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Pellucid degeneration

The OPD is useful to characterize the optical consequences of illnesses that cause a deformation of the cornea. Pellucid marginal degeneration is a disease that, as the Keratoconus is a non inflammatory disease that affects the cornea (more information on the pellucid marginal degeneration of the cornea). It is not hereditary, and its causes are unknown;  Allergic land and of repeated eye rubbing may be involved in the genesis of this affection.  It is characterized by the appearance of a astigmatism reverse direction, bilateral (both eyes are reached), and which is progressing over time. The discovery of this affection is often made by chance by corneal topography. This review is prescribed in patients with astigmatism is changing, either accompanied by Visual symptoms such as a duplication of the images, an impression of blur despite the correction in glasses of astigmatism.

Corneal topography shows level anterior curvature map a characteristic appearance in "wings of mill", or "Gallic moustache", which corresponds to the presence of a reverse irregular astigmatism. This aspect is actually secondary to what seems to be the distinctive feature of this affection: a lower corneal thinning located and marked. This thinning is objectified by the realization of a tomographic examination (product optical by collection of corneal elevation anterior and posterior).

degeneration pellucid marginal topography

Specular corneal topography earlier (Placido disc) revealing an aspect of pellucid marginal degeneration of the cornea. The software "Corneal Navigator" (OPD SCAN 3) suggests this diagnosis in an automated way. Note the characteristic appearance in "whiskers" conferred by areas of accented curvatures. These areas surround a "corridor" vertical central where the flatness is maximum, up to an area of "casesure" (maximum thinning area) where the curvature increases again.

In OPD, it is interesting to inspect the map OPD, which shows (in diopters) refraction in the pupil. Astigmatism caused by corneal deformation is "irregular"; Indeed, we observe that there is a difference of distribution of optical power between the halves upper and lower of the pupil.

map OPD pellucid marginal degeneration

Map OPD, which shows the distribution of the optical power in the pupillary area (studied diameter: 6 mm). The diopter power variations ("local myopia", since the numbers are all negatives) are stackable to changes in curvature of the anterior cornea. There is the superior pupil hemi presents less less than the hemi pupil average refractive power.

The OPD map restricted to the only aberrations of high degree to objectify their effect on local refraction in the pupil. This map shows the residual defects of powers, when we correct the eye of myopia and regular astigmatism (cylindrical sphero 'ideal' bezel glass). In the absence of optical aberrations of high degree, this map would show no residual refractive error. In the case of a pellucid marginal degeneration, the asymmetrical deformation of the cornea engenders a high rate of optical aberrations; so there is still a wide uncorrected refractive error rate:

map OPD HO pellucid marginal degeneration

Map OPD for the only aberrations of high degree. Despite an optimal correction of myopia and the regular component from astigmatism, there is still a significant refractive error that is related to the existence of the irregular deformation of the cornea; observed fluctuations whose amplitude reaches almost 10 diopters between certain areas of the pupillary area. Those errors explain that the optical quality of the eye should be amended, and some visual symptoms.

The joint study of the ocular Wavefront of the entire eye and corneal wave front is instructive. She obviously confirms corneal origin of wave-front aberrations; the phase shifts of the Wavefront eye total (entire eye) looks quite superimposable than previous ocular wave-front. The 'internal' wave front, calculated by subtraction, reveals a "reverse" aspect There is a slight mitigation of previous corneal aberrations internally. This is actually related to the posterior side of the cornea. If the deformations at this level are parallel to those of the front (less pellucid degeneration corneal thinning has consequences on the corneal wall and so on both corneal faces), the change of index of refraction between the corneal stroma and aqueous humor ' separated by the posterior face of the cornea) is of opposite sign that separates air from the cornea. "Equal" geometry, as the index of refraction of the cornea is higher than that of aqueous humor, the effects on the phase of the wave front are opposed.

Wavefront aberrations pellucid degeneration

The total Wavefront maps (entire eye) and corneal are nearly identical. On the other hand, the present internal wave front opposite a topology; the posterior face of the cornea being distorted jointly to the front, it affects the rate of eye optical aberrations, but in the sense of a reduction of the effect of the deformation of the front (the gradient index between cornea and aqueous humor bathing the anterior Chamber of the eye is of opposite sign communities separated by the front of the cornea - air and horny face).

 

The pellucid marginal degenrerescence of the cornea is a condition which induced a remarkable corneal distortion, because its axis is very close to the vertical direction (90 °). Thinning corneal lower causes a local "casesure", with camber marked next to it. Since the casesure is very less, the laws of the conservation of the average curvature of the cornea dictate the occurrence of a flattening joint, which predominates in the vertical corridor in the case of this affection. The induced aberrations are predominantly type coma and trefoil. The axis of these aberrations is close to 90 ° (it comes terms Z (3, -1) and (Z3, 3).)

Here's a representation separate from these modes coma and trefoil, extracted from the eye full, corneal and internal wave fronts (dominated by the effect of the posterior cornea). For each, a PSF is calculated (black and white box). The combination of these aberrations (lower line) provides a distortion of the Wavefront who marries one of the tested eye. Beware, the PSF generated by this combination is not equal to a simple "superposition" of the respective PSF of the aberrations, coma and trefoil.

coma trefoil pellucid marginal degeneration

Representation of phase shifts caused by aberrations of type coma (upper line), trefoil (Central line) and the combination coma + trefoil (lower line). These aberrations are typically high in case of pellucid marginal degeneration. They are responsible for the degradation of the quality of the retinal image, which persists despite the correction of the regular component from astigmatism by glasses. Rigid contact lenses can neutralize some of these aberrations, but "unmask" their internal component (last column).

 

3 responses to "pellucicide degeneration"

  1. Nabil says:

    Hello
    I am now 41 years old. I have a keratoconus been detected for 20 years. I wear semi rigid lenses but recently I learned that I had a form of keratoconus marginal pellucicide degeneration with a large astigmatism because when I remove my lenses I could read a book before but since I can no longer. can tell me what the consequences and is what this can lead to a corneal transplant knowing that the correction with lenses is done very well.
    Thanks

  2. Dr. Damien Gatinel says:

    It is probable that you present a form of Keratoconus whose topographical characteristics evoke a "marginal pellucicide degeneration" of the cornea (lower thinning with characteristic deformation in topography). It is necessary to monitor the possibility of eye friction (to interrupt) and the sleep position (belly or side, to be modified for sleep position on the back). For the correction of vision, it is always better to wear lenses than to undergo a corneal transplant.

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