Corneal Topography Aim The aim of corneal topography is to obtain detailed, accurate data about the corneal contour and display it in a clinically useful format. Topography display Measurements Height A more sensitive way of presenting height data is to plot the difference in height from a sphere of known size or from an idealized cornea shape. Elevation maps Astigmatism interprtation Flat axis appearance BFS Flat axis Steep corneal appearence Steep axis BFS Curvature It is a sensitive measure of variation in contour across the corneal surface Global (axial/sagittal): Measurements are made relative to visual axis. Less accurate in the corneal periphery and for irregular surfaces. Local ( instantaneous / tangential ) ` Curvature of each point is calculated with respect to its neighbours More accurate Power Refractive power is a slightly less accurate measure of corneal contour than curvature Display Three dimensional wire mesh representation, for detection of gross distortion. Two dimensional colour coded contour mapping: Colours Warmer colours (red, orange, yellow) Represent the steeper areas. Cooler colours (green, blue) Represent the flatter areas. Scales Absolute/standardized scale There is a fixed colour coding system. The same colours always represent the same curvature of powers. Normalized/relative scale Uses a Fixed number of colours (11) which are automatically adjusted to fill the range of diopteric values for that single map. Adjustable scale It enables the operator to select the step interval and diopteric range of the contours so the topographic information can be displayed to optimum effect to emphasize a certain information, used to compare the clinical situation. Axes Axis is the meridian of either greatest or least slope Orthogonal axes Represent the major and minor meridians.they are determined by averaging the power from the rings within the central 3mm zone along every meridian Zonal axes Are the steepest and flattest meridian within the 3 mm, 5 mm, and 7 mm zones Instantaneous axes Are the true major and minor axes.they are continuos lines joining points of max.or min. power on each ring axes Statistical indices Corneal asphericity Q Classification of normal corneas Round: 22.6% Classification of normal corneas Oval : 20.8% Classification of normal corneas Symmetric bow-tie: 17.5% Classification of normal corneas Asymmetric bow-tie: 32.1% Classification of normal corneas Irregular:7.1% Artifacts of corneal topography Alignments and focusing Artifacts of corneal topography Tear film irregularities Pooling of tears in lower meniscus lead to focal steepening Artifacts of corneal topography Tear film irregularities Thinning of the tear film by drying lead to localized flattening keratoconus Topographic features VKS mires are distorted They lie closest together in the inferiocentral region where the cornea is steepest and furthest apart superiorly where the cornea is flattest Colour coded curvature map Show an asymmetric bow-tie corresponding to the exaggerated prolate shape and irregular astigmatism of the corneal surface Diagnostic features of keratoconus Central corneal power greater than 47 D A difference of 3 D or more in corneal power comparing points 3 mm inferior to the center to points 3 mm superior to the center Asymmetry between central corneal power of the fellow eyes in excess of 1 D Keratoconus detection indices Keratoconus predictability index(KPI) KPI is derived by inputting eight quantitative indices into an automated keratoconus detection algorithm Sensitivity 68% specificity 99% accuracy 90% Keratoconus detection indices Keratoconus predictability index(KPI) Classification of keratoconus Severity Is related to the difference in power between the steepest and flattest portions of the cornea. The more sever cases are also associated with increasing surface irregularity Location Most commonly inferiorly or inferiocentrally Location Rare - central Location Rare - superior Shape Most cones have an oval shape involving one or two quadrants Globus cone,cone is extensive and involve up to three quarter Nipple cone,much more localized and are completely surrounded by relatively flat (normal cornea)