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Corneal asymmetry

The asymmetry of the cornea is a feature seen in corneal topography. It is important to detect and quantify, because it is a marker for the early detection of Keratoconus (Keratoconus fruste), especially in its orientation vertical or oblique forms.

Definition of asymmetry

Corneal asymmetry is defined by a  marked curvature difference between opposites hemi-meridians (ex: hemi-Meridian 90 °, hemi-Meridian 270 °): an area of the cornea is more arched than the opposite sector.

Three-dimensional schematic example of corneal surface asymmetrical:

Asymmetry of the cornea

The corneal surface is represented by the Green mesh: asymmetry is related to a camber increased to half level below: this type of asymmetry is characteristic of Keratoconus.

However, the cornea has a physiological asymmetry: she is usually less arched in nasal and temporal. In part this is because of the nasal gap of the corneal vertex, which is centered in topography specular review, and thecorneal Asphericity physiological prolate.

 

Topographic translation of asymmetry:

In curvature

 A skewed distribution of the gradient of topographic curvature reflects the difference in Camber between certain opposites hemi-meridians.

variations of the curvature in corneal topography

Schematic representation of an asymmetric distribution of curvature (here quantified by the osculating circles, which are not drawn to scale) along a meridian of the anterior corneal surface. In this example, the apex, which is defined as the point of maximum curvature, is lateralized (biased in lower) towards the geometric center of the cornea. The latter marks the dividing line between the two meridians hemi (upper and lower) constituent of the explored Meridian. This type of asymmetry is often observed in patients with Keratoconus (apex moving typically turned-temporal relation to the geometric center of the cornea)

Here is an example of corneal topography obtained in a patient of Keratoconus.

axial topography of Keratoconus

Specular topography in axial mode (custom relative scale) of a cornea with Keratoconus. Note the strong vertical curvature gradient, and the lower displacement of the apex in axial curvature. The asymmetry of curvature between the half top of the cornea and half lower of the cornea is obvious.

 

The asymmetric curvature distribution is often associated with a displacement of apex who moves away from the central region, in the direction of the curved meridians hemi. It is important to keep in mind that this map (axial curvature) arises from the study of the local radius of curvature of the cornea (and not a representation of the spatial form of the cornea). The areas represented in warm colors are "over arching" (the small radius of curvature).

The cornea is usually slightly flat in his nasal portion, a certain degree of asymmetry horizontal (vertical axis) is commonly found for normal corneas, and it manifests in cooler colors on the nasal side of the side temporal of the cornea, while the apex remains relatively central.

physiological nasal corneal asymmetry

Representation of the curvature in the axial mode a normal cornea (right eye, custom scale). There is a slight asymmetry vertical, but of low magnitude. The Asphericity is of type prolate, the cornea is overall more arched in the Center and towards the edges. Note the peripheral flattening more marked in nasal (arrow), which causes an asymmetry of horizontal type: this asymmetry is physiological, and partly related to the nasal offset of the vertex, which is by definition in the center of the topographic map, which should not be confused with the geometric center of the cornea.

All increase in asymmetryespecially when she is oblique or vertical and associated with a displacement of the apex must be suspect of a breach of the cornea such as the Keratoconus or the pellucid marginal degeneration.

In the early stages of Keratoconus, the asymmetry is often characterized by an aspect of bowtie asymmetrical ("asymmetry bow tie") whose orientation follows the direction of the more arched part of the cornea. At the earlier stages (Keratoconus subclinical form fruste), only the presence of a slightly more arched area in lower answers can be observed, with a deviation from the direction of one of the arched hemi-meridians relative to each other (radial throw axes: SRAX).

asymmetry corneal fruste Keratoconus and anomaly of type SRAX

Axial curvature (custom scale) topographic representation of the other (adelphe, right eye) of the patient with Keratoconus featured on the previous figures. The cornea of the eye is one of the early topographical signs of Keratoconus: a 'bow tie' asymmetrical with meridians hemi of maximum curvature (red segments) of axis deflected. You can use the acronlyme "AB-SRAX" to refer to this aspect: AB for asymmetry Bowtie, SRAX for radial throw axes). The appearance in bow tie is linked to the conjunction of the Asphericity and the corneal toricite; the deviation of the hemi-axis is the result of the asymmetrical deformation of the cornea. In this context, when we know that the other eye has a Keratoconus, it is possible to formally diagnose Keratoconus fruste, despite the minor nature of these anomalies. Repeated eye rubbing are originally of asymmetrical deformation of the cornea, as well as the compression of the eye at night by hand or the pillow.

At advanced stages, extreme asymmetry is combined with a negative Asphericity (prolate) more important and result in an aspect where is evident a more arched off-center area.

Emphasize that it is important to detect and quantify the asymmetry in order to develop tools for screening of Keratoconus in refractive surgery. any asymmetry is not necessarily synonymous with Keratoconus fruste, and is made up of other clues to accredit this diagnostic hypothesis (clinical context, corneal thickness, etc.). The eye rubbing repeated explain a large proportion of the topographic case of asymmetry (corneal)According to the author of the site, the eye rubbing explain the occurrence of Keratoconus). It is also striking to observe the correlation between Forms of isolated corneal asymmetries and sleeping on your stomach or side, pressing hold the eye (pillow or hand).

 

Apart from friction and these traumatic causes chronic, the cornea may be due to an infection (abscess) acute deformation, or the issuance of an off-center laser correction.

The following example corresponds to a shift of the correction of myopia in laser surface (PKR), occurred in the mid-1990s, when the first lasers excimers of had not in routine of enslavement to the eye (eye-tracker) prosecution system. During the surgical recovery through personalized treatment (topographically-guided), to relieve the patient of symptoms of double vision, again in PKR, there well offset by lower treatment area. The original PKR have taken locally the layer smooth Bowman, the most superficial stroma of the cornea (putting naked after removal of the epithelium), is observed through the reduction of the reflection photoablation in lower area offset after removal of the epithelium.

shift photablation corneal asymmetry

Curvature in map k axial asymmetry linked to a shift of laser correction. The warm colored area corresponds to the fitting with the untreated area. Flattening induced by the laser correction is shifted to lower. After removal of corneal epithelium during surgery performed 20 years after the initial procedure, observed through the loss of the reflection of the layer of Bowman that the treatment is biased significantly in lower.

Various indices from the maps of curvature and quantifying the asymmetry have been described, they are based on the differences of curvatures noted between horny hemimeridiens or hemi upper and lower (ex: I-S value, which is used in the calculation of the value of the Score with the) software SCORE Analzyer). These indices are useful for the screening of Keratoconus (see the) site dedicated to SCORE Analyzer software)

Elevation

In topography of elevationphysiological horizontal asymmetry is responsible, in the normal corneas, a slight temporal shift and differences between points respectively in temporal and nasal in the relative distribution of their respective elevation with respect to the sphere of reference.

In the early stages of Keratoconus, the asymmetry translates also in elevation by a reduction of the axial symmetry with respect to the horizontal axis. A slight shift to the bottom of the bunch of horizontal elevation ("promontory") on the maps of elevation of the anterior cornea can be observed.

corneal asymmetry: appearance in elevation

Elevation of the anterior corneal surface topography (the sphere of reference is free: float mode). The cornea is prolate and o-ring)astigmatism (in line), and presents a significant vertical asymmetry, resulting in a shift of the landscape promontory down (note the difference between the distance upper and lower at the geometric center of the cornea). The corneal surface analyzed here in elevation is represented in curvature on the previous Figure - vertical asymmetry. In elevation, the asymmetry is less obvious than in curvature; This is especially of the to the sphere of reference shifts to marry the best studied surface, which minimizes topographical translation of asymmetry.

Corneal asymmetry may be better revealed through the use of an alignment constraint (for example, "axis" mode) for the calculation of the reference sphere (Best Fit Sphere - BFS) since "float" mode allows the sphere away from Center, therefore reducing the apparent asymmetry on the maps of elevation. The "axis" mode forces the sphere to remain aligned with the geometric center of the cornea.

asymmetry corneeenne elevation: using axis mode

Representation in elevation, using a constraint of alignment for the sphere of reference (axis mode). The sphere is forced to align themselves with the geometric center of the cornea. This constraint allows to underline the asymmetry of the explored area. The arched hemi-meridians are located in lower, and lie under the sphere of forced reference. Senior, less arched, hemi-meridians are logically located above.

 

In practice, the asymmetrical component is only rarely isolated, with varying degrees of toricite and Asphericity depending on the clinical circumstances. In Keratoconus, the toricite and the negative Asphericity are often more pronounced than for a normal cornea.

The following figure, based on a mathematical simulation software (Maple) summarizes the topographic features observed in curvature and elevation when the asymmetry is combined with the Asphericity and the toricite.

corneeennes, toricite, Asphericity asymmetries: comparison between the curve elevation maps

Computer simulation of the representation in elevation (E) and (C) curvature of corneal surface according to their geometric properties elementary. The surfaces were generated by a mathematical simulation software, and a calculation algorithm is designed for the calculation of the sphere of reference (needed for the representation of the elevation), and the southern curve of type.

 

 

 

It is important to note that the clinical interpretation of topographic asymmetry is based on the clinical context, and other topographical settings like scalability, and the existence or not of a comparable contralateral asymmetry (preservation of the enantiomorphism, that characterizes the symmetry between right eye and left eye mirrored).

 


Maeda N, SD, MK, Thompson HW Smolek Klyce. Automated keratoconus screening with corneal topography analysis. Invest invest Vis Sci. 1994; 35 (6): 2749-57.

 

Rabinowitz YS, Rasheed K, Yang H, Elashoff J. Accuracy of ultrasonic pachymetry and videokeratography in detecting keratoconus. J Cataract Refract Surg. 1998; 24 (2): 196-201.

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