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A modified version of a commercially available high-definition optical coherence tomography (OCT) device, adapted for imaging the anterior chamber angle, was able to provide higher-resolution images than current OCT devices and help investigators identify a new anatomic landmark. If this instrument is developed commercially, it could become a new gold standard for diagnosis of angle-closure glaucoma.
Fort Lauderdale, FL-A modified version of a commercially available high-definition (HD) optical coherence tomography device (Cirrus OCT, Carl Zeiss Meditec), adapted for imaging the anterior chamber angle, was able to provide higher-resolution images than current OCT devices and help investigators identify a new anatomic landmark. If commercially developed, such a device would be a promising tool for the detection of eyes at risk of angle-closure glaucoma, according to Hon-Tym Wong, MBBS, MMed (Ophth), FRCS(Ed), who presented the findings of research with the device at the annual meeting of the Association for Research in Vision and Ophthalmology.
Dr. Wong, clinical department head, ophthalmology, Tan Tock Seng Hospital, Singapore, was a member of a team working with the prototype HD-OCT device in a prospective observational case series in patients recruited from the hospital's glaucoma clinic.
"No one has actually ever seen a high-definition optical biopsy of the angle, although there are already instruments using OCT technology to image this area of the angle," he said. The high proportion of angle-closure glaucoma cases in Singapore and other Asian locales makes it imperative to have improved imaging technology for better diagnosis, Dr. Wong added.
This instrument's ability to visualize the scleral spur, a crucial element for diagnosis of angle-closure glaucoma, was unsatisfactory, however, he said. The scleral spur is a landmark used to help define the parameters within the angle, but the older-model OCT device is unable to visualize it in at least 20% to 30% of the scans it performs, according to Dr. Wong.
"Inability to discern the scleral spur in this group of eyes makes it impossible to quantify the angle width, because the spur is the vital reference point for many standard angle parameters," he said.
For this investigation, a technician from the device's manufacturer traveled to Singapore to modify one of the clinic's newer HD-OCT instruments, designed for posterior imaging, for use in anterior imaging. HD-OCT has a scanning rate 50 to 60 times faster than other current OCT devices and an axial resolution of 3 to 5 µm.
The instrument's optics were changed with a lens adaptor that focused the laser beams onto the anterior segment of the retina, where the angle is located. A 60-D aspheric lens (Volk Optical) was mounted over the OCT's imaging aperture to increase the transverse scan range.
Investigators enrolled 45 phakic subjects who underwent gonioscopy and HD-OCT. Their mean age was 62.5 years. The majority (91.1%) were Chinese, and 62.2% were female. Anterior segment imaging of the nasal and temporal quadrants also was performed in a subset of 31 patients with the anterior segment OCT (AS-OCT) device.
A single examiner conducted all of the gonioscopies and was masked to the OCT findings. Angle closure in a quadrant was defined by Scheie grade III or IV (posterior trabecular meshwork not visible).
A second investigator examined the patients with the modified HD-OCT; he was masked to the results of the gonioscopy exams. Scans were performed of the temporal and nasal quadrants of each eye. When the trabecular meshwork (TM) was visible in an image, angle-closure glaucoma in one of the quadrants was diagnosed if there was contact between the iris and the TM. When the TM was not seen clearly, the diagnosis was based on contact between the iris and the angle wall superior to the scleral spur.
Good agreement was seen between gonioscopy and HD-OCT for angle-closure diagnosis by quadrant. The modified OCT device also had much finer resolution than the current AS-OCT, which allowed very detailed analysis of the structures, Dr. Wong said, adding that the device enabled the observer to identify the scleral spur more frequently and with greater confidence. The investigators found that in vivo visualization of the scleral spur was possible in 71 of 90 quadrants (78.9%) with the HD-OCT.