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).
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.
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:
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.
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.