Fourier-domain optical coherence tomography allows detailed visualization of the retinal layers, especially the photoreceptor layer.
So says Susanna S. Park, MD, PhD, who with colleagues used for research a high-resolution Fourier-domain OCT instrument developed by the Vision Science and Advanced Retinal Imaging Laboratory at the University of California Davis. The system differs from those that are commercially available, she said, in that it uses a different-wavelength light source. In addition, it allows immobilization of the patient's head with a bite block during imaging. The researchers use extensive data processing to achieve images with maximal quality, she said. Dr. Park is associate professor, Department of Ophthalmology and Vision Science, University of California Davis, Sacramento.
"With this system, by increasing the light source band width and by increasing the scan rate by 25-fold, the entire macula can be imaged with an axial resolution of 4 to 4.5 µm. This increased axial resolution allows the individual layers of the retina to be visualized better," she said.
Patient best-corrected visual acuity ranged from 20/20 to 20/80. All patients had a central scotoma that had been present from 2 weeks to 3 years. Maculopathy was diagnosed in six of the nine eyes.
Illustrative of these patients is the case of a 67-year-old woman with a history of breast cancer and subsequent treatment with tamoxifen for 4 years. She reported a centrally located blur in the right eye that had been present for 3 months; in the left eye, a similar blur had been present for 3 years. Visual acuity (VA) was 20/30 and 20/80, respectively, in the right and left eyes; microperimetry showed a central scotoma in both eyes. Results of funduscopy and fluorescein angiography were unremarkable. Time-domain OCT examination showed minute microcystic changes in the fovea; initially, Dr. Park noted, clinicians were uncertain whether the microcystic changes were artifacts.
"High-resolution Fourier-domain OCT clearly showed the microcystic changes. In addition, focal patches of photoreceptor loss were seen bilaterally that extended to the subfoveal zone," she reported. The microcystic changes and patches of photoreceptor loss were considered to have been related to the extended tamoxifen therapy, although funduscopy did not reveal retinal crystals typically associated with tamoxifen maculopathy. Dr. Park pointed out that similar morphologic changes in the macula were diagnosed using time-domain OCT in other patients in whom crystalline maculopathy due to tamoxifen was diagnosed.
Another case was that of a 60-year-old woman who had had a central scotoma in the left eye for 3 years. VA was 20/25, but an examination with the Amsler grid indicated that the scotoma involved the central 4 ՠ4 squares. Time-domain OCT was the only examination that showed an abnormality, that is, a mild epiretinal membrane on a macula that appeared normal. High-resolution Fourier-domain OCT, however, showed a subtle foveal detachment and possible traction of the overlying epiretinal membrane.
A third case was that of a 60-year-old woman had had a scotoma in the left eye for 2 years. VA was 20/20, but a scotoma was identified on testing with the Amsler grid. A similar problem had developed in the fellow eye a few weeks earlier. Results of routine diagnostic testing, including time-domain OCT, were unremarkable. Only high-resolution Fourier-domain OCT showed a focal disruption of the photoreceptors outer segment layer under the fovea in both eyes.
"Fourier-domain OCT also detailed visualization of the retinal layers, especially the photoreceptor layer. This technology allowed diagnosis of maculopathy in eyes with unexplained vision loss using the routine diagnostic tests," Dr. Park said.