Keratometry Readings From Different Devices Not Interchangeable for Patients With Keratoconus

Keratometry readings can vary significantly between patients with and without keratoconus when examined using anterior segment optical coherence tomography (AS-OCT) or a Scheimpflug-based imaging device, according to findings published in BMC Ophthalmology. Researchers report that the AS-OCT had higher readings than the Scheimpflug and that these devices cannot be used interchangeably.

Researchers conducted a prospective, observational study to assess the level of agreement of keratometry readings, central corneal thickness (CCT), and thinnest corneal thickness (TCT) measurements obtained by an AS-OCT and a Scheimpflug-based imaging device in eyes with and without keratoconus.

The study included 62 eyes with topographic evidence of KC and 48 normal eyes. Participants underwent cycloplegic refraction, spectacle best-corrected distance visual acuity (BVCA), comprehensive slit-lamp biomicroscopy, and fundoscopy. Participants also underwent corneal topography.

The study found that BCVA, intraocular pressure, and CCT measurements were significantly lower in the KC group compared with the control group. TCT measurements in patients examined with Scheimpflug-based imaging and AS-OCT were lower among patients in the keratoconus group compared than those in the control group (470.9 and 455.7 µm vs 541.9 and 518.7 µm, respectively).

Researchers also report a significant difference between patients with and without keratoconus regarding anterior keratometry readings detected by Scheimpflug-based imaging (47.1 vs 43.6 D, respectively), and between patients with and without keratoconus detected by OCT (52.3 vs 47.8 D, respectively).

Comparing keratometry readings among the KC group by both techniques shows significant differences between OCT and Scheimpflug-based imaging (52.3 vs 47.1 D, respectively). The same significance was found in the control group, according to the study authors.

The researchers explain that it can be difficult to differentiate subclinical forms for keratoconus for normal corneas, despite the fact that moderate to severe keratoconus is easily recognizable by the characteristic topographical pattern and classic clinical signs. For this reason, AS-OCT is often used to produce reliable pachymetry maps to identify the disease, as well as corneal thinning, prior to laser refractive surgery.

Regarding the results, the study notes that Scheimpflug-based imaging and AS-OCT provided comparable corneal pachymetry readings with good agreement in the study group with accurate identification of KC eyes. However, due to the observed difference for keratometry readings, there are clinical implications of note.

“Our findings suggest that most of the keratometric values cannot be used interchangeably between two devices with significant inter-method variability,” the researchers report.

Study limitations include relatively small sample size and the lack of inclusion of different grades of KC or eyes with different prior treatments.