Article Date: 9/1/2006

DIAGNOSTICS
Advanced Anterior Segment Imaging

From measuring flap thickness to calculating angle depth, the new OCT offers versatility.
Scott F. Lee, O.D., and Ella G. Faktorovich, M.D.

In recent years, Optical Coherence Tomography (OCT) has been used with increasing frequency to visualize and measure the posterior pole structures. We now have the same advanced diagnostic capabilities for the anterior segment with the Visante OCT from Carl Zeiss Meditec. Detailed pachymetry maps, high resolution corneal images and precise anterior chamber depth calculations increase precision accuracy in diagnosis and follow-up of patients with corneal thinning disorders, narrow angles and those presenting for refractive surgery.

Just as with the posterior segment, the OCT is now an invaluable tool for advanced anterior segment diagnostics.

Detailed pachymetry map

When examining corneal thickness, probe placement and repeatability can be an issue in detecting or following keratoconus. If a patient's corneal thickness shows a decrease a year later, is it because the probe was placed in a different spot or is it true thinning? The OCT maps the corneal thickness in 25 spots across the

1. Corneal topography (top) shows flat, but symmetric corneas. In the absence of a detailed pachymetry map, this patient might be considered for LASIK.
2. The OCT map (bottom), however, shows a localized area of corneal thinning, raising the suspicion of forme fruste keratoconus.

cornea and has great repeatability. It can also create a differential map so you can compare past readings and detect subtle changes involved in corneal thinning conditions. This is very useful in following a keratoconus patient, for example.

The OCT is also great tool in refractive surgery screening. Even if the corneal topography is symmetric and central ultra-sound pachymetry is normal, the OCT pachymetry map can reveal an abnormal pattern of corneal thickness, raising suspicion for forme fruste kerato-conus (see figures 1 and 2).

The OCT pachymetry map is especially useful for screening patients with thinner corneas by ultrasound pachymetry. Even if their topography is symmetric, the OCT can reveal an asymmetric pattern of corneal thinning. Consider a patient with a symmetric topographic map and central ultrasound pachymetry of 490 microns. When examining this patient you may suspect PRK, but you would like a little more information about the overall corneal thickness pattern, especially inferior vs. superior cornea. The OCT pachymetry map reveals inferior thinning that may sway us to not perform any corneal refractive surgery on this patient.

High resolution corneal scan

Postoperatively, high resolution corneal scans detail the actual thickness of the flap and the residual stroma. This is useful in considering an enhancement to ensure enough residual stroma remains after the enhancement. For instance, this patient (see figure 3, below) had LASIK in Peru six years prior to presenting to us with -2.00D. Her medical file was unattainable so there was no preoperative or intraoperative information available to us. We could perform the ultrasound pachymetry, but there would be no differentiating between the flap and the stromal bed thickness. Alternatively, we could take this patient into the operating room, lift the flap, perform the ultrasound pachymetry on the stromal bed, and if there was enough stromal bed, we could then perform the excimer enhancement. On the other hand, if the stromal bed was insufficient, we would have had to replace the flap and send the patient home. With the OCT scan, we can measure the residual stroma non-invasively. We performed an OCT scan on this patient and were able to locate her flap-bed interface. Using the measuring tools in the OCT software we measured the residual stroma at 351 microns, enough for a -2.00D enhancement.

Another patient (see figure 4) underwent LASIK in Shanghai four years ago and presented to us for a possible enhancement of -5.00D. In her case, there was too little stroma left (255 microns) and enhancement would put her at risk for ectasia. Another option to consider would be to perform PRK. By measuring the flap and epithelium thickness we could see whether there was enough stroma within the flap to treat the residual refractive error without creating a buttonhole.

3. The OCT (top) shows this patient's flap thickness is 148 microns with stromal bed thickness of 351 microns, enough for enhancement.
4. This patient's stroma (middle) is too thin for further LASIK enhancements.
5. Angle depth calculation (bottom) in a patient with +4 angle at the slit lamp.

Angle depth calculation

With this new device, you can also visualize the anterior chamber in great detail. This has some very useful applications in evaluating glaucoma suspects (see figures 5). Angle depth measurements can vary from doctor to doctor and exam to exam. Using the slit lamp to grade the Van Herrick angle is a good gauge, but it's prone to observer bias. Gonioscopy, although necessary to directly visualize the angle, may introduce not only observer bias, but the compression bias as well. With the OCT scan we can visualize the angle in multiple cross sections of the anterior chamber. You can then use a measuring tool to calculate a definitive angle depth in degrees. We can now monitor more closely patients at risk for angle closure glaucoma as the crystalline lens matures. An image of the angle may also make you feel more at ease about dilating a patient whose angles may appear borderline narrow on slit lamp exam and gonioscopy. The scan is detailed enough to even visualize Schlemm's canal and its patency.

Visante can measure every aspect of the anterior chamber accurately. Prior to placement of the intraocular lens, you can view and evaluate the chamber depth, diameter and the sulcus. This can aid in your work-up of surgical candidates and help with proper lens selection.

These are just a few of the many applications for the new anterior segment OCT. Whether it's measuring flap thickness or calculating angle depth, the OCT is a versatile supplement to our everyday slit lamp examination.

The authors have no financial interest in any of the products mentioned in this article.

Dr. Lee serves as the Director of Clinical Care at Pacific Vision Instititue (PVI) in San Francisco. Send e-mail to drlee@pacificvsion.org.

Dr. Faktorovich is Director of Corneal and Refractive Surgery at PVI. Send e-mail to ella@pacificvision.org.

 

 

 

 



Optometric Management, Issue: September 2006