Comparative analysis of corneal thickness parameters using Scheimpflug imaging system and HD-OCT in keratoconic eyes

Introduction
The Scheimpflug camera and optical coherence tomography (OCT) are important tools in diagnosis and follow-up of keratoconus. In our study we evaluated and compared the automatic measurement results provided in keratoconic eyes with the Scheimpflug camera and HD-OCT. Central corneal thickness (CCT), thinnest corneal thickness (TCT) and Y-coordinate of the thinnest point (Y-TCT) were analyzed.

Material and methods
66 eyes of 33 patients diagnosed with keratoconus were included in the study. Each patient underwent examination with a Scheimpflug camera (WaveLight Oculyzer II) and HD-OCT with cornea lens attachment (Zeiss Cirrus 5000) on the same visit.

Results
We did not find a statistically significant difference in CCT value derived with the two imaging systems: 478 ±47 µm (Oculyzer), 477 ±47 µm (HD-OCT). TCT measured with the HD--OCT (455 ±45 µm) was thinner than measured with the Oculyzer (466 ±46 µm). Y-TCT and Δ CCT-TCT were not well correlated between imaging systems and characterized by greater variance for HD-OCT. No statistically significant difference in the number of eyes counted in the study group was obtained using the cut-off points values recommended by 22. Ambrósio et al. for the diagnosis of keratoconus for CCT, TCT, Y-TCT, and Δ CCT-TCT.

Conclusions
We do not recommend using the Scheimpflug camera and HD-OCT interchangeably in the diagnosis and monitoring of patients with keratoconus. Although the corneal thickness measurement results are highly correlated, these cannot be used interchangeably in clinical practice except for CCT. Vertical TCT decentration and Δ CCT-TCT show a weak correlation. Analysis of the number of eyes using the cut-off point values published for data derived with the Scheimpflug camera suggests that these could also be used for data derived with HD-OCT.