Keratoconus is characterized by the thinning of the cornea and irregularities of the cornea’s surface. The cornea is the clear, outer layer at the front of your eye. The middle layer is the thickest part of the cornea, mostly made up of water and a protein called collagen.
Collagen makes the cornea strong and flexible, and helps keep its regular, round shape. This healthy cornea focuses light so you can see clearly. With keratoconus, the cornea thins and bulges into an irregular cone shape, resulting in vision loss.
A cone-shaped cornea causes blurred vision and may cause sensitivity to light and glare. Keratoconus usually affects both eyes, though it often affects one eye more than the other. It generally begins to affect people between the ages of 10 and 25. The condition may progress slowly for 10 years or longer.
Keratoconus generally begins at puberty and progresses into the mid-30s. There is no way to predict how quickly the disease will progress, or if it will progress at all. Keratoconus typically affects both eyes, with one being more severely affected than the other.
Although keratoconus has been studied for decades, it remains poorly understood. The definitive cause of keratoconus is unknown, though it is believed that the predisposition to develop the disease is present at birth.
A common finding in keratoconus is the loss of collagen in the cornea. This may be caused by some imbalance between production and destruction of the corneal tissue by the corneal cells.
Other corneal changes typical of keratoconus that can be seen during an eye exam include iron deposits in the cornea that form a yellow-to-brownish ring, called the Fleischer ring, surrounding the colored part of the eye (iris).
Affected individuals may also develop Vogt's striae, which are thin, vertical, white lines in the tissue at the back of the cornea.
Cataract surgery can improve best corrected visual acuity (BCVA) in all severities of keratoconus without significant corneal change.
Clear-cornea phacoemulsification with intraocular lens implantation improved the BCVA in all severities of keratoconus, with a mean postoperative visual acuity of 20/32.
When preparing to perform cataract surgery in a keratoconus patient, there are several considerations that need to be taken into account. Most of these are refractive in nature, as the cataract procedure itself bears very little difference than a non-keratoconic eye.
It is tempting to try to correct in known keratoconus patients the cylinder in the refractive error with a toric IOL in order to enhance their uncorrected visual rehabilitation to the maximum. But there are several considerations that need to be taken.
Testing - We estimate that one out of 20 people, including those we perform cataract surgery on, have keratoconus. This number includes mild subclinical cases that have never been diagnosed before — doing so prior to cataract surgery may enhance a patient’s refractive outcome!
Keratoconus appears to be endemic in countries like Greece, — a stark difference compared to the patients we encounter and operate on in New York City, where keratoconus is usually reported in one in 500 to 2,000 patients.
Of course, this data represents older clinical paradigms when corneal imaging was not as popular and readily available in most clinical settings. I am sure that keratoconus “suspicious” corneas, even in the United States, will be surprisingly higher than most clinicians expect.
So, as a precaution, we screen all of our cataract population — especially at the time of planning to undergo cataract surgery — with Scheimpflug tomography (ie, the Pentacam (Oculus)) to test for keratoconus.
This is extremely important, in my opinion, because it helps to avoid the postoperative surprise of pseudophakia that does not reach an uncorrected or best-corrected visual acuity at the level that we had planned and hoped for and both we and our patients desire.
Contact lens use - An initial important parameter to consider is whether a keratoconic patient planning to undergo cataract surgery is a rigid gas permeable (RGP) lens or scleral lens wearer.
This poses a special consideration since a decision, after careful discussion with the patient, yields whether the patient will continue to wear RGP or scleral lenses after the cataract procedure or prefer to avoid that.
After a toric IOL is implanted, if a patient desires to go back to a RGP and/or scleral lens, then they must wear over the contact lenses, a spectacle correction that neutralizes the power of astigmatism present in the Toric IOL that has been implanted.
In general, patients who can only function with RGP or scleral lenses and desire to do so after their cataract procedure are probably best treated with a simple, non-toric, monofocal lens.
They can continue to wear those lenses as they are already comfortable with them while also enjoying improved visual acuity.
IOL calculations and considerations In my experience, most patients desire to explore the possibility of correcting not only their invariable myopia (keratoconic eyes are usually myopic and with deeper anterior chamber), but also astigmatism with the use of a toric IOL during cataract surgery.
The IOL calculation is quite complicated due to variability of keratometries that can be registered in keratoconus, especially if the cone is oblique (which is the most common kind — 80% of cases).
We have to consider that the interferometry device readings that we use to calculate IOLs assess the 2.8-mm to 3.2-mm anterior central corneal keratometry (along with axial length).
We have come to find the 5-mm total cornea (anterior plus posterior) keratometries and axis from a Pentacam that calculates the anterior and posterior cornea power altogether is probably the most accurate.
In our practice, the “go-to” IOL formula for keratoconus is the Holladay 1. This is due to the invariably large axial length, which is more importantly combined with an invariably larger than usual anterior chamber depth in these eyes, as we have recently reported in the European and the American meetings (European Society of Cataract and Refractive Surgeons, ASCRS and AAO).
Our study is currently under review for publication. As the majority of our patients in Athens are intolerant to contact lenses, 95% of our cataract patients enjoy the options of a T2 to T9 range of toric IOL correction.
Our go-to IOL is the AcrySof IQ Toric IOL (Alcon) with range of inclusive cylinder as noted above from 1 D to 6 D — a shorter range of toricity is approved and available in the United States.
As mentioned previously, we use the Holladay 1 formula and the keratometries and axis from a 5-mm total cornea power from a Pentacam measurement, and we found that visual correction has been very rewarding in these patients.
We are cautious, however, in adding pilocarpine drops despite intraoperative acetylcholine to attain miosis because of the hovering danger of Urrets-Zavalia Syndrome of a permanently dilated pupil that can be associated with an intraocular procedure in keratoconus patients.
The last consideration is that if a femtosecond laser will be used to enhance the cataract procedure, the clinician needs to carefully screen for any corneal opacities (common at the peak of advanced cones) that may interfere with the optical clarity needed for the femto-assisted procedure
The Athens Protocol - Normalizing the cornea optics with the Athens Protocol corneal cross-linking (CXL) procedure (which we introduced in 2005) can be viewed as a prequel to a cataract procedure, as it will drastically improve the visual function and likely the spectacle-free lifestyle.
For many years, we have advocaed that CXL can become far more effective when combined with a minimal-in-thickness topographic-guided photorefractive keratectomy (PRK) procedure.
That needs to be viewed more as a phototherapeutic keratectomy (PTK) procedure aiming to reduce the cornea refractive coma induced by the corneal ectasia. We have named this CXL procedure the Athens Protocol.
Since introducing this procedure, we have reported close to 50 peer-reviewed papers and more than 1,000 eyes very carefully studied, some of them recently, with 10 years follow up, establishing the safety and efficacy of this technique.
Of course, this approach makes more sense when attempting to visually rehabilitate patients in an environment where rigid contact lenses and scleral lenses are very difficult to tolerate, such as in southern Europe.
So, for us, this approach usually includes besides the topography-guided normalization, the refractive treatment of 1.0-2.0 D of astigmatism and 1.0 to 2.0 D of myopia — most of the time only a fraction of the actual refractive errors that the patients have strictly on the topography-defined astigmatic axis and not the manifest refraction axis.
The advantage of combining this partial in refraction, topography-guided PRK of maximum 50 μm ablation at the minimum thickness of the cornea is that it appears to achieve unique robust synergy with the CXL procedure.
We have found that this fluence offers the best cross-linking effect in these patients as it achieves the deepest cross-linking line studied with anterior segment optical coherence tomography (OCT); it reaches more than 60% of the total cornea thickness and, with the Athens protocol, appears to be very uniform in depth throughout the 9 mm of cornea that usually are started with this technology .
This is in great contrast to a classic standalone CXL procedure that usually achieves a more in-depth cross-linking effect centrally and less peripherally.
This is the reason why the cross-linking — combined with the partial-in-refractive error corrected minimal thickness PRK — is able to invariably achieve 5.0 D to 15.0 D of flattening, where the laser alone would achieve maximum 1.0 D or 2.0 D of flattening.
CXL as a standalone procedure usually achieves the same 1.0 D to 2.0 D of flattening. This greater flattening achieved in these patients at the peak of their cone or their ectasia underlines the previously noted synergistic effect of using both procedures together.
We have theorized that this probably happens for two reasons. First, the larger riboflavin molecule better penetrates a cornea that has been partially ablated in its surface.
In addition, UV light is able to better penetrate the cornea and achieve a better cross-linking effect, thus offering a more stable and dramatically reduced steepest keratometry.
Cross-linking rather than stabilizing oaths We have also applied this procedure in the United States since topography-guided ablations became FDA-approved in 2013, and CXL later became approved as well. It is an off-label combined procedure and needs to be consented with patients appropriately, but our US experience has been very rewarding.
A large number of US surgeons, especially cornea specialists, are very familiar with specialized cornea work with the excimer laser and keratoconus management. They have been able to duplicate these results and offer the benefits of employing the Athens Protocol in US patients as well.
When used with caution, this procedure can become a great tool to add stability and to dramatically visually rehabilitate these patients. It should, nevertheless, not be viewed as a pure refractive procedure.
Discussing the refractive target with patients The Athens Protocol CXL may be a pre-cataract surgery step to consider with advanced keratoconus patients, as it may enable them to approach emmetropia following a cataract procedure several months later and after their corneas have been significantly normalized.
It makes cataract surgery a longer path, but some patients may be interested in this approach. We have used the vast experience accumulated from mostly younger keratoconus patients that are commonly treated with the Athens Protocol CXL.
Evaluating a family for keratoconus - This work has also made us very astute of carefully investigating possible subclinical keratoconus.
We like to evaluate the whole family — and we have presented a 100% yield of finding one of the two parents of a known keratoconic young patient having a trait of keratoconus.
This becomes valuable for us to evaluate the other young adults in that family. A thorough ophthalmic evaluation to rule out keratoconus can never exclude a cornea topography or tomography.
A 20/20 patient with perfect corneas in slit-lamp biomicroscopy may still harbor an obvious keratoconus when cornea topography or tomography is employed, and it is even more accurate when anterior segment OCT is employed that can also offer a 3D image of the cornea epithelium that may mask the actual expression of ectasia in early stages.
We have reported extensively on this in both meetings and in the peer-reviewed literature. A cataract surgery candidate likewise having a grandchild with known keratoconus raises the suspicion for closer evaluation of subclinical irregular topography that may aid in a better refractive outcome in their cataract surgery.
Integrating laser work Although there are currently many venues of employing CXL, such as epithelium-on (sufficient to stabilize the keratoconus short term), it has shown in our hands and in the literature that it is not as robust as the standard epithelium-off cross-linking.
Iontophoresis to deliver the large riboflavin molecule transepithelially into the cornea was very popular a little more than 5 years ago, but it is fading away as a popular clinical option, as other techniques appear to be as effective and simpler.
Since 2013 we have also introduced — in collaboration with Avedro (now part of Glaukos) — advanced UV projection technology (the KXL II device CE marked in 2013, later named Mosaic) that can allow us to use a pattern of different fluence and energy delivered into the cornea to attain reproducible refractive changes in keratoconic and even normal corneas.
Ray tracing excimer customization - We recently reported in Cornea the use of ray tracing as a means of measuring total eye aberrations (low and high) in combination with CXL.
This means using a ray-tracing customized ablation instead of a topography-guided partial refraction excimer laser ablation and higher fluence cross-linking.
This employs data from the Sitemap device, an adapted Pentacam AXL Wave (offering HR Pentacam, Lenstar interferometry and Hartmann-Shack wavefront measurements along with pupillometry and limbal and iris recognition landmarks) and the Innoveyes software (WaveLight, Alcon), which is undergoing FDA trial in the United States and was CE-marked in Europe in 2019.
Ray tracing offers the attractive option of measuring total eye aberrations in a keratoconic or an ectasia patient, in normalizing the cornea and treating some of the higher aberrations as well, in combination with CXL.
On a practical level, ray tracing appears to view the ectatic corneas and tilted cornea, thus normalizing that cornea with treating mainly a superior cornea hyperopic arc.
This approach helps to avoid treatment in the very debatable area of the thinnest part of the cornea, which is the area of the steepest part of the cone.
This is a new, exciting frontier in the treatment of keratoconus. Ray-tracing customization in therapeutic excimer ablations used in keratoconus management may become an application used by more clinicians globally and help establish whether this will be more beneficial than the topography-guided ablation, which has become the standard of care when trying to not only stabilize the cornea but also improve visual function globally and within the United States.