Keratectasia can present months after uneventful LASIK

Several promising treatments are available for post-LASIK ectasia.

Issue: April 1, 2006

ByAmar Agarwal, MS, FRCS, FRCOphth

The anterior cornea is the major stress-bearing layer of the cornea, as it is composed of alternating collagen fibrils with a more complicated interwoven structure than the deeper stroma. The flap used for LASIK is made in this layer and thus results in a weakening of that strongest layer of the cornea, which contributes the most to the biomechanical stability of the cornea. This cornea is not able to withstand the normal IOP of the eye and becomes progressively ectatic at the weakest area, leading to worsening myopia and irregular astigmatism. The process is irreversible once it begins. Corneal ectasia occurs insidiously after ablative refractive surgery and may be seen months after an originally uncomplicated refractive procedure.

Etiology

Amar Agarwal

Two well-known contributing factors are an excessively deep ablation and LASIK in a previously undiagnosed forme fruste keratoconus patient. The lamellar cut in the cornea and the decreased residual bed thickness both contribute to the decreased biomechanical stability of the cornea after LASIK. Larger ablation diameters result in lesser residual bed thickness postoperatively and a larger area of thin cornea. The residual bed thickness should not be less than 250 µm to avoid subsequent iatrogenic keratectasia. Factors such as drying of the stromal bed may result in an ablation depth more than intended. Normal IOP, inadvertent excessive eye rubbing, prone position sleeping, and the normal wear and tear of the cornea all play a role in the progression of ectasia.

Patients with thin corneas less than 500 µm, primary posterior corneal elevation and forme fruste keratoconus are at greater risk of post-LASIK ectasia. In some cases no preoperative risk factor can be identified. Structural rigidity of the individual cornea and IOP may play major roles in these cases. Attempted correction, optical zone diameter and flap thickness are other parameters that have to be considered. Flap thickness may not be uniform throughout its length.

Corneal topography

Topography is invaluable for preoperative ophthalmic examination of LASIK candidates. Topographic analysis with Orbscan (Bausch & Lomb) uses three-dimensional slit-scanning technology (Figures 1 and 2). Three-dimensional imaging allows us to predict which candidates would do well with LASIK and also confers the ability to screen for subtle configurations that may be contraindications to LASIK.

Figure 1. A patient with iatrogenic keratectasia after LASIK. Note the upper right-hand corner pictures shows the posterior float has thinning, and this is also seen in the bottom right picture in which pachymetry reading is 329 µm.

Figure 2. The same patient with iatrogenic keratectasia after LASIK in a 3-D pattern. Notice the ectasia seen clearly in the bottom right picture.

Images: Agarwal A

Clinical features

A patient with post-LASIK ectasia presents with progressively increasing myopia, irregular astigmatism, fluctuating refraction, difficulties in scotopic vision, glare, halos, ghosting of images and loss of best corrected visual acuity weeks, months or even years after an uneventful LASIK. Detection of mild keratectasia requires knowledge about the posterior curvature of the cornea. The earliest changes are detected on the posterior corneal surface as posterior corneal bulging. Increased negative keratometric diopters and oblate asphericity of the posterior corneal curvature are seen. An eccentric posterior bulge below the center of the laser-ablated area is most ominous. Later, a central or paracentral area of steepening, which is seen to progressively worsen on follow-up evaluations, is seen. Decreased pachymetry is seen in the area of steepening. Increasing amounts of irregular astigmatism are also seen in these patients.

Treatment

There have been numerous advancements in the treatment of post-LASIK ectasia.

Rigid gas permeable lenses can be worn to slow down or halt the process of ectasia, and they may delay the need for any surgical intervention.

Topical ocular antihypertensives have been used and act by relieving the biomechanical strain on the cornea.

Intacs (Addition Technology), or intrastromal corneal ring segments, are clear, microthin PMMA intracorneal inserts, hexagonal in cross-section. Intacs act by distending the peripheral cornea and hence flattening the central cornea; thicker segments produce a greater effect. For central ectasia, two segments can be inserted, and in cases of inferior keratectasia, irregular astigmatism can be corrected with a single Intacs segment placed at the site where corneal flattening is needed, that is, inferiorly or inferotemporally. The placement of a single Intacs segment prevents overcorrection of the myopia. The exact role of Intacs in slowing or halting the progression of ectasia is still not known. A unique characteristic of the Intacs refractive surgical procedure is its potential reversibility.

New bonds between adjacent collagen molecules are created by collagen cross-linking with riboflavin, or the C3-R treatment (Figure 3). This increases the stiffness of the cornea 1.5 times, making it less malleable. The procedure involves application of 20% riboflavin over the de-epithelialized cornea, followed by irradiation of the cornea with UVA light for 30 minutes. Cessation of keratectasia has been noted with an improvement in best corrected visual acuity and maximal keratometry values in about 50% of patients. The C3-R treatment can be combined with Intacs.

Deep anterior lamellar keratoplasty (Figure 4a-4c) is a new technique based on adding tissue to strengthen the cornea. Here, a host bed consisting of Descemet’s membrane and endothelium is created into which a full-thickness corneal stroma and epithelial button is placed. The recovery time is faster and visual recovery quicker than a penetrating keratoplasty. There is no risk of endothelial rejection.

Penetrating keratoplasty is the last resort for a patient with post-LASIK ectasia.

Collagen cross-linking with riboflavin (C3-R treatment). In this, an application of 20% riboflavin in dextran solution on the cornea is done, followed by irradiation of the cornea with UVA (365 to 370 nm, 3 mW/cm²) at a distance of 1 cm for 30 minutes.

Deep anterior lamellar keratoplasty. (4a) Injection of air bubble starting so that one can dissect until Descemet’s membrane.

Image: Pfeifer V

Anterior cornea removed. Only Descemet’s membrane and endothelium left behind.

Donor cornea placed on the recipient bed and sutured.

Conclusion

We see that post-LASIK keratectasia is a potentially devastating complication developing in a previously normal patient who has undergone an elective, usually cosmetic procedure. It is a preventable complication provided adequate care is taken.

For Your Information:

Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Group of Eye Hospitals, trains surgeons internationally in phacoemulsification, phakonit, LASIK and retinal lasers. Dr. Agarwal is author of several books published by SLACK, Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery, and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.

References:

Agarwal A. Refractive Surgery Nightmares. Thorofare, NJ: Slack Incorporated; in press.

Agarwal S, Agarwal At, Agarwal A. Textbook on Corneal Topography. India: Jaypee Brothers Medical Publishers; 2005.

Agarwal A. Handbook of Ophthalmology. Thorofare, NJ: Slack Incorporated; 2006.