Anterior segment imaging is a rapidly advancing field of ophthalmology. Corneal topography and aberrometry are well established diagnostic technologies that today represent gold standards in clinical practice, especially for practices concentrating on refractive surgery. New technologies offering the possibility of noncontact cross-sectional imaging of the cornea and the whole anterior segment have recently become commercially available. These new approaches, based on rotating Scheimpflug imaging and anterior segment optical coherence tomography (AS-OCT), supplement the more established imaging modalities of scanning slit corneal topography and ultrasound biomicroscopy.1
In this article, we present some of the emerging applications of AS-OCT (Visante OCT; Carl Zeiss Meditec, AG, Jena, Germany) with particular emphasis on the imaging of corneal pathologies and related surgeries.
AS-OCT uses the physical principle of low-coherence interferometry to generate images of the anterior segment. OCT systems for posterior segment imaging use a laser diode emitting at 830 nm. In contrast, AS-OCT uses a longer wavelength (1310 nm) that allows deeper penetration through the highly reflective tissues of the sclera. Use of this wavelength also reduces retinal exposure and scanning time, which minimizes motion artifacts in images.2
With standard software on the Visante OCT, lateral resolution of acquired images is 60 µm, and axial resolution is 18 µm. With high-resolution cornea software, the axial resolution improves to 8 µm (data on file with Carl Zeiss Meditec AG). The system provides anterior segment images up to 6 mm in depth and 16 mm in width. Therefore, this methodology allows clinicians to perform with great reproducibility morphologic and qualitative as well as morphometric and quantitative analyses of the anterior segment,3-5 the angle structures, and the cornea, even when these anterior chamber structures cannot be detected at the slit lamp because of loss of corneal transparency.
Initial clinical use of AS-OCT was mainly focused on applications such as measurement of phakic IOL placement, angle analysis in glaucoma, and flap detection after LASIK.6-8 As clinical research progresses, the applications of this technology in clinical practice are continuously expanding. AS-OCT is now a valuable tool for the detection, staging, and follow of several types of corneal diseases.
This article discusses the use of AS-OCT in the identification and diagnosis of corneal thinning disorders, as well as its ability to image anterior segment structures despite the presence of an opaque cornea.
CORNEAL THINNING DISORDERS
Corneal thinning is a clinical condition characterized by localized reduction of corneal thickness, with or without corneal ulceration. Commonly observed nonulcerative corneal thinning conditions include keratoconus, corneal marginal degeneration, and corneal ectasia following LASIK.
Keratoconus. Keratoconus is characterized by a degenerative, progressive corneal thinning in the central or paracentral cornea producing ectasia and irregular astigmatism. Diagnosis and staging are commonly based on clinical and corneal topographic elements. Standard corneal topography furnishes data on only the anterior curvature of the cornea, and ultrasound pachymetry cannot precisely detect areas of maximal thinning, especially in advanced keratoconus. In contrast, AS-OCT provides precise topographic mapping of the entire extent of the ectatic cornea and details of the corneal structure.
Morphologic and morphometric analysis of keratoconic corneas with AS-OCT may assist with surgical planning, whether the choice is collagen cross-linking, intrastromal corneal ring segment implantation, deep anterior lamellar keratoplasty, or penetrating keratoplasty. AS-OCT can provide documentation of corneal parameters and objective evidence of progression of the disease over time—important elements in making the surgical choice, particularly if automated deep lamellar keratoplasty is planned (Figures 1 and 2).
Peripheral corneal degenerations. In the clinical evaluation of peripheral corneal degenerations, corneal topography is the mainstay for the diagnosis and detection of irregular astigmatism, but it may fail to provide reliable follow-up images when the thinning disorder is associated with alteration of the corneal surface. Moreover, conventional ultrasound pachymetry may not be applicable to detect progressive thinning in peripheral areas.
In such cases, AS-OCT is particularly useful to evaluate corneal thickness and irregularity of the corneal surface, as well as the progression of the ectatic disorder, for example, in pellucid marginal degeneration (Figure 3).
Corneal ectasia after LASIK. One undesirable and potentially sight-threatening complication of LASIK surgery is the occurrence of late-onset progressive corneal ectasia,9 which is characterized by progressive corneal thinning and ectatic changes of the cornea in the ablated area starting from the posterior surface. It is frequently associated with high myopic corrections, thick flaps, and low residual stromal thickness.
Clinical signs of regression of myopia and topographic anterior corneal changes are typical, but they may appear late in the course of pathology. Moreover, it has been shown that scanning slit corneal topography presents several limitations in analyzing the posterior corneal changes after LASIK, which may precede the anterior surface modifications.
AS-OCT can document topographic and refractive elements that are useful for the clinical diagnosis and follow-up of LASIK-associated ectasia: It provides morphometric data relative to flap profile and thickness, residual stromal bed thickness, and the posterior profile of the cornea. It can also generate pachymetric maps of the post-LASIK cornea, as shown in Figure 3.
ASSESSMENT WITH LOSS OF CORNEAL TRANSPARENCY
One of the most valuable diagnostic capabilities provided by AS-OCT is its ability to produce a reliable representation of anterior chamber structures even when there is total loss of corneal transparency.10 This is of particular help when there is a need to plan surgery on the affected eye and knowledge of the anatomical status of the entire anterior segment is needed in order to anticipate possible negative prognostic factors.
The 1310-nm wavelength used by the Visante OCT allows deeper penetration through the reflective sclera, and it is minimally affected by corneal opacifications that can mask anterior chamber details during slit-lamp examination (ie, angle closure, anterior and posterior synechiae, IOL displacement, regularity and depth of the anterior chamber, pupillary block). Moreover, the depth of corneal opacities and the condition of the posterior corneal layers can be assessed with great precision. Some representative cases are shown in Figure 4.
Roberta Calienno, MD, practices in the cornea and ocular surface unit of the Ophthalmology Clinic at the Regional Center of Excellence in Ophthalmology, University G. D'Annunzio of Chieti and Pescara, Italy. Dr. Calienno states that she has no financial interest in the products or companies mentioned.
Manuela Lanzini, MD, practices in the cornea and ocular surface unit of the Ophthalmology Clinic at the Regional Center of Excellence in Ophthalmology, University G. D'Annunzio of Chieti and Pescara, Italy. Dr. Lanzini states that she has no financial interest in the products or companies mentioned.
Leonardo Mastropasqua, MD, is the Director of the Ophthalmology Clinic, Regional Center of Excellence in Ophthalmology, University G. D'Annunzio of Chieti and Pescara, Italy. Dr. Mastropasqua states that he has no financial interest in the products or companies mentioned.
Mario Nubile, MD, practices in the cornea and ocular surface unit of the Ophthalmology Clinic at the Regional Center of Excellence in Ophthalmology, University G. D'Annunzio of Chieti and Pescara, Italy. Dr. Nubile states that he is a paid consultant to Carl Zeiss Meditec AG. He may be reached at mnubile@unich.it.
