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
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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.
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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.
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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.
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Angle depth calculation
With this new device, you can also visualize the anterior cham