The popularity of LASIK—and the number of surgeons performing LASIK—continues to flourish, despite a slight recent decline. Likewise, the range of indications for LASIK has expanded and also become more specific. The thickness of the residual corneal bed is directly related to the magnitude of the ametropia corrected. Therefore, the treatment of high myopia produces the greatest change in the corneal biostructure.1
Irreversible alterations and changes in the equilibrium of the corneal biostructure following LASIK can lead to corneal ectasia, which is described as a topographic change with a steeper slope that develops in conjunction with a drastic reduction in visual acuity. What factors contribute to the development of corneal ectasia following LASIK surgery? It has been reported that the risk for ectasia increases as the amount of ablation increases.
Even now, years after the introduction of LASIK, corneal ectasia is still the most feared complication related to this refractive procedure. Finding and analyzing the most influential variables implicated in the development of ectasia may help us to decrease its incidence. The portion of corneal tissue containing the flap does not fully reintegrate into the total corneal structure, and therefore, the structural resistance of the cornea is reduced to that of the residual corneal bed. Hence, it is important to establish a minimum residual bed thickness and further explore other parameters that may intervene in postoperative corneal stress. Keeping this in mind, pachymetry is mandatory when considering LASIK for high myopia.
Authors have proposed a variety of risk factors for post-LASIK corneal ectasia,2-19 including a high degree of tissue ablation, thin residual stromal bed, low preoperative pachymetry, and asymmetric astigmatism. In terms of quality of influence, asymmetric topography is a significant factor. Isolated keratometry values are not consistently useful as a screening criterion; however, abnormal topography with an inferior-superior difference value more than 1.40 D represents an effective screening parameter.20-22
There are many unanswered questions regarding specific causes of corneal ectasia. For instance, what influence does the optical zone have on the development of corneal ectasia? Does intraocular pressure (IOP) play a role in the process of ectasia? The diameter of the optical zone and the IOP have not been considered consistently as a variable in the development of this complication. Therefore, we developed a computerized simulation study7 and employed a finite element mathematical corneal model (Table 1).8
We analyzed a computerized simulation of the cornea (Figure 1). and upon simulating the elasticity of collagen fiber and its resistance to changes in force vectors (ie, ablation), we observed that modifications of these properties increased as the size of the optical zone decreased.8 When we introduced IOP as a variable, the displacement produced in an eye with an IOP of 21 mm Hg was 70% higher than that produced in an eye with an IOP of 12 mm Hg.8
To prevent the development of this unfortunate complication, we must carefully examine preoperative corneal topographies and use superficial ablation (if required) to preserve at least 280 µm of residual stromal bed as frequently as possible23 and employ moderate to large optical zone diameters. It is helpful to apply antiglaucomatous drugs, such as timolol, latanoprost, brimonidine, and dorzolamide, after the diagnosis of corneal ectasia.24
Post-LASIK corneal ectasia is a potentially serious complication that has, unfortunately, become a frequent occurrence. In decreasing order of influence, the development of ectasia is closely bound to preoperative abnormal topography, thin pachymetry, and residual corneal bed thickness, the last of which is related to an excessive amount of ablation in terms of biomechanic instability. Residual stromal bed thickness is an essential factor because the posterior and anterior parts of the stroma differ structurally. If LASIK affects the corneal structure, posterior keratocyte density is reduced.20
It is crucial to perform intraoperative pachymetry, at least in borderline cases at risk of ectasia. Factors for the surgeon to consider for the prevention of corneal ectasia include high IOP values and a small diameter optical zone. Other considerations are family history of corneal degeneration, any eye-rubbing habits, and if the patient's BCVA is less than 20/20.
The principal approaches to managing corneal ectasia include optical restoration, treatment of irregular astigmatism, and surgical restitution of the cornea's tectonic integrity. Using rigid gas permeable contact lenses allows a nonsurgical reinstatement of quality vision. In our experience, most cases of corneal ectasia can be managed in this fashion; however, a diversity of other treatments have appeared, including corneal collagen crosslinking, intrastromal rings, and lamellar keratoplasty.
In the presence of post-LASIK keratectasia, the use of ultraviolet-A riboflavin collagen crosslinking helps to freeze progression. Alternatively, intrastromal segments provide a reversible surgical approach with acceptable outcomes in moderate cases of corneal ectasia. For severe cases, keratoplasty remains the best option. Lamellar keratoplasty with an automated microkeratome and use of excimer and femtosecond lasers shows promising visual and refractive outcomes.25-28 Manual lamellar techniques permit complete replacement of corneal tissue; however, they depend significantly on surgical skills. For patients with difficult astigmatism, a peripheral lamellar procedure that incorporates a compressive C-shaped lamellar graft may be a viable option.29,30
It seems that no one feature can be used to detect all candidates who are at risk for corneal ectasia after LASIK. As a general rule, special consideration should be placed on topographic analysis. Reports and personal experience have shown that even in cases without excessive ablation or thin residual bed, corneal ectasia may still develop.19 Intraoperative pachymetry must be performed in cases when a borderline residual bed thickness is expected. When considering the indication for refractive surgery, the patient's optical zone and IOP may be taken into account as risk factors for ectasia.
If the variables mentioned in this article are considered, corneal ectasia may be avoidable.
María José Cosentino, MD, is the Associate Medical Director, Instituto de la Vision, the Vice President of the Asociación Latina de Catarata, Refractiva & Segmento Anterior (ALCRS) and the Vice President of the Sociedad Argentina de Córnea, Refractiva y Catarata (SACRYC). Dr. Cosentino states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +54 11 4827 7900; e-mail: majocosentino@fibertel.com.ar.
