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Corneal ectasia

Ectasia is a known risk of laser vision correction. It is mostly encountered after LASIK surgery for myopia and compound myopic astigmatism. Ectasia, also called keratectasia or “post LASIK ectasia” is characterized by a progressive deformation of the cornea which is very similar to that of keratoconus, leading to irregular astigmatism along with progressive central thinning.

It is very rare, and estimations for its incidence vary from 1/2000 to 1/5000 cases.

In the case of keratectasia, the patient will experiment a deterioration of its uncorrected vision, due to the reoccurrence of myopia, astigmatism, and high order aberrations.

In the vast majority of the cases, the occurrence of ectasia can be retrospectively explained (or at least not considered as a fully unexplainable outcome without gray zone) by a conjunction of risk factors, which had not been properly  taken into account by the surgeon. However, this does not mean that ectatic complications after refractive surgery must be blamed on poor preoperative screening. Preoperatively, refractive surgery candidates exhibit continuum of clinical and topography findings that range from “normal” to the “clearly pathologic” cornea. Between these two extremes, it remains difficult to predict which patients would develop this dreaded complication. This explains why some patients were proposed LASIK despite minor abnormalities that were judged compatible with the realization of this technique.

Identified risk factors for post LASIK ectasia can be divided into two categories.

The first category corresponds to factors related to a possibility of underlying early undetected form of keratoconus. Factors predisposing to overlook such conditions include age (the lower, the higher the risk), and certain corneal topography features that are frequently associated with keratoconus “forme fruste”: asymmetry, irregularity, and central thinning. Hence, proper interpretation of corneal topography is key. The use of automated screening tool is encouraged, but there are no perfect (100% sensitive and specific) algorithm. Placido based specular topography is sensitive, but does not suffice to explore the corneal shape and thickness. The use of posterior elevation, and tomography (full cornea thickness maps) should be part of every LASIK candidate evaluation. Abnormal topography is the main risk factor for post LASIK ectasia.

The second category comprises risk factors that relate to the risk of excessive “weakening” of the corneal wall: high myopia, thick flaps, “thin and/or weak” cornea. When these factors are present of associated, there is a risk of induction of a “low” residual bed thickness (RBT). The RBT is equal to the preoperative corneal thickness value (the minimum value) minus the sum of the thickness flap and the depth of the profile of ablation. Traditionally, the minimal residual bed thickness value has been set to 250 microns (300 microns for conservative surgeons). The author of this website would add to this list chronic “eye rubbing”, which seems an underestimated risk factor of keratoconus, and ectasia in general (be it post LASIK or “spontaneous”). Explaining to each operated patient that vigorous eye rubbing should be stopped is mandatory. Patients suffering from allergies tend to rub their eyes several times a day, sometimes using their knuckles, and this may cause progressive corneal weakening by disorganizing the structure and arrangement of collagen fibers.

The risk of ectasia may peak when factors belonging to category 1 and 2 are combined: high myopic correction on an inherently weaker cornea.

 

Despite this very low incidence when proper screening is performed, it is important to learn how to prevent ectasia. This can be achieved by taking several actions. Developing tools to improve the senstitivity and specificity of forme fruste keratoconus detection is valuable for the LASIK-surgeon community. Recently, we have developed a new automated screening tool name “SCORE Analyzer” for the Orbscan topograph. Currently, we orient our efforts to adapt this method to other corneal topographers.

Another way to teach and learn on how to prevent ectasia is to share and analyze previous documented cases of this affection. From the inspection of topography maps, in the light of the clinical context, one may become more familiar with the traits of corneas that my fall into the “at risk of ectasia” category. The goal of this page and subpages where it links to is to share some documented cases of keratectasia, to better learn about this complication, and subsequently reduce further its very low incidence

Each report of ectasia brings some interesting lessons to learn. It is important to bear in mind that the first report of corneal ectasia was published in the medical literature in 1998. Over time, increased awareness from the medical community of this severe complication led to better identify its risks factors… It remains important to collect and analyze data obtained from ectasia cases, to become increasingly familiar with the topographic patterns and clinical features that may predispose to this rare complication.

 

References

Seiler T1, Quurke AW.Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg. 1998;24(7):1007-9.

Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery.Ophthalmology. 2008;115(1):37-50

Saad A, Gatinel D. Topographic and tomographic properties of forme fruste keratoconus corneas.Invest Ophthalmol Vis Sci. 2010;51(11):5546-55

 

 

 

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