Contrasting Views on Contrast Sensitivity & Multifocals

Cataract surgeons acknowledge its role but differ somewhat on how it may impact lens selection and postop management.

By René Luthe, Senior Associate Editor

While reduced contrast sensitivity (CS) is a fact of life with multifocal IOLs, the good news is that in healthy eyes, it doesn't have to be a major problem. Cataracts reduce CS far more than a multifocal lens will, after all. The bad news is that in not-so-healthy eyes, or the wrong personality profile, the CS loss is a major problem. Fortunately, there's another dose of good news—with careful preoperative screening and the right technology, you can lessen the chances of implanting multifocal IOLs in candidates who can ill afford decreased CS.

Looking at CS

The well-known trade off with multifocal IOLs is that they reduce contrast sensitivity somewhat in exchange for a more spectacle-free existence. While that's a trade many patients are willing to make, others receive an unhappy surprise after surgery, when they find that images appear washed out, or “ghosty.” Their surgeon learns of their unhappiness in short order—after the fact, unfortunately, when remedies are harder to come by.

Yet presurgical evaluation of contrast sensitivity in cataract patients is not routinely possible due to the absence of appropriate technology in most practices and inability to distinguish how much deterioration is due to the cataract and how much to another, as yet undetected, problem.

Cataracts scatter the light traveling towards the retina in a forward direction, explains Jay Pepose, MD, PhD, of St. Louis. This causes the point spread on the retina to become diffuse rather than small and crisp. “Sometimes we can't appreciate that as ophthalmologists, because we are looking at the backscatter. When examining a patient at the slit lamp and looking at a cataract, we are really seeing the light that's coming back at us; we're not really seeing the light that reaches the retina.”

Thus, a patient who seems to have a very mild cataract may at times complain much more than seems warranted. The problem is, Dr. Pepose says, that the clinician is not actually assessing their retinal image. “And we are not having patients do low-contrast testing, or contrast sensitivity testing.” Instead, he believes, many cataract surgeons are recording the patient's high-contrast vision on an ETDRS chart, or a Snellen acuity chart.

He evaluates the retinas of his patients with the Optical Quality Analysis System (Visiometrics), in which a small, circular light source is imaged on the retina, and the size and the shape of the light spot is analyzed using a dual-pass system. “It measures optical scatter and the retinal point-spread function,” he explains. “We can derive many things from the point-spread function, such as they eye's modulation transfer function and Strehl ratio. These metrics reflect how good the optical system is in providing image contrast with respect to object contrast.”

Unfortunately for clinicians, devices that measure contrast sensitivity are not usually found outside of research facilities. Glenn Pomerance, MD, of Chattanooga, Tenn., however, uses Vector Vision's CSV-1000 for his evaluations. The device, he explains, presents sine-wave gratings with a specific luminance that remains constant in order to obtain repeatable results. He uses the device at nearly every visit because it demonstrates the crucial loss of function.

It has proved helpful in detecting and quantifying cataract vision loss in patients with “early” cataracts, he says. “Patients come in and tell me they're doing great,” Dr. Pomerance says. “But due to the cataract, patients with what appear to be minimal lens clouding through the slit lamp, frequently already have a reduction in contrast sensitivity, which is a measurable reduction in the quality of vision rather than quantity of vision.”

To demonstrate this deterioration, the CSV-1000 offers an image that Dr. Pomerance notes is very persuasive with patients: a picture of a child chasing a ball in the street, shown at various levels of contrast sensitivity (Figure 1). “Patients really learn the lesson that when their contrast sensitivity is reduced, they might not be able to detect enough detail to function well in bright light situations. It's obscured by glare,” says Dr. Pepose. “But their deterioration is so gradual that they are really not perceiving that loss. You can demonstrate to them that this is what happens over time with a cataract.”

Figure 1. The image on the left illustrates normal contrast sensitivity and what this enables the patient to see. The image on the right shows what reduced CS might cause them to miss.

Dr. Pepose notes that such tests still don't tell the clinician why the patient's ability to perceive contrast is reduced, or whether something besides cataract is at work. “Preoperative screening for contrast sensitivity is not very predictive of comorbidity.” Instead, as he puts it, “You take out the cataract and, lo and behold, now you have a better view of the retina, and now you see that they had an epiretinal membrane or some macular degeneration that you couldn't see too well because of that dense cataract.” Dry eye can also reduce contrast sensitivity.

Should a multifocal IOL be implanted in an eye with an undetected comorbidity, Dr. Pepose says the patient will then have two sources of contrast sensitivity loss: the multifocal lens and the pathology.

Dr. Pepose finds that a more helpful screening is to measure the patient's potential retinal acuity. The device he favors, the Retinal Acuity Meter (AMA Optics), helps the surgeon find the place in the cataract that is least dense and, through pinhole glasses, beam the eye chart onto the patient's retina. “You're trying to obviate the cataract, to get an idea of what the retina or optic nerve function would be like without it.” While it is not helpful for patients with very dense cataracts, he says, it is for most others.

“I use it on every patient that I can't refract down to 20/20, because it's got pretty good predictive value,” he explains. Should a patient with a moderate cataract read only to 20/30 or 20/40 on the RAM test, Dr. Pepose knows he needs to reduce patient's expectations for their IOL options. “I may start to counsel that patient that, while this test isn't 100% predictive, most of the time, in someone with a normal retina or normal optic nerve function, we're going to get significantly better than 20/40 on this test, as long as the patient has better than 20/200 best-corrected vision.” The inability to do so, he tells the patient, means that retinal potential may be less than what was anticipated.

In addition to reconsidering implanting a multifocal lens, the test results prompt Dr. Pepose to possibly do more testing, such as an OCT of the macular or optic nerve and/or perimetry. “Maybe I'll want to refer that patient to a retina specialist before we do the cataract surgery. But, either way, that is a red flag for me.”

Thus, preoperative screening is crucial to avoid the patients who will not enjoy optimal vision with a multifocal.

Johnny Gayton, MD, of Warner Robins, Ga., notes that this group includes those with:

• severe dry eye
• those who have previously undergone LASIK that resulted in higher-order aberrations
• a history of RK or corneal transplant
• epiretinal membrane
• diseases such as glaucoma, macular edema or AMD

According to Dr. Gayton, the surgeon should evaluate the tear film, magnitude of astigmatism and the condition of the retina. Patients getting any sort of premium IOL at his practice receive a special work-up that includes Lenstar IOL power calculation, corneal topography, possibly wavefront analysis, and Cirrus OCT of the macula and posterior pole.

“I would rather miss putting one of these lenses in four people who would have done OK with them than to have put it in one person who doesn't do OK,” he explains, “because that one person who doesn't do well can frankly make your life miserable. But more than that, it makes their life miserable.”

Then there are the borderline multifocal candidates to consider. These include dry eye patients and those whose maculas are not “absolutely pristine,” Dr. Gayton says. Other patients may seem to be good candidates at the moment, but that status might not last long term. “If you are looking at someone who has a significant depression of their island of vision from glaucoma, even if they seem to be an ideal candidate, they are probably not going to have a successful experience.”

CS Post-Implantation

After the multifocal has been implanted, Dr. Gayton notes, the surgeon has only three options: (1) Attempt to treat the pathology in order to improve the quality of vision, (2) try to help the patient adjust to the drawbacks of their new vision or (3) perform an IOL exchange.

When checking for pathology or anatomical issues, Dr. Pepose eliminates the following: “You want to be sure they don't have dry eye, which could be treatable; you want to be sure they don't have any uncorrected astigmatism; you want to be sure that the lens is centered—that there's no mechanical problem with the lens. You want to be sure that they don't have any significant posterior capsular opacity and that they don't have cystoid macular edema.” However, as the surgeon goes through this diagnosis of exclusion, if there is nothing to be remedied, the only alternative left may be IOL explantation.

Should you have to resort to option number two, helping the patient adjust to the IOL's visual drawbacks, you will have occasion to find out how accurate your preoperative assessment of the patient's expectations and how effective your patient education had been. Patients who have been prepped about the multifocal's drawbacks and how these might affect them in their daily lives, these surgeons agree, are much more positive about adjusting.

“I give them tricks on how to do it,” says Dr. Pomerance. “For instance, when a patient complains that he just can't read the menu in a restaurant, I remind him that the pupil size affects function with multifocal IOLs. You are mixing too much distance light with your near light, so you need brighter light in the restaurant. We joke about taking a flashlight along, but sometimes it really does help.”

Similarly, he explains to patients who have large pupils that they will have more problems with contrast sensitivity than those whose pupils become very small when they read or move into bright light, and should compensate by wearing sunglasses.

The IOL exchange option holds a good many dangers, Dr. Pepose notes. Capsular tears or capsular ruptures are two, especially if the eye has already been YAGed. “If you've opened the capsule, an intraocular lens exchange has more risks in terms of vitreous prolapse, macular edema, things like that,” he explains.

Yet while IOL exchanges are not an easy solution, Dr. Gayton says that they are sometimes the only one that will make the patient happy. A patient who after surgery was seeing 20/20 at both distance and near was recently referred to him. Despite what would seem to be an excellent result, she was unhappy because she could not drive at night, he recounts. “And she couldn't adjust—at that point she'd had the IOLs for several months. In cases like that you might as well pull the trigger and exchange the lens. When I did, she was ecstatic.”

Looking Down the Road

Another potential post-implantation issue is the longevity of outcomes.

Regardless of thorough preoperative screening and patient education, the patient still may not live happily ever after with multifocal IOLs. As Dr. Pepose notes, today's longer life span means that patients are more likely to later develop a complicating comorbidity that reduces contrast sensitivity.

Ten or 15 years after a successful implantation, Dr. Pepose warns, the patient could develop glaucoma, epiretinal membranes or macular degeneration, all diseases that will cause further deterioration of CS. While there is no way to predict that, the patient is then unhappy with their vision. Or the surgeon may put a multifocal into a suitable 60-year-old candidate without accounting for anatomical changes likely with advancing age.

For instance, that 60-year-old might have pupil diameters of 5 mm in dark light and 2 mm in bright light, but by age 80 those may reduce to 3 mm and 1.5 mm, respectively. “For the multifocal lens, that completely changes the dynamics,” Dr. Pepose explains. “A small pupil can sometimes limit available light to as little as 20% at some focal lengths and up to 20% of light will be simply lost to useless foci. There's no way that the contrast provided by 0% of light could be equal to the contrast of 100% of light that an accommodating IOL transmits, for example,” he says. That makes is difficult to predict both the short-term and long-term outcomes, and performance of multifocals, with much precision.

Putting it in Perspective

Frank Bucci, MD, of Wilkes-Barre, Pa., allows that patients with a compromised macula or other retinal disease are certainly not good candidates for a multifocal and the additional loss of contrast sensitivity it induces, but contends that in otherwise healthy eyes, the contrast sensitivity issue is usually unimportant. The slight loss the multifocal causes, he reminds, is negligible compared to that caused by the cataract.

Moreover, contrast sensitivity can't be precisely quantified, nor is there a standard established for just how much is enough with regards to multifocals, he points out. “Attempts at quantifying contrast sensitivity are usually reserved for clinical trials. There is no routine test or machine in the clinic that tells us how much loss of contrast is present, nor can we clearly delineate when to use a multifocal and when not to based on CS status.”

After implanting 2,500 presbyopia-correcting IOLs over the past 10 years, Dr. Bucci is convinced that a patient's desire to ditch their reading glasses is the “single strongest factor correlating” with successful wear of multifocals.

“If they pull their bifocals off and wave them at me and tell me that they hate them, I don't have to worry about contrast sensitivity or halos,” he says. In Dr. Bucci's experience, haloes are far more likely to cause patient dissatisfaction than contrast sensitivity. Yet motivated patients usually find even those more bearable than issues related to glasses.

“If as a result of the preop consultation you become convinced that they have a strong desire to lose their glasses, that they are reasonable people, they understand what you've explained to them about halos and they are willing to take that risk, then contrast sensitivity is pretty far down the list of what I'm worried about.”

Worth Keeping an Eye On

Dr. Pomerance agrees that for healthy eyes, the slight contrast sensitivity loss that comes with multifocal IOLs is not really a problem. However, he still finds attention to CS helpful in facilitating patient adjustment to multifocals post surgery. In addition to explaining to them the way multifocals divide light and helping him coach patients through the neuroadaptation process, frequent testing allows him to demonstrate where their contrast sensitivity was at the previous visit versus where it is now. “I can explain to them, ‘This time you are doing better on contrast, so your brain is getting used to it.'” Another crucial benefit to contrast sensitivity testing, though, Dr. Pomerance explains, is that it enables the clinician to identify the subtle changes in function that occur long before the anatomic changes that accompany disease occur. Research he has done on glaucoma patients has led him to believe that testing could be very valuable regarding that disease. Contrast sensitivity tests, he points out, ask the patient to look at the gridlines, defined as sine-wave gradings of various frequencies.

“It's three, six, 12 and 18 cycles per degree that we are asking patients to discriminate for us. Eleanor Faye originally described this, but we discovered that we could actually detect glaucoma before it was evident by any other technique by looking at the contrast sensitivity and noticing a peculiar drop in contrast (a notch) on the six-cycle per degree grid in patients with early glaucoma. Once we initiated treatment, we could see that resolve with adequate treatment.”

Dr. Pomerance also says that CS testing helps to detect optic neuropathy in sleep apnea patients long before it is sufficiently developed to be measured on OCT. “Many people don't even realize they have it, and sleep apnea patients are at high risk of comorbidities such as atrial fibrillation and stroke,” he says.

So while proponents of premium IOLs should be mindful about loss of contrast in multifocal patients, contrast sensitivity is well worth the clinician's attention in the presurgical work up—both for successful vision postop, and for the patient's overall health. OM