6 Visante Anterior-Segment Optical Coherence Tomography in Evaluation of Patients for Refractive Surgery
Optical coherence tomography (OCT) is a technique that uses light to create a two-dimensional cross-section image of the eye. 1 This technique allows imaging of either the anterior segment or the posterior segment of the eye. Decision making for refractive surgery is aided by evaluations in either the preoperative or postoperative stages. The main focus of this chapter is to discuss the Visante anterior-segment OCT, its properties, capabilities, and functions in evaluation of patients being considered for excimer laser treatment and phakic intraocular lens (IOL) implantation. Although one of the most common refractive surgery procedures is cataract extraction, that topic is outside the scope of this chapter.
6.1 Visante Anterior-Segment OCT
Optical coherence tomography is a noncontact, real-time technique that uses low-energy infrared laser energy to image structures. The most commonly used retinal OCT uses 820-nm light, which allows excellent tissue penetration to the level of the retina. The Visante anterior-segment OCT (Carl Zeiss Meditec, Dublin, CA) uses 1310-nm light, which has greater absorption, resulting in limited penetration, allowing increased intensity of the light as decreased amounts reach the retina. As such, in the Visante, the light is 20 times more intense, giving a much greater signal-to-noise ratio. This increased intensity allows imaging speed to be increased by 20 while retaining signal-to-noise ratio levels similar to those with retinal OCT but with resultant decreased motion artifact. Additionally, the 1310-nm light has reduced scattering and therefore better penetration through opaque tissue, such as an opaque cornea and sclera, thereby resulting in better evaluation of the anterior segment and visualization of the angle and, to a lesser degree, the ciliary body.
The Visante OCT has two modes: standard-resolution imaging and high-resolution imaging. Standard-resolution imaging provides a broader view of the anterior segment with a 16-mm-width and 6-mm-depth image, providing a full overview of the anterior segment, including the cornea, anterior chamber, iris, and both angles. The high-resolution imaging mode (“high-res mode”) provides a more detailed image with dimensions of 10-mm width and 3-mm depth. High-res mode is more appropriate for imaging of the cornea and any segment in need of detailed evaluation.
In the standard-resolution mode, the Visante performs 256 scans assessing the 16 × 6-mm area in 0.125 seconds. In high-res mode, the Visante performs 512 scans to assess the 10 × 3-mm area in 0.250 seconds. The resolution of the Visante images is limited by the spacing between the scans performed. The resolution of the Visante reaches 18 µm axially and 60 µm transversally.
In addition to a single scan, the operator has the option of selecting automatic dual or quad scans. The dual-scan mode performs two scans, one at a selected orientation between 20 and 200 degrees, the second scan between 160 and 340 degrees. The quad-scan mode performs four scans between axes of 0 to 180, 45 to 225, 90 to 270, and 135 to 315 degrees. All scans in all modes can be rotated manually to any of the 180 axis lines at the discretion of the operator performing the imaging.
The corneal pachymetry mapping (“pachy map”) module in the standard software performs eight modified high-resolution scans of the cornea in preset radial axis lines starting at 0-degree axis and rising at 22.5-degree intervals. The high-res mode allows for the pachy map to be performed in 0.5 seconds with little artifactual distortion. It produces a 10 × 10-mm pachymetry map of the cornea, revealing the corneal thickness in all locations. The preset grid separates the cornea into the central 0- to 2-mm area along with eight radial regions at 2 to 5 mm, 5 to 7 mm, and 7 to 10 mm concentric to the central region. All areas produce three numbers: the thinnest area, noted as the top number; the average, noted as the middle number; and the thickest area, noted as the bottom number. The global pachy map in the Visante 2.0 software performs 16 modified high-resolution scans of the cornea in 1.0 second. The preset radial axis lines start at 0 degrees and rise at 11.25-degree intervals. The global pachy map is able to provide twice as many data points and therefore a more detailed evaluation. The repeatability of the Visante pachy map in the standard mode was noted at 7-µm standard deviation in the center and 14 µm standard deviation in the periphery.
The Visante is also equipped with an optometer capable of changing focus from a +20 to −35 diopters. As the optometer changes focus, accommodation can be induced; and, as such, dynamic changes of the anterior segment can be quantitatively measured.
In the Visante 2.0 software, there is an additional enhanced mode for anterior-segment mode scan and the high-res mode scan. In the enhanced mode, four consecutive scans are performed and compressed into a single image to produce a higher-density, higher-contrast image. Additionally, new software tools to produce a phakic IOL template and measurement tools for endothelial clearance and lens vault distance, along with more sophisticated angle measurement tools, are among the few enrichments that have been devised.
Visante anterior-segment OCT images can be used in evaluating the refractive patient both preoperatively and postoperatively. It is capable of providing broad overview images of the anterior segment and more detailed high resolutions of specific areas. Pachymetry mapping, angle evaluations, corneal flap evaluations, and phakic IOL templates are some of the useful features that are demonstrated in examples in the remainder of this chapter.
6.2 LASIK Patients
In the evaluation of a patient for LASIK (laser-assisted in-situ keratomileusis) eye surgery, the Visante is able to provide preoperative pachymetry mapping of the cornea, evaluation of the angles, and any corneal pathology that is present. In evaluation of post-LASIK surgery patients, detailed anatomical data may be obtained to further guide in clinical decision making.
6.2.1 Pachymetry Mapping
Pachymetry mapping of the cornea, combined with the corneal topographic map, provides a powerful set of information, which may be especially important in cases where possibility of corneal ectasia is raised. A normal pachymetry map of the cornea (Fig. 6.1) has the thinnest portion in the center with concentrically thicker tissue as it approaches the periphery.
A 19-year-old man was being considered for LASIK eye surgery; he had refractions of −1.75–0.25 × 111 in the right eye and −2.00 sphere in the left eye, with best-corrected vision a weak 20/20−. His corneal topographies (Fig. 6.2) were interpreted by the computer software as normal (PathFinder Corneal Analysis Software, Atlas Topographer, Carl Zeiss Meditec, CA). Slight interior steepening of the right eye prompted concerns of possible forme fruste keratoconus. A global pachy map of both eyes (Fig. 6.3 , Fig. 6.4 , Fig. 6.5) revealed an inferotemporal thinning, consistent with bilateral inferotemporal steepening seen on topography. Diagnosis of forme fruste keratoconus was confidently made, and the patient was encouraged to continue with his regular corrective methods and not to consider excimer laser therapy at that time.
To counterbalance the previous example, a 13-year-old adolescent boy with rapidly progressive keratoconus had undergone standard penetrating keratoplasty 11 months earlier in his left eye. In the span of 11 months, the keratometric measurement in his right eye had progressed from a mean of 48 to 58 diopters. His vision had also declined from best-corrected 20/20 to 20/400. In his preoperative evaluation for IntraLase-enabled keratoplasty, the pachy map (Fig. 6.6) revealed a perfectly normal pattern of pachymetric distribution, despite the clearly pathologic cornea. This case demonstrates that one must consider pachymetry mapping only as an augmentation to other information in considering LASIK surgery and that it may appear normal despite such advanced pathology.
6.2.2 Angle Evaluation
In the care of a refractive patient, one must never overlook the ophthalmic care that may be required. A 55-year-old woman with 2.0 diopters of hyperopia planned to undergo LASIK eye surgery. Preoperative Visante evaluation (Fig. 6.7) revealed narrow angles of approximately 10 degrees. Moderate anterior bowing of the posterior iris pigmented line was noted. Peripheral iridotomies were performed weeks before her scheduled LASIK eye surgery. After iridotomy (Fig. 6.8), despite the fact that the pupils were larger compared with the preoperative images, the angles were significantly more open at approximately 25 degrees with near complete flattening of the posterior pigmented iris lines. Compared with standard slit-lamp gonioscopic examination of the eye, the angle is identified quantitatively with multiple methods of calculated angle area and distance measurements. Trabecular-iris space area (TISA) and angle-opening distance (AOD) are both measured at 500 and 750 µm away from the scleral spur. The AOD is defined as the distance perpendicularly away from the trabecular meshwork/endothelial surface to the surface of the iris. The TISA is a trapezoidal area bordered by the AOD at 500 or 750 µm and a line drawn perpendicularly from the scleral spur to the iris. The ability to acquire and present all this information in real time to a patient represents a major advance in both diagnosis and patient education.