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1From the Laboratory of Experimental Ophthalmology, School of Behavioral and Cognitive Neurosciences, University of Groningen, The Netherlands; 2Department of Ophthalmology, University Hospital Groningen, Groningen, The Netherlands; 3Vision Rehabilitation Center, Bartimeus Utrecht, The Netherlands; the 4Vision Rehabilitation Center, Sensis Eindhoven, The Netherlands; and the 5Vision Rehabilitation Center, Visio Loo Erf, Huizen, The Netherlands.
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Abstract |
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METHODS. Twenty night-blind subjects with retinitis pigmentosa were requested to walk predetermined routes at night with and without NVGs. The number of unintended contacts with obstacles (hits) and the percentage of preferred walking speed (PPWS) en route were assessed in three different situations: a darkened indoor corridor; a moderately lit outdoor residential area; and a well-lit outdoor shopping area. Assessments were performed before and after a 5-week training period, during which the subjects practiced using NVGs in their own surroundings, registered their experiences in a journal, and filled out questionnaires.
RESULTS. The mean number of hits in the darkened corridor declined from eight to two when NVGs were used. Mean PPWS (34%) did not improve. In the residential area, mean hits declined from eight to practically zero and mean PPWS increased from 60% to 72% (after training to 78%). In the shopping area, subjects walked at 93% PPWS without any hits and showed no improvement with NVGs. Subjective scores revealed a good sense of orientation, feelings of safety and tranquility and an increase in independent mobility when NVGs were used.
CONCLUSIONS. Using NVGs seems to improve nighttime mobility in dark outdoor conditions by decreasing unintended contacts with obstacles and increasing walking speed. Use of NVGs increased independent activities in these subjects and was generally positively evaluated for everyday outdoor use.
A luminance-enhancing vision aid may be of great help to night-blind individuals. In the past, light-enhancement devices were large, heavy monocular instruments,4 that later evolved into smaller,5 more compact, hand-held devices.6 These were then followed by head-mounted binocular instruments, known as night-vision goggles (NVGs). Today, a newer version of these spectacles, called the Multi-Vision (Trivisio, Taegerwilen, Switzerland), is available (Fig. 1) . It has the added advantage of a higher resolution and an improved automatic light-adapting system. Two studies investigating the potential benefit of NVGs were performed recently.7 8 The one by Friedburg et al.7 found that night-blind subjects could improve their visual functions in a laboratory design using NVGs. The second study8 was conducted in a real-life situation, in which participants subjectively evaluated the device after walking a designated route. It reported that a small majority of participants was positive about the instrument. Although both studies provided new information, they did not show data about the influence of such a device on orientation and mobility in a realistic outdoor situation and they were both based on single experiences. To evaluate the usefulness of such a device adequately, we believe that more data should be assessed from real-life outdoor situations: not only from a single use, but also after a period with ample opportunity to practice using the instrument.
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Methods |
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1.5 hours when the infrared light is switched on).
Routes
The two indoor laboratory walking routes consisted of a darkened corridor with floor-level illumination (at +20 cm) between 10–3 and 10–2 lux, and a route length of 36 meters. Ten artificial rectangular obstacles were placed along the route at different heights (foot, knee, shoulder, and head). The four outdoor walking routes (>0.5 hour after sunset) in the residential area each covered 330 meters and had floor-level illumination between 10–2 and 10–1 lux. The four routes at the outdoor shopping areas were 187 meters long and had illumination levels between 10–1 and 10 lux. The outdoor routes had comparable amounts of obstacles: curbs, public gardens, lampposts, and poles, for example.
Test Protocol
All subjects were consecutively invited to our laboratory in Groningen, twice during the dark winter season of 2002 to 2003. The first visit started with instructions on the use of the Multi-Vision. Then, the individual preferred walking speeds (PWSs) were assessed by measuring the walking speed of each participant three times along an unobstructed, straight path (17 m).9 This was followed by orientation and mobility tests first without and second with use of NVGs along the three dark walking routes (indoor corridor, outdoor residential area, and shopping area). Every test route differed and thus was unknown to the participant. An initial walking route with NVGs without scoring was performed before starting the first test. Furthermore, the order of routes randomly changed between subjects. During the test routes, subjects had to follow a predetermined route, of which we had measured the exact distance. To follow the route, subjects were instructed by the investigator to turn either to the right or left at the next crossing. This information was given long before a particular junction was reached, so the subjects had to find their own way.
All the subjects were lent a Multi-Vision to use during the 4 to 6 weeks between visits. They were requested to practice using the device in their own surroundings every evening and to register their experiences in a journal. The subjects also received weekly mobility instructions and feedback from a professional mobility trainer. After this training period, the subjects returned to Groningen for their second visit. There, the orientation and mobility tests were repeated and personal experiences discussed. All subjects were requested to fill out a questionnaire regarding their nighttime walking experiences at start and after the training period.
Scores: Objective
Orientation and mobility performance along the routes was scored as hits and percentage of preferred walking speed (PPWS). Hits were scored as the number of unintended contacts with obstacles along the walking route (i.e., curbs, poles, garden fences, and public gardens). This score reflects the level of "risk" on a route.10 PWS assumes that subjects walk at their optimal walking speed when they do not have to worry about obstacles or dangerous conditions en route. Walking speed (meters per second) on a test route is calculated as the percentage of the preferred walking speed (PPWS) and is a measure of "walking efficiency."11 The number of unintended contacts with obstacles and the time to cover a route were recorded by the investigator. All data were collected and converted on computer (SPSS for Windows; SPSS Science, Chicago, IL).
Scores: Subjective
Questionnaires consist of 22 questions considering "specific problems with mobility," "bumping into obstacles," and "independent travel." The experienced trouble on these items was scored with help of a 5-point Likert scale: 1, never; 2, sometimes; 3, regular; 4, often; and 5, always. Research by Turano et al.12 has shown that a similar questionnaire is a valid way to measure perceived ability for independent mobility in persons with RP. However, this questionnaire has not been validated for the particular nighttime condition. Therefore, we will confine our results to a descriptive evaluation of median scores.
The journal consists of judgments about difficulty, recognition, orientation, and feelings of safety and tension after every daily walking-route with use of NVGs. Values on these items were scored by use of an appreciation scale form 1 (very low appreciation) to 10 (very high appreciation).
Statistics
The significance of the differences in PPWS and hits with and without NVGs was tested using the nonparametric Wilcoxon test at the 0.01 level. The relation between the ophthalmic features and the results was tested at the 0.01 level with the Spearman’s correlation test. Visual acuity was expressed as logarithm of the minimum angle of resolution (logMAR)13 for the analyses. Visual fields were calculated as the visual field score according to Colenbrander.14 (The visual field score replaces the Esterman grids and is proposed by Colenbrander,14 accepted by the International Council of Ophthalmology (http://www.icoph.org/pdf/visualstandardsreport.pdf), and included in the fifth edition of the AMA Guides to the Evaluation of Permanent Impairments.15 See also Table 1 .)
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Results |
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The increase in walking speed (r = 0.572) and for a great part the hit-score (r = 0.54) was related to the dark-adaptation thresholds but not to age, sex, visual acuity (logMAR), or visual field score (Table 2) .
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Discussion |
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Using NVGs on the outdoor residential test route clearly improved orientation and mobility and was expressed as a decrease in hits and an increase in walking speed.
Results were already highly significant at the first visit and therefore were independent of mobility training. Most striking was the immediate change in obstacle avoidance by every subject, resulting in practically no contacts with curbs, poles, fences, or other obstacles. Use of the goggles therefore is considered to improve safety while walking, since the risk of injuries or accidental falls caused by hits is likely to be reduced. At the second visit, walking speed and hit score without use of NVGs had not changed. However, with the use of NVGs, the general walking speed improved further after just a few weeks of practice, indicating an additional positive effect of training on mobility with NVGs.
At the start of the study, we did not know at which level of artificial streetlight vision would be sufficient for night-blind people. Our test showed the street-lighting levels at the shopping street (between 10–1 and 10 lux) were strong enough for "normal" mobility (no hits on obstacles and a normal walking speed). Also, our results showed no benefit from NVGs in this particular condition, which is in line with a recent evaluation study on NVGs by Bowers et al. (Bowers AR, et al. IOVS 2003,44:E-Abstract 2772).
The indoor corridor was the darkest of the three test conditions, and the walking speed there was extremely low. Because there was no improvement in walking speed when the goggles were used, the low walking speed was probably mostly due to the numerous artificial obstacles placed over the short distance. Binocular depth perception is not possible with the Multi-Vision, since both eyes receive an image from the same camera. This seemed to cause people to walk slowly in anticipation of reaching an object. This problem with distance estimation together with the intensive scanning needed to detect the obstacles, randomly placed at head and feet height, also is probably the reason that the hit score did not reach zero in the indoor corridor as it did in the outdoor environment. Furthermore, its field of view is rather small, which at the short distances existing under indoor conditions, limits the opportunities to anticipate obstacles along the route. We presume that an overview is achieved easier on outdoor streets with larger distances. In other words, the instrument is considered to be less effective under indoor conditions.
For the measurements in all conditions it should be noted the without NVGs route always proceeded the with NVGs route, which may have biased our test results in favor of the goggles. It would have been more correct if we had changed these conditions. Yet, we believe a great consequence from a learning effect is implausible, because all performed test routes were different and thus new to the participant. Also, an initial walking route without scoring with NVGs was performed before starting the first test, which is considered as the primary practice route. Furthermore, at the second visit, walking speed and hit score without use of NVGs had not changed, indicating no effect from learning.
The mainly positive subjective evaluations given by night-blind people after several weeks of intensive use imply that the instrument is not only effective, but is also appreciated in practical use. The questionnaires and journals also revealed more independent travel during the dark evening hours. This does not, however, mean that there is no room for improvement. The most pronounced problem with the Multi-Vision involved experiences with light sources (e.g., car headlights, lit shop windows), which were perceived as unpleasant sparkling light spots within an otherwise intact view. Another frequently reported difficulty was the fact that no depth perception could be experienced using the instrument. Participants reported that they became accustomed to the two-dimensional view in many situations after several weeks of practice, but that this definitely did not apply to situations in which they had to estimate the distance to approaching cars. Some subjects also mentioned difficulties with the restricted visual field. Although these individuals already had constricted fields due to their disease, with use of NVGs, the perceived view can be increased only by head movement. Without the goggles, eye movements could accomplish this increase easier and faster. During mobility training, the participants were trained to enlarge their visual field by scanning the environment with systematic movements of the head. Experiences and the successful application of this scanning method, however, differed between the subjects. To increase comfort and facilitate utilization, future improvements to NVGs should include development of a better automatic light-adapting system, the implementation of binocular vision, and the enlargement of the visual field.
Our study was designed primarily to indicate the potential effectiveness of NVGs. As we were restricted by the number of devices available and the limited period with dark evenings in the winter season, we could include only 20 participants. We selected participants with constricted central visual fields, because the study by Rohrschneider et al.8 showed a better outcome within this group. We cannot, therefore, make any statements regarding the potential benefits for night-blind people with normal visual fields (e.g., those with congenital stationary night blindness) or for people with impaired central visual fields.
This study found no relationship between the results (objective and subjective) and visual acuity, visual field, sex, or age. The only relationship found was that subjects with more impaired light-sensitivities had more hits while walking unaided and, therefore, showed a larger reduction in the number of hits plus a higher increase in walking speed when using the goggles. The subjective improvement in independent mobility, however, did not differ between people with different levels of impairment.
Other available night-vision aids are the white cane and the wide-beam flashlight.6 A comparison between NVGs and these instruments can be interesting and might be a subject for future research. From our study results, we consider NVGs as an alternative vision aid that has also been proven effective.
In our opinion, a potential NVGs candidate is a night-blind subject who declares him- or herself unable to move safely and independently under dark conditions. These individuals should be given the opportunity to practice using the instrument for 2 to 3 weeks to assess individual benefit.
In conclusion, at dark, outdoor conditions in which night-blind subjects have been shown to have considerable mobility problems, the NVGs seemed to improve mobility by decreasing hitting of obstacles and increasing walking speed. Use of the goggles increased independent nighttime activities in our subjects and was generally positively evaluated for everyday outdoor use. In very well-lit outdoor environments as exists in shopping streets, luminance is sufficient for normal mobility, and NVGs were of no additional value. The instrument may be less effective in indoor environments in terms of gain in walking speed, though it decreased the number of hits on obstacles considerably.
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Footnotes |
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Submitted for publication September 25, 2003; revised December 22, 2003, and February 4, 2004; accepted February 12, 2004.
Disclosure: D.T. Hartong, None; F.F. Jorritsma, None; J.J. Neve, None; B.J.M. Melis-Dankers, None; A.C. Kooijman, Trivisio (F)
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Dyonne T. Hartong, Department of Ophthalmology, University Hospital Groningen, PO Box 30001, 9700RB Groningen, The Netherlands; d.t.hartong{at}ohk.azg.nl.
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