Mario Giovanzana Milano, 14 nd october 01 KERATOCONUS INTRODUCTION The keratocunus is a deformation of the cornea that tends to assume the shape of a cono. The genesis is substantially uncertain. It is manifested generally with a progressive character, especially in a young subjects and it s characterized by a slow and underhand development. If it s manifested in advance age, above 30 years, the evolution is less serious and often the alteration doesn t progress. Not always the deformation happens in the centre part, so the apex of the keratocunus it cannot correspond to the corneal centre. In the greater part of the cases it is an alteration that is present bilaterally although it s manifested before in an eye and then in the other. In the most advances cases, the corneal deformation can be easy observed fig. 1. Fig. 1 SYMPTOMS AND SIGNS The development of the keratoconus can be outlined in four foundamentals stages. The first stage is substantially without symptoms unless for a light reduction of far visual acuity. It s characterized from the appearange of irregular astigmatism that can be corrected with astigmatic lenses, the sights of ophtalmemeter appear already light deformed. 1
In the second stage the patient feels a sensation of a strange body under the upper eyelid, photophobia and sometimes lachrymation. Irregular astigmatism reaches much elevating values and the sights of the ophtalmometer introduce a strong deformation. In the more advanced phases, at the biomicroscopic examination, can appear some signs of a corneal suffering, that is the endotelio presents a translucent shagreen (orange peel) and can appear fine striae on the Descemet (figure 2). In the third stage a serious reduction of visual sharpness is manifested that is not correctable in satisfactory way except with a contact lenses and it is present an intense ittitative symptomatology. With the ophtalmometer examination the sights appear completely deformed, the typical deformation cono shape of the corneal apex is visible also with naked eye and it is obviuos a remarkable corneal thinning. The fouth stage is characterized subjectivement by a very reduct visus and objectivement by a strong deformed and thinning cornea with presence of leucomi expecially in apex zone. In this phase the only theray is often a corneal transplant. Fig. 2 SURVEY INSTRUMENTS Ophtalmometer It is a traditional instrument to measure corneal curvature and its diopters. In presence of keratoconus the instruments sights manifest a deformation that is so much elevated as more the stage of the pathology evolution is advanced. The instruments that allows to appreciate in more substantial and obvious way the deformation of the sights is the Helmholtz s ophtalmometer, characterized by sherical sights. Corneal topographer It s the instrument that is able to reconstruct exactly the corneal shape. It allows to know the corneal geometry, to analyze the minimus details and to supply a refractive interpretation of the corneal surface, moreover it is in a position to calculate the space coordinates and to supply an altimetrical map of the surface. 2
CORNEAL TOPOGRAPHER KERATRON Fig. 3 Fig. 4 3
SPHERICAL ABERRATION OF SPHERICAL AND MULTIFOCAL LENSES The shperical aberration of a lens depends on the power increase, passing from the parassial zone to the marginal one. Fig. 5 Aberration of Sf.-4.50 spherical lens in mm2.50 from 4.50 to -6.00 Fig.6 4
Modifying in opportune way the curvature radius of external surface of a contact lens it s possible to increase or to reduce the spherical aberration. Aberration of Sf. 4.50 aspherical lens in mm 2.50 from 4.50 to 8.50 Fig7 Aberration of Sf. 5.00 aspherical lens in mm 2.50 from 5.00 to 5.50 Fig. 8 5
Multifocal lenses With this type of lenses it s possible to increase or to reduct the power towards the external optic zone (see diagram Fig. 10), this allows us to move the focals as to increase or to reduce the power. Fitting progressive eccentricity contact lenses to keratoconus patients, we reduce the distance between fires with consequent increase of the contrast and visus. Fig.9 Fig.10 Speric lenses Normal ocular condition in which a spherical lens allows to bring the focus image on the retinic plane. SPHERICAL LENS IN NORMAL OCULAR CONDITION Fig.11 6
Regarding keratoconus abnormality it has a concentric shape and the consequence of wearing contact lenses is an insufficient focalization, because there isn t any plane (retina) on which we can find a definite image (see Fig. 12). F1 focus is generated from the central zone rays and F2 focus from peripheric ones. Adding or removing spheric power we obtain a shifting of both focuses but doesn t change their reciprocal position, that is, the distance between them remains always the same. SPHERICAL LENS IN ABNORMAL OCULAR CONDITION Fig.12 In Fig. 13 we make a coincidence with F1 focus, caused by the central zone rays, and the retinic plane.as a consequence F2 focus, caused by the peripheric zone rays, moves away from the retinic plane. We can obtain the reduction of this distance, as from Fig, 15, only reducing the lens power in the peripheric zone, in a progressive way. SPHERICAL LENS IN ABNORMAL OCULAR CONDITION Fig.13 7
In Fig. 14 we make a coincidence with F2 focus, caused by the peripheric zone rays, and the retinc plane. In this case F1 focus, caused by the central zone rays, goes away from the retinic plane. We can obtain the approaching of F1 and F2 focuses only with lower progressive power in the central zone. SPHERICAL LENS IN ABNORMAL OCULAR CONDITION Fig.14 Multifocal lens MULTIFOCAL LENS IN ABNORMAL OCULAR CONDITION Fig.15 The excellent correction is the one which, with a progressive lens, brings the focuses on the retinic plane. 8
Determination of the correction with spheric lens Fix the power of the spheric lens - ottotipo for far vision - full field - to take note of the visus obtained with shperic lens Fig.16 Determination of central zone power Fix the correction power of the lens with peripherical diaphragm - ottotipo for far vision - diaphragm with 9 mm diameter placed to 35 cm approximately Procedure To observe for far ottotipo through the diaphragm, only central letters can be seen, we can determine so the spherical power ideal for far. In this way it is possible to determine the better visus in central zone. Fig.17 9
Determination of the peripherical zone correction Fix the lens power with central diaphragm - ottotipo for far - the diaphragm with a hole of 40 mm diameter with central cover of 9 mm placed to 35 cm approximately Procedure To observe for far ottotipo through this type of diaphragm that gives visible the peripheral part and covers the central zone of the ottotipo. In this way I determine the necessary power variation to obtain the focusing in the peripheral zone. Fig.18 Using these diaphragms the two focal plans (central and peripheral) are in evidence in order to establish the power variation between the central zone and peripherical one in the optic zone. 10
ABERROMETRO The Aberrometro is a laser high precision instrument for measuring refractive error by identifying aberrations, and then providing the optimal corrective solution. The instrument operation is based on Hartmann Shack principle which allows to determinate a wavefront geometrically re-formed by Zernike s polynomials. This uniquely powerful wave-front diagnostic tool allows to measure with precision the total optical system aberration. Procedure of spherical aberration valutation using aberrometro instrument. After have fitted a spherical contact lens to a keratoconus patient, we proceed to measure the spherical optical system aberration with aberrometro instrument. It is not necessary to dilate the pupil to obtain significant data, because the natural pupil diameter in normal conditions is enough to use the instrument. The use of aberrometro allows us to prove what we said before, that is in keratoconus cases the rays coming from infinitive fail to focalize in a regular way on the retinic plane and so using a spherical lens it is not possible to obtain an optimal visus cause the spherical aberration of the system. The picture on the left, through colour variations, shows as well as, also in a specific case, the rays coming from infinitive fail to focalize in a regular way on the retinic plane and therefore it shows the optic system spherical aberration. The picture on the right is an elaboration given by the instrument and it shows the teorical correction nearer to the correction requirement of the optical system itself with a sphero-toric lens. Fig.19 11
Procedure of spherical aberration reduction through multifocal contact lens fitting Picture 19 shows a spheric contact lens fitting. This spheric contact lens has a power of 2.00 D and the visual acuity obtained in this specific case is of 5 tenths. Picture 20 shows on the contrary a multifocal contact lens fitting. This multifocal contact lens has a power of 1.00 D with an addiction of 1.00 D. This kind of lens permits a reduction of system spherical aberration of about 40% and allows an increase of the contrast and consequently an increase of visual acuity at least of 3 tenths. Picture 20, left image, shows a colour uniformity greater than picture 19; this uniformity is just determined by the reduction of system spherical aberration obtained through a progressive eccentricity multifocal contact lens fitting. Fig.20 12
Further reduction of spherical aberration by reducing the diameter of central spherical zone Picture 20 shows a multifocal contact lens fitting. This multifocal contact lens has a power of 1.00 D with an addiction of 1.00 D. Picture 21, left image, shows a multifocal contact lens fitting. This multifocal contact lens has a power of 1.00 D with an addiction of 1.00 D, the only variation introduced in this lens is the reduction of central spherical zone diameter. This variation allows a color uniformity greater than picture 20. The reduction of spherical aberration is only possible through a progressive eccentricity multifocal contact lens fitting, consequently obtaining an increase of contrast and visual acuity. Fig.21 13