KERATOCONUS: « ECTASIA » or « WARPAGE » ?
« Inferior steepening » should not be confused with and is not synonymous with « ectatic protrusion ».
A mild inferior steepening is often found as a topographic sign in early forms of keratoconus:
The curvature map (left) shows a mild inferior steepening. One should not interpret this as an area of local protrusion. On the contrary, this area of the cornea is located in a more posterior plane than its superior counterpart, as shown on the vertical crossectional image taken by the Scheimplfug camera (red arrow), and also demonstrated on the elevation map, which reveals a more negative elevation relative to the best fit sphere. This change in corneal curvature occurred in a patient after a number of years of vigorous eye rubbing.
Even when it becomes more accentuated, the presence of a « red zone » on a curvature map in corneal topography should not be interpreted as a protruding zone. The red area corresponds to a zone where the local curvature is more pronounced:
The orange-red area seen in the axial map of the left eye corresponds to a zone where the local curvature is more pronounced (this area is located in a plane posterior to its superior counterpart). The elevation map reveals that the elevation is more negative inferiorly. There is no protrusion here.
This patient sleeps on his left side, with his head pressed against his hand and forearm. As a result, he developed chronic ocular irritation, which in turn prompted him to rub his left eye vigorously with the knuckles of his index finger, often upon awakening. The repeated trauma accounts for the corneal deformation seen. From our clinical evaluation of hundreds of keratoconus patients, the first perceived visual symptoms leading to the diagnosis of early keratoconus occurs after 2 to 3 years of intense and repeated eye rubbing.
In response to eye rubbing, the buckling and flexure of the corneal fibrils may occur in association with the slippage between collagen fibrils at the cone apex. The repetitive local trauma alters the viscosity of the ground substance and initiates a temporary displacement of ground substance from the corneal apex. The zone exposed to the trauma from the fingers is often located inferiorly with regards to the geometric center of the cornea. This is because of the Bell’s phenomenon (upward and outward movement of the eye, when an attempt is made to close the eyes) and the greater exposure of the inferior rather than the superior part of the cornea, which is protected from compression by the fingers by the superior orbital margin. (more about keratoconus genesis)
In response to the forces of intraocular pressure, corneal buckling ensues around the weakest zone of the cornea. As this zone is located in the paracentral inferior area, this results in a superior flattening and inferior steepening. These changes should not be considered ectatic, as they are isometric, as illustrated in the diagrams below.
Schematic representation of the change in the corneal profile caused by repeated trauma. The initial corneal profile is depicted by an arc of a circle (A). It has a constant curvature (the green arrow corresponds to the radius of curvature). Repeated trauma exerted on the corneal surface results in asymmetric curvature redistribution (B). This incurs not only an inferior steepening (decreased local radius of curvature: red arrow), but a concomitant superior flattening (extended local radius of curvature: blue arrow) This process should not be considered ectatic, as it is isometric (no change of the corneal surface area as there is no distension of the corneal tissue), at least in its initial stages.
Schematic representation of the approximation of the corneal profile (vertical cross section obtained with the Scheimpflug camera of the Pentacam topographer). The profile is symmetrical and can be grossly approximated by a circle (the cornea is naturally slightly aspheric, but we will neglect this aspect here).
The vertical profile of a keratoconic cornea cannot be grossly approximated by a circle. The vertical asymmetry is the result of the concomitant superior flattening and inferior steepening. It does not correspond to an ectatic protrusion. The biomechanical alteration causes buckling of the corneal dome, which results in a relative backward position of the inferior (steepened) cornea (arrow). On the axial map, the red zone is not protruding: it is simply steeper, and as such located in a more posterior plane than the flatter superior corneal area.
This figure shows the vertical cross section of a keratoconic cornea, for which two different circles have been used to approximate the superior (flatter) and inferior (steeper) hemi-corneal profile.
After years of vigorous rubbing, the corneal distortion can become very pronounced, leading to severe irregular astigmatism. While the alteration of the corneal profile is difficult to appreciate in the magnified picture, it is clear that it does not resemble a major protrusion or an ectasia. The inspection of the corneal profile of most cases reported on this site should be sufficient to convince the most skeptical that the notion of « protrusion » does not correspond, at least from a macroscopic point of view, to the reality of keratoconical deformation.
Rather, the buckling of the corneal dome results in a « flexure » of the area which contains the thinnest point of the cornea (orange arrow). While the inferior part of the cornea sags and steepens, the superior part of the cornea flattens. The sagging of the cornea contributes to the vertical inferior displacement of the thinnest point, which is one of the earliest signs of the permanent warpage caused by eye rubbing.
This patient was referred for the management of mild keratoconus detected during a recent refractive surgery suitability assessment for low myopia. Scheimpflug corneal topography revealed the presence of bilateral inferior steepening, more pronounced in the right eye. This patient admitted to rubbing his eyes frequently with his right hand, massaging vigorously in a horizontal motion. He would do so particularly when tired, in the mornings, and under the shower. The presence of a “ red zone ” located in the inferior part of the curvature map should not be interpreted as bulging or ectasia. There is no protrusion here, as demonstrated by the raw image of the vertical slice taken by the Scheimpflug camera. On the contrary, this is more indicative of warpage of the corneal wall, caused by repeated trauma inflicted by the particular rubbing technique. The corneal central zone is flattened (larger radius of curvature, lower keratometry) to compensate for the inferior third of the corneal surface which is concomitantly steepened (shorter radius of curvature, higher keratometry). There is no stretching or corneal tissue distension. At this stage, there is no marked corneal thinning. When this particular technique of rubbing is employed, the cornea rubbed by the thumb is usually more affected. The pulp of the thumb and index finger is softer than the knuckles, and one could predict that if a different technique had been used (grinding movements with knuckles), the deformation and thinning would have been more pronounced.
Based on the current definitions in Medicine and supported by an in-depth analysis and interpretation of topographic curvature and elevation maps, the term “ECTASIA” does not accurately describe the typical morphological changes seen in keratoconus. The inferior steepening seen in keratoconus should not be confused with and is not synonymous with ectatic protrusion. Keratoconus should be described more as a specialized form of corneal “WARPAGE”, at least in the early and moderately advanced stages of the disease.
Many cases of keratoconus are reported in this website. They all share a history of eye rubbing which preceded the onset of keratoconus. The diagnosis of keratoconus can be life-changing but should not be regarded as tragic. The disease can be prevented and arrested by the cessation of eye rubbing.
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