Article
Contrast sensitivity metrics extend beyond measure of vision
Contrast sensitivity is a more valuable metric than many ophthalmologists realize, with applications in preoperative and postoperative management of corneal and refractive surgery patients and routine screening of patients’ quality of vision.
Take-home message: Contrast sensitivity is a more valuable metric than many ophthalmologists realize, with applications in preoperative and postoperative management of corneal and refractive surgery patients and routine screening of patients’ quality of vision.
By Nancy Groves; Reviewed by Gregory J. Pamel, MD, and Prof. Dan Reinstein
Contrast sensitivity provides an objective measurement of quality of vision to complement the standard routine measurement of visual acuity, or quantity of vision, and is a useful metric in routine care as well as preoperative and postoperative patient management.
Routine contrast sensitivity testing gives the ophthalmologist, and particularly the refractive surgeon, another string to their bow and provides a more complete understanding of the patient’s visual function.
For both corneal and intraocular refractive surgery applications, it is important to measure contrast sensitivity routinely to provide a baseline measurement so that a comparison is always possible postoperatively, said Prof. Dan Reinstein, medical director, London Vision Clinic, London.
Preoperatively, contrast sensitivity can help pick up signs of deteriorating vision earlier than visual acuity testing.
For example, patients with early cataract might be diagnosed earlier by a drop in contrast sensitivity before the visual acuity is affected.
This could help a surgeon decide whether corneal or intraocular refractive surgery might be best for the patient, said Prof. Reinstein, who is also adjunct professor of ophthalmology, Department of Ophthalmology, Columbia University Medical Center, New York, and professor associé, Centre Hospitalier National D’Ophthalmologie, Paris.
Postoperatively, contrast sensitivity is used to assess the quality of vision.
In cases of patients complaining of poor vision, a postoperative contrast sensitivity measurement can be compared to the baseline measurement to evaluate the change in the quality of vision. This information can then be used along with topography and wavefront data to make a diagnosis of the cause of a drop in quality of vision.
The surgeon can then consider the different options available to treat the patient, Prof. Reinstein said.
For corneal surgery, in the past, older-generation excimer lasers have been shown to reduce contrast sensitivity. However, modern excimer lasers have been shown to maintain or even slightly increase contrast sensitivity.
“For example we have previously reported no reduction in contrast sensitivity at 3, 6, 12, or 18 cpd for myopic and hyperopic LASIK with the Carl Zeiss Meditec MEL80 excimer laser,” Prof. Reinstein said.
“In comparison, a statistically significant decrease in contrast sensitivity has been reported after multifocal IOL surgery,” he added. “Studies such as these help to educate ophthalmologists on the changes in vision that might be expected for different refractive procedures.”
Finally, contrast sensitivity is a useful measurement to consider as part of the diagnosis of optical complications following refractive surgery. Contrast sensitivity has been shown to be correlated with wavefront measurements.
For example, in a previous study, Prof. Reinstein and colleagues compared contrast sensitivity between 2 groups of patients: a repair group of eyes selected for wavefront-guided repair because of debilitating night vision disturbances following primary myopic LASIK, and a control group matched for spherical equivalent treated but with no night vision complaints.
In the control group, there was an increase in spherical aberration but no change in contrast sensitivity. In the wavefront-guided repair group, there was a reduction of spherical aberration by 27%, and although spherical aberration was still much higher than that of the normal group, contrast sensitivity returned to normal.
In this example, contrast sensitivity is a useful measurement to assess the impact of aberrations on visual quality.
Similarly, with new laser surgery platforms-such as topography-guided LASIK-clinicians could use contrast sensitivity to show patients that their quality of vision has improved because treatment was customized to their needs, and corneal aberrations were reduced to improve quality of vision.
Contrast sensitivity has perhaps been underused in ophthalmology practices despite the wealth of data on its application in various procedures.
“In intraocular optics, any time you attempt to expand the range of vision with a multifocal intraocular lens you’re going to lose some contrast sensitivity,” said Gregory J. Pamel, MD, a LASIK and vision correction surgery specialist in New York and clinical assistant professor of ophthalmology at New York University School of Medicine. “That’s been well documented in the literature.”
He added that when a patient is dissatisfied following a multifocal lens implant despite excellent visual acuity on the Snellen chart, ophthalmologists already understand that vision has been impacted by the procedure and don’t need contrast sensitivity testing to prove it.
Consequently, they may not consider it necessary as a routine component of postoperative patient management.
Another reason why contrast sensitivity testing has so far had a limited postoperative role is that it is challenging to determine when it might be worthwhile.
“We’ve all had patients who can see very well postoperatively, seeing 20/20 at distance and J1 up close with a multifocal lens who are unhappy, and there are those who don’t necessarily see 20/20 at distance and don’t necessarily read 20/20 at near after a multifocal lens implant who are very happy,” Dr. Pamel said. “Contrast sensitivity doesn’t always differentiate those patients.”
The explanation may lie in part with the neuroadaptivity of the brain and how well it is allowing patients to see to their level of satisfaction. However, contrast sensitivity testing may still play a role by a circuitous route.
“We know that contrast sensitivity improves in the first 6 months after implantation of a multifocal lens as a result of neuroadaptation,” Dr. Pamel said. “So if a patient is not happy at 1 month we could take their contrast sensitivity and check it again at 6 months to show them that it has improved. That may be a way to continue to educate the patient after surgery.”
Contrast sensitivity may be even more valuable as advancements continue in the technology of multifocal lenses, Dr. Pamel said.
Soon, surgeons may perform comparative studies of older versus newer lens technologies and use the contrast sensitivity results to show future cataract patients the dramatically better real-world results they are likely to have with the improved lenses.
Contrast sensitivity could also be helpful when performing enhancements on patients who may have had laser surgery a decade or more ago with earlier versions of technology and are experiencing problems such as glare and halos or poor night vision. Often these patients have gone to a number of physicians who have corrected their vision in a darkened exam room to see the 20/20 or 20/25 line, yet they are still unhappy with their vision and its impact on their quality of life.
“This really is a function of their loss of contrast sensitivity, and the laser technology that is out there now can improve upon that dramatically,” Dr. Pamel said. “Contrast sensitivity would be very beneficial in explaining to the patient why they’re not seeing well.”
Yet another aspect of vision correction in which contrast sensitivity could be helpful is corneal collagen crosslinking for keratoconus. Sometimes patients subjectively report improved vision after this procedure despite modest gains in visual acuity, and this may be due to enhanced contrast sensitivity.
Dr. Pamel and colleagues are performing a study on contrast sensitivity before and after crosslinking to determine if they can better understand potential benefits and apply this knowledge to future cases.
Contrast sensitivity testing has been part of the clinical trial process for presbyopic corneal inlays. Results have shown that contrast sensitivity does not change significantly after corneal inlays have been implanted.
Contrast sensitivity testing will be important for physicians to perform both preoperatively and postoperatively to demonstrate to patients that their visual quality has not changed, Dr. Pamel suggested.
Gregory J. Pamel, MD
Dr. Pamel has no financial interests in any of the products mentioned in this article.
Prof. Dan Reinstein
Dr. Reinstein is a consultant for Carl Zeiss Meditec.
