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12.2 Simplifying preoperative keratoconus screening
by Prof. Michael W. Belin, Prof. Renato Ambrósio Jr,
Andreas Steinmüller, MSc
The original version of the Belin/Ambrósio Enhanced Ectasia Display changed the way we screened
patients for sub-clinical ectatic disease. It was the first screening tool to fully exploit the benefits of
Scheimpflug derived optical cross-sectioning tomography. It has been shown that anterior curvature
and ultrasonic pachymetry, alone, do not provide enough information to detect early disease. The
Belin/Ambrósio Enhanced Ectasia Display (BAD display) was designed to utilize the data supplied by a
Pentacam® rotating Scheimpflug camera and provide a comprehensive keratoconus screening display.
The display combines the anterior and posterior elevation and pachymetric data into one all-inclusive
display giving the clinician a more complete overview of the corneal shape and allowing for quick
and effective screening of refractive surgery patients. The combination of anterior and posterior
elevation and complete pachymetric data gives the clinician a more complete view of the structure
of the cornea and allows for earlier detection and more effective screening than was possible with
previous systems.
The original display (Belin/Ambrósio Enhanced Ectasia Display – software release # 1-16b96)
showed both anterior and posterior elevation data relative to a standard BFS calculated at a fixed
optical zone of 8.0 mm. Fitting a BFS to the central 8.0 mm zone has been shown best for clinical
interpretation and allows for the generation of standardized normal values. The original display
also showed anterior and posterior elevation values relative to the ‘enhanced reference surface’
computed by determining the BFS from the central 8.0 mm zone after excluding all the data from a
3.5 mm optical zone centered on the TP of the cornea. In the case of keratoconus or ectasia, the cone
will have the effect of steepening the BFS. This steepened BFS will actually minimise the elevation
difference between the apex of the cone and the BFS. By eliminating the conical portion of the
cornea from the BFS computation, the “enhanced reference surface” serves to further accentuate
ectatic or conical protrusion, while having little if any effect on normal corneas. The Belin/Ambrósio
Enhanced Ectasia Display then computes the change in elevation values going from the standard BFS
and the enhanced BFS. This change (elevation change between the standard BFS and enhanced BFS)
has been shown to be a key differentiator between normal and ectatic corneas.
The second component of the Belin/Ambrósio Enhanced Ectasia Display is a comprehensive
pachymetric evaluation. Both pachymetric values at the apex and TP are displayed and the
displacement of the TP from the corneal apex is calculated along with the direction of the
displacement. The distance between the TP and the geometric central point is significantly higher in
keratoconus. Graphical representations of the progressive thickening of the cornea from the TP to
the periphery are depicted in the CTSP. The PIT refers to the percentage of increase in thickness from
the TP to the periphery. The data from both graphs are calculated from the pachymetric values at 22
concentric rings centred on the TP. Corneas with ectatic disease (e.g. keratoconus, post LASIK ectasia)
show a more rapid progression of thickening from the TP to the periphery. This increase follows a
normal pattern and is a strong differentiator between normal and keratoconic corneas.
A more intuitive way of saying the same is that ectatic corneas thin more rapidly than normal eyes
going from the periphery to the thinnest part of the cornea. The CTSP and PIT display provides the
average progression derived from a normal population (centre line) and 95% confidence interval
(upper and lower black lines) against the patient’s own data shown in red. This allows the clinician
to differentiate a normal thin cornea from one with early ectatic disease. The ‘progression index’ is
calculated as the progression value at each meridian from the TP. The average of all meridians and
the meridian with maximal and minimal progressions are displayed. These parameters allow for the
differentiation of a normal thin cornea versus one with ectasia, as well as of a normal thick cornea
versus one with early edema.
12 Belin/Ambrósio Enhanced Ectasia Display
