Risk Factors for Ectasia after Laser In Situ Keratomileusis

Significant strides have been made recently to preoperatively identify patients who are most at risk of developing corneal ectasia after laser in situ keratomileusis (LASIK) and to avoid surgery in these patients. This is imperative, because post-LASIK ectasia can cause progressive corneal steepening inferiorly, resulting in increased myopia and astigmatism, loss of uncorrected visual acuity, and often loss of best corrected visual acuity (BCVA) that can present days to years after uncomplicated LASIK. The specific mechanisms resulting in extreme corneal warpage postoperatively remain undetermined, but better understanding of biomechanical factors such as corneal plasticity and viscoelasticity (J Refract Surg. 2005;21:186-190) and corneal parameters such as Young’s Modulus, Poisson’s ratio, and curvature radius (J Refract Surg. 2005;21:176-185), may provide insight in the future. This article will review known risk factors for ectasia after LASIK and discuss current and future strategies for avoiding this complication.

Known Risks

Since the first reports by Seiler and colleagues in 1998 (J Refract Surg. 1998;14: 312-317 & J Cataract Refract Surg. 1998;24:1007-1009) corneal ectasia has remained a serious concern after LASIK. The actual incidence of this condition remains undetermined, but it is probably less than 1 in 2,000 uncomplicated cases. Specific patient characteristics, including high myopia, low preoperative corneal thickness, low residual stromal bed (RSB) thickness, and preoperative topographic abnormalities, definitely increase the risk of postoperative ectasia (Ophthalmology 2003;110:267-275). Nevertheless, some patients have been known to develop ectasia without such recognized preoperative risk factors (J Cataract Refract Surg. 2000;26:967-977 & Cornea. 2006;25:388-403).

Treatment of high myopia with LASIK naturally results in ablation into deeper layers of the cornea than LASIK for more moderate prescriptions. Therefore, patients treated for high myopia generally have lower RSB thickness after appropriate tissue ablation than their low myopia counterparts. Consequently, RSB and high myopia may be interrelated rather than independent risk factors for ectasia.

In addition to high myopia, factors contributing to low postoperative RSB include excessive flap thickness, deeper than expected stromal ablations, and multiple LASIK enhancements. There can be significant variability in the measurement of corneal thickness, flap thickness, and ablation depth measurements, and while most microkeratome plate markings overestimate actual flap thickness, excessively thick flaps still occur with regularity. Thicker than expected flaps can also occur with femtosecond laser flap creation (J Refract Surg. 2006; 22:556-561). Therefore, the single best way to avoid laser ablation after thick flap creation is to routinely measure RSB intraoperatively.

Corneal measurements taken months after initial LASIK usually overestimate RSB thickness at the time of LASIK retreatment (J Refract Surg. 2003;19:113-123 & Ophthalmology. 2005;112:98-103). Since accurate assessment of actual RSB thickness is crucial to avoid excessive ablation of the posterior stroma prior to performing LASIK enhancements, physicians should utilize confocal microscopy prior to surgery or intraoperative pachymetry measurements prior to laser ablation. The confocal microscope is ideal, as it allows physicians to accurately measure RSB thickness without ever lifting the flap (J Refract Surg. 2002;18:S378-381).

In general, RSB thickness less than 250 microns is a risk factor for post-LASIK ectasia. However, many patients with RSB less than 250 micorns have stable outcomes, while numerous ectasia cases have been reported with much thicker calculated RSBs. This suggests that RSB measurements may not be the most important predictor of post-LASIK ectasia.

Topographic Abnormalities

Most authors recognize preoperative topographic abnormalities as uniquely indicative of increased risk for post-LASIK ectasia. However, the definition of “abnormal” remains a source of great debate. Patients with early keratoconus, forme fruste kleratoconus, or pellucid marginal corneal degeneration are all definitely at significantly increased risk. The significance of more subtle topographic abnormalities, such as asymmetric bowtie patterns, mild inferior steepening, or isolated increased posterior float values, remains to be determined. Based on an extensive review of the literature, the members of the joint committee of the American Academy of Ophthalmology (AAO), International Society of Refractive Surgery (ISRS), and American Society of Cataract and Refractive Surgery (ASCRS) recommend avoiding LASIK in patients with asymmetric, inferior corneal steepening or asymmetric, bowtie patterns with skewed steep radial axes above and below the horizontal meridian (J Cataract Refract Surg. 2005;31:2035-2038).

Age and Other Potential Risk Factors

Some recently reported cases of ectasia in very young patients have raised questions as to whether age may be a factor to consider during preoperative planning. Theoretically, if LASIK is performed at younger ages, keratoconus suspects may not be diagnosed, because they have yet to manifest topographic abnormalities. According to Klein and colleagues, patients who developed ectasia without recognized risk factors were significantly younger than those in previously published reports of corneal ectasia with identifiable risk factors (Cornea. 2006;25:388-403). Other potential risk factors include a family history of keratoconus, variable or unstable refractions, or increasing astigmatism over time.

Although most reported cases of post-LASIK ectasia involve patients with at least 1 of the aforementioned risk factors, there have been patients who developed this condition without any these risk factors (J Cataract Refract Surg. 2000;26:967-977 & Cornea. 2006;25:388-403). The position of the AAO/ISRS/ASCRS joint committee is that no single risk factor stands alone as an absolute predictor of post-LASIK ectasia (J Cataract Refract Surg. 2005;31:2035-2038).

Future Considerations

As practice patterns evolve and at-risk characteristics are better defined, borderline patients, especially those with high myopia, thinner corneas, or subtle topographic abnormalities, should be presented surgical options other than LASIK such as surface ablation or phakic intraocular lens (IOL) implantation.

Furthermore, newer screening techniques like corneal interferometry, corneal hysteresis measurements, and dynamic corneal imaging may allow identification of at-risk patients with normal topographies but reduced biomechanical integrity preoperatively, thus allowing the surgeon to avoid operating on these patients. This will be an improvement upon current screening strategies that rely on relatively late indicators of reduced corneal integrity, namely topographic evidence of corneal weakness.

In conclusion, future alterations in practice patterns and technological advancements should improve recognition of at-risk patients and prevent the development of post-LASIK ectasia. In the meantime, surgeons should carefully analyze preoperative topographies, measure intraoperative pachymetry to avoid ablation after excessively thick flaps, and utilize surface ablation, phakic IOLs, or avoid surgery altogether for borderline LASIK candidates. In cases where cornea ectasia has occurred, visual rehabilitation may be achieved via rigid gas permeable contact lens fitting, intracorneal ring segment placement, or in extreme cases penetrating keratoplasty.

References

Author Disclosure

The author states that he has no financial relationship with the manufacturer or provider of any product or service discussed in this article, or with the manufacturer or provider of any competing product or service.