman T. Important causes of visual impairment in the world today. JAMA 2003;290(15):2057‐60. [PubMed] [Google Scholar]

Corn 1990

• Corn AL, Lippmann O, Lewis MC. Licensed drivers with bioptic spectacles: user profile and perception. Review 1990;22:221‐30. [Google Scholar]

Deeks 2009

• Deeks JJ, Higgins JPT, Altman DG (editors). Chapter 9: Analysing data and undertaking meta‐analyses. In: Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.0.2 [updated September 2009]. The Cochrane Collaboration, 2009. Available from www.cochrane‐handbook.org.

EDPRG 2004

• Congdon N, O'Colmain B, Klaver CC, Klein R, Munoz B, Friedman DS, et al. The Eye Diseases Prevalence Research Group. Causes and prevalence of visual impairment among adults in the United States. Archives of Ophthalmology 2004;122(4):477‐85. [PubMed] [Google Scholar]

Feinbloom 1997

• Feinbloom W. Driving with bioptic telescopic spectacles. American Journal of Optometry and Physiological Optics 1977;54(1):35‐42. [PubMed] [Google Scholar]

Geruschat 1989

• Geruschat DR, Del'Aune W. Reliability and validity of O and M instructor observations. Journal of Visual Impairment and Blindness 1989;83(9):457‐60. [Google Scholar]

Geruschat 1998

• Geruschat DR, Turano KA, Stahl JW. Traditional measures of mobility performance and retinitis pigmentosa. Optometry and Vision Science 1998;75(7):525‐37. [PubMed] [Google Scholar]

Gillespie 2009

• Gillespie LD, Robertson MC, Gillespie WJ, Lamb SE, Gates S, Cumming RG, et al. Interventions for preventing falls in older people living in the community. Cochrane Database of Systematic Reviews 2009, Issue 2. [Art. No.: CD007146. DOI: 10.1002/14651858.CD007146.pub2] [PubMed] [Google Scholar]

Glanville 2006

• Glanville JM, Lefebvre C, Miles JN, Camosso‐Stefinovic J. How to identify randomized controlled trials in MEDLINE: ten years on. Journal of the Medical Library Association 2006;94(2):130‐6. [PMC free article] [PubMed] [Google Scholar]

Golledge 1991

• Golledge R. Tactual maps as navigational aids. Journal of Visual Impairment and Blindness 1991;85(7):296‐301. [Google Scholar]

Guth 1997

• Guth DA, Reiser JJ. Perception and the control of locomotion by blind and visually impaired pedestrians. In: Blasch BB, Wiener WR, Welsh RL editor(s). Foundations of Orientation and Mobility. 2nd Edition. New York: The American Foundation for the Blind, 1997:9‐38. [Google Scholar]

Higgins 2009a

• Higgins JPT, Altman DG (editors). Chapter 8: Assessing risk of bias in included studies. In: Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.0.2 [updated September 2009]. The Cochrane Collaboration, 2009. Available from www.cochrane‐handbook.org.

Higgins 2009b

• Higgins JPT, Thompson SJ, Spiegelhalter DJ. A re‐evaluation of random‐effects meta‐analysis. Journal of the Royal Statistical Society 2009;172(1):137‐59. [PMC free article] [PubMed] [Google Scholar]

Hollis 1999

• Hollis S, Campbell F. What is meant by intention to treat analysis? Survey of published randomised controlled trials. BMJ 1999;319(7211):670‐4. [PMC free article] [PubMed] [Google Scholar]

Hoover 1946

• Hoover RE. Foot travel without sight. Outlook for the blind and teachers forum 1946; Vol. 40:244‐51.

Ioannidis 2007

• Ioannidis JP, Patsopoulos NA, Evangelou E. Uncertainty in heterogeneity estimates in meta‐analyses. BMJ 2007;335(7626):914‐6. [PMC free article] [PubMed] [Google Scholar]

Jacobson 1993

• Jacobson WH. The art and science of teaching orientation and mobility to persons with visual impairment. New York: AFB Press, 1993. [Google Scholar]

Korb 1970

• Korb D. Preparing the visually handicapped person for motor vehicle operation. American Journal of Optometry and Archives of American Academy of Optometry 1970;47(8):619‐28. [PubMed] [Google Scholar]

Kuyk 1996

• Kuyk T, Elliott JL, Biehl J, Fuhr PS. Environmental variables and mobility performance in adults with low vision. Journal of the American Optometric Association 1996;67(7):403‐9. [PubMed] [Google Scholar]

LaGrow 1994

• LaGrow SJ, Weessies MJ. Orientation and mobility: techniques for independence. Palmerston North, NZ: Dunmore Press, 1994. [Google Scholar]

Langelaan 2007

• Langelaan M, Nispen RMA. Multidisciplinary rehabilitation and monodisciplinary rehabilitation for visually impaired adults. Cochrane Database of Systematic Reviews 2007, Issue 2. [DOI: 10.1002/14651858.CD006543] [CrossRef] [Google Scholar]

Levy 1949

• Levy WH. On the blind walking alone and of guides. Outlook for the blind and teachers forum 1949;43:103‐10. [Google Scholar]

Long 1996

• Long RG, Boyette LW, Griffin‐Shirley N. Older persons and community travel: the effect of visual impairment. Journal of Visual Impairment and Blindness 1996;90(4):302‐13. [Google Scholar]

Marsh 2000

• Marsh RA, Hartmeiser F, Griffin Shirley N. Legal issues for orientation and mobility specialists minimizing the risk of liability. Journal of Visual Impairment and Blindness 2000;94(8):494‐5. [Google Scholar]

Massof 2001

• Massof RW, Rubin GS. Visual function assessment questionnaires. Survey of Ophthalmology 2001;45(6):531‐48. [PubMed] [Google Scholar]

Novi 1998

• Novi RM. Orientation and mobility for sight deficients. Proceedings: the 9th International Mobility Conference, Atlanta, Georgia. Washington, DC: Department of Veterans Affairs Rehabilitation Research & Development Center, 1998:89‐91.

Peterson 1998

• Peterson L, Cory D, Wiener W, Brooks A. O&M services for all individuals with functional mobility limitations. Proceedings: the 9th International Mobility Conference, Atlanta, Georgia, 1998. Washington, DC: Department of Veterans Affairs Rehabilitation Research & Development Center, 1998:366‐70.

Ross 1984

• Ross MA. Fitness for the aging adult with visual impairment. New York: American Foundation for the Blind, 1984. [Google Scholar]

Seybold 1990

• Seybold D. Accessing the "MET". International Conference on Low Vision, 1990: Low Vision Ahead II, Conference Proceedings. Melbourne: Association for the Blind, 1990:282‐5.

Soong 2000

• Soong GP, Lovie‐Kitchin JE, Woods R, Arnold N, Byrnes J, Murrish J. Preferred walking speed for assessment of mobility performance: sighted guide versus non‐sighted guide techniques. Clinical and Experimental Optometry 2000;83(5):279‐82. [PubMed] [Google Scholar]

Soong 2001b

• Soong GP, Lovie‐Kitchin JE, Brown B. Does mobility performance of visually impaired adults improve immediately after orientation and mobility training?. Optometry and Vision Science 2001;78(9):657‐66. [PubMed] [Google Scholar]

Turano 1999

• Turano KA, Geruschat DR, Stahl JW, Massof RW. Perceived visual ability for independent mobility in persons with retinitis pigmentosa. Investigative Ophthalmology & Visual Science 1999;40(5):865‐77. [PubMed] [Google Scholar]

West 2002a

• West SK, Rubin GS, Broman AT, Munoz B, Bandeen‐Roche K, Turano K. How does visual impairment affect performance on tasks of everyday life? The SEE Project. Salisbury Eye Evaluation. Archives of Ophthalmology 2002;120(6):774‐80. [PubMed] [Google Scholar]

West 2002b

• West CG, Gildengorin G, Haegerstrom‐Portnoy G, Schneck ME, Lott L, Brabyn JA. Is vision function related to physical functional ability in older adults?. Journal of the American Geriatric Society 2002;50(1):136‐45. [PubMed] [Google Scholar]

White 2008

• White 2008 White IR, Higgins JP, Wood AM. Allowing for uncertainty due to missing data in meta‐analysis ‐ Part 1. Two‐stage methods. Statistics in Medicine 2008;27(5):711‐27. [PubMed] [Google Scholar]

WHO 1992

• World Health Organization. Related Health Problems, Tenth Revision. Vol. 1, Geneva, Switzerland: World Health Organization, 1992. [Google Scholar]

Wiener 2000

• Wiener WR, Siffermann E. Development of orientation and mobility certification examination. Journal of Visual Impairment and Blindness 2000;94(8):485‐94. [Google Scholar]