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:
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:
A skewed distribution of the gradient of topographic curvature reflects the difference in Camber between certain opposites hemi-meridians.
Here is an example of corneal topography obtained in a patient of Keratoconus.
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.
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).
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.
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)
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 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.
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.
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).
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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.