Figures 1A and 1B. Axial map (A) and tangential map (B) of the same eye. The cursor denotes the location of the corneal apex. Note the corneal apex on the axial map appears further from the centre of the cornea as compared to the tangential map. The tangential map reveals the curvature of the apex to be 7.47 mm which is steeper than that of the axial map (7.66 mm).
Richard Lindsay and Associates, East Melbourne
Over the past 25 years, videokeratoscopy has transformed the measurement of corneal topography. One of the key advantages of being able to measure corneal topography is that it now allows us to detect keratoconus at an earlier stage.
Prior to the introduction of videokeratoscopy, it was probably thought that about 10 per cent of cases of keratoconus were unilateral. We now know that this is simply not correct. Subsequent evaluation of corneal topography shows us that nearly all of these ‘unilateral’ cases are bilateral with early but definite signs of keratoconus noted in the better eye. In fact, clinical experience tells us that true unilateral keratoconus is extremely rare and probably occurs in less than 0.5 per cent of cases of keratoconus.
Detecting early or sub-clinical keratoconus by assessment of corneal topography can still be challenging, especially when only very subtle corneal changes are noted. One way practitioners can improve their chances of diagnosing keratoconus is to use the ‘tangential’ (also known as ‘instantaneous’ or ‘true’) radius to measure the corneal shape.
Two types of radii are used in videokeratoscopy: axial and tangential. The axial radius (also known as ‘sagittal’) is also the radius that is measured in keratometry. It is the distance from a point on the cornea to the optic axis of the videokeratoscope when it is aligned with the cornea. The axial radius tends to be the default setting for most clinicians when using videokeratoscopy.
The tangential radius is the other type of radius and it is independent of any axis; rather it is based on only the local curvature at each corneal point and therefore it is often referred to as the ‘true’ or ‘instantaneous’ radius).
With peripheral corneal flattening, the tangential radius will always be longer than the axial radius for each peripheral corneal point. Conversely, for peripheral corneal steepening the tangential radius will always be shorter (steeper) than the axial radius for each corneal location as you move away from the visual axis.
Local changes in corneal shape, such as those occurring in keratoconus, are seen most clearly when using tangential radius measurements. Figure 1A shows the axial map of a patient with very early ‘sub-clinical’ keratoconus in his right eye. Note that the patient has advanced keratoconus in his left eye. With this corneal map, it is not clearly evident that this eye has keratoconus. Figure 1B shows the tangential map of the same cornea. It can be seen from this corneal map that the patient definitely has keratoconus in this eye.
Note also that when comparing the two corneal maps, it can be observed that the tangential map reveals the corneal apex to be closer to the centre of the map and also shows the true central curvature, which is much greater (steeper) than that obtained with the axial map.