science of soft lens vision, The
Ridder, William H III
Fluctuating vision is a troublesome possibility.
Soft contact lenses provide major benefits — both cosmetic and functional – for many patients. Activities that can interfere with spectacle wear are now open to them. Practitioners can also prescribe soft contact lenses to cover a disfigured iris, facilitating social acceptance for some. And they can improve the quality of life for high myopes because the field of view they provide is significantly greater than that provided by spectacles.
Inevitably, there can also be disadvantages to soft contact lens wear. The patient must invest the time to keep the lenses clean to maintain ocular health and optimal vision. He also needs to see his eyecare practitioner more frequently than the spectacle wearer does. Another disadvantage for some patients concerns adaptation: Not all adapt to the foreign-body sensation of the soft contact lens. Additionally, some patients report that their vision fluctuates when they blink. The last is a problem that may be difficult to replicate in the exam room with the Snellen acuity chart. Here’s a look at the variables that cause fluctuating vision.
AFTER THE BLINK
Frequent blinking is necessary to maintain the health and visual function of the pre-ocular surface of the eye. However, it may produce a decrease in visual sensitivity that results from blink-induced suppression, temporal changes in tear film structure or contact lens movement. Blink suppression is a neural phenomenon associated with the act of blinking; when we blink there is a neural suppression of incoming visual information. Suppression starts about 100 millisecond (msec) before each blink and lasts about 100 msec.
Specifically, low spatial frequency and high temporal frequency visual information (i.e., large, rapidly moving objects) is suppressed. A short interruption in visual perception occurs during eye blinks at about 15 times per minute. The function of blink suppression is to suppress the perception of the eyelid closing and opening. Because blink suppression is present during spectacle and contact lens wear, it should not be a major cause of fluctuating vision in soft contact lens wearers.
WHAT OPTOTYPE AND CONTRAST CHARTS CAN TELL YOU
It’s possible that patients who wear hydrogel contact lenses, in addition to the decrease in visual sensitivity as a result of blink– induced suppression, may have other factors that affect their vision. For example, the tear film is less stable during contact lens wear.
Several techniques (e.g., optotype acuity, variable contrast acuity charts and contrast sensitivity) have been used to assess this problem. They have been used to examine the effect of soft contact lens wear either on the minimum resolvable object (i.e., visual acuity [VA]) or on contrast sensitivity.
* Optotype acuity charts. Optotype acuity typically employs a Snellen letter chart; it yields the minimum resolvable object under conditions of maximum contrast. Patients often demonstrate similar acuities on the Snellen chart with soft contact lenses and spectacles. This suggests that Snellen acuity cannot easily demonstrate changes in VA following blink in contact lens wearers.
* Variable contrast charts. Variable contrast charts such as the Bailey-Lovie chart have been used to determine if a decrease in contrast sensitivity might explain the subjective complaint of patients. (Bailey– Lovie uses several Snellen-type charts of different contrast levels.)
Studies with variable contrast charts don’t suggest that vision with spectacles is any better than with hydrogel contact lenses; in fact, there have even been reports that spectacles yield worse vision than do hydrogel contact lenses. However, because this technique uses only a few contrast levels, it doesn’t allow for a thorough investigation of the effect of soft contact lenses on contrast sensitivity.
* Complete contrast sensitivity functions. Complete contrast sensitivity functions are able to determine differences in vision between soft contact lenses and spectacles. You can also use this technique to examine the effect of hydrogel contact lenses and spectacles on resolution (i.e., VA). In early studies, contrast sensitivity functions with hydrogel contact lenses and spectacles did not yield clinically significant differences.
However, previous investigators may have failed to fully define the nature of the subjective difference between hydrogel contact lenses and spectacles because stimuli were presented for too long a time period (similar to acuity measures in the exam room).
In all of the early studies, patients had extended viewing periods something they don’t always have the luxury of in their everyday experiences. It’s possible that the patients’ subjective complaint is the result of a transient loss in contrast sensitivity. This loss would have its maximum effect in situations where the visual stimulus is only presented for a brief period (e.g., reading freeway signs while driving a car).
MEASURING VISUAL PERFORMANCE RAPIDLY
In the past decade, a technique has been designed to measure transient fluctuations in contrast sensitivity. It allowed the dynamic effects of blinking on vision with contact lenses and spectacles to be investigated. In studies of the technique, the target was presented for a brief duration (e.g., 16.7 msec) at a specific time after blink detection. Investigators found that there was a significant loss of contrast sensitivity while wearing conventional spherical soft contact lenses in comparison to spectacles. Similar effects were observed with other types of soft lenses, including disposable lenses and toric soft lenses.
* The role of blink-induced contact lens movement. These results suggest that blink-induced contact lens movement may contribute to the transient reduction in contrast sensitivity. With each blink, the contact lens is pulled superiorly by the upper lid. This displaces the optical center of the contact lens from the ocular visual axis and the off-centered lens induces a prismatic shift of the retinal image and thus reduces contrast sensitivity. The extent of this temporal blurring depends on the magnitude of the lens movement and the lens power. Loss of visual performance increases as the contact lens prescription and the movement increases.
* How soft contact lenses measure up. Toric soft lenses resulted in the greatest loss in sensitivity. In addition to the vertical movement of the contact lens there is also the rotational movement affecting vision. With well-fit spherical soft contact lenses, this movement-induced reduction in visual sensitivity lasted about 100 msec after each blink. Typically, the longer the lens took to stabilize after the blink, the longer this decrease in visual sensitivity was observed. The decrease in visual sensitivity is followed by a period of optimum visual performance.
Figure 1 displays the results of this blink-induced contact lens movement on contrast sensitivity for one subject. This figure is re-drawn from Ridder, et al. The stimulus was a vertically oriented sine wave grating of 11.4 cpd (i.e., roughly equivalent to a 20/53 Snellen letter).
The post-blink stimulus onset time is plotted on the horizontal axis and the contrast threshold (1/contrast sensitivity) is plotted on the vertical axis. The error bars are standard errors of the contrast threshold.
The thresholds obtained with spectacles are depicted by the open squares, those for the optimally fit contact lens by the filled triangles, and those for the loose fitting contact lens by the filled circles. For post-blink, stimulus times greater than or equal to 100 msec, the spectacle and contact lens thresholds were identical and did not change as a function of post-blink time.
* Consistent with previous studies. This is consistent with earlier reports (using long viewing durations), which demonstrated that contact lenses and spectacles yield similar contrast sensitivities and visual acuities. As the post-blink stimulus onset time is decreased, the threshold with the contact lens and the spectacles increased (from blinkinduced suppression); however, the threshold for the contact lenses increased more than for the spectacles.
Furthermore, the threshold for the loose-fitting contact lens increased more than did the threshold for the optimal-fitting contact lens. At a post-blink stimulus onset time of 50 msec, the threshold for the loose fitting contact lens is significantly different from the threshold for the optimal– fitting contact lens and the spectacles. At the 25-msec post-blink stimulus onset time, all three conditions are significantly different from one another with the spectacles having the lowest threshold and the loose fitting contact lens having the highest.
* Decrease in contrast sensitivity is temporary. Figure 1 thus indicates that there is a transient decrease in contrast sensitivity following a blink. Looser fitting contact lenses, which move more with the blink, result in a greater decrease in contrast sensitivity.
Other studies also indicated that the loss in contrast sensitivity increased as the power of the contact lens increased. After about 100 msec following the blink, the contrast sensitivity stabilized at its highest level. This is when the patient’s vision is the best and acuity measurements will be optimal. Blink– induced contact lens movement accounts for the majority of vision loss immediately following the blink.
CHANGES IN VISION WHILE THE LIDS REMAIN OPEN
As the contact lens re-centers after the blink, visual performance will be optimized, but this optimal period will depend on the length of time before the pre-lens tear film breaks up. The break-up time for the pre-contact lens tear film is significantly less than that for the precorneal tear film; typically pre-lens break-up times are approximately six seconds. Drying of the pre-contact lens tear layer is the cause of poor visual performance as the lid remains open.
Some contact lens patients complain that their vision gradually fades as they view a stimulus; blinking tends to improve vision for these patients. Recent studies indicate that visual performance of soft contact lens wearers, measured with low contrast visual acuity techniques, decreases when blinking is interrupted. Blink-induced contact lens movement could not account for this decrease in visual sensitivity.
Contact lens wear results in a reduction in the pre-lens tear film lipid layer and an increase in tear film evaporation; thus, tear breakup time is much shorter in the presence of a contact lens. A decrease in retinal image quality has been inferred from the modulation transfer function calculated from the aberrations induced from the drying tear film observed by the Shack– Hartmann aberrometer.
* Investigators examine light scatter. Lohmann, et al. and Timberlake, et al. have suggested that variations in visual performance with soft lenses may caused by light scatter produced by the changes in the hydration levels of the lens or changes in the quality of the tear film. It’s possible that the tear layer disruption that accompanies contact lens wear is responsible for the fading of vision observed by some patients if their inter-blink interval exceeds the pre-lens tear film break up time.
* Contrast sensitivity and precontact lens tear break-up. By employing the same technique outlined for the data in Figure 1, Thai, et al. examined the effect on contrast sensitivity of the pre-contact lens tear layer break up. Figure 2 is a re-plot of their summary data (i.e., their Figure 4) for one subject.
The horizontal axis plots post– blink stimulus onset time and the vertical axis displays the contrast sensitivity. Immediately after the blink, there is a decrease in contrast sensitivity that results from blink suppression and blink-induced, contact lens movement.
Approximately 100 msec after the blink, vision rises to a level that is relatively constant for a period of up to four seconds.
At this stage, the pre-contact lens tear film begins to break up and visual performance starts to de.fade. This degradation continues as the open eye condition is prolonged for another four seconds. This is the vision cycle experienced by all soft contact lens wearers in the intervals between blinks. The blurring of vision may provide one of the stimuli to blink in those patients in whom the pre-contact lens tear film break-up time is less than the inter-blink interval.
REVIEWING THE VARIABLES
Remember, several variables affect vision in soft contact lens wearers. Immediately following the blink, vision is reduced because of:
* blink-induced neural suppression
* a prismatic shift of the retinal image resulting from blink-induced contact lens movement.
The magnitude of the loss in vision depends on the amount of lens movement and the power of the contact lens. The vision then quickly stabilizes at an optimal level and remains this way for several seconds.
This is the condition under which visual acuity is typically measured in the exam room. The tear layer eventually destabilizes, resulting in a blurring of the retinal image. The blurred retinal image may be caused by distortion from the tear layer drying on the front surface of the contact lens and/or light scatter resulting from a dehydrated contact lens. It may then be one of the factors involved in initiating the next blink, and the cycle starts again.
CONSIDER THESE TREATMENTS
The treatment for the patient who has fluctuating vision will depend on when the blur is noted and the disruptive nature of the blur. Education concerning the nature of the blur may be all that is required to satisfy the patient. If the blur is unacceptable and is noted immediately following the blink, then, if possible, the contact lens fit should be adjusted to decrease the amount of lens movement; a tighter fitting contact lens fit may be indicated.
The improvement in visual performance with the decrease in lens movement must be weighed against the potential corneal problems resulting from a too tight lens fit. If the patient notices a gradual blur that occurs several seconds after the blink, he may have a poor pre-lens tear layer. Treatment would be the same as for any dry eye patient that is, artificial tear replacement therapy or possibly punctal plugs. The use of these may result in the inter-blink interval being shorter than the pre-contact lens tear layer break-up time and stabilization of vision will result.
WILLIAM H. RIDDER III, O.D, M.S., PH. D., F.A.A.O
Dr Ridder received his Doctor of Optometry degree from the Illinois College of Optometry, his Master of Science in Biology from the Southern Illinois University and his PhD in Physiological Optics from the University of Houston. He is a tenured professor at the Southern California College of Optometry where he teaches ocular physiology, neurophysiology, and runs the electrodiagnostic service. He has published more than 30 papers in refereed journals and has given more than 60 invited lectures at major meetings in the United States, Asia and Europe. Dr Ridder has been a member of the American Academy of Optometry (AAO) for more than 15 years. He is currently the chair of the Vision Science Section of the AAO. He’s also a member of the Association for Research in Vision and Ophthalmology and the International Society for Clinical Electrophysiology of Vision.
References available on request.
Copyright Boucher Communications, Inc. Jan 2003
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