Assessment of benthic foraminiferal assemblages

While there are multiple elevation topographers available for clinical use, there is marked variability in the their respective elevation and pachymetry measurements.^3,4 The Orbscan (Bausch & Lomb), a slit scanning topography system, has been shown to underestimate corneal thickness measurements, specifically in post LASIK eyes.^5-7 This, by definition, translates to errors in the elevation measurement, as pachymetry is calculated directly from the elevation data.^8,9 The Pentacam uses Scheimpflug imaging and, like the Orbscan, measures both elevation and pachymetry using the same data set. The Pentacam has been shown to have excellent agreement with the ultrasound pachymeter with regard to central corneal pachymetry in both pre and post LASIK eyes¹⁰ (FIGURES 3, 4 –pachymetry graphs). Because pachymetry measured by the Pentacam is accurate it can be inferred that the corneal elevation measurements, both anterior and posterior, are accurate, in pre and post LASIK eyes. Unfortunately, this limits the normative data to the Pentacam Eye Scanner and potentially other Scheimpflug devices as long as the same reference shape (i.e. sphere) and area (i.e. central 8.0 mm zone) are used.

In prior work we defined normative anterior and posterior corneal elevation values at the corneal apex and thinnest points for the Pentacam Eye Scanner (Oculus Optikgeräte GmbH).¹¹ Data was initially generated from a retrospective review of 100 eyes of 50 myopic patients who presented for refractive surgery evaluation. This data was later validated on a database of over 1,200 myopic patients. Anterior and posterior corneal elevation measurements as well as corneal pachymetry measurements were documented at the corneal apex and thinnest point. The elevation data used to calculate the reference surface (best fit sphere) was gathered from a fixed 8.0 mm diameter zone centered on the corneal apex.

Corneal maps used to generate the normative values had at least 9.0 mm of corneal coverage

with no extrapolated data in the central 8.0 mm zone. As previously noted, using elevation data from this fixed 8.0 mm area creates an elevation map which allows for easier detection of qualitative and quantitative abnormalities. Additionally, using a defined 8.0 mm zone allows us to standardize the elevation data and generate a normative data set.

The average pachymetry in this study was 550µm at the apex and 547µm at the thinnest point. This is consistent with previously published data.¹² Average anterior elevation values (off the BFS) at the apex and thinnest points were 1.6µm and 1.7µm respectively. Average posterior elevation values at the apex and thinnest points were 0.8µm and 3.6µm respectively.

The range and standard deviations of the elevation values are shown in Table 1.

Detecting subtle, preclinical corneal topographic abnormalities in patients being evaluated for refractive surgery is challenging. Up until this point, the lack of consistent standardized normal elevation values along with numerous elevation imaging modalities have limited the interpretation of elevation maps.^3,4 We have determined that in myopic patients anterior elevation values greater than 5.5µm at the apex or 7.7µm at the thinnest point occur in less than 0.3% of normal myopic patients. Posterior elevation values of greater than 9.8µm at the apex and 17.7µm at the thinnest point also occur in less than 0.3% of normal myopic patients. (The numbers for 2 standard deviations (4.2 and 5.7 anterior & 6.8 and 13.0 posterior) would occur in less than 5 % of normal individuals). Any anterior or posterior elevation values greater than three standard deviations above normal at the apex or thinnest point should raise suspicion of a corneal irregularity.

Just as normal corneal curvature values are different in myopic and hyperopic patients so are normal corneal elevation values. In hyperopic patients average anterior elevation values (off the BFS) at the apex and thinnest points were 0.4µm and -0.1µm respectively. Average posterior elevation values at the apex and thinnest points were 5.7µm and 10.6µm respectively.

Figure 4. Bland-Altman graph of Pentacam vs. ultrasound pachymetry measurements. The plot demonstrates that 95% of the eyes differed in their measurements with the two instruments by +21.8 and -18.9 µm.

The anterior range and standard deviations of the elevation values are shown in Table 2. Although the differences in hyperopic and myopic anterior measurements were significantly different, they are unlikely to appreciably alter our screening parameters as the +2 SD and +3 SD values, often used as screening gates, between the myopic and hyperopic groups are similar (within 1.4µm). For the posterior surface, however, the typically used screening gates (+2 SD/+3 SD) vary greatly as they differ by 6.1 & 6.8µm for the apex and 9.1 & 10.1µm for the thinnest point. These differences must be accounted for when screening hyperopic refractive surgical candidates, and this highlights the need for a normative database specific for a hyperopic population.

TABLE 1 - Normal Corneal Elevation Values for Myopic Patients

Elevation +

TABLE 2 - Normal Corneal Elevation Values for Hyperopic Patients

Elevation + 3 SD (µm)

It is important to note that while the data apply to normal corneas, elevation was measured at the corneal apex and thinnest points only. These are two points that can be easily and consistently identified when looking at multiple elevation maps from different patients.

Only elevation measurements at these two points can be compared to the normative data above. These values should not be compared to areas of elevation in the peripheral cornea that are a function of normal astigmatism.

When evaluating a map for subclinical disease an accurate assessment of image quality must also be made. All Pentacam images should have elevation data taken from a fixed 8.0 mm zone (BFS set to Manual, Float, Sphere, Diameter = 8.0 mm) centered on the corneal apex. This fixed zone standardizes BFS calculation so that a valid comparison of elevation data can be made across multiple patients and images. An 8.0 mm zone for BFS calculation prevents excessive flattening or steepening of the reference surface. Additionally, no extrapolated data should be used to calculate the BFS. As previously noted, extrapolated data is demarcated with either black dots or white areas (user selectable) on the topographic maps. If extrapolated data is used in the BFS calculation this may change the BFS radius of curvature and will change the elevation measurements. This introduces error into the relative elevation measurements and can lead to errors in interpretation. We routinely reimage patients if there is any extrapolated topographic elevation data within an 8mm zone centered on the cornea.

The presentation of myopic and hyperopic normative elevation data for both the anterior and posterior corneal surfaces adds a quantitative component to the typical qualitative interpretation. While pattern recognition remains the quickest and easiest method of interpretation, there are many cases where a closer more meticulous evaluation of the elevation values is necessary. If the elevation measurements fall outside the normal range, further evaluation and subsequent testing may be warranted.