Normal Belin–Ambrosio display difference map in keratoconus

Dear Editor, Corneal tomography is one of the most important diagnostic modalities used to screen candidates for refractive surgery.[1] The Belin–Ambrosio enhanced ectasia display (BAD) was developed as a screening tool to diagnose cases with early keratoconus (KC) and to detect cases potentially at risk of developing ectasia post-refractive surgery.[2,3] The display was designed to combine the elevation data from the anterior and posterior cornea and the pachymetry data to provide a more comprehensive display of ectasia screening. One of the earliest features developed in the BAD map was the difference map display which uses an enhanced reference surface (ERS). ERS excludes 3.5–4 mm area surrounding the thinnest point and modifies the standard best-fit sphere (BFS), which is usually of 8 mm. In an unaltered cornea, ERS is flatter than the standard BFS and results in an amplification of the elevation points on the anterior and posterior cornea. When the difference between the two comparisons in the anterior and posterior elevation maps is 5 and 12 microns respectively, the map displays the suspicious area in yellow, and when it is more than 7 and 16 microns respectively, the map displays the ectatic area in red. This legend helps to screen the candidates easily. Fig. 1 shows the Pentacam quad map and the BAD display difference map of the right eye of a patient who came for refractive surgery screening to the LV Prasad Eye Institute. This 24-year-old patient was diagnosed with KC based on the typical tomographic findings. The flattest keratometry value (K1) was 42.9 D, the steepest keratometry (K2) was 45.0 D, the mean keratometry (Km) was 43.9 D, and the maximum keratometry (Kmax) was 46.1 D. The thinnest location was displaced from the apex, and the thickness at the thinnest point was 477 µm. Significant keratometric asymmetry, displaced location of the thinnest point from the apex, and posterior elevation helped clinch the diagnosis of KC. However, the BAD difference map was normal, showing no difference in elevation points.Figure 1: (a) Pentacam quad map (above) and the enhanced ectasia display (below) showing typical features of keratoconus with a significant inferior–superior keratometric asymmetry and displaced thinnest location on the pachymetry map. (b) Belin–Ambrosio display showing normal difference map, normal curve in CTSP, and normal curve in the PTI graph. Df, Db, Dp, and Dt are normal, with Da being in the suspicious range resulting in a suspicious D-value highlighted in yellow, instead of the D-value in red zone as would be expected. CTSP = corneal thickness spatial profile, PTI = percentage thickness increaseRecently, a report was published where a normal cornea had an abnormal BAD-D display and was deemed unsuitable for refractive surgery.[4] In the case reported, the cornea was thick and had a lower-than-normal horizontal white-to-white (HWTW) diameter resulting in an abnormal display. There are studies that have reported that the BAD display is influenced by the HWTW diameter.[5] HWTW in the eye being reported here was 12.5 mm. To know whether a slightly larger HWTW influences the BAD map to display a normal difference map, larger studies need to be conducted. A recent study by Ding et al.[6] also reported that the BAD map might underestimate the risk of corneal ectasia in larger corneas. This case highlights that while BAD display is known to be sensitive in the diagnosis of early KC, certain unexplained factors may still lead to a false-negative display, and the factors responsible need to be further explored. Financial support and sponsorship: Nil. Conflicts of interest: There are no conflicts of interest.