OPD and Keratoconus
These cards OPD SCAN 3 have been identified in a patient of 30 years reached bilateral Keratoconus. The patient was rubbing the eyes since adolescence (responsible for Atopic field of eye pruritus chronic: the) eye rubbing enough to cause marked, corneal deformation that is labeled "Keratoconus"). Visual acuity corrected in glasses was 10/10: the patient reported the impression of a double vision, and the impression that vision "dropped" to the night, with night-time halos.
The optical aberrations of high degree are high in the presence of a Keratoconus. They result essentially from the deformation of the anterior corneal surface (negative aspherisation, asymmetry, paracentrale hypercambrure). The back is also a concomitant deformation. However, the negative sign of the difference in index between aqueous and corneal stroma assigns a role of compensation to the effects induced by the front (note the opposite aspect of phase shift type aberrations-dominated coma of the wave front internally, with respect to the corneal Wavefront). Adaptation of a rigid lens sometimes unmasks the internal quota of aberrations of high degree.
It is possible to display the values of the RMS of each optical aberration (represented by a = Zernike polynomial term). Note the elevation of the aberrations of type coma, trefoilClassic in the case of the extent of an affected eye of Keratoconus.
The retinal image quality can be calculated from these data aberrometriques. The image of a point source on the retina (Point Spread Function) allows the reduction of the stigma. The convolution of the PSF with a reference image provides a representation of the image projected on the retina by the eye dioptres. It allows to objectify the existence of a reduction in the contrast of images, duplication, etc. These demonstrations are common in case of Keratoconus. The MTF is a curve that explores the contrast of the retinal image reduction, it is represented for different pupillary diameter: in case dilation pupillary, optical aberrations rate increases and decreases the contrast.
This card can be recalculated to show the effect of the only aberrations of high degree; It is the optical quality of the eye once nearsightedness and astigmatism corrected at best (with the best possible glasses). "" Even if there is logically a significant improvement of the optical quality of the studied eye, the effect of the "irregular astigmatism ' remains (aberrations of high degree), and must be reconciled with the Visual complaints of the patient (a split and less contrasted vision feeling).
In particular, the study of these symptoms can be performed by a convolution operation: it comes to calculate the effect of aberrations of high degree on the vision of the patient, in "convoluant" the reference with the PSF image, she even calculated from the aberrations of high degree.
Representation of the star of Siemens ("Siemens star") allows to study the effect of the aberrations of optical high spatial frequencies, in terms of contrast but also of 'phase' more independently (gap between black and white areas adjacent). Nearsightedness and astigmatism induce inversions of phase more marked on some spatial frequencies, which makes the picture particularly blurred. This is a "reference" image of a Siemens star:
For the affected eye of Keratoconus, we noted that there (in the absence of any lens correction) some spectrum that can be identified, according to an oblique axis: this axis is the result of the interaction between type coma aberration and astigmatism:
Here, 'Siemens Star' map is recalculated for only aberrations of high degree. It reveals the predominance of an overall reduction of contrast, and the absence of phase inversion.