Tsag-Nsag is the new parameter added to the ESP software that will quantify scleral asymmetry and give eye care givers better control over lens fits.
Literature describes the flattening of the sclera on the nasal side of the eye, compared to the temporal side which shows no such behaviour, which introduces an asymmetry to the eye surface. A pronounced asymmetry between the nasal and temporal scleral curves will impact lens fit.
In the image above you see a horizontal cross section of a typical eye surface. The sagittal height is calculated over a total chord of 16mm in this example. Tsag and Nsag each get attributed exactly half of the total chord length and the respective sagittal heights are determined from the apex outwards.