Review aberrometrique (OPD)
Reviewing aberrometrique allows the objective study of the optical quality of the eye.
It allows the collection of the Wavefront, and study the optical aberrations that distort it, depending on the diameter of the pupil.
The OPD includes optical analysis techniques, which were initially used in non-medical disciplines such as astronomy in order to better understand and correct the imperfection of optical telescopes. The reduction in the cost of the opto-electronic components has allowed to develop over the past 10 years of the aberrometres easy to use and affordable for the study of the optical properties of the eye. The aberrometer is a measuring instrument which, applied to the eye, for measuring the optical quality.
Benefits of the measure by OPD
In ophthalmology, the aberrometric will detect and quantify some optical defects whose eye is tainted even when is corrected with glasses or contact lenses. "" Not measurable and quantifiable before the appearance of the OPD in ophthalmology, these optical defects were grouped under the term of "irregular astigmatism. Their presence was suspected when a patient complained to ' blur ', especially in certain circumstances, despite an optimal correction in its refractive defect bezel (ex: myopia and astigmatism). Reviewing aberrometrique allows you to correlate visual symptoms (like double vision by monocular) with a special optical anomaly (ex: coma). The OPD is based on the study of ocular wave-front. His interest is twofold: qualify and quantify aberrations of high level measured at the breast of the wave-front.
Indications of the OPD
The OPD is indicated to explore the causes of reduced quality of vision: it allows the exploration stage optical vision, which begins with the corneal penetration of the light emitted by the object observed and ends with its interaction to the level of the photosensitive retinal pigments. The symptoms of appeal are related to the degradation of the optical quality of the eye corrected for the refractive optical defect (myopia, hyperopia, astigmatism). Their cause may be made of the cornea or the lens.
Symptoms induced aberrations of high degree
Not correctable by glasses optical defects are essentially perceived by the patient during certain vision conditions)vision in the dark = mesopic vision). They induce the perception of Visual as symptoms the halos, the presence of "bright spicules" goshawks of bright lights, the perception of"ghosting". "(ex: a line of subtitle of film in VO is seen as split). They are most frequently found in patients with abnormalities of curvature or regularity in eye refractive surfaces (horny, crystalline). They increase when the pupil expands (which occurs when the ambient brightness decreases: the night of course, but also in a room little lit, etc...).
Causes of aberrations of high degree
Deformations of the cornea are a classic source of high degree aberrations: patients with old techniques of refractive surgery such as the radial keratotomy, patients with Keratoconus, corneal trauma victims, or who received a cornea transplant are often those who found a higher rate of optical aberrations of high degree.
Optical aberrations of high degree can also come from the lens, especially when it is clouds. Nuclear cataract is a common cause of elevation of certain optical aberrations of high degree (negative spherical aberration, trefoil, etc.). The conventional OPD is based on the study of ocular wave-front. Instruments coupled with a topographer (ex: OPD SCAN, Nidek) to separately calculate the contribution of the cornea to the optical aberrations of high degree.
Characterization of the aberrations of high degree
The optical aberrations of high degree are quantified by an RMS (Root Mean Square) rate. This figure is calculated from the residual phase of ocular wave-front when the best correction bezel (the sphero-cylindrical defocus correction) is obtained. If it is zero (which never happens in practice), the eye is devoid of optical aberrations of high degree; There are no irregular astigmatism. The optical quality of such an eye is limited only by diffraction.
A RMS of aberrations of high rate zero corresponds to the presence of an irregular astigmatism. This rate depends on the pupillary diameter at which it was recorded. It tends to increase with this diameter, as pupillary dilation unmasks these refractive anomalies on the outskirts of the pupil in general.
The presence of aberrations of high degree corresponds in practice to the existence of a focus of the imperfect light and even in optimal correction of myopia and astigmatism regular case. The effect of these aberrations appears especially in case of pupillary dilation. Positive spherical aberration induced for example a deterioration in the quality of the retinal image resulting in practice in the induction of Visual "halos".
Aberrometry and custom laser corrections
The realization of a photoablatif laser treatment (ex: conventional myopic LASIK) on a optical size box too reduced or offset can also be the source of an increased rate of optical aberrations of high degree. Performed LASIK procedures there are ten years did not have the same degree of passive safety that today ' today, and were often issued on optical areas closer than today ' hui. At the time, there were few aberrometres to achieve common measures in clinical practice.
Today, the measure of the IRIS pupil is performed accurately by the aberrometer, as well as the rate of optical aberrations not correctable by glasses (ie the aberrations of high degree) from the preoperative consultation. When they have a higher than normal rate and / or are responsible for a major gene that is perceived by the patient, a treatment of "personalized" or "custom". can be proposed. It is carried out in order to process these pre-existing optical aberrations in addition to the error of initial refraction (myopia, astigmatism, ect...).
In all case, the same treatment not personalized aims to minimize the induction of aberrations of high degree ; optimal balance, choice of the most suitable optical area, the pupil and the depth of ablation of laser treatment ect... The OPD used however to increase this level of customization and correct optical aberrations of high degree pre existing.
A mapping of the iris allows for example to access the photoablation with Iris recognition (ex: Zyoptix system). This technology is particularly interesting for the treatment of the strong astigmatismes. The footprint of the iris is scanned, then sent to the laser. This uses the map of the iris to align and Center the treatment during the intervention, for even greater accuracy of delivered treatment. Currently, it is possible to compensate for real-time movements recorded during the phase of laser correction, eye of translation and rotation.
Clinical examples of the interest of the OPD in ophthalmology are available on this link: OPD examples