Assessing Accuracy of Total Corneal Astigmatism Measurement in the Absence of Lenticular Astigmatism

2017

Author: Ben R. LaHood, FRANZCO, BM BCh

Contributors: Michael Goggin, FRANZCO, FRCOphth

Purpose:

The only source of refractive astigmatism in eyes with spherical IOLs is assumed to be the cornea. A device that measures corneal astigmatism accurately should provide values that closely agree with refractive astigmatism in such eyes. This study compares measurement of corneal astigmatism by 3 devices to refractive astigmatism in eyes with spherical IOLs to determine the most accurate device.

Methods:

40 consecutive eyes implanted with spherical IOLs and corrected visual acuity 6/7.5 or greater were examined. Eyes with corneal, capsular bag or macular pathology were excluded. Manifest refraction was carried out by two experienced clinicians with refinement of the cylinder power to 0.12D. Corneal power was measured with the Cassini and the Pentacam with CorT values derived using iASSORT software. These devices measure the posterior cornea. Automated keratometry values using a Nidek ARK-510 were also obtained. The vector difference between the corneal plane refractive astigmatism and the various keratometric measures was derived. The method that produces the lowest difference is likely to be the most accurate corneal astigmatism measure for toric IOL calculation.

Results:

The summated vector mean (or centroid) difference between the refractive and corneal stigmatism values were as follows. Refraction v. Cassini Total Corneal Astigmatism, 0.08D @ 131 degrees; Refraction v. Pentacam anterior sagittal 3 mm zone 0.52D @ 6 degrees. Refraction v Pentacam CorT anterior surface 0.51D @ 1 degree. Refraction v Pentacam CorT total (anterior and posterior surfaces combined) 0.45D @ 180 degrees; Refraction v. Nidek ARK, 0.54 at 3 degrees.

Conclusions:

The Cassini was most accurate at measuring total corneal astigmatism. Pentacam CorT total approached 0.5D which is clinically significantly larger than the ideal, zero. The Cassini may help to produce better refractive astigmatic outcomes with toric IOLs. Measurement of total corneal astigmatism is progressing but until greater evidence exists, we should persist in anterior cornea nomogram adjustments.