A comprehensive corneal assessment is fundamental before both refractive and cataract surgery. Detecting corneal irregularities before surgery using a combination of corneal topography or tomography in addition to epithelial mapping is essential. Using both of these technologies drives decisions with regards to performing corneal cross-linking, selecting the appropriate type of refractive surgery, and selecting the intraocular lenses that provide the patient with their desired outcome.
Placido disk technology is an example of corneal topography which provides a 2D map showing variations in corneal curvature. A placido disk topographer projects concentric rings onto the eye to evaluate the cornea shape. Placido disk topographers depend on a stable tear film, therefore patients with dry eyes may be challenging to image. Furthermore, this technology only focuses on the anterior cornea and is unable to provide posterior cornea structure information. Posterior corneal elevation changes can precede anterior changes, therefore corneal tomography which provides a three-dimensional assessment of the cornea may be a superior option. Scheimpflug photography and Optical Coherence Tomography (OCT) are examples of corneal tomography technologies that generate curvature, power, elevation and pachymetry maps. The added benefit of Optical Coherence Tomography is epithelial mapping as it can detect anterior stromal change through epithelial thinning. This occurs prior to any change in the surface curvature which is needed for detection by Placido or Sheimpflug.
In this study, images from three different imaging modalities were acquired on 28 eyes of 14 patients. Subjects were imaged on Zeiss Atlas with Placido technology, Oculus Pentacam with Scheimpflug technology, and Optovue Solix with OCT technology. The first image for each device was compared. The population included: one normal cornea, two contact lens wearers, one dry eye, five keratoconus, three irregular steepening, one post-LASIK, and one flat cornea.
The cornea elevation maps were compared between the three devices and visually demonstrated comparable presentations. The values compared between the three devices were: K1 and K2. K1 and K2 represent the dioptric power of the flattest and steepest meridians of the cornea and can help in diagnosing keratoconus and ectasia. Overall, values were found to be comparable between the three devices for K1. For K2, Optovue Solix and Pentacam were comparable (p=0.07), however both devices were statistically different from Atlas (p=0.00029, p=0.0314). The outliers were all patients with moderate to severe keratoconus. Though statistically different, the clinical interpretation of K2 between the three devices was comparable.
Additionally, pachymetry apex was compared between the Pentacam and Optovue Solix and showed that the Optovue Solix pachymetry apex was on average 20 microns thinner than Pentacam. The difference may be attributed to differences in axial resolution between the two devices. OCT has a 5-micron axial resolution while Scheimpflug has a 15-micron axial resolution.
Overall, the Optovue Solix is able to demonstrate both epithelial mapping in addition to tomography, thereby providing a doctor with comprehensive data to help plan for a favorable surgical outcome. While corneal topography and tomography can demonstrate keratoconus, forme fruste keratoconus can be challenging to distinguish without the use of epithelial mapping. Steepening of the cornea with associated thinning of the epithelium is consistent with forme frustre keratoconus and may prompt a corneal cross-linking procedure. If a patient with such epithelium presentation is interested in refractive surgery, an ICL procedure with topography-guided PRK touch-up may be suggested. The image below demonstrates the epithelial map with pachymetry on the far-left side next to the associated tomography maps for axial, true net power anterior elevation, and posterior elevation maps. In addition to epithelial mapping, posterior elevation maps can also show early ectasia.
In conclusion, combined imaging modalities can be used to detect early corneal irregularities. Optovue Solix provides anterior elevation maps and posterior elevation maps like other devices; however, the added epithelial thickness map is specifically useful in early keratoconus detection.
Figure 1. Corneal Map Topography from the Optovue Solix showing the epithelial map with pachymetry on the far-left side next to the associated tomography maps for axial, true net power, anterior elevation, and posterior elevation maps.
See the Optovue Solix live at the American Academy of Optometry Booth #702 and the American Academy of Ophthalmology Booth #1441.
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