Thin Corneas in LASIK: How Low Can You Go?

The refractive surgeon has multiple options for delivering good visual outcomes.

By Amin Ashrafzadeh, MD

Patients with thin corneas present a difficult challenge to many refractive surgeons. How thin is too thin to rule out refractive surgery? How normal or abnormal, pathologically, is the thin cornea? What corneal abnormalities contraindicate refractive surgical correction? Are there any signs of ectasia or irregular corneal topographic behaviors that may suggest ectasia? Many opinions exist, and a consensus is probably elusive. Context counts as well. A 520-μm cornea is not thin for a −1.00 D ablation but it is for a −10.00 D ablation.

This article explains my approach to managing the thin cornea in refractive surgery. It may present a reasonable approach for you.

Questions to Consider

Before performing surgery on thin corneas, I recommend you ask yourself these questions:

■ Regardless of the corneal thickness and patient data, is the cornea normal? Anterior surface corneal topography is without question a standard for refractive surgery consideration. If the cornea is completely normal, several imaging options for a confirmatory posterior surface evaluation are available: optical coherence tomography, Scheimpflug imaging or slit imaging are the accepted common methods of posterior surface evaluation. With all systems, a 10-μm posterior surface protrusion anteriorly should raise major concerns about possible corneal abnormality and ectasia risks. If the cornea is normal, excimer ablation remains in the algorithm. Otherwise move on to the intraocular intervention discussion in the sections on phakic IOLs and clear-lens exchange, below.

■ How thick is the cornea and what is the ablation depth? As I mentioned, the ablation depth and corneal thickness are essentially integrated. As a medicolegal standard, a 250-μm residual stromal bed (RSB) is fairly accepted as the minimum tissue that must remain in place. Some surgeons are more conservative and require a 300-μm RSB. In the Leaming Survey 2011, 99% of surgeons surveyed would not operate on a cornea thinner than 250 μm of RSB, yet 37.7% place their cut off at 300 μm.¹

Delivering monovision with phakic IOLs, such as the Verisyse, is one way to correct presbyopia for those with thin corneas.

Unfortunately, I have seen a few referred patients who had a residual stromal bed of 300 μm or greater and went onto develop post-LASIK ectasia nonetheless. Some surgeons use a ratio of the original corneal thickness to the RSB. Some use the minimum total corneal thickness as a secondary guide. In the Leaming survey, 45.9% of the respondents said 500 μm was their cut-off for offering LASIK.

■ How well do you trust your technology? If you have a blade microkeratome, what is its median flap size and standard deviation? Are you prepared to deal with a thick flap? The standard of care clearly dictates intraoperative pachymetry before ablation. How well do you trust your pachymeter on a stromal bed? With the femtosecond laser, the predictability factor is much higher than the blade microkeratome. The variability in the flap thickness is greater with the blade microkeratome as compared to the femtosecond laser. This variability may affect the safety of the procedure as it may result in reduced RSB with a thicker flap. Some pachymeters have a more variable reading of the stromal thickness once the flap is lifted. After about 100 intraoperative pachymetry and postoperative Visante anterior segment OCT (Carl Zeiss Meditec, Dublin, Calif.) imagings of the flap and the RSB, I have since abandoned intraoperative pachymetry due to the reliability of the femtosecond laser.

I perform the vast majority of my excimer ablations with a 100-μm IntraLase-made flap. As such, with the flap created, there are about 50-μm of epithelium and 50 μm of stromal tissue within the flap. If my standard to use excimer laser is based on the marginal 50 μm of extra stromal tissue available through PRK, I usually abandon excimer ablation.

■ What are the status of the natural corneal cross-linking and the patient’s age? A consensus seems to exist that young corneas are less naturally cross-linked, making them more prone to ectasia. I would be far more cautious with a 21-year-old than a 36-year-old. No hard data supports such claims, but this seems to be well accepted among refractive surgeons.

■ Does the patient have ocular anti-hypertensive treatment? Ocular hypertension is another major factor to weigh in your assessment. For years, anti-hypertensive agents have been advocated for ectasia, keratoconus or other conditions. A recent study showed patients on timolol 0.5% developed less regression after excimer ablation.² These findings have suggested that myopic regression may be a mild, uniform, subclinical ectasia. In these patients, the protective effect of timolol therapy remained six months after they discontinued drops. Therefore, timolol therapy for six months may provide some additional reassurance to surgeons and patients.

■ How old is the patient? Presbyopia and the patients age are also paramount in the decision making. If you are considering refractive surgery for patients age 50 or older, I would, without hesitation, move the discussion to clear lens extraction with presbyopia-correcting IOL. For patients age 39 or younger, I would give strong deference to excimer. If that were not possible, I would choose phakic IOLs.

Patients in the 40-50 age group are the most difficult to address. Their motivation and purpose in seeking refractive surgery will largely guide the decision-making. Many accept presbyopia easily.

I had a 49-year-old patient — a general surgeon and avid water skier — who was perfectly happy to give up his −4.00 D OU for full distance vision and reading glasses for near vision. Monovision did not appeal to him. A 48-year-old photographer who was −13.00 OU chose to have Verisyse (AMO, Santa Ana, Calif.) phakic IOLs with monovision for now and wait for better presbyopia-correcting technology in future. The visual aberrations that can come with multifocal IOLs did not appeal to her.

The Methods at Our Disposal

Our choices of treatments are ever-evolving. As newer techniques and safety features emerge, the decision-making tree changes. Following are some current options, as well as ones that may soon be available:

► Excimer laser. The classic laser will remain an important modality. It has many advantages in terms of cost, familiarity, recognition, safety, results and recovery. Disadvantages remain in the physical suitability of patients, such as normality of the cornea (no ectasia), sufficient corneal tissue for ablation, potential ectasia formation and difficulty in predicting IOL powers in the years to follow. LASIK is highly predictable and excellent results have catapulted this procedure to the most popular elective surgery in the United States with nearly 1 million procedures a year.

► Collagen cross-linking. Although not yet FDA-approved, the promise of collagen cross-linking is to prevent or even treat ectasia. It is available in centers participating in clinical trials.

► Phakic IOLs. They provide the ability to treat much higher levels of myopic correction (up to −20.00 D). They typically cost about twice as much as LASIK and most surgeons implant them one eye at a time. The cost is a barrier for many patients. Potential for damage to the crystalline lens as well as cataract formation and corneal endothelial damage influence many surgeons when they consider this option. Only a few thousand such procedures are done each year in the United States, and few surgeons perform three or more cases a month. Among the advantages phakic IOLs offer are preserving the corneal stromal tissue, ability to perform in the setting of mild ectasia and the ability to preserve the natural accommodation of the crystalline lens. This may be a great option for patients in their 40s who want to wait for future presbyopia-correcting technology and not deal with troublesome IOL calculations.

► Clear-lens exchange or refractive lens exchange. The surgery is identical to cataract surgery, so many surgeons are comfortable with it. Presbyopia-correcting IOLs have made this procedure much more attractive, and many surgeons now perform it on a monthly basis. The procedure is typically performed unilaterally and costs about the same as a phakic IOL surgery, or twice what LASIK costs. Keep in mind potential complications in the early and late postoperative phases, including retinal detachment. The major advantage of this procedure is its ability to treat virtually most refractive errors — hyperopia and myopia as well as most regular astigmatism.

Creativity Required

The patient inevitably presents for whom one can combine the technologies such as phakic IOL or clear lens exchange with a planned second-phase LASIK procedure. As an example, I implanted phakic IOLs OU and then performed LASIK six weeks later in a 42-year-old woman. Her refraction was −11.00 −2.75 × 021 OD and −9.75 −3.75 × 176 OS, with corneal pachymetry of 556 μm OD and 550 μm OS. Her astigmatism limited the results of a phakic IOL. Her refraction and corneal thickness ruled out LASIK alone. However, combining two modalities overcame the limits of each procedure on its own.

The ability to think outside the box is essential in cases of thin corneas. Recognizing patient’s motivation and matching that with applicable technologies and techniques will be essential in the care of such patients. OM

References

1. Leaming DV, Duffey RJ. Leaming 2011 Survey of American Society of Cataract and Refractive Surgery Members.
http://www.analeyz.com/AnaleyzASCRS2011.htm. Accessed Jan. 25, 2013.

2. Shojaei A, Eslani M, Vali Y, Mansouri M, Dadman N, Yaseri M. Effect of timolol on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis: a prospective randomized clinical trial. Am J Ophthalmol. 2012;154:790-798.

Amin Ashrafzadeh, MD, is a cataract, corneal and refractive surgeon in private practice at Modesto Eye Center, Modesto, Calif. He is also a consultant to Carl Zeiss Meditec.