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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.

Keratoconus OPD SCAN 3 Overview

The "Overview" map is an overview of the analyzed eye. It allows to enjoy the refraction (correction glasses) measured by the instrument (red box), the degree ofastigmatism irregular (blue box), the corneal Asphericity (green box), and the distribution of astigmatism (total, corneal, internal) (orange box). Colors cards are from left to right: map OPD (refraction in the pupil), the axial topographic map, and the map internal OPD (effect on the refraction of the internal components: front posterior cornea and crystalline)

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.

aberrations Keratoconus

Calculation of aberrations optical total (left), corneal (in the middle), and internal (right). Under each colorful map (wave front limited to aberrations of higher degree), there is the list of the optical aberrations grouped into major categories. In the box to the right of the wave front maps is plotted the Point Spread Function (PSF, the point spread function)

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.

keratoconus HOA Zernike

Decomposition of the total, internal and corneal Wavefront (Zernike polynomials). The analyzed diameter is 5.65 mm.

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.

Keratoconus quality optical MTF convolution

Evalution of optical quality and the retinal image (contrast, sharpness)

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).

Keratoconus optical quality HOA MTF

The optical quality of the eye for the high degree only aberrations (HOA)

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.

keratoconus HOA convolution acuity

It is interesting to compare the reference (optically perfect eye) with the convoluee (right) image image. Note the reduction in contrast, and the fact that the image "bleed" way slash. These are the type coma aberrations, including orientation wife this "distortion" of the image (the inversion of the retinal image close). However, it is possible to recognize the letters, at least until the line corresponding to the acuteness of 10/10. Contrast reduction does not exclude the possibility of the pattern recognition, which is facilitated by cognitive factors (pattern recognition which we know priori characteristics)

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:

Siemens star star

Siemens Star in the absence of optical aberrations.  Closer to the Center, and more frequencies correspond to patterns "purposes", so a higher resolving power (until 20/20 or 10/10). In case of aberrations, this representation shows the effect of optical aberrations on the contrast (mitigation) and phase (move) the spatial frequencies, based on value and orientation

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:

Keratoconus siemens star total aberrations

This map reveals the optical aberrations to induce greater interference of the spatial frequencies located on an axis in line with the type coma aberrations.

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.

Keratoconus siemens star HOA

The Siemens Star pattern for the only aberrations of high level reveals the marked reduction in the contrast of the retinal image, even if it is possible to discern these frequencies up to the equivalent to 10/10 resolution (i.e. Visual acuity which was measured with a correction in glasses).

3 Responses to "Aberrometry and Keratoconus"

  1. Dr. derhy says:

    beautiful very useful work and is not too simple understanding SEO

  2. Martin says:

    Hello, a patient with kératocone with 12/10 weak to both eyes with soft lenses, but clinical aberrations, can he find a profit with rigid lenses? Thank you.

  3. Dr. Damien Gatinel says:

    The rigid lenses are able to suppress certain aberrations related to keratoconus, as they compensate for the distortion of the cornea (the soft lenses "absorb" less this type of deformation).

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