Refractive: Beyond the routine
Fall 2025

by Ellen Stodola
Editorial Co-Director

Keratoconus is a unique disease in that its consequent vision loss, in most of its stages, involves corneal optics, according to Peter Hersh, MD. “The biomechanical weakness of the cornea in keratoconus causes a focal misshaping of the cornea, compromising its optical quality,” he said. The cornea supplies two-thirds of the total refractive power of the eye, so having distortions in the corneal optical surface will diminish vision. Dr. Hersh and Beeran Meghpara, MD, discussed finding the right refraction in patients with keratoconus, what to take into account, and considerations if patients are seeking surgery.

In a perfect eye with a perfect cornea, refraction involves the normal Cartesian optics of sphere and cylinder, and those are called lower order aberrations, Dr. Hersh said, noting that these define normal refraction. The keratoconic cornea, because it is optically distorted, also has higher order aberrations. “When I’m discussing these with patients, I explain it’s akin to visual static. It’s like having static on a television. These are aberrations that are different from the standard sphere and cylinder that you have in a normal optical system. These impact patients’ functional vision because they can’t be corrected with spectacles,” he said. “Spectacles can only correct sphere and cylinder, that is myopia, hyperopia, and astigmatism.”

It’s important to consider these higher order aberrations, and it’s important to determine a good starting point, Dr. Hersh said.

Finding the right refraction can be tricky in keratoconus patients, Dr. Meghpara said, because they often have high amounts of astigmatism, and additionally, the astigmatism is irregular. “A lot of times the amount of astigmatism in the cornea from the keratoconus doesn’t necessarily match up to the amount they’ll get in a refraction,” he said. The amount of cylinder we measure in glasses often isn’t as high as what you’re getting in the cornea, he said. The surgeon needs to know that you don’t necessarily want to give all the astigmatism that you’re measuring in a glasses prescription because it’s so high, and it can be intolerable to wear.

There are many great diagnostic devices available now to measure refraction, Dr. Meghpara said, adding that two popular devices are the iTrace (Tracey Technologies) and the OPD-Scan III (Nidek). These can provide a good starting point to use to refine the refraction in the phoropter. “Getting a good starting point is one of the keys to making this an efficient and accurate process. You could even go old school and use a retinoscopy to get a starting point,” he said.

An autorefractor is not as good in the circumstance of keratoconus, Dr. Hersh said, because of the higher order aberrations, so there are a few instruments that are good adjuncts to help get started. The first is corneal topography because it can give you a map of the corneal surface and show you the optical irregularities and the irregular astigmatism that is present. “I even prefer in these cases videokeratography; that is a topography system that’s based on keratoscopy or the video image of the Placido disc. Looking at the Placido image itself allows you to see the general axis and magnitude of the astigmatism over the entrance pupil.” He added that you can also use optical aberrometry, which will define the most important aberrations of the cornea. Thus, inferring the astigmatism from the corneal topography analysis is often a good starting point.

If the patient has old glasses, Dr. Hersh said you can start with that. But often old glasses don’t bear a resemblance to what their actual refraction should be because when they’re looking out of the cornea, they are looking through multiple refractive spots, and each spot is focusing light in a different location. Using topography is a good starting point for the refractive cylinder, and having them choose among a wide range of spheres, often starting with 3 diopter increments, along with this is also a good starting point for the spherical correction; refining from that gives the best results.

Another important factor, Dr. Hersh said, is pupil size. In a pristine cornea, pupil size doesn’t make a difference because it’s always a similar sphere and cylinder, but as the pupil size changes in keratoconus, the type and magnitude of aberrations can change. That can change subjective refraction in dark and light. “I think it is advantageous to do refraction in both photopic and scotopic conditions,” he said. Then you might take into account what the patient is doing most or give two pairs of spectacle correction, one for daytime and one for night driving.

As far as correction non-surgically, the mainstay is contact lenses, Dr. Meghpara said. Patients are always going to see better in contacts compared to glasses because you can treat higher amounts of refractive error with a contact lens.

Options depend on the severity of keratoconus the patient has and how irregular the cornea is. “If it’s a milder form where the astigmatism is regular and you put them in the phoropter and can refract them pretty well, often these patients will do well in a soft contact lens, or there are special contacts that are soft but meant for keratoconus patients,” he said. “The progression beyond that goes to typical RGP lenses.” Dr. Meghpara said these are often smaller in diameter, and to try to get them to fit on a steep, protruding cornea is difficult, so wider diameter contact lenses are typically what he’s using. These could be hybrid lenses that are soft on the outside and hard in the middle. However, if you’re not careful with the fit, Dr. Meghpara said patients can run into issues with lenses that are fit too tightly, and they can get corneal surface issues, signs of contact lens overwear, or neovascularization of the cornea.

“The gold standard now is the scleral contact lens,” Dr. Meghpara said, adding that there are many options. There are regular scleral lenses, customized scleral lenses, and PROSE lenses. “One of our optometrists is using EyePrintPRO [EyePrint Prosthetics],” he added. This involves taking a mold of the eye and using a customized process to get a contact that mimics the surface of the patient’s eye. “I think the revolution in contact lenses has reduced the amount of corneal transplants that I’m having to do for keratoconus.”

If a keratoconus patient wants or needs surgery, Dr. Meghpara said there aren’t many refractive options. The problem is having keratoconus is a contraindication to having corneal refractive surgery, whether it’s LASIK, PRK, or SMILE, because the risk of ectasia is high. “You need to think outside the box when it comes to surgical options for these patients. They’re going to be younger so not necessarily cataract age,” he said.

Outside the U.S., there is a treatment algorithm where patients will get crosslinked first to strengthen the cornea to potentially make them a candidate for corneal refractive surgery, then possibly do PRK on top of that to reduce risk of ectasia. Dr. Meghpara said this may be catching on in the U.S., but outcomes are still unpredictable, and the risk of haze in the cornea after that is high.

“If someone with keratoconus comes in interested in refractive surgery, I’m usually doing ICL surgery [STAAR Surgical], and it’s a different mindset and counseling for that,” he said. The expectation with refractive surgery is to see great without glasses or contacts. “When it comes to keratoconus patients, we have to tell them it’s unlikely they’ll be perfect, and make sure to tell them that vision will likely be better in refractive contact lenses compared to with refractive surgery. Our goal is to debulk that prescription and reduce it as much as possible but not necessarily eliminate it.”

This goes back to the whole refraction discussion, Dr. Meghpara said. It’s important to get an accurate refraction but also know when adding refractive error into the refraction is worth it or not. It’s easy to overcorrect and put too much. “You want to correct as much as possible but not overcorrect, and you have to learn when you’re at a happy medium,” he said.

“When I’m talking to a patient, I tell them there are three things we need to discuss in the treatment,” Dr. Hersh said. The first is that keratoconus is a progressive disease, and aberrations will change over time, so refractions can change over time. “Because it’s a progressive disease and we now have a treatment for progression that is corneal crosslinking, that is first and foremost what’s on our mind when we’re seeing a keratoconus patient. If they’re progressive, they should have crosslinking.”

The second thing is getting the best optical vision, Dr. Hersh said. We’re trying to get the best spectacle correction, but they’re always going to get their best vision with a rigid or scleral contact lens. In those cases, the fitting is entirely different from the refraction because these lenses cover the cornea and give a new optical surface, so they negate the lower and higher order aberrations.

The last thing to discuss is surgery, he said. Many patients are diagnosed with keratoconus when they come in seeking refractive surgery, and he agreed that procedures like LASIK or PRK are contraindicated in keratoconus because you don’t want to thin the cornea and further weaken it in what is a progressive disease caused by weak cornea biomechanics. However, there are some interventions that can be helpful for patients surgically. Some are candidates for topography-guided PRK, which takes the corneal topography map and feeds these aberrations into the laser to correct some of the topography irregularities. “This will not prevent progression and should be done with crosslinking, and we limit the amount of tissue removed,” he said. Another option is CTAK, first originated by Dr. Hersh in 2015, which uses a custom femtosecond laser-cut inlay of corneal preserved tissue, using various topography and tomography inputs that are specific to the patient. CTAK can improve corneal topography and, most importantly, patients’ visual function in many cases.

Dr. Meghpara also mentioned CAIRS or CTAK as options, which involve the addition of corneal segments into the patient’s cornea to try to thicken it and make it more regular. By doing those, you’re also reducing refractive error. You may not be eliminating contacts but are reducing the prescription or making it easier to correct.

“As corneal surgeons, we’d offer crosslinking, which is great and it’s revolutionized the treatment of keratoconus, but all crosslinking does is stabilize the disease,” Dr. Meghpara said. It doesn’t make the patient better, but it hopefully prevents corneal transplant in the future. “We’re doing more and learning more,” he said, adding that he thinks there will be a shift in the next 5–10 years to focus on trying to get these patients to see better.

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About the physicians

Peter Hersh, MD
Cornea and Laser Eye Institute
Rutgers New Jersey Medical School
Princeton University
Teaneck, New Jersey

Beeran Meghpara, MD
Director of Refractive Surgery
Co-Chief of the Cornea Service
Wills Eye Hospital
Philadelphia, Pennsylvania

Relevant disclosures

Hersh: CorneaGen
Meghpara: Glaukos

Contact

Hersh: phersh@vision-institute.com
Meghpara: bmeghpara@willseye.org