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.

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 cham

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.