In a field in which there are high patient expectations and a consistent desire by the practitioner to provide the most up-to-date interventions, new diagnostic modalities in the anterior segment have emerged to aid us in confidently diagnosing and managing corneal conditions.

This article presents the recent advancements of different corneal imaging modalities, as well as each technique’s principle and current applications.

Corneal Topography and Tomography

Corneal topography and tomography are used in conjunction, as they are complementary modalities in caring for patients who have anterior-segment issues.

Corneal topography is a computer-assisted tool that creates a curvature map of the anterior surface of the cornea (Figure 1).¹ It was first introduced in the mid-1980s² and has gained popularity with the rise of refractive surgery and premium intraocular lenses (IOLs).

Placido disc-based corneal topography provides an accurate assessment of the anterior curvature of the cornea, which is the most powerful refractive surface of the eye. This imaging shows lacrimal film alterations and corneal irregularities due to lesions, such as pterygium.³ Detecting these surface irregularities has become an essential step in the preoperative patient selection for premium IOLs. These topographers can also measure angle kappa, which can be used to assess the risk of developing optic phenomena after trifocal IOL implantation.⁴ Additionally, placido disc-based topographers provide real keratometry values, aiding in toric IOL alignment planning, and guide suture removal after keratoplasty,⁵ making them indispensable tools for cornea specialists.

Corneal tomography provides a comprehensive overview of the cornea and anterior segment. These devices generate simulated keratometry values. Corneal tomographers, including Scheimpflug-based devices, such as Pentacam (Oculus), Sirius (Costrizione Strumenti Oftalmici Florence), and Galilei (Ziemer), have gained popularity and become the standard of care for the comprehensive corneal evaluation.

Scheimpflug-based corneal tomography is now widely accepted as the gold standard for the diagnosis of keratoconus, given its ability to reveal corneal thinning and abnormal posterior elevation even during the early stages of the disease (Figure 2).⁶ These abilities make it an essential tool for screening patients for refractive surgery, such as laser-assisted in-situ keratomileusis (LASIK) and photorefractive keratectomy.⁶ Additionally, Scheimpflug-based tomographers are used to monitor post-LASIK complications, such as corneal ectasia, and the wide pachymetry map that can be obtained from corneal tomography assists in corneal intrastromal ring implantation planning in keratoconus patients and the simulation of phakic IOL positioning in eyes with high myopia. Higher- and lower-order aberrations, grading of lens opacity, and lens thickness measurements are valuable pieces of information that can be obtained. Further, these devices can be used to optimize the results of toric IOLs, as they quantify posterior corneal astigmatism.⁷

Optical Coherence Tomography

Anterior-segment optical coherence tomography (AS-OCT) is a noncontact device that provides cross-sectional images of the anterior segment with resolutions down to a few microns.⁸ It generates 2- or 3- dimensional images of the cornea, conjunctiva, anterior-chamber structures, and anterior-segment vascular supply. As a result, AS-OCT devices are valuable in the evaluation of pterygia, ocular surface lesions, ectatic disorders, and the status of endothelial corneal grafts in Descemet stripping automated endothelial keratoplasty (DSAEK) and Descemet membrane endothelial transplantation.

Additionally, AS-OCT is a valuable complementary imaging tool to Scheimpflug-based tomography, aiding in the evaluation of suspicious tomography. Specifically, apical epithelial thinning, detected using Fourier domain OCT mapping, is diagnostic of keratoconus,⁹ as well as subclinical keratoconus.¹⁰

Also, AS-OCT is capable of showing interface debris, epithelial ingrowth, fluid collection, flap amputation, and stria following LASIK, and it is used to manage such complications.¹¹ Further, AS-OCT was shown to be an accurate tool in detecting the corneal stromal demarcation line following corneal crosslinking.¹²

In addition, AS-OCT provides an optical biopsy of the ocular surface, making it an essential tool for the diagnosis and management of ocular surface squamous neoplasia (OSSN) and degenerations. Specifically, the technology can reveal pathognomonic signs of OSSN, namely thickening and increased hyper-reflectivity of the ocular surface epithelium (Figure 3)¹³ It can also aid in differentiating OSSN from pterygia, among other benign lesions.¹⁴ Moreover, AS-OCT can be used to guide the treatment of OSSN.¹⁴

AS-OCT also reliably shows the thickness of corneal pathology, making it indispensable to corneal surgeons deciding among phototherapeutic keratectomy, deep anterior lamellar keratoplasty (DALK), and penetrating keratoplasty to rehabilitate the vision of their patients (Figure 4). Additionally, it allows the surgeon to intraoperatively measure residual corneal thickness during DALK or after the removal of ocular lesions.¹⁵ Finally, AS-OCT is very useful in detecting endothelial graft detachment following DMEK and DSAEK.¹⁶

Confocal Microscopy

Confocal microscopy (IVCM), although mainly used in research, is employed to visualize the cornea at a cellular level.¹⁷ As a result, it is valuable in the diagnosis of microbial keratitis.

Using IVCM, acanthamoeba, fungi, and bacteria exhibit different specific cellular features, such as bright spots, honeycomb, and anterior stromal bullae, respectively.¹⁸

Detecting these cellular features can be helpful in determining the etiology of microbial keratitis and thus in selecting the most appropriate treatment.

Ultrasound Biomicroscopy

Ultrasound biomicroscopy (UBM) provides information about the anatomical structure of the anterior segment regardless of the transparency of the optic media. The device has a high-frequency B-scan mode with a lateral resolution of 25 µm and 50 µm.¹⁹ It can provide valuable qualitative and quantitative information about multiple structures of the anterior segment, such as the conjunctiva, cornea, anterior chamber angle, iris, zonules, ciliary body, lens, and anterior vitreous. These abilities make it an essential tool in the management of patients who have congenital corneal opacities, such as Peters anomaly, dermoids, and sclerocornea.²⁰ In these patients, UBM usually allows the surgeon to evaluate the extent of the congenital anomaly and thus plan the management accordingly.²⁰ For the cataract surgeon, UBM is important for evaluating the anterior chamber and the position of IOL haptics, which can lead to uveitis-hyphema-glaucoma syndrome.²¹

Specular Microscopy

Specular microscopy (SM) is a technique for imaging the corneal endothelial cells. Specifically, the technology reveals the cell shape, number, and morphology.²² As a result, it is an important diagnostic tool to facilitate the diagnosis of corneal endothelial diseases, such as Fuchs endothelial dystrophy, posterior polymorphous dystrophy, and iridocorneal endothelial syndrome. On SM, these conditions exhibit endothelial cell features, such as black holes (guttae), dark rings, and bright margins with a central spot, respectively.^23,24

Additionally, several parameters obtained from SM have been used to predict whether endothelial decompensation would occur after intraocular surgery. This prediction is especially important for a cataract surgeon operating on patients who have Fuchs dystrophy or who have undergone previous multiple surgeries. An endothelial cell count <1,000/ mm2, polymegathism/coefficient of variation >0.40, or number of hexagonal cells <50% carry a high risk of corneal decompensation following intraocular surgery.¹⁹

Corneal Visualization Scheimpflug Technology

The Corvis ST (Oculus) utilizes Scheimpflug imaging to record the deformation of the cornea caused by an air puff indentation. Biomechanical indices and tomographic (Pentacam) data integration, known as the “tomographic and biomechanical index,” show high accuracy in aiding in the detection of keratoconus and forme fruste keratoconus.²⁵

Optimal Care

As corneal specialists, we are very fortunate to live in the high-tech world of 2020, when we have such a broad spectrum of devices that help us to make the most accurate diagnosis and design the most precise treatment regimens for our patients. CP

References:

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17. Tavakoli M, Hossain P, Malik RA. Clinical applications of corneal confocal microscopy. Clin Ophthalmol. 2008;2(2):435-45.
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21. Piette S, Canlas OA, Tran HV, Ishikawa H, Liebmann JM, Ritch R. Ultrasound biomicroscopy in uveitis-glaucoma-hyphema syndrome. Am J Ophthalmol. 2002;133(6):839-41.
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23. Bourne WM, McLaren JW. Clinical responses of the corneal endothelium. Exp Eye Res. 2004;78(3):561-72.
24. Laganowski HC, Sherrard ES, Muir MG, Buckley RJ. Distinguishing features of the iridocorneal endothelial syndrome and posterior polymorphous dystrophy: value of endothelial specular microscopy. Br J Ophthalmol. 1991;75(4):212-6.
25. Zhang M, Zhang F, Li Y, Song Y, Wang Z. Early Diagnosis of Keratoconus in Chinese Myopic Eyes by Combining Corvis ST with Pentacam. Curr Eye Res. 2020;45(2):118-23.

 

DR. ABOU SHOUSHA specializes in laser surgery and external disease at the Bascom Palmer Eye Institute. Additionally, he is an associate professor of Ophthalmology, Electrical and Computer Engineering and Biomedical Engineering at Bascom Palmer Eye Institute.

 

 

DR. ÖZCAN is a European Board Certified ophthalmologist in Turkey. He is currently a research fellow in Cornea and Refractive Surgery at Bascom Palmer Eye Institute, Miller School of Medicine, University of Miami, Florida.