Use of anterior segment OCT growing

With this easy non-contact technique, the images are taken quickly, and the technician decides which axis to explore.

Issue: January 1, 2007

ByGeorges Baikoff, MD

Amar Agarwal

Introduction

Until recently, most efforts made in ocular imaging concerned posterior segment exploration. As the anterior segment could be directly observed with the slit lamp, until now extensive research was not a priority. With the development of sophisticated surgical techniques, it became essential to obtain elaborate static and dynamic measurements of the anterior segment in order to meet modern safety requirements. Today, anterior segment OCT usage is expanding rapidly. My special guest in this column is Georges Baikoff, MD, of France, who explains this development.

Amar Agarwal, MS, FRCS, FRCOphth

The Visante OCT from Carl Zeiss Meditec uses a 1,310-nm wavelength laser, but in its present form, the infrared light is blocked by pigments. However, the non-pigmented opaque structures are permeable, and images can be obtained through a cloudy or white cornea, the conjunctiva and the sclera. Axial resolution is 18 µm and transverse resolution is 50 µm. The procedure is non-contact and easy. Because of its simplicity, a technician can be rapidly trained to carry out the examinations. It is possible to choose the axis to be explored or carry out an automatic 360° exploration along the four meridians. There is an optical target that can be focused or defocused with positive or negative lenses. Natural accommodation can be stimulated, and anterior segment modifications during accommodation can be explored in vivo.

Phakic implants

Until recently, measuring the depth of the anterior chamber and checking the endothelial cell count with a specular microscope were considered sufficient when performing phakic IOL implants. With the development of techniques such as OCT, surgical indications can be streamlined and a regular checkup of the anterior chamber after such an intervention is mandatory. Figure 1A shows a posterior chamber Visian ICL (STAAR Surgical) inserted in a patient over the age of 45 years who had developed cataract and severe optical problems. Although the ICL has been placed in the posterior chamber, on the endothelial safety scale we note that the edges of the optic are approximately 1 mm from the endothelium. This distance is insufficient, as it has been proved that a minimum safety distance of 1.5 mm is necessary between the edges of the lens optic and the endothelium.

In Figure 1B, pigment dispersion syndrome was observed after insertion of an Artisan hyperopic implant (Ophtec). Compared with a normal anterior segment, the iris is thin and pigment cysts have developed on the pupil between the implant and the patient’s anterior capsule. A convex iris, which is a contraindication for Artisan implants, can be evaluated in a precise way using the crystalline lens rise method (distance from the crystalline lens’ anterior pole to the internal diameter of the iridocorneal angle). When the crystalline lens rise is greater than 600 µm, the risk of developing pigment dispersion syndrome with a drop in visual acuity is probable in 70% of cases.

Phakic implants. (1A) Posterior chamber Visian ICL implant. Note the edges of the optic are inside the 1.5-mm endothelial safety zone. (1B) Artisan hyperopic implant with pigment dispersion in the pupil area. Note the flattening of the iris and the pigment cysts in the pupil area behind the implant.

Images: Baikoff G

Corneal imaging

For the moment, with the prototype we used, corneal imaging is essentially qualitative. Although resolution is not sufficient enough to determine intracorneal disorders or specify different types of dystrophies, in Figure 2 three obvious pathologies are visible: Figure 2A, Terrien’s degeneration with peripheral thinning; Figure 2B, old central descemetocele; and Figure 2C, keratoglobus with considerable thinning of the entire cornea. In Figure 3, one is able to see the presence and the position of intracorneal rings used to treat myopia.

Glaucoma

Visualizing angle structures and exploring the sclera and conjunctiva are possible with the anterior segment OCT. The risk of angle closure (Figure 4) is easy to evaluate and can be objectively and precisely measured.


	Different aspects of corneal pathologies. (2A) Terrien’s degeneration. (2B) Descemetocele. (2C) Keratoglobus.
Intracorneal rings to treat myopia.	Appearance of angle-closure glaucoma.

Future of OCT exploration

This general overview should give an idea of the importance of this imaging technique. The main thing to remember is how simple this equipment is to use. Once the patient has fixated on the target, manipulation is as easy as corneal topography. There is no contact, the images are taken quickly, and the technician decides which axis he wishes to explore.

Resolution is similar to the ultrahigh frequency scanner, but the zones explored are easier to find because the fixation point is on the optical axis.

The iridocorneal angle is perfectly visible, and to evaluate the measurements or to check the evolution of an anterior segment, either the iridocorneal angle sinus or the scleral spur area can be used as a reference point because both remain constants of the anterior chamber anatomy during dynamic or senile modifications.

As it is possible to measure multiple meridians, 3-D reconstruction of the anterior segment could perhaps be the next step.

A 10-mm diameter pachymetric mapping of the cornea is already possible. Another objective for the near future would be to estimate the quality of a LASIK flap over a certain length of time.

Finally, in the laboratory, with a more appropriate wavelength and/or a modification of the power of the laser, it has been possible to obtain images close to histology. On pseudophakic cadaver eyes, one is able to show cell proliferation on the posterior capsule. The images obtained with high-resolution OCT are similar to a pathology study. Dr. Reijo Linnola of Finland has been working on images close to histology for studying posterior capsule cell proliferation.

The technological evolution of the Visante OCT for exploring the anterior segment is something to look forward to. Hopefully, in the near future, these improvements will be similar to those of OCT for exploring the posterior pole: more precise images, resolution of a few microns and 3-D reconstruction of the structures under study. It is quite certain that this imaging system will, in daily practice, shortly replace ultrasound equipment for anterior segment exploration.

For More Information:

Amar Agarwal, MS, FRCS, FRCOphth is director of Dr. Agarwal’s Group of Eye Hospitals. Dr. Agarwal is author of several books published by SLACK, Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery, and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.

Georges Baikoff, MD, can be reached at Centre DOphtalmologie Cinique Monticelli, 88 Rue du Commandant Rolland, 13008 Marseille, France; 33-49-116-2223; fax: 33-49-116-2225.

References:

Agarwal A. Handbook of Ophthalmology. Thorofare, NJ: SLACK Incorporated; 2005.

Agarwal A. Refractive Surgery Nightmares. Thorofare, NJ: SLACK Incorporated; 2007 (in press).

Agarwal S, Agarwal A, Agarwal A. Four volume textbook of ophthalmology. India: Jaypee; 2000.