Anterior Segment Optical Coherence Tomography

Introduction

Optical coherence tomography (OCT) was initially developed by Fujimoto, Huang, et al. as a way to obtain near-histologic–resolution images of tissue without biopsy. Izatt et al. reported the first application of OCT in imaging of the cornea and anterior segment in 1994. The earliest generation of OCT systems used time-domain technology that scans one depth at a time. Current OCT systems use Fourier-domain technology, which detects signal from all depths in parallel, resulting in greatly improved speed and signal-to-noise ratio. Table 17.1 lists the commercially available OCT platforms for anterior segment imaging. An important consideration is the wavelength; longer wavelength light penetrates deeper into the sclera and iris but has coarser resolution, and the best operating wavelength will depend on the intended application. Several commercial OCT systems operating in the 840-nm wavelength range can provide an axial resolution of 5 μm (in tissue) or even better and are capable of visualizing fine corneal structures such as the corneal epithelium, Bowman layer, and corneal endothelium ( Fig. 17.1 ).

Detection and Classification of Keratoconus and Other Corneal Irregularities

Keratoconus is an ectatic disorder of the cornea characterized by progressive stromal thinning and steepening of the cornea leading to a loss of visual acuity. Moderate to advanced keratoconus is easily recognizable by several distinctive clinical features, but the diagnosis of forme fruste keratoconus can be challenging. Keratoconus evaluations, done through corneal topography-based algorithms, can sometimes be ambiguous. When a patient presents with normal vision and shows only a slight inferior steepening on topography, the clinician is left to wonder whether subclinical keratoconus is present.

OCT-generated pachymetry (corneal thickness) ( Fig. 17.2 ) could help to confirm the diagnosis by detecting the eccentric focal corneal thinning characteristic of keratoconus. Studies used five pachymetric variables to evaluate the presence of asymmetry in corneal thickness and thinning: minimum, minimum-median, superior-inferior (S-I), superonasal-inferotemporal (SN-IT), and the vertical location of the thinnest cornea (Ymin). A keratoconus risk score table (downloadable version available at www.coollab.net/resources ) was designed based on these pachymetric variables.

OCT also has sufficient resolution to map the corneal epithelial thickness (see Fig. 17.2 ), which is not possible with slit scanning and Scheimpflug camera technologies. In keratoconus, the epithelium thins at the apex of the cone to reduce focal steepening. ^, This compensatory focal thinning reduces the surface distortion detectable by corneal topography. An epithelial thickness–based pattern standard deviation (epi-PSD) variable is shown to be very sensitive for detecting uneven epithelium in any type of corneal shape irregularities. ^, Analyzing the corneal and epithelial thickness patterns together can further distinguish keratoconus from other corneal irregularities (see Fig. 17.2 ). In corneal irregularities due to secondary epithelial compensation, which include keratoconus, stromal scars, and stromal dystrophies, areas of topographic steepening are associated with focal epithelial thinning . In contrast, in primary epithelial deformations due to contact lens-related warpage, dry eye, or epithelial basement membrane dystrophy, areas of topographic steepening are associated with focal epithelial thickening (see Fig. 17.2 ).

Refractive Surgery Evaluation

LASIK Flap Evaluation

OCT allows for the visualization of laser in situ keratomileusis (LASIK) flaps in the immediate postoperative period and is useful for the evaluation of microkeratome performance ( Fig. 17.3 ). Flap morphology analyzed with OCT demonstrated that flaps created with a femtosecond laser are more uniform in thickness whereas those created with a mechanical microkeratome are thinner in the center.