THE CORNEAL THICKNESS SPATIAL PROFILE (CTSP) AND THE PERCENTAGE THICKNESS INCREASE (PTI)

The CTSP graph displays the sequence of pachymetric values along concentric circles of increasing diameter, beginning at and centered on the corneal thinnest point TP. The original analysis was performed using twenty-two circles centered on the thinnest point with increasing diameters at 0.4mm-steps (FIGURE 2).¹⁴

The Ambrósio2 color palette (FIGURE 3) was developed for pachymetric maps, considering statistical values found in a study comprising of 226 normal corneas and 88 keratoconic corneas (Ambrósio, Caiado & Bonfadini, unpublished data 2009). In the normal population, the mean thinnest point (TP) value was about 550µm and standard deviation (SD) of 30 µm. The green color was centered on the 550 and the shades of darker and lighter green were calculated to be within 1SD. The best cut off value in the Receiver operating characteristic (ROC) curve for keratoconus and normals was around 500µm, which was set at the yellow threshold. Finally, the mean TP was about 450µm for keratoconus corneas, which was set as the threshold for the red color. In a cohort of 34 corneas with Fuchs’ Endothelial Dystrophy mean TP value the best cut off value in the ROC curve were 625µm and 600µm, which was set for the green to blue threshold.

The experience with the Ambrósio2 pachymetric scale was designed to facilitate the differentiation of thin and thick corneas, as well as to provide important information regarding the thickness profile. The distribution of the colors around the TP correlates well with pachymetric distribution graphs (FIGURE 4).

The initial study involved 46 eyes with mild to moderate keratoconus and 364 normal eyes. Significant differences were found between normal eyes and those with keratoconus (p<0.01), along all positions of the CTSP. Eyes with keratoconus had much lower (thinner) values. It was estimated that keratoconic corneas were, in average, 27.3 micron thinner than normals.¹⁴

Figure 3. Ambrosio2 color palette suggested for use on corneal thickness maps.

Figure 4. Corneal thickness spatial profile (CTSP) and percentage thickness increase (PTI) graphs.

The data for the patient’s corneal thickness is plotted in red while the population average and 95% confidence interval values are plotted in black.

The percentage of thickness increase (PTI) from the corneal thinnest point (TP) is calculated using a simple formula: (CT@x - TP)/TP, where x represents the diameter of the imaginary circle centered on the TP with increasing diameters as provided by the CTSP. In the original study, significant differences were also found for all positions of the PTI between normal eyes and those with keratoconus (p<0.0001), in which keratoconus had much higher rate of increase in thickness toward the periphery.¹⁴ The CTSP and PTI graphs present the patient’s data in red. The three dark broken lines in the graph represent the upper and lower double standard deviation (95% confidence interval) and the average values from a normal population (FIGURE 4).

The CTSP and PTI graphs provide information which allows the clinician to differentiate a normal thin cornea from one with early ectatic disease (FIGURES 5A and 5B).

Figures 5A & B. Scheimpflug images (upper) and CTSP and PTI graphs (lower) comparing a cornea with normal thickness progression (A) to one with abnormal thickness progression (B).

The picture and graphs on the right show a cornea with ectatic disease.

Figure 6. Scheimpflug image (upper) and CTSP and PTI graphs (lower) showing an abnormal thickness progression secondary to corneal edema due to endothelial dysfunction. The PTI graph shows a flattening typically seen with early edema.

In addition, the thickness profile also enables the clinician detect early edema, in which the change in thickness from the center towards the periphery is decreased (PTI graph is flattened) (FIGURE 6).

Usually the thickest hemi-meridian is nasal and the thinnest is temporal and inferior. The averages of the pachymetric values along each meridian enable the detection of the meridians with maximal (fastest) progression and minimal (slowest) progression. The pachymetric progression indexes (PPI) are calculated for all hemi-meridian 360º, starting from the thinnest point. For this calculation, the increase in thickness from the thinnest at each point of the cornea is referenced to a mean normal population database.¹⁴ The meridians with maximal (PPI Max) and minimal (PPI Min) pachymetric increase are noted along with their axes (FIGURES 5A-B and 6). The arithmetic average of thickness on the 1mm, 2mm,

3mm, 4mm and 5mm diameter rings is represented as the average progression index (PPIavg).

This metric has a statistical significance when comparing normal patients (0.91 ± 0.23mm) to those with keratoconus (1.81 ± 1.16mm) (p<0.05). Normal corneas typically have an average progression index lower than 1.2 and a CTSP and PTI lines within the 95% CI limits. However, there is an overlap between normal and keratoconic eyes. For example, 7% of normal eyes have an average progression index between 1.2 and 1.8. In addition, 10% of the cases with clinical keratoconus have an average progression index lower than 1.2 and may have a CTSP and PTI within the normal limits. Long-term longitudinal studies to evaluate ectasia progression will be required to further refine our observations.

Rarely, the CTSP and PTI lines may appear aberrant. This may occur with early or atypical corneal edema. In such cases, it is important to evaluate the Scheimpflug images, looking for signs of edema (higher reflectivity) or the “Camel Sign” - a second hump on the densitometry graph, at the level of the Descemet’s Membrane, which correlates with corneal guttata (FIGURE 7).

Figure 7. Scheimpflug image of an edematous cornea with corresponding densitometry graph (green). The densitometry graph shows the “camel sign”, an increase in the reading at the level of Descemet’s membrane which correlates to corneal guttata. To the right of the densitometry graph is a specular microscopy image showing a reduced endothelial cell count and corneal guttata consistent with Fuch’s Dystrophy.