[Corneal asphericity in myopes]. / Etude de l'asphéricité cornéenne chez le sujet myope.

PURPOSE: To study the variations of corneal asphericity in a population of myopic patients. METHODS: One hundred consecutive myopic patients were included in this study. The EyeSys videokeratoscope was used to assess the corneal topography of these patients seeking refractive surgery. We compared the results of cycloplegic refractions with the values of the corneal asphericity and mean central keratometry. RESULTS: Mean corneal asphericity was -0.09. Eighty per cent of the myopic patients had a prolate corneal contour, whereas 20% had an oblate corneal contour. No significant relationship was found between the corneal asphericity value and the mean keratometry value or the mean refractive error. CONCLUSION: The mean corneal asphericity in our myopic population was -0.09. This is slightly more than previously reported data in similar studies. No statistically significant relationship was found between corneal asphericity, mean refractive error, and mean keratometry.

[A review of mathematical descriptors of corneal asphericity]. / Etude des paramètres permettant la description mathématique de l'asphéricite cornéenne.

PURPOSE: Corneal asphericity may be modeled on a conic section which can be described by the apical radius of curvature in the meridian studied and by a measure of the degree of asphericity. MATERIAL AND METHODS: Through an extensive review of the literature, we expose the principles, the population variations and report the application of such corneal modeling. RESULTS: The aspheric anterior corneal surface can be described by a conic section, defined by its radius of curvature and by a parameter measuring asphericity. We analyse the various parameters used in the literature to determine their usefulness. Conic sections, obtained by cutting a cone by a plane, include ellipses, hyperbolas and parabolas. Two useful parameters are the apical radius of the ellipse and its eccentricity defined in Cartesian terms by a second order equation where the apical radius is R and the eccentricity is e: The apical radius is that of the circle tangent to the apex of the conic section and e describes the variation of this curve with distance from the corneal apex. Baker introduced the form factor p making the equation: with It is easier to understand the effect of alteration of p than of e on corneal curvature: There is a relation between the horizontal, a, and the vertical, b, hemi-axes and R The advantage of this notation is that e(2) can be greater than 1 When p=0 the conic section is a parabola, when p<0 it is a hyperbola. Kiely et al. studied corneal asphericity by photokeratoscopy and introduced the parameter Q, where Q=p-1. Q, the asphericity factor, is used by the Eyesis and Orbscan systems; when Q=0 the cornea is spherical. Thus different parameters describe variations in corneal curvature along any meridian. Average anterior corneal asphericity using various keratometric systems is p=0.8, making the corneal section a prolate ellipse. However there is great individual variation, 20% of normals exhibiting oblate (p>1), paraboloid (p=0) or hyperbolic (p<0) corneas. all becoming more spherical with age. Little connection between asphericity and ametropia is reported, except for a tendency to flattening in myopia and towards oblateness in progressive myopia. Direct measurement of denuded cadaver corneas gave a prolate elliptical profile although calculation after deduction of epithelial thickness measured by ultrasonic biomicroscopy suggested p=-0.22, a hyperbolic profile. The few reports on the posterior surface suggest it to be hyperbolic or prolate. Increasing distance from the corneal apex worsens the comparison to a conic section as flattening increases. Precision can be improved by adding polynomial coefficients above the second degree to the equation of the section: The non-toric 3D corneal surface can be described by the following equation for the revolution of a conic section about the optic axis: x(2)+y(2)+pz(2)-2rz=0 where z is the axis of revolution. Since the mean value of p is 0.8 this corresponds to a sphere stretched along one axis, as is a rugby ball. Each meridian has the same radius of curvature and the value of p is constant. For a toric cornea the radius and value of p must be defined for two meridia at right angles. This corresponds to an elongation on an axis different from that of revolution. Similarly a toric ellipsoid is generated by rotation of an arc around an axis at right angles to its elongation. Because of its asphericity, representation of the corneal surface depends on the direction in which its curvature is measured: In the ellipsoidal model this depends on the principal meridians, the tangential, in the plane of the axis of symmetry and the saggittal, perpendicular to this. These may define two radii of curvature, the saggital (axial) and the tangential. Most algorithms assume these properties of ellipsoids. Asphericity is translated into variations in radius of curvature from apex to periphery, increasing for a flat periphery, decreasing for a steep one. Associated to toricity, it gives rise to the common butterfly pattern. Spherical aberration is minimal through a small pupil but becomes significant the larger the aperture, with deterioration of image quality. Raytracing allows analysis of the effects of non-axial rays. The mean value of Q, at -0.26 thanks to the naturally prolate asphericity of the cornea reduces spherical aberration by half. The relaxed form of the crystalline lens further reduces it by inducing the opposite spherical aberation to that of the cornea. This is important in accommodation and presbyopia. The use of an aspheric corneal surface in the schematic eye allows calculation of the ideal asphericity, which corresponds quite well with clinical findings. Radial keratotomy reverses the natural asphericity of the cornea. Photorefractive keratotomy (PRK) also changes asphericity, Q increasing to an oblate value. These changes might increase spherical aberration, explaining some postoperative deficiencies. Current excimer laser protocols ignore asphericity, relying on paraxial algorithms alone. New strategies to control asphericity in order to diminish spherical aberration may offer solutions. The original conic section models were made to improve the geometry of contact lenses. Understanding of asphericity is important in adaptation after refractive surgery. Modification of spherical aberration by contact lenses and corneal warpage induced by rigid lenses have also been studied. CONCLUSION: The approximation of the corneal surface by a conic section allows understanding of corneal asphericity and offers a quantitative description. This allows a more precise description of the corneal surface and of the genesis of certain optical aberrations of the eye.

[Central serous chorioretinopathy and systemic steroid therapy]. / Choriorétinite séreuse centrale et corticothérapie systémique. A propos de 14 cas.

BACKGROUND: The manifestations of the ocular toxicity of systemic corticosteroids include posterior subcapsular cataracts and glaucoma. We describe 14 cases of serous detachment of the macula due to central serous chorioretinopathy in patients given long-term steroid therapy, which may be another potential ocular side effect of corticosteroid. CASES REPORT: The 14 (9 men and 5 women) patients were aged from 39 to 55 year old. Their systemic diseases were allergic thrombopenic purpura, optic neuritis, kidney or heart transplant, Churg and Strauss vasculitis, facial palsy, rheumatoid arthritis, systemic lupus and a kidney tumor. None of the patients had hypertension. RESULTS: Serous detachment occurred between 6 days and 10 years after the start of steroid treatment. The higher the doses, the earlier the onset of ocular disease. All patients were symptomatic, with rapid onset of blurred vision. Serous detachment was bilateral in two cases. The fluorescein angiographic finding was in most cases a single small focal hyperfluorescent leak from the retinal pigment epithelium which appeared early in the angiogram and increased in size and intensity. No diffuse degradation of the retinal pigment epithelium was seen on the fluorescein angiogram. Five patients underwent laser photocoagulation of the leaking area followed by resorption of subretinal fluid. In other patients, the symptoms disappeared as the doses of steroid were reduced. CONCLUSION: The pathogenesis of central serous chorioretinopathy remains unclear and is controversial. Corticosteroids are known to worsen the prognosis of idiopathic central serous chorioretinopathy, and serous detachment has been reported after renal transplantation. In most of these cases, chorioretinopathy was combined with diffuse leakage from the choriocapillaris. We discuss the relationship between steroid therapy and focal leakage as seen in idiopathic central serous chorioretinopathy. In conclusion, we describe 14 cases of central serous retinopathy whose clinical and fluorescein angiography were fairly typical, without obvious diffuse degradation of the retinal pigment epithelium. All these patients had been given long-term steroid therapy for various diseases.