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Corneal ectasia

Corneal ectasia: introduction

The induced corneal ectasia (or iatrogenic Keratoconus) is a rare complication of the LASIK that brings into play the Visual function. His sometimes insidious and delayed start in time, its irreversible character and its pejorative prognosis (she can motivate the indication of a corneal transplant) are all characteristics that make a dreaded complication, and give an opposing or even pernicious dimension.

The main risk of the ectasia factor is the presence of a form not detected in Beginner infra clinical Keratoconus at the level of the operated cornea. As for Keratoconus, the notion of Repeated eye rubbing is also a risk factor, the friction that led to reduction of the corneal resistance before the intervention, and in the course of it if the friction were pursued.

The corneal topography is a essential review to eliminate the presence of what is referred as a very beginner of Keratoconus infra clinical form, whose diagnosis is based on the identification of asymmetrical changes in corneal curvature, and the use ofautomated for Keratoconus screening indices.

According to the author of this site, the notion of 'Keratoconus fruste' might be a topographical picture dominated by the presence of corneal irregularities induced by repeated mechanical stress that constitute the eye rubbing. Whatever the cause, this table represents a contraindication to the realization of LASIK and also the procedures of type SMILE. Indeed, these topographical anomalies appear to reflect the reduction of the biomechanical resistance of the cornea. In this context, and for low and medium myopia, surface techniques (PKR) can however be made in some case.
This page concerns the main clinical and topographical features of this complication: Particular attention will be given to the study of the factors that promote the prevention.

Diagnosis of corneal ectasia

Definition of the corneal ectasia

The induced corneal ectasia can be defined as the apparition within a variable period after completion of a procedure LASIK or SMILE of a topographic of evolutionary Keratoconus and clinical picture. The cornea is a progressive deformation with central arch and irregularity accented.

OCT in sagittal cut of a cornea with a post LASIK ectasia. Note the Central deformation with pronounced camber irregular. The thickness of the flap is estimated at an average of 120 microns. At the Center, the thickness of the flap seems to be reduced, and the total thickness of the cornea is estimated close to 350 microns.

OCT in sagittal cut of a cornea with a post LASIK ectasia. Note the Central deformation with pronounced camber irregular. The thickness of the flap is estimated at an average of 120 microns. At the Center, the thickness of the flap seems to be reduced, and the total thickness of the cornea is estimated close to 350 microns.

-Clinical signs of this complication associate a progressive deterioration of Visual function with the re-appearance of myopia and astigmatism (often oblique direction astigmatism), lower acuity uncorrected Visual, then loss of best lines Visual acuity corrected (MAVC): even with correction glasses, Visual acuity remains less than 10/10. The occurrence of an ectasia may occur some weeks after LASIK, sometimes a few years.
– The specular corneal topography (curvature) reveals the appearance of a area of arch High usually localized in the half lower part of the anterior corneal surface. The appearance of an irregular curvature after refractive surgery, corneal must always be thinking about the possibility of an ectasia. The ectasia is often confused with a greater shift.
– The elevation topography underscores an asymmetric distribution of the elevation of the anterior face of the cornea and allows to locate the top of the ectasia which is most often located in the lower central or paracentral region. The elevation of the posterior face vis-à-vis its calculated reference sphere is often highly accentuated in relation to the minimum pachymetry zone.
– The optical pachymetry allows the identification of the coordinates and the value of the minimum thickness point of the residual corneal wall. This is generally low, and its location depends on the initial position of the most fine point and the centering of the ablation profile. It is possible that inflammatory mechanisms induce a decrease in the number of collagen fibers during the occurrence of a iatrogenic ectasia of the cornea, and accentuate the thinning of the cornea.

Ectasia appeared two years after the release of a LASIK patient of 36, for a myopia of about 6 D. In preoperative, there is an irregular astigmatism of the right eye, with elevation marked to the back. It was an atopic ground with notion of digital eye rubbing repeated. The post operative map reveals an aspect of marked central thinning, with asymmetrical deformation of the front, and pronounced rear elevation

Ectasia appeared two years after the release of a LASIK patient of 36, for a myopia of about 6 D. In preoperative, there is an irregular astigmatism of the right eye, with elevation marked to the back. It was an atopic ground with notion of digital eye rubbing repeated. The post operative map reveals an aspect of marked central thinning, with asymmetrical deformation of the front, and pronounced rear elevation

-Aberrometrique review says the rise in the rate of optical aberrations of high degree. The asymmetric corneal deformity causes an increase of odd optical aberrations (coma, trefoil). The rate of these aberrations increases exponentially with the pupillary diameter, and these may be responsible for a decrease of the contrast sensitivity, then a reduction of the MAVC. Similarly, the feeling of a "double vision" at the level of the eye reached is frequent.

It should be noted that none of these signs are specific of the induced corneal ectasia (see differential diagnosis). Their conjunction and their worsening/increase over time After a LASIK is however very evocative, and eliminates the presence of a shift.

Causes and risk factors for corneal ectasia

A review of the literature concerning the ectasia post-LASIK corneal was published recently (1). Eighty-five case of post LASIK ectasia were analyzed. At the end of this analysis, the presence of a fruste Keratoconus or turned out was proven in about 28 case, undocumented in 18 case, and regarded as absent in the remaining case 39.

Apart from the possible existence of factor (s) risk (s) currently unidentified, two main etiologies are conventionally invoked to explain the occurrence of an ectasia secondary in patients operated in LASIK: theexcessive corneal posterior wall thinning in normal subjects, or the presence of a subclinical keratoconus unidentified among the case considered normal (2). In these two situations, the ectasia occurs by excessive weakening of the corneal wall with Biomechanics decompensation and/or at the end of the evolution of a progressive disease process accelerated by LASIK (3).
In fact, the study of preoperative characteristics of patients with Ectasia after LASIK reveals the presence of more pronounced myopia and astigmatism than in the free subjects, as well as the calculation of a finer thickness of the wall posterior residual.

Mechanism of corneal ectasia

If it makes sense to apply that the biomechanical behavior of a cut on the surface then thinned surgically cornea may differ from that of a Virgin and healthy cornea on the surgical plan, the elements involved in the control of corneal Biomechanics and the process leading to the appearance of the ectasia are certainly not all identified.

The study of corneal ectasia physio-pathological mechanism is paradoxically being few experimental studies, which would be necessary in order to clarify the influence of pre operating parameters such as average keratometry, the modulus of elasticity of the cornea (Young module), or the eye pressure. A study on histology a corneal button trepanated before keratoplasty keratoplasty has highlighted the presence of macro-stries at the level of the stroma posterior (4).

The cornea is a shell which is subject to a force exerted on its posterior surface by pressure that prevails inside the eye, and its biomechanical properties allow it to keep a harmonious shape in physiological conditions. It is recommended, on the basis of empirical data, not thin excessively in LASIK cornea and preserve a wall residual posterior stromal of at least 250 microns, or still do not induce a posterior residual wall less than half the original corneal thickness, to avoid a Biomechanics decompensation and the ectasia.

SMILE technique, originally positioned as less likely to cause an ectasia, seems to be avoided in case of doubt. case of ectasia post SMILE have been reported, occurring for corneas with a topgoraphiques in preoperative atypies. The communication issued around the SMILE emphasizes the absence of flap cutting but often obscures the necessary removal of a lens in depth, depth also often more extensive than in LASIK where it is possible to achieve purposes flaps.

 

Differential diagnosis of corneal ectasia

The trap main diagnosis is the decentering of the photoablation, topographic signs may be neighbours, but the terms of support are obviously different from that of the induced ectasia (where any supplementary photoablation after re-uprising of the flap is of course prohibited) (Figures 1-3).

Frequently, the ectasia is observed after "editing" for a correction, or what was diagnosed as a shift.

Any shift or under correction - rapid regression of the effect of the correction after LASIK should be considered an ectasia until proof to the contrary.

KC1

Figure 1: Orbscan (Quad Map view) pre operative, Œil left: presence of a Keratoconus fruste

KC2

Figure 2: map postoperative Orbscan, seven days after LASIK (Hansatome ring 9.5 / head 180, photoablation:-5.75 D on optical zone of 5.5 mm / depth of ablation maximum: 84 microns, Technolas 217 Laser)

KC3

Figure 3: map Orbscan conducted after surgical for supposed shift: resoulevement and issuance of a supplementary photoablation. Note the emphasis of the corneal asymmetry and the accentuation of the ectasia.

Comments:

This case of induced ectasia occurred in a patient of 42 operated in LASIK for myopia average is particularly illustrative. He presented an aspect of Keratoconus subclinical, undiagnosed before surgery. The accentuation of the initial asymmetry (who translated the 'biomechanical weakness' preoperative linked to beginner Keratoconus) after surgery has been regarded wrongly as a greater tilt of the photoablation. It is likely that a rear wall of at least 250 microns was respected in this case, the being the consequence of excessive weakening of the wall of corneal ectasia. In our experience, the ectasia resulting from the cutting of 'fragile' corneas comes quickly after LASIK, as excessive thinning of an initially normal cornea occurs later.
On pre-operative topographic map (Figure 1), we note the asymmetry present at the anterior face, which is responsible for asymmetric astigmatism in map axial specular, and an asymmetric distribution of The elevation. The scale used on the specular map is however too wide and "crushes" the curvature variations of the anterior face of the cornea, which blurs the asymmetric aspect of the vertical hourglass.
The appearance of the posterior elevation map is particularly suspicious (increased and downward shifted elevation vis-à-vis the calculated reference sphere). Finally, if the central mean pachymetry has a normal value (541 microns), there is a point of minimum thickness (thinnest point) measured at 518 microns and the location of which is shifted below. All these signs retrospectively evoke the presence of an infra-clinical keratoconus (keratoconus fruste see keratoconus suspect). The patient was however considered fit for a refractory LASIK correction by his surgeon.

It is important to consider that any topographic irregularity observed after LASIK done without problem should eliminate the presence of a debutante corneal ectasia.

case of diagnostic doubt, spontaneous evolution allows to decide. case beginner ectasia, she is always towards theprogressive worseningWhile scar corneal remodeling will often reduce topographic the decentering and clinical symptoms. The evidence provided by the topographical examinations and aberrometrique are particularly useful in this context because they allow monitoring and the comparison of quantitative objectives.

It is crucial to keep in mind that any regression after LASIK correction refractive effect must do eliminate the diagnosis of ectasia induced. I have observed in many case where a beginning of ectasia, wrongly considered one under correction or a shift, did ask the indication of an editor that has unfortunately accelerated the ectasia.  The appearance of a myopic astigmatism of oblique direction is particularly evocative of ectasia, especially when it tends to worsen. Aberrometrique reveals a gradual rise in the rate of coma and trefoil. The realization of control repeated over time with study of differential maps allows to confirm or disprove the diagnosis.

Treatment of the ectasia

Therapeutic support to an ectasia depends on its severity and its impact on the Visual function.

Eye rubbing is crucial.

The optical correction caused by the asymmetric corneal deformation is best done by adaptation of a rigid contact lens When this is possible (5,6). Intra-stromaux rings segments laying, initially proposed for the treatment of Keratoconus (7), also represents an interesting alternative to transplantation of cornea in case of ectasia induced (8). Limiting the placement of sutures between the flap, the posterior stroma and the ring could help in some case to limit the development of the ectasia (9).

The realization of techniques of cross linking (polymerization of the corneal stroma by application of riboflavin A and exposure to UVA) is a technique whose beneficial effects remains to be demonstrated, especially in the case of the ectasia after LASIK. Claimed stabilization is difficult to characterize, in a context or the realization of the CXL changes the topography of the cornea (epithelial regrowth), and that the judgment of the eye rubbing is in itself an effective way to not to increase the deformation of the cornea.

The insertion of segments intra corneal (rings) allows a certain Visual rehabilitation for some patients. The failure of the previously mentioned methods led to consider the realization of a corneal transplant, keratoplasty, or lamellar deep, whose details and prognosis are similar to that of primitive Keratoconus.

Prevention of the corneal ectasia, LASIK-induced

Treatment options are currently limited in terms of induced ectasia prevention of this complication has importance. It is based on two pillars: the respect of a residual posterior wall of sufficient residual thickness, and ESPECIALLY the exclusion of the corneas 'at risk '. Furthermore, it is important to explain to the patient that should vigorously rub their eyes under any circumstances after corneal refractive surgery; the eye rubbing repeated favour the onset and evolution of Keratoconus and corneal ectasia.

See: Detection of early forms of Keratoconus (Keratoconus frustrated)

Limit the thinning of the rear wall in LASIK:

The determination of the minimum value of the thickness of the wall to preserve residual stromal is not determined with certainty. Different values have been proposed, between 200 and 300 microns, or equal to half of the original central product. A consensus around a value of 250 microns seems to be for stromal thickness after preserve (1). In fact, this value depends on many factors and is certainly specific for each patient (10).
The calculation of the residual posterior wall and the choice of a limit not to be crossed concern only subjects free of pre-existing corneal biomechanical fragility.

 

ectasia insufficient posterior residual wall

Image cut OCT of the corneal wall of an affected eye of ectasia after LASIK. On the outskirts, the thickness of the flap is measured at 144 Micron, while the thickness of the cornea at the Center is 306 microns. The posterior residual wall is estimated at about 150 microns, which is insufficient for the biomechanical balance of the corneal dome.

The existence of a Keratoconus fruste reflecting a structural fragility of the corneal wall, end same part is to be avoided as likely to train, with the issuance of the photoablation, biomechanics decompensation.
The theoretical calculation of the posterior residual wall thickness depends on the initial pachymetry, the predicted thickness of the flap, and the announced maximum depth of photoablation. However, there is a significant dispersion around the average predicted value of canopy thickness obtained with most first-generation mechanical microkératomes (Automated Corneal Shaper * – Bausch and Lomb, One *, Cariazo Barraquer * Manual and Electrical-Moria). The thickness of the corneal flap retained for the calculation of the posterior wall may significantly deviate from the thickness actually obtained after mechanical cutting, as demonstrated by several studies using a peroperative pachymétrique measurement (11.12 ) For example, flaps of a thickness equal to 220 microns could be obtained... for a expected thickness of 160 microns. An case of immediate ectasia after cutting an excessively thick flap without photoablation in the course of an aborted LASIK procedure (microkeratome mounting error) was even reported! 13)
Calculated from the predicted theoretical thickness of the flap, the residual posterior wall thickness could thus have been overestimated in some case of ectasia corneal without an "apparent" violation of the 250-micron rule. However, several LASIK case with real measurement of the thickness of the flap allowing the calculation with a posterior residual wall less than 250 microns were followed for several years without a ectasia to develop (14).

The reduction of dispersion around the Predictor of thickness and increased cutting regularity are part of benefits of the use of the femtosecond laser in LASIK. Indeed, the accuracy of the thickness of the cut by the femtosecond laser flap is better than that of the microkeratomes.

In practice, a pachymetrique intraoperative measurement is recommended when the theoretical calculation of the posterior wall provides a value close to the limit. It is a measure of the Central corneal thickness before and after cutting and uprising of the pane, the thick of it being calculated as the difference between the two measurements. The EX500 laser is equipped with a system to measure the thickness of the cornea in time real (OCT online).

If the obtained cut turns out to be thicker than expected, reduction of the optical zone scheduled for correction excimer can allow obtaining a residual wall enough through a reduction of the maximum depth of photablation (a reduction of about 30% is for example obtained by reducing the diameter of optical zone 6 to 5 mm for a same magnitude of treatment).

case sous-correction recovery, increased security could be obtained through the use of imaging techniques allowing fine visualization and measurement of the respective thicknesses of the different coats of the corneal wall. L ultrasonic ultrasound at high frequency, or the study in "Optical Coherence Tomography" (OCT3) of the anterior segment allow the direct estimation of the thickness of the component and the residual posterior wall obtained after the initial LASIK. This allows then to establish the maximum depth value of additional ablation allowed for a limit value of thickness of the residual rear wall data.

Forms frustes of Keratoconus detection and corneas 'at biomechanical risk for LASIK.

Beginner infra-clinic Keratoconus detection must be one daily concern in refractive surgery. The frequency of beginner infra clinical Keratoconus is higher in the population of myopic refractive surgery candidates (2 to 5%) than in the general population (0.05%), due to selection bias related to frequent intolerance to contact lenses and the poor quality of vision obtained by additive correction in this type of patient.
The diagnosis of the proven and evolved forms of keratoconus does not pose any problems in specular vidéokératoscopie.

On the other hand, It is more difficult to eliminate the presence of a fledgling form of Keratoconus, that diagnosis must recuse LASIK (10.15). Indeed, Keratoconus subclinical (Keratoconus which only corneal topography betrays the existence) corresponds to a 'fragile' corneal biomechanical implications and as already pointed out, the passage from a fruste shape to a proven form (ectasia) can be accelerated by the simple realization of a corneal flap cutting without laser photoablation.
To date, the precise limit between "normal" and "early stage keratoconus" remains blurred. Labeling "Keratoconus fruste" any cornea with topographic features such as moderate asymmetry or below-average thickness is certainly "semantic abuse". The association between eye rubbing, or prolonged compression of the cornea (s) during sleep (sleeping patients on the belly or side, the head in the pillow), and the "suspicious" forms of keratoconus is troubling.

We devote a significant part of our research work to prevent the ectasia post LASIK, and these efforts were recently rewarded by prizes international and incarnate in a clinical decision support software, using data from the corneal topography Orbscan IIz for better screening of debutantes () subclinical formsSCORE Analyzer). The following example of map "score" corresponds to the cornea having presented an above ectasia. The calculation of the score were made of course of retrospective material (the score did not exist at the time or that eye was operated).

score analyzer

Map 'Score analyzer', obtained retrospectively for a cornea submitting a post LASIK ectasia. A positive score means a degree of susceptibility to the ectasia to be reconsider the indication of a LASIK

We have recently proposed to clarify the terms used to describe the beginner Keratoconus: Keratoconus fruste, suspect Keratoconus Keratoconus subclinical, etc. We have contributed to "move the lines" with respect to the means to detect these abnormalities: despite the fact that screening current 'official' is based on the use of corneal topography earlier (Placido disk), we recently showed that the data collected at the level of the posterior and the product "point by point" are useful to better discriminate the early Keratoconus forms :

Saad, Gatinel D. Topographic and tomographic properties of form fruste keratoconus corneas. Invest invest Vis Sci. 2010 Nov; 51 (11): 5546-55. EPUB 2010 Jun 16.

(article download): Topographic and Tomographic properties of FFKC Saad Gatinel IOVS ).

Keratoconus: suspicious forms and frustes

To help surgeons to diagnose, there are software programs that analyze the topographic maps of the anterior face of the cornea. The term Keratoconus subclinical includes forms «» suspicious. ", which are detected as such by screening software automated by the use of indices (ex: Rabinowitz indices, indices of Klyce Maeda) and forms «» frustes", which are considered objectively 'normal' topographic 'classics' detection systems, but have some special features for an expert eye.

Observed from the point of view of the surgeon of myopia, this distinction has a more pragmatic dimension or dichotomous (installation of an indication of LASIK or abstention / either use a technique of surface as the PKR?), that diagnostic (establishment of a differential diagnosis between Keratoconus subclinical forms, minimal pellucid marginal degeneration or noninflammatory degenerative type Pathology) , assignment of a specific evolutionary grade, ect... ).

In this context, 'suspicious' forms cause no particular concern because they are by definition detected by automated detection systems (some forms "suspicious" are sometimes classified as such by excess: the consequences of this error of false positive classification are less serious than an error in false negative.) It is therefore crucial to better detect the Keratoconus "frustes." that is to say the earliest forms of infra clinical Keratoconus.
We will thus group under the label of "Keratoconus fruste" (Infra clinical beginner) The corneas to the intermediate topographic aspects between normality and keratoconus suspect. These are "biomechanically risky" corneas for LASIK, which often show in our experience only a few minor anomalies (moderate thinning, slight asymmetry). This approach is simplifying but based on the fact that the retrospective study of case Ectasia reveals the presence of these often discreet signs on preoperative topographic maps. Similarly, the careful study of the eye judged as "normal" in patients with primitive keratoconus ^ "unilateral" is used to detect this type of anomaly.
The identification of these topographic "anomalies" is therefore paramount after the pre-operative evaluation of a LASIK patient and has been the subject of the recently published research work (see above). The results of our study show that data from the posterior side of the cornea should be used, and the data from the "point-by-point" map (optical pachymetry). Our conclusions allow us to envisage the short-term realization of a new screening test for Keratoconus fruste with Orbscan, clearly more sensitive and specific that current software, using all our clues within a discriminant function.

 

Patients who present evocative topographic anomalies of the so-called "Keratoconus fruste" often admit to be regularly rubbed eyes due to allergies, or Visual efforts extended on screen. Some patients are not aware of rubbing the eyes, even when these frictions are vigorous. A special form of asymmetry of curvature with preserved thickness meets patients who sleep in a position where the eye is "compressed" by the pillow, mattress, hand, front arms, etc. This association is unknown, but it is remarkable in the clinic for who wants to search for it.

It is possible that Keratoconus is not a genetic disease, but simply a clinical and topographic table corresponding to repeated mechanical damage to the corneas. This hypothesis is difficult to prove, but no clinical or experimental data can either overturn it, and it has the advantage to explain certain "mysteries" in the exploration of this pathology.

In all case, the presence of topographical anomalies conveys a State of biomechanical weakening and should reconsider the indication of LASIK in this kind of situation.

Topographic characteristics to be reconsider the indication of a LASIK:

 Topographical aspects

In practice, he must recuse the indication of LASIK in case of positivity of one or several of these classic signs of detection (form Keratoconus suspect).  This indication may only be reconsidered once the differential diagnosis with certainty (corneal warpage) and suspicion of "horny biomechanically fragile for LASIK" eliminated.
None of the "topographic quantitative indices" has a predictive value (positive or negative) equal to 100%. This lack of specificity reflects the importance of the standard deviation of the distribution of the numerical values of the different parameters corneal (keratometry mean, value of corneal astigmatism, ect...). In other words, for the practitioner who would only rely on the only result of this type of indices, the respective risks of operating a patient with keratoconus fruste in case of test, however negative, or of not operating a patient without of keratoconus Beginner in case of test yet positive, would not be null. The risk associated with the false negativity of the test is obviously much more detrimental to the patient, and the value predictive negative testing of Keratoconus is very important.
The negativity of one or more screening tests therefore does not exclude with certainty the diagnosis of keratoconus fruste and the collection and analysis of anamnestic elements (family history of keratoconus, atopic terrain) and clinical (examination Biomicroscopic) and the qualitative analysis of posterior topographic maps of elevation and thickness are currently indispensable in this context.

Randleman offered a score to quantify the risk of ectasia or age, the preoperative corneal thickness, the value of the thickness of the rear wall, preoperative topographic aspects and the degree of nearsightedness to treat are the main criteria (21). The interest of this type of classification is disputed by some authors, who have doubts when his sensibility.

The presence of a asymmetrybe reported to the level of a cornea (toricite asymmetric, asymmetric rise, asymmetrical distribution pachymetrique, ect..), or between two corneas (loss of symmetry in mirror = enantiomorphism) must always be evoke the possibility of a fruste Keratoconus, even if the classic tests are negative.
It is often useful to use the different scales (individualized, absolute) and to know the peculiarities of the Modes of topographic representation (axial mode, tangential, Gaussian...). The tangential curvature algorithm is inherently more sensitive to rapid local curvature variations and has a greater sensitivity to the axial curvature algorithm in this context. The diagnosis of marginal pellucida degeneration is easily performed at this stage by acknowledging the typical appearance in "Gallic whiskers" of the axial topographical map.
A study designed to define the most appropriate scale step for the detection of keratoconus in elevation topography concluded an optimum compromise between specificity and sensitivity equal to 10 microns of elevation for the anterior face, and 20 microns for the Posterior side of the cornea (16). The positivity criterion used in this study was the presence of more than three colors in the central 3 millimetres. The default use of the float mode for the calculation of the reference sphere results in a reduction in the expression of the elevation asymmetry. It may be useful to use the "axis" or "pinned" (15) modes which enhance the elevation differences associated with asymmetric distortion of the cornea.

The actual existence of an ectasia limited to the posterior side of the cornea (the term usually used to describe a central rise marked the back compared to the calculated reference surface) has been a controversial time, accused to be linked to an artifact of reconstruction, or a subjective interpretation of the images to rise due to the presence of a physiological Asphericity more prolate at this level. However, the existence of a marked rear elevation or overall distribution different from that of the front is very often found in Keratoconus fruste case in our experience. A rear elevation increased the longer end is a useful factor to discriminate fruste Keratoconus (Saad A, Gatinel D. Topographic and tomographic properties of shape fruste keratoconus corneas. Invest invest Vis Sci. 2010 Nov; 51 (11): 5546-55). This index is one of the criteria for the graphical presentation of the lsoftware SCORE Analzer.

We published the results of a study conducted in subjects who showed a suspected form of Keratoconus on the topographical appearance in specular topography (corneal anterior), and whose conclusions confirmed the presence of abnormalities of the posterior face of the cornea (17).
The re-calculation of a reference surface for the representation of each examination does not allow the measurement of position deviations over time with certainty (18). Thus, even in the absence of other anomalies, a "posterior ectasia aspect" associated with a low minimum pachymetry is suggestive of keratoconus fruste. This suspicion will be reinforced by the measurement of a particularly prolate asphéricité at the anterior face (asphéricité index Q less than-0.10).
A recent computer modeling work explains why in case of Keratoconus beginner, the posterior elevation is an early marker of the changes that the cornea undergoes (download this article:  Gatinel and al. Best Fit Sphere Asphericity Topography Elevation

In practice, in case of doubt, it is recommended to defer the realization of a photoablative refractive surgery and perform a delayed topographic control in time. It is then possible to perform one or more differential maps whose data are very useful in this context. These maps may indeed reveal the presence of evocative topographic changes of 'biomechanical fragility' (scalable asymmetric camber, thinning, ect...)

Aspects pachymetriques (tomogaphie)

The optical product is reliable in preoperative, and can be calibrated on the ultrasonic product by adjustment of the acoustic factor with the Orbscan (19) remember that the conventional ultrasonic product does not measure with certainty the value and location of the minimum product. However, the topography of direct rise by scanning allows the collection of useful data such as contact information and measure pachymetrique next to the point of lower thickness, usually located in the lower temporal quadrant. This location is still preferred in case of proven Keratoconus or fruste, but its eccentricity (its distance from the center of the cornea) is statistically more important in this pathology (19) (see Figure 1). The collection of the value and details of the minimum product is also useful for the theoretical calculation of the residual corneal thickness after LASIK. Above all, the gradient of thinning toward the Center is increased in case Keratoconus fruste, regardless of the value of the minimum thickness. Even if a corneal thickness less than the average (ex: less than 500 microns) is more common in case of Keratoconus infra clinical, it is above all the existence of a more rapid thinning toward the center of the cornea (difference between the peripheral and Central thickness).

Clinical signs associated with the risk of ectasia

The age and the Allergic ground a factor of risk for the presence of infra-clinic form of Keratoconus. A patient is young, more likely to be confronted with a latent form of Keratoconus. The notion of eye rubbing repeated also pejorative (and might be adversely affected in the genesis of the topographical anomalies such as the asymmetry or the corneal thinning).

The place of topographic screening of Keratoconus indices

The specular videotopographie is limited to the study of the curvature of the anterior face of the cornea but benefits from a higher elevation topography seniority. The first indices available for the detection of Keratoconus are curvature data (index Klyce-Maeda, index of Rabinowitz, ect...). They derive from the cross curvature data collected on large samples and detection algorithms (artificial intelligence). These indices have been validated scientific publications (Klyce Maeda, ect) (20).

Other clues were subsequently proposed from elevation data provided by the Orbscan (Rousch criteria, calculation of the ratio between the radius of the spheres of respective references of the anterior and posterior face of the cornea, ect...) but they did have the no published studies or accurate assessment of their sensiblilite and specificity. Finally, we have demonstrated that these data had a utility to track with a sensitivity and increased specificity forms frustes of Keratoconus: Topographic and Tomographic properties of FFKC Saad Gatinel IOVS.

A new test of the results of this work is underway.

How to prevent the risk of corneal ectasia in practice...?

When the cornea is of normal thickness and topographic anomalies (absence of signs of Keratoconus fruste), but the ametropia of the patient implies a depth of significant ablation, cutting an end component is recommended in order to reduce the risk of a breach of the rear wall. If the defective vision is too large for the original cornea thickness, forbearance, or a no corneal technique can be proposed (phakic implant, or pseudophake depending on the age, the State of the anterior segment, etc...).

Our experience facing the comprehensive study of the case published in the literature currently prompts us to reject candidates for refractive surgery LASIK (in addition to the classic contraindications for this indication as identified proven Keratoconus or fruste) with one or more of the following signs, even if automated Keratoconus detection indices are negative (no suspicion) :

-topographical asymmetric or irregular astigmatism same moderate, especially if its axis is oblique or reverse, no or weakly correlated to the axis of the eye adelphe (low enantiomorphism) and if the optical product or ultrasonic Central average is less than 510 microns.

-a central product average less than 500 microns and/or a minimum product less than 500 microns, whatever the topographical appearance and the value predicted by the predictive calculation of the residual thickness of the rear wall.

-a loss of the marked enantiomorphisme (loss of appearance of symmetry mirror between the two eyes), no history of corneal traumatic or infectious.

-eccentricity increased the lower temporal direction of the thinnest point.

-a (greater than 25 microns) increased elevation of the posterior face to the calculated reference sphere.

The software SCORE Analyzer has been designed to collect this type of topographic anomalies automatically and calculate a score to be objectively studied cornea (topographer Orbscan). When this score is positive, the indication of LASIK must be reviewed.

These recommendations are based on our experience and in part their validity has been demonstrated. A pragmatic attitude dictated by prudence and to minimize the risk of surgery a patient of Keratoconus fruste not detected or a "cornea to biomechanical risk" for LASIK. In this context,. It is better to recuse that include excessive !
Patients disqualified for LASIK can be reoriented according to case in Contactology, or to an additive surgery (implant, rings, ect...) or surface photoablative (PKR, LASEK), especially if the programmed ablation depth is Moderate (less than 70 or 80 microns). The surface technique must be carefully carried out in order to reduce the risk of haze for deep surface ablations.
The realization of a personalized treatment surface guided by the topography is particularly indicated in case of irregular refractive astigmatism of corneal origin (increase in the rate of optical aberrations of high degree) (22). The results published in the literature about the fate of patients with Keratoconus fruste with stabilized refraction in PKR are satisfactory and do not reveal a pejorative influence of this technique on the evolution of Keratoconus fruste (23,24). Recently, the occurrence of an ectasia has been reported 22 months after completion of a LASIK on the left eye of a myopic patient with Central corneal thickness of 500 microns and the topographical aspect marked by the existence of an asymmetric astigmatism. While presenting a more thin cornea (485 microns), a topographic aspect and equivalent myopia, the right eye that had been operated in PKR at the same time has not developed ectasia (25).

The PKR does face a risk of ectasia in patients with suspicious forms of Keratoconus?

This issue stems from the fact that the PKR (technical surface without flap) includes a step of photoablation and therefore represents a potential risk of corneal weakening. The number of case of ectasia after surface technology is low, and their analysis reveals virtually every time the presence of a form while rookie but proved of Keratoconus (more rarely the presence of a subclinical or suspicious form), which could have anyway evolve without realization of the photoablation.

Conversely, there are series of photoablation of surface in patients with suspicious corneas that show no negative effect of this technique.
A simple reasoning also makes it possible to refute the harmfulness of surface techniques for suspicious forms. PKR is an older technique than LASIK, and has therefore been performed in many patients with suspicious corneas (although not identified as such at the time because the realization of a topography was not systematic before surgery, And the screening criteria for the Keratoconus plus permissive). In addition, the magnitudes of the treatments delivered were more important than today. Lastly, there were no special precautions with regard to the prevention of a ectasia, as this complication was not described... until 1998, or at the beginning of the period of LASIK growth. and, If the PKR significantly heightened risk of progressing to the ectasia, then there should be as much or much more case of Keratoconus iatrogenic post-PKR as post-LASIK !

 

The cross linking is it advisable after ectasia?

The author of this site does not consider that the cross linking has proved effective in restoring or even partial improvement of the corneal biomechanics.  A page focuses on the reasons for scepticism. No studies showed that the corneal biomechanical resistance increased in vivo after cross linking (CXL), despite the use of various instruments (Ocular Response Analyzer, CorVIS, etc.).  Changes observed after cross linking are very likely related to scar epithelial changes, and the issuance of a UV irradiation on an already weakened by the ectasia and LASIK cornea does not seem desirable. No ectasia has regressed spontaneously, or after cross linking. Stabilisation of corneal topography may be obtained if the patient stops rubbing the eyes and intra ocular pressure is under control. In addition, case of "haze" have been observed after cross linking for ectasia... without reduction none of the ectasia.

The prescription of a CXL in the presence of a corneal ectasia induced by surgery is often justified as a measure to slow the ectasia. The CXL is sometimes advocated on the side of the eye not reached in case of unilateral ectasia. Given the lack of clinical evidence of the effectiveness of this technique, and the fact that it fails to induce a measurable hardening of corneal rigidity, it seems logical to explain to the patient that should not be (more) rub their eyes, and undertake topographic surveillance close. The CXL is a technique that induces a major oxidative stress of the corneal wall, and after refractive surgery, the risk of inflammation of the tissue corneal stromal is important (haze).

In the following example, unilateral enforcement of a bilateral crosslinking after ectasia has not changed significantly the values of biomechanical parameters measured by the Ocular Response Analyzer (ORA). On the other hand, he trained a bilateral haze with reduction of vision and gene in high brightness for several months.

Makgre the realization of a CXL, no improvement of corneal rigidity is measured, or on the side of the ectasia without ectasia.

Makgre the realization of a CXL, no improvement of corneal rigidity is measured, or on the side of the ectasia without ectasia.

 

Conclusion: LASIK and risk of ectasia

LASIK is the Act of the most popular refractive surgery for the correction of the sphero-cylindrical ametropia. In addition to its comfort, this supremacy reflects the important security and predictiblite of this technique, acquired over time through growing experience surgeons and successive improvements of vested material to the cutting of the stromal (laser femtosecond, new generation microkeratomes) pane and the issuance of the photoablation. Thus,. Today most per or post operative complications can be prevented by careful selection of patients or the implementation of prophylactic and therapeutic measures adapted.
The realization of a surface technique is preferable in case of clinical doubt, because there are no arguments based on the iatrogenic character of this technique vis-à-vis the suspicious corneas. The expected functional results are identical to those of LASIK performed in subjects with normal corneas.

The induced corneal ectasia should not derogate from this development and its impact should be reduced to a minimum by strict compliance with the rules and principles discussed in this article.

Summary / key Points

The realization of thin components (90 to 110 microns about) is recommended to reduce the risk of ectasia in strong short-sighted in LASIK.

Creating a flap (without photoablation) performed on a fragile cornea (Keratoconus fruste) biomechanical implications can lead to the emergence of a progressive corneal ectasia.
A suspicion of keratoconus fruste must have the indication of LASIK reconsidered. Its confirmation or the absence of formal elimination of this diagnosis against-indicates the realization of a LASIK.

There is no test of sensitivity and absolute specificity for the screening of Keratoconus frustes forms: some policies, including a large number of indexes allow to increase the sensitivity and specificity of screening.

Topographical testing negative, especially when it is limited to the study of the only front of the cornea does not formally exclude the presence of a fruste of Keratoconus form whose evolution could accelerate after LASIK.

The asymmetry between the two corneas (loss of the enantiomorphisme), asymmetric or irregular astigmatism associated with a thin cornea (Central average thickness less than 510 microns, minimum thickness less than 500 microns, fast thinning to a fine point in temporal lower) must reconsider the indication of LASIK.  In case of doubt, it is preferable to opt for a technique without lamellar cutting (surface technology, implant, intra-corneen ring insertion, ect... depending on the ametropia of the patient).

References

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