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Mapping of corneal birefringence in thin and asymmetric keratoconus corneas with ultrahigh-resolution
Rahul Patil1, Rohit Shetty, Raghav Narasimhan
1From the Imaging, Biomechanics and Mathematical Modelling Solutions Lab, Narayana Nethralaya Foundation, Bangalore, India (Patil, Narasimhan, Patel, Roy); Department of Cataract Surgery, Narayana Nethralaya, Bangalore, India (Shetty, Khamar); Department of Cornea and Refractive Surgery, Narayana Nethralaya, Bangalore, India (Shetty, Khamar); Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria (Pircher, Hitzenberger); University Eye Clinic Maastricht, Maastricht University Medical Center, Maastricht, the Netherlands (Nuijts).
Purpose:
To evaluate phase retardation (PR) across healthy eyes and eyes with thin corneas (<500 μm) and with asymmetric and bilateral keratoconus (KC).
Setting:
Narayana Nethralaya Eye Hospital, Bangalore, India.
Design:
Observational cross-sectional.
Methods:
There were 4 eye groups: healthy eyes (Group 1; n = 10 eyes), eyes with thin corneas and no clinical disease (Group 2; n = 10 eyes), eyes with asymmetric KC (Group 3; n = 5 eyes), and eyes with clinical KC (Group 4; n = 15 eyes). All eyes were imaged with polarization-sensitive optical coherence tomography (PS-OCT), MS-39, and Corvis-ST. Using PS-OCT, PR was analyzed in annular regions. The anterior (A-E) and Bowman (E-B) wavefront aberrations, epithelial Zernike indices (EZI), total corneal thickness, Corvis biomechanical index (CBI), total biomechanical index (TBI), and Belin-Ambrósio overall deviation index (BAD-D) were analyzed.
Results:
Only CBI, TBI, BAD-D, A-E and E-B aberrations, EZI, and total corneal thickness distributions of Groups 1 (n =10), 2 (n =10), and 3 (n =5) were similar ( P > .05) but not CCT ( P < .05). PR distributions clearly showed that the eyes in Groups 1, 2, and 3 had a normal corneal birefringence unlike Group 4 (n = 10) eyes ( P < .05). The PR map was similar to the preferred orientations of collagen fibers seen in X-ray diffraction ex vivo studies of corneal stroma.
Conclusions:
PR distributions may eliminate the uncertainty associated with the stromal status of thin and asymmetric KC corneas. Group 2 and 3 eyes appeared as healthy because of normal corneal birefringence at the time of imaging, but a longitudinal follow-up of these eyes with PS-OCT may assist in early detection of onset of disease.
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