The Real World of Toric Contact Lens Wear
Addressing everyday situations in the exam room can lead to a more successful toric lens fitting.
By Kurt Moody, OD, FAAO, FBCLA
Approximately one-third of potential contact lens wearers require astigmatic correction, according to a recent analysis. From a database of 11,624 spectacle prescriptions, Young and colleagues (2011) determined that 47.4 percent of patients have clinically significant astigmatism (≥0.75D) in at least one eye and 24.1 percent have it in both eyes. The percentage of people who have higher amounts of astigmatism (≥1.00D) is lower, but still quite significant, with a prevalence rate of 31.8 percent in at least one eye and 15.0 percent in both eyes.
The good news is that fitting trends seem to be catching up with prevalence. The share of toric soft lens fits has been increasing over the past decade, up to approximately 25 percent of soft lens fits in the United States (Nichols, 2009). Worldwide, some reports suggest that soft toric lenses now represent more than 35 percent of all soft lenses prescribed (Efron et al, 2011).
“The quality and predictability of toric lenses has improved to the point that we are able to quickly help patients reach a higher level of performance, just as we do with a spherical lens,” says John Todd Cornett, OD, a practitioner in Amarillo, Texas. Reducing the chair time and number of follow-up visits needed to fit an astigmatic patient is important not only to the practice, but to patient satisfaction, he says.
Yet, there remain challenges in successfully fitting astigmats. A recent study demonstrates that clinicians' assessment of the stability of toric lenses in the chair is not well correlated to lens stability in the patient's life beyond the chair (Chamberlain et al, 2011). This parallels what another group of authors found when fitting presbyopes. Woods and colleagues (2009) reported that patients' modality preferences changed depending on what real-world tasks they were engaged in, from using a mobile handheld device to low-light near tasks to nighttime driving. They concluded that “making a prediction of success or not based on consulting room acuity tests alone is probably unwise.”
If you think about it, this shouldn't be surprising. The exam lane—with its static, high-contrast eye chart viewed in primary gaze—is an extremely artificial setting. Outside the office, patients are faced with variable lighting conditions, lots of different spatial frequencies and degrees of contrast, and most importantly, a world that is in motion.
The eye movements and changes in head position required to interact with that world pose particular challenges for toric lens wearers. Depending on the lens design, the effect of gravity, eyelid vectors, and blink forces can cause unpredictable degrees of rotation. For example, when an individual tilts his head, gravity influences the lens differently compared to when the head is in an upright position. Ballasted lenses that depend primarily on gravity for stability are particularly prone to rotate with head tilt. Studies have shown that they can rotate as much as 30 degrees with a 90-degree head tilt, while lenses with more stability points designed to reduce the effect of gravity (Blink Stabilized Design), rotate just 11 degrees (McIlraith et al, 2010).
Large versional eye movements, such as the motion required to glance into a rearview mirror and then quickly back at the road, can also dramatically affect the forces acting on the lens. Again, Blink Stabilized Design has been shown to provide greater stability than prism-ballasted lenses do for large versional tasks (Zikos et al, 2007).
Impact of Toric Lens Rotation
The problem with misrotation of a toric lens is that it causes a transient reduction in visual acuity that patients find bothersome. During normal activities, vision may fluctuate from 20/15 to 20/30 to 20/20 as a toric lens rotates with changes in gaze.
“That instability of vision is what patients find most concerning,” says Mile Brujic, OD, who practices in Bowling Green, Ohio. “In fact, when it comes to toric lenses, stability of the lens trumps every other quality, even comfort. If the patient doesn't see clearly, he won't have much motivation to wear them.”
One tricky aspect in getting to the bottom of dissatisfaction with toric lenses is that patients often ascribe their visual fluctuations to dryness. “They know that blinking makes it better, but they don't really know whether that's because the blink is lubricating the contact lens or re-aligning it,” Dr. Brujic explains.
In the office, he uses a handheld trial lens with the correct prescription and then turns it to show patients how the vision starts out clear but blurs as the lens rotates. “I want them to understand the inherent challenges in how a toric lens works,” he says. “That way, at follow-up visits, patients can give me a more accurate assessment of how often they experience that transient blur during their daily activities.”
Practitioners might assume that sports or other very active tasks pose the greatest problem for toric lens wearers. “But for many people, the visual demands aren't as great when they are doing something like playing soccer for fun, even though the latter involves a lot of eye movements,” says Charissa Lee, OD, who practices in Irvine, Calif. She finds that visual fluctuations at work typically pose the greatest challenge for her toric patients.
“They might struggle with alternating between two monitors or between a screen and the papers on their desk,” she says. Dr. Lee had a lot of trouble finding the right toric lens for one of her patients, a police officer who works the night shift. “His visual demands are very high, so he doesn't want to be dealing with temporary blur when he's trying to read the license plate on a moving car or judge his surroundings during an arrest,” she points out. “He has a low astigmatic prescription, but spherical lenses did not give him the visual clarity he needed.” After trying several different lenses, Dr. Lee found a two-week silicone hydrogel toric lens featuring Blink Stabilized Design that gave him consistent vision to meet his job demands.
So how can you do a better job of assessing that stability in the chair, before going through multiple pairs of trial lenses?
One way is to have patients mimic more natural eye and head movements rather than just looking straight ahead at an eye chart. “I find out what kinds of activities the patient does at work and we try to re-create that in the exam room,” says Dr. Lee. For a surgical nurse, she might instruct the nurse to hold the near chart about where a medical chart or vitals monitor might be, then look over to the “patient” on the bed, then up at a “monitor” above her head. “For others we might simulate a desk and computer,” she says.
Another quick, low-tech test is to present the near acuity chart sideways. Almost universally, people will tilt their head to view something written sideways. Testing acuity in this position is a good indication of how much visual acuity is lost with lens rotation during common actions such as watching TV on the couch, aligning a golf ball, or looking under a bed. All toric lenses will rotate with this action, but the clinician should look for lenses that rotate as little as possible and—just as importantly—quickly rotate back when the patient returns to primary gaze.
The VANT Chart
Working with researchers at the University of Manchester in England, we developed a novel chart for assessing near vision in soft toric contact lens wearers. The Vision Assessment at Near for soft Toric lenses (VANT) chart is intended to serve as a practical tool for testing dynamic vision in the office. It consists of a central, color-coded logMAR eye chart and eight peripheral letter targets set on a white background measuring 60cm x 40cm (Figure 1). Looking at the horizontal and diagonal targets from the logMAR central panel creates a versional movement of about 40 degrees while the vertical targets create versional movements of about 25 degrees.
Figure 1. The VANT chart for testing dynamic vision.
In our initial testing with this chart, we found that the horizontal and vertical versions cause lesser degrees of lens misalignment than traditionally thought. This would explain why the traditional instructions to “Look up, look down, look left, look right” that we were all taught in optometry school don't necessarily predict success with toric lenses. This research indicated that the four diagonal movements (e.g., up and to the right) create the most significant lens rotation. This corresponds well to our patient feedback of difficulty during these types of versional tasks.
In a study that looked at just the diagonal versions, 35 subjects (68 eyes) were fitted with four toric lenses in random order (Chamberlain et al, 2011). After a settling period of five minutes, standard high- and low-contrast distance logMAR acuity measures were taken, as well as standard lens fit assessments of centration, movement, and corneal coverage using a five-point grading system.
Patients were instructed to look at a diagonal target in the corner of the VANT chart, then look back at the eye chart in the center and read the smallest line they could see clearly. Investigators recorded the rotation (the angle between the vertical lens scribe marking and the 6 o'clock position) and stability (the maximum excursion of the lens in degrees), as well as the near acuity, following these versional movements. All measurements were monocular.
The typical soft toric lens wearer underwent a reduction of about one line in near visual acuity after diagonal eye versions, with some wearers experiencing much larger reductions of up to eight lines of acuity (Chamberlain et al, 2011).
An Unconventional Approach
The VANT study verified what practicing clinicians have long realized anecdotally—that conventional approaches to measuring acuity do not fully replicate the real-world experience of soft toric lens wearers. If clinicians take nothing else from our experience with this novel chart, they can at least incorporate simple diagonal movements into evaluations of lens rotation. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #195.
Dr. Brujic has lectured, acted in an advisory capacity, or performed research for Bausch + Lomb, Ciba Vision (now Alcon), CooperVision, and Vistakon. Dr. Cornett is a consultant for Vistakon and Heidelberg Instruments. Dr. Lee is a speaker and professional affairs consultant for Vistakon.
|Dr. Moody is director of Design; Research & Development for Vistakon, Division of Johnson & Johnson Vision Care Inc., Diplomate in the Cornea and Contact Lens Section of the American Academy of Optometry, and a Fellow of the British Contact Lens Association. Contact him at (904) 443-3088 or KMoody1@its.jnj.com.|