Presbyopic Lens Performance and the "Real World"
BY ERIC PAPAS, PHD, MCOPTOM, DIPCL
As reported by Nichols in "Contact Lenses 2009," (January 2010 issue), multifocal lenses accounted for only 15 percent of fits during 2009; at the same time, around 33 percent of the U.S. population was aged between 40 and 65 years (U.S. Census Bureau, 2010). For clinicians trying to close this prescribing gap, the paramount factor in successful fitting is lens performance. Unfortunately, what this really means and how it can be meaningfully assessed are not necessarily straightforward concepts.
For example, while just about everyone will include some form of visual acuity testing in their routine, this may not be all that useful in predicting a successful outcome. Asking wearers to rate their satisfaction with various multifocal lens types shows that while significant changes can occur over the assessment period, these do not seem to be reflected in corresponding visual acuity shifts at either distance or near (Papas, 2009).
Looking Beyond Visual Acuity
Faced with this perspective, several researchers have turned to occupational-based methods to gain additional insight into how presbyopic lenses affect wearers' ability to function. In the past, performance on tasks such as index card filing, letter editing, and placing cocktail sticks into an array of drinking straws have all been used (Sheedy et al, 1988). The last few years have seen something of a resurgence in this type of research, with technological developments permitting the use of rather different methods and emphasis to those previously employed.
Navigating a Room Arguably the most fundamental task for any human being is that of moving around successfully from place to place. When asked to do this under laboratory conditions, individuals used to wearing monovision show subtle changes in gait compared to when they use a full distance correction. To be specific, they walk more slowly and have less toe clearance (i.e. their feet get closer to the obstacle) when negotiating steps (Chapman et al, 2010). This suggests that the visual compromise associated with monovision introduces a degree of uncertainty to the task of walking around, and this in turn causes wearers to change their behavior. Although it seems likely that subjects will quickly adapt to their new environment, clinicians may feel that there is sufficient evidence to advise extra care during the early stages of wear and take action to educate new wearers on the potential for falls, etc.
Driving For many people, being able to drive is a valuable activity that contributes significantly to quality of life. Safety while using the road is paramount, and good vision is critical to achieving this. Conducting research into how the type of vision correction affects driver performance and behavior is tricky; factors such as the need for safety and continually changing traffic conditions limit an experimenter's ability to control the relevant variables so that meaningful comparisons can be made among situations.
One way to overcome this problem is to use a driving simulator. Here video of real scenes and situations can be repeatedly projected to test subjects seated in a "car" equipped with all the usual instruments, steering wheel, foot pedals, etc. Additional equipment can be added to allow measurements of behaviors such as eye or head movement. Work with this kind of setup has demonstrated that when presbyopic subjects wear contact lenses, they make smaller movements of both their head and eyes than they do with spectacle-based corrections (Chu et al, 2009). It didn't make any difference in this study whether the lenses were fitted as multifocals or for monovision. Less restriction of the visual fields during lens wear was presumably largely responsible for this effect, as the number of eye movements remained unchanged.
For all their strengths, simulators lack the crucial factor of reality. Driving involves at least four of the five senses, and it seems reasonable to expect that the link between events on the road, either predictable or unexpected, and responses such as steering changes, braking, or acceleration will be influenced by factors other than just visual cues.
Efforts to understand such circumstances have put subjects in control of real cars on real roads, although these necessarily must be closed circuits. A series of hazards and situations that require drivers to either control the vehicle or indicate their awareness of an item are introduced at various points around the route. Having subjects do all this at night adds an additional element of stress.
Comparisons of various presbyopic corrections made under these conditions show some interesting outcomes (Chu et al, 2010). While the recognition of road signs seems unaffected in terms of reading the content during the journey around the course, drivers had to get significantly closer to signboards when wearing contact lenses than they did with spectacles. On average, monovision wearers allowed their vehicles to get about 10 meters closer, and multifocal wearers 20 meters closer, to signs before being able to identify the content. Perhaps as a result of this need for proximity, these two groups also hit slightly more hazards than spectacle wearers did. Although this particular observation did not reach statistical significance, there was an associated reduction in road speed that did—meaning that lens wearers drove more slowly and took longer to complete the course. On the other side of the coin, dashboard instrument legibility was superior for the lens wearers when compared with single vision spectacles, indicating an enhanced ability to recognize targets at closer distances, as would be expected given the refractive elements involved.
It is worth recalling that the experimental conditions for these studies required that the subjects were previously inexperienced with contact lenses. There was also no scope to adjust near power or refractive configuration to optimize vision, as would be the case in a normal clinical setting. It is quite possible that performance may be different under circumstances in which adaptation has occurred and/or a more customized fitting was employed. This would certainly be a useful and relevant area for future research. Nevertheless, the data do suggest that individual behavior can be influenced by the visual correction worn in ways that are detectable by objective methods.
A "Real World" Test
Driving simulators are not standard equipment in most consulting rooms, so it would be helpful to have a more practical way of accessing behavioral information. Woods et al (2009) asked young presbyopic subjects to perform a series of activities while wearing various corrections. Tasks included driving to a local coffee shop after dusk, then reading the menu and a newspaper, walking around a preset route to access information, and simply being at home watching television, reading, or using the computer. While doing each procedure, participants fed back their impressions of satisfaction using simple 0 to 100 scales through a customized interface based on a cell phone. In this way there was no time lag between the perception of the event and the recording of the response.
One outcome of this work was that conventional visual acuity poorly predicted performance in these "real-world" tasks, confirming the view to this effect mentioned earlier. However, the results further indicated that the subjective satisfaction scores were able to discriminate between monovision and multifocal performance on most of the tasks investigated, with the multifocal correction generally coming out on top. Again though, it seems that individual optimization with respect to near addition was absent, potentially leading to diminished performance particularly for monovision correction.
Reflecting on this investigation, the discipline of getting wearers out of the consulting room and into an environment consistent with their normal daily lives seems a good way to generate feedback that is more helpful in indicating the performance of various lens-based modalities than may be the case with conventional clinical testing. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #180.
Associate Professor Papas is executive director of Research & Development and director of Post Graduate Studies, Institute for Eye Research and Vision Cooperative Research Centre, and senior visiting fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. He has received research funds from Ciba Vision, Alcon, AMO, and Allergan. You can reach him at E.Papas@brienholdenvision.org.