Focus on Cornea

Clinical applications of AS-OCT for corneal disease

Bringing measurement of corneal thickness and pathology to the micron level.

By Syril K. Dorairaj, MD, and Michael W. Stewart, MD

Detailed assessment of the anterior structures of the eye is an integral part of the ophthalmic examination. Physicians usually evaluate the anterior segment and iridocorneal angle with the slit lamp and gonioscope, but new imaging technologies are enabling us to visualize these structures with micron-level precision.

Anterior segment optical coherence tomography (AS-OCT), which is based on low-coherence interferometry, provides both quantitative and qualitative information regarding the cornea, anterior chamber angle, iris and crystalline lens. Since it was introduced in 2006, clinicians have frequently used AS-OCT in a variety of clinical situations.

Rapid acquisition of high-resoultion images

OCT rapidly acquires high-resolution images of the anterior segment — up to 2,000 A-scans per second and 18-25 µm in axial resolution — without contacting the eye or disturbing tissues. Its measurements of corneal thickness, the anterior chamber angle, anterior chamber depth and lens opacities are both highly accurate and reproducible.

With real-time video recording, AS-OCT effectively captures dynamic changes of the iris and lens in response to light and accommodation. With this information, we can better understand the physiological and pathological changes of anterior segment structures.

Non-contact AS-OCT visualizes spatial relationships within the anterior segment and, in glaucoma, can objectively measure the anterior chamber angle. In addition, the infrared laser combined with the real-time eye position monitor permits the precise capture of angle morphology in the dark.

This article reviews the indications and uses for AS-OCT in management of corneal diseases and dystrophies. A later article will explore how clinicians can use AS-OCT to aid in the management of glaucoma.

MEASURING CORNEAL THICKNESS

Central corneal thickness (Figure 1) is important not only for planning and performing refractive surgery, but also for assessing corneal diseases, as well as monitoring glaucoma progression in patients with ocular hypertension and primary open-angle glaucoma.¹

Figure 1. Anterior-segment OCT can measure a number of important parameters of the anterior chamber, such as depth and volume and pupil diameter, and cornea, including thickness, radius of curvature and distance between scleral spurs.

Advantages of AS-OCT vs. pachymetry

Ultrasound pachymetry has been the gold standard for measuring central corneal thickness because it is economical, easy to perform and reproducible. Because it contacts the corneal epithelium, however, it requires topical anesthesia and cannot make continuous measurements.

Complications due to pachymetry include patient discomfort, corneal epithelial damage, infection and erroneous measurements because the probe had been misplaced or the corneal surface disturbed or compressed, or both.

An error of a few microns may not affect the treatment of a patient with glaucoma, but it may lead to iatrogenic ectasia in a patient undergoing refractive surgery. For these reasons, high speed, non-contact AS-OCT appears to be a promising alternative method for measuring central corneal thickness.

A group in Hong Kong demonstrated the excellent reliability and reproducibility of central corneal thickness measurements with the Visante AS-OCT (Carl Zeiss Meditec, Dublin, Calif.) and slit-lamp OCT (SL-OCT), with intersession (within the group) coefficient variable and intraclass correlation coefficient values less than 1.4% and larger than 0.94, respectively.²

Fourier domain OCT (FD-OCT) has better intersession reliability (coefficients of reliability less than 1.5% vs 4.5% of time-domain OCT) with smaller 95% limits of agreement range than TD-OCT.³

USE OF OCT FOR MANAGEMENT OF CORNEAL DISEASE

Ectatic corneal disorders

Keratoconus, the most common corneal ectatic disorder, is a leading indication for corneal transplantation surgery. Though moderate and advanced keratoconus, as well as some cases of subclinical keratoconus, can be easy to diagnose by clinical examination and computerized videokeratography, forme fruste cases may be difficult to recognize.

AS-OCT produces a detailed pachymetric map, comprising 10-mm radial lines along eight meridians centered on the vertex, which may aid in the diagnosis of early corneal ectasias that still have normal visual acuity and minimal clinical signs of the disease.^2,4

OCT for DALK and DSAEK

Recent advances in lamellar keratoplasty have made it an attractive alternative to conventional full-thickness penetrating keratoplasty (PKP). These have led to the development of customized component surgery, which involves targeted replacement of diseased corneal tissue while retaining healthy layers.

Corneal surgeons have used anterior lamellar keratoplasty techniques, such as deep anterior lamellar keratoplasty (DALK), to manage conditions affecting the anterior layers. Nearly all corneal stromal can be removed with retention of Descemet’s membrane and host endothelium, eliminating the risk of endothelial rejection.

Surgeons have also treated endothelial dysfunction with posterior lamellar procedures, such as Descemet-stripping automated endothelial keratoplasty (DSAEK) (Figure 2) and Descemet’s membrane endothelial keratoplasty (DMEK), in which only the posterior corneal stroma and Descemet’s membrane/endothelium are replaced.

Figure 2: AS-OCT images of a dislocated lenticule after Descemet stripping automated endothelial keratoplasty (DSAEK).

Accurate measurement of opacities and rings

AS-OCT scanning allows surgeons to accurately determine the depth of corneal opacities, enabling them to choose the most appropriate treatment: excimer laser⁵ or lamellar⁶ or penetrating keratoplasty. Investigators in Singapore⁶ found that AS-OCT can help predict the success of lamellar transplantation surgery and aids in the management of postoperative complications.

Insertion of shallow intrastromal corneal ring segments increases the incidence of epithelial and stromal breakdown and ring extrusion. AS-OCT can assess the depth and position of intracorneal rings to determine the risk of extrusion.⁷

To improve the results of scleral expansion band (SEB) surgery for correction of presbyopia, a team of German investigators⁸ measured scleral thickness preoperatively and controlled the depth of the SEB implantation with SL-OCT.

Imaging for pathology

Sequential imaging can monitor the progression and response to treatment of microbial keratitis-related corneal stromal infiltrates, which appear as hyper-reflective areas on AS-OCT scans.⁹ AS-OCT can also detect the retrocorneal pathologic features and anterior chamber inflammatory cells in these eyes.

Fourier-domain OCT creates detailed cross-sectional images of biological tissues.^10,11 One study¹² reported that FD-OCT images of the anterior segment identified normal anatomic details (eg, Bowman’s layer), as well as pathologic findings (eg, corneal guttatae, lattice lines or irregular Bowman’s layer) that other imaging modalities could not.

CRYSTALLINE LENS AND IOL

Evaluation of lens opacities

Most AS-OCT studies of the crystalline lens focus on anatomic changes with accommodation. Investigators in France observed that the anterior radius of the curvature of a crystalline lens in unaccommodated eyes was less in myopic than in hyperopic eyes.¹³

The Lens Opacity Classification System Version III (LOCS III) has become the standard for evaluating cataract morphology and density, but it relies on subjective observations. Scheimpflug photography quantitatively identifies lens changes, but light-scattering measurements at the posterior cortex and posterior capsule can be difficult to obtain, even through a dilated pupil.

Using anterior segment-OCT, ophthalmologists at the Chinese University of Hong Kong developed a novel method to objectively assess and quantify lens opacities.¹⁴

Post-cataract management

Imaging of the IOL after cataract surgery is becoming increasingly important. The position and tilt of the IOL and any postoperative fibrosis may affect the patient’s refractive outcome.

Investigators have used AS-OCT to study the development of posterior capsule opacification,¹⁵ the relationship of the IOL to the posterior capsule, and the posterior movement of the IOL after laser posterior capsulotomy.^16,17

AS-OCT is particularly useful for analyzing the in vivo position of the IOL and its relationship to the iris.¹³ Imaging of phakic IOLs during accommodation has demonstrated contact of the IOL with the anterior surface of the crystalline lens and its proximity to the corneal endothelium.¹⁸

Studies have looked at the use of AS-OCT to assess the capsular block syndrome¹⁹ and reverse pupillary block after implantation of scleral-sutured posterior chamber IOLs.²⁰

POSTOPERATIVE APPLICATIONS

Wounds and trauma

AS-OCT plays a valuable adjuvant role in the detection of small anteriorly located ocular wounds, foreign bodies, tumors, and trauma. Investigators in Poland²¹ evaluated the utility of AS-OCT for diagnosing and monitoring ocular trauma.

Clinicians have used high-resolution AS-OCT to evaluate wound dynamics immediately after small-incision, clear cornea phacoemulsification surgery.^22,23 These reports observed corneal wounds gaping in the immediate postoperative period, possibly allowing bacteria to access the anterior chamber.

Corneal transplantation

The introduction of anterior and posterior lamellar corneal transplantation has increased the importance of AS-OCT.^24,25 Surgeons can use AS-OCT to monitor the postoperative position and thickness of the donor lenticule after DSAEK.²⁶

Others have used AS-OCT to detect localized detachment of Descemet’s membrane and found it superior to slit-lamp microscopy.²² AS-OCT can precisely characterize the configuration of a Descemet’s membrane detachment after deep lamellar keratoplasty.²⁷

Hong Kong researchers²⁸ used AS-OCT to study changes in optical performance of the cornea after Descemet stripping endothelial keratoplasty with cataract extraction and IOL implantation. All eyes in this series developed a postoperative hyperopic refractive error together with an increase in the posterior corneal curvature, similar to results other groups have reported.^23,29–32

THE FUTURE OF AS-OCT

As valuable as in macular disease

Some experts believe that the new anterior segment OCTs will have as profound an impact on the diagnosis and management of anterior segment disorders as the SD-OCTs have on macular diseases. OCT imaging does not aim to replace conventional slit-lamp biomicroscopy, but it will supplement and augment clinical practice, and will become an invaluable tool for ophthalmic research.

The major advantages of the newer devices include non-contact imaging, high-speed scanning, good reliability and reproducibility for quantitative and qualitative measurements, and cross-sectional visualization of anterior segment structures. Because AS-OCT can simultaneously visualize the entire anterior chamber, we can see all the essential parameters for detection of angle closure/narrow angle in a single scan.

In glaucoma, AS-OCT may become an essential tool for screening of primary angle closure, making screening programs for primary angle closure glaucoma more feasible. AS-OCT has led to a better understanding of several anterior segment diseases. We can monitor treatment outcomes, without discomfort or risk of iatrogenic inflammation.

A new era in OCT imaging

The new devices may provide insights regarding the limitations of current surgeries. They promise to improve the safety of phakic IOL implantation and overcome problems with IOL power calculations in patients who have undergone prior corneal surgery. They may also further our understanding of crystalline lens changes during aging and accommodation.

We are just beginning to explore the clinical applications of OCT imaging, and the full range and depth of information it may yield has yet to be determined. The use of the newer anterior segment OCT imaging devices could represent the beginning of a new era for ophthalmic diagnosis. OM

ACKNOWLEDGEMENT

The authors would like to acknowledge the contributions of Vishal Jhanji, MD, from the Chinese University of Hong Kong for the initial draft of the article and Alison Dowdell, of Mayo Clinic in Florida, for editing the manuscript.

REFERENCES