Study: Not all topography devices are alike

Take-home message: Subclinical haze after CXL can affect corneal topography measurements; a comparison shows that measurements with four different devices cannot be used interchangeably.

By Vanessa Caceres; Reviewed by Aarti Agrawal, MD

Bangalore, India-Devices used to measure corneal topography after collagen crosslinking (CXL) should not be used interchangeably, said Aarti Agrawal, MD.

In a study, it was found that four commonly used devices had variable measurement results, and that subclinical corneal haze can affect measurements obtained with commonly used topography devices, said Dr. Agrawal, Narayana Nethralaya, Bangalore, India.

Dr. Agrawal and fellow researchers included 60 eyes from 60 patients with keratoconus who had undergone accelerated CXL a year before. All eyes were measured with three Scheimpflug-based devices and one slit-scanning imaging device. The devices used were the Orbscan II (slit-scanning beam), Galilei G6 (Placido disk with dual Scheimpflug), Sirius (single Scheimpflug with Placido), and Pentacam HR (single Scheimpflug).

Each eye followed a standard methodology-it was aligned to the visual axis by the central fixation light, and a single operator captured three measurements from each eye from all machines. Measurements captured were thinnest corneal thickness as well as steep (K1) and flat (K2) keratometry and astigmatism.

Patients with previous lamellar surgery, corneal scarring, penetrating keratoplasty, and corneal ring implantation were excluded from the study.

Researchers found that the thinnest corneal thickness was most variable with the Orbscan as compared with other devices.  The within-subject standard deviation and coefficient of repeatability with Orbscan showed the highest variability for all the corneal parameters in comparison with other devices.

Among the Scheimflug, Galilei was found to be the most repeatable and in agreement for the various corneal parameters, followed by Sirius and Pentacam, Dr. Agrawal said.

One clinical implication to keep in mind is that post-CXL, the cornea behaves differently, and the interpretation of topography should be done with caution, Dr. Agrawal said.

“There can be subclinical haze which might not be evident on slit lamp examination,” Dr. Agrawal noted.

 Corneal haze was quantified using the densitometry software in Pentacam HR.

Dr. Agrawal and fellow researchers are working on a correction factor that can be applied in each of the device in presence of haze.

Aarti Agrawal, MD

E: aartiagrawal1988@gmail.com

This article was adapted from Dr. Agrawal’s presentation at the 2016 meeting of the American Society of Cataract and Refractive Surgery. Dr. Agrawal did not indicate any financial interest in the subject matter.