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Up Front | Jul 2006

Keratectasia After LASIK

Corneal ectasia (Figure 1) — often inadvertently induced by the surgeon — is a well-recognized complication following ablative refractive surgery that may not be seen until months after the original procedure. Common causes of iatrogenic keratectasia preceeding LASIK include (1) the presence of undiagnosed forme fruste keratoconus and (2) removal of excessive posterior stromal tissue.

The anterior cornea is the strongest and stress-bearing layer of the cornea; it boasts an interwoven structure that is more complicated than the deeper stroma. Because LASIK flaps (Figure 2) are created in the anterior cornea, biomechanical stability is compromised. Not only does the anterior corneal layer weaken, but the entire cornea also weakens. In return, normal IOP becomes unbearable for the cornea, and outward corneal bulging at the weakest area occurs. Once initiated, this process cannot be reversed. Clinical presentation of keratectasia (ie, progressively increasing myopia, irregular astigmatism, fluctuating refraction, difficulties in scotopic vision, glare, halos, ghosting of images and loss of BCVA) may not be noticeable until weeks, months or possibly years after surgery.

The first step in LASIK surgery is determining if the patient is a good candidate. Using topography (ie, a 3-D slit scan system such as the Orbscan [Bausch & Lomb, Rochester, New York]) during the preoperative examination identifies corneal thickness, posterior and anterior surface and predicts the shape of the cornea after LASIK. It is also used to screen for contraindications to LASIK.

WHAT CAUSES KERATECTASIA?
There are several factors that predispose a patient to this condition. They include decreased residual bed thickness (ie, <250 µm), contributing to the biomechanical stability of the cornea;1-3 dandelion keratectasia (ie, increased myopia after an operation); ablation diameter <5.5 mm, whereas larger ablation diameters result in more tissue removal and thinner residual bed thickness; and laser ablation depth, resulting from drying of the stromal bed.4

Patients with high myopia (≥ -8.00 D), high astigmatism, thin corneas (<500 µm) and primary posterior corneal elevation are also at greater risk for post-LASIK ectasia. Further consideration should be given to patients with forme fruste keratoconus and a family history of keratoconus. Additional parameters for consideration are age, attempted correction, the optical zone diameter and the flap thickness.5,6

KERATECTASIA DIAGNOSIS
To detect mild keratectasia, one must understand the posterior curvature of the cornea. Posterior corneal surface topographic changes occur after LASIK (Figures 3 and 4); increased negative keratometric diopters and oblate asphericity of the posterior corneal curvature, correlating with the intended correction, may lead to mild keratectasia.4,7

Presence of post-LASIK ectasia on topography is a central or paracentral area of steepening. This area will progressively worsen on follow-up evaluations and is associated with a decreased pachymetry in the area of steepening. The earliest change — detected on the posterior corneal surface — is posterior corneal bulging, and an eccentric posterior bulge below the center of the laser ablated area is most threatening.8

POST-LASIK TREATMENTS
Rigid gas permeable (RGP) lenses. RGP lenses, which slow down or halt ectasia (Figure 5), are a safe and reversible option for ectatic corneas. Their use may delay surgical intervention.9 Other options are nonventilated modern gas permeable scleral lenses10 and multicurve and reverse geometry lenses11 for extreme cases of kerataconus.

Topical ocular antihypertensives. These agents relieve biomechanical strain on the cornea, particularly in patients with early ectasia. The same relief is not seen in patients with long-standing ectasia. Hiatt et al12 hypothesized that the cornea may be malleable during early and active ectasia and therefore amenable to flattening from IOP reduction.

Intrastromal corneal rings.
Intacs (Addition Tech-nology, Des Plaines, Illinois), clear microthin PMMA intracorneal inserts, have a hexagonal cross-section and an arc length of 150º. They work by distending the peripheral cornea (ie, flattening the central cornea). Two positioning holes are used to implant the device (angulation 31º), following the corneal curvature. The ring segments are placed at two-thirds corneal depth (Figure 6). A single Intacs segment, placed at the site for corneal flattening (ie, inferiorly or inferotemporally),13,14 prevents overcorrection of a myopic patient as well as corrects irregular astigmatism in inferior keratectasia. Two segments may be inserted in patients with central ectasia.

It is unknown how Intacs slow or halt ectasia progression: Two-year follow-up is available from 393 patients (ie, 363 keratoconus and 30 postsurgical ectasia) using the steepest refractive axis incision technique.15 All mild and 55% of moderate-to-severe cases achieved UCVA of ≥20/40. All patients gained lines of vision, and patients with stage three keratoconus achieved maximum benefit. Martiz et al16 placed two intracorneal ring segments in 25 keratoconus patients (40 eyes). Improvements in BCVA and UCVA (ie, two to nine lines of vision), contact lens tolerance and irregular astigmatism were noted. See article on page 45 for more information on Intacs.

Ferrara rings. This device produces a prismatic effect (ie, directs reflected light away from the visual pathway) because of its triangular cross section.17 Halos and edge glare are prevented, and there is a smaller optical zone compared with Intacs. Ferrara intrastromal corneal rings reduce corneal steepening, normalize the central cornea in eyes with keratoconus17 and may likely be an effective treatment for post-LASIK ectasia.

Collagen cross-linking with riboflavin (C3-riboflavin). C3-riboflavin remodels the cornea by creating bonds between adjacent collagen molecules and increasing the stiffness of the cornea (1.5 times). The process is as follows: the epithelium is removed; 20% riboflavin is placed on the cornea; and the cornea is irraditated with UVA light (365 to 370 nm, 3 mW/cm2) at a distance of 1 cm for 30 min (Figure 7).

Approximately 50% of patients achieve an improved BCVA, maximum keratometry values and cessation of keratectasia.18 This treatment has been combined with single-segment Intacs, and the results have been promising.19 For more information on C3-riboflavin, see article on page 49.

Deep anterior lamellar keratoplasty (DALK). This new technique, although it requires further study, appears to boast faster and quicker visual recovery times versus penetrating keratoplasty (PK). By placing a full-thickness corneal stroma and epithelial button into a host bed (ie, Descemet's membrane and endothelium) (Figure 8), the cornea is strengthened, and there is no risk of endothelial rejection.

PK. This invasive procedure (Figure 9) should be the last resort for post-LASIK ectasia because it interrupts the structural and immunological integrity of the eye. PK is associated with long rehabilitation and donor cornea rejection.

Cornea-sparing LASIK. Applying part or all of the treatment on the undersurface of the flap may also decrease the incidence of ectasia. It is useful in patients with thin corneas, high power or in enhancement procedures where flap thickness is unknown. Mark the center of the pupil and cut the flap, reflecting it backward onto a dome-shaped flap-holding instrument. Do not breach Bowman's membrane; it may lead to haze. Take the eye tracker off and apply only the spherical treatment to the undersurface.

PREVENTION
Preoperative anterior and posterior corneal evaluation as well as corneal pachymetry should be performed. Use the Orbscan to diagnose LASIK contraindications including thin cornea and forme fruste keratoconus. Furthermore, intraoperative pachymetry to measure flap thickness and posterior stromal thickness before and after ablation is important to avoid ectasia. Continuous noncontact corneal pachymetry with a high-speed reflectometer20-22 has been proposed, and intraoperative real-time optical coherence pachymetry23 may be an important safety mechanism. Real-time pachymetry should become a routine step in all LASIK procedures.

Amar Agarwal, FRCS, FRCOphth, MS, practices at Dr. Agarwal's Eye Hospital, in Chennai, India. Dr. Agarwal states that he has no financial interest in the companies or products mentioned. He may be reached at dragarwal@vsnl.com.

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