Electrophysiological Changes in Cases of Different Stages of Keratoconus

Overview

Keratoconus is a bilateral and asymmetric disease which results in progressive thinning and steeping of the cornea leading to irregular astigmatism and decreased visual acuity.A typical patient with keratoconus presents in the teens or early twenties with complaints of blurring or distortion in vision and having to change glasses frequently due to changes in refractive error. Corneal thinning frequently precedes ectasia. In moderate and advance cases, a hemosiderin arc or circle line, known as Fleischer's ring, is frequently seen around the cone base. Vogt's striaes, which are fine vertical lines produced by Descemet's membrane compression, is another characteristic sign. Most patients eventually develop corneal scarring. Munson's sign, a V-shape deformation of the lower eyelid in downward position; Rizzuti's sign, a bright reflection from the nasal area of the limbus when light is directed to the limbus temporal area; and breakages in Descemet's membrane causing acute stromal oedema, known as hydrops, are observed in advanced stages. Corneal blindness is one of the major causes of reversible blindness, which can be managed with transplantation of a healthy donor cornea. It is the most successful organ transplantation in the human body as cornea is devoid of vasculature, minimizing the risk of graft rejection. Preoperative clinical evaluation of visual potential after corneal transplantation in the patient with dense corneal opacity remains difficult using only history and biomicroscopic examination. Some studies found that electrophysiologic tests were good predictors of visual prognosis prior to corneal transplantation. Electrophysiology is an objective functional test of the visual pathway and allows the location of visual dysfunctions to be detected. Full-field electroretinography (ERG) is an established clinical technique used to evoke and record mass responses of the retina to flashes of light. The ISCEV Standard full-field ERGs assess generalized retinal function under dark-adapted (DA) and light-adapted (LA) conditions.The visual evoked potential (VEP) is a cortical response and serves as a functional test of the entire visual pathway from the eye to the visual system of the brain.

Aim of the Study To assess the effect of different stages of keratoconus on electrophysiology test results. All patients will undergo a comprehensive ocular examination including: 1. Best corrected visual acuity (BCVA). 2. Anterior segment examination using slit lamp bio microscopy. 3. Posterior segment examination using a +78 D condensing lens and peripheral retinal examination by means of binocular indirect ophthalmoscopy (keeler ophthalmoscope). 4. Penatacam examination to define grade of KC and corneal topography. 5. OCT-ONH to assess the function of optic nerve. 6-Electrophysiological tests: All subjects will be examined using: 1. FVEP: via the assessment of the amplitude \&latency of P2 wave 2. PVEP:via the assessment of the amplitude \&latency of P100 wave 3- Full field flash ERG: via the assessment of the amplitude \& implicit time of a\&b waves