Chapter 16: Topographic Disparity

Topographic Disparity, asymmetric, nonorthogonal bow tie, irregular astigmatism, double-angle vector diagram With so many different corneal parameters displayed on the various topographers and tomographers which do you think most accurately describes the corneal irregularity? Well I use the I-S index or inferior/superior index on a daily basis to quickly determine inferior steepening as one indication of possible keratoconus. I also use the topographic disparity or TD. This is an excellent vectorial quantification of irregularity. It looks at the vectorial difference between the superior and the inferior arms of the meridian and magnitude of the astigmatism and so in a perfect world the topographic disparity would be 0, that means you have no irregularity. In some eyes you can go up to 5 and so it is quite a significant range. Every topographer has a different way of measuring irregularity so the TD is a standardized parameter that can be displayed on every topographer. You mentioned that the topographic disparity is available on topographers. How does the doctor access this? The potential for availability is there. You need to connect the topographer or tomographer with the iASSORT software and that gives you topographic disparity and a number of other parameters such as ORA, Ocular Residual Astigmatism. The iAssort software uses all the measured data acquired from the device in the calculations. How does addressing the topographic disparity or TD reflect on visual outcomes? It is like looking through a distorted or wonky window. When you have a clear glass plane you see more clearly than you do through a distorted window and in the same way, if you reduce topographic disparity you reduce irregularity and you are going to have an improvement in your best corrected vision. It is important to note that the TD is related to the ORA so that the greater the TD the higher the ORA