Fourier-domain optical coherence tomography and time-domain OCT have both showed that the retinal nerve fiber layer is significantly thinner in glaucomatous eyes.
The RTVue-100 is unique in its ability to measure the ganglion cell complex (GCC), compared with other FD-OCT instruments that are commercially available, and the GCC has also been found to be significantly thinner in glaucomatous eyes, making it potentially valuable for assessing patients with glaucoma, Sunita Radhakrishnan, MD, reported at the annual meeting of the American Glaucoma Society.
Measuring the macular thickness for assessing glaucoma is not a new idea. It has been done using TD-OCT, said Dr. Radhakrishnan, research director, Glaucoma Research and Education Group, San Francisco, and an associate at the Glaucoma Center of San Francisco.
Macular segmentation also has been performed with TD-OCT, but "compared [with] TD-OCT, FD-OCT permits more specific segmentation and is therefore potentially superior," according to Dr. Radhakrishnan and colleagues. In a retrospective study, they evaluated the correlation between GCC and RNFL thicknesses in normal eyes, eyes with suspected glaucoma, and glaucomatous eyes by comparing the RNFL thickness with TD-OCT and FD-OCT in the three groups of eyes.
The charts of all patients who underwent scanning with TD-OCT and FD-OCT at the Glaucoma Center of San Francisco were reviewed. TD-OCT was used to measure RNFL thickness; FD-OCT was used to measure the RNFL and the GCC thicknesses. Thirty-eight eyes of 38 subjects were included in the study; 12 eyes were normal, 13 eyes had suspected glaucoma, and 13 were glaucomatous.
Dr. Radhakrishnan said that the RNFL values measured by TD-OCT and FD-OCT in the normal and suspected glaucoma groups were very similar to each other, but the RNFL measured by TD-OCT was significantly thinner than the FD-OCT measurement in the eyes with glaucoma. The average RNFL thickness was significantly thinner when the glaucomatous eyes were compared with the eyes with suspected glaucoma and the normal eyes using both OCT devices.
One caveat-the subjects with glaucoma were significantly older than subjects in the other groups. However, she noted that the RNFL thickness decreased more in the glaucomatous eyes than would normally occur during the course of aging.
The investigators also found that the GCC was significantly thinner when the glaucomatous eyes were compared with the eyes with suspected glaucoma and the normal group. Another finding-the thickness of the GCC was well correlated with the thickness of the RNFL among the three study groups.
"Although both [types of OCT] showed a thinner RNFL in glaucomatous eyes, the actual measurements were significantly different, and, therefore, one cannot be substituted for the other," Dr. Radhakrishnan concluded. "The GCC thickness showed good correlation with the RNFL values and may be an alternative in eyes in which a good RNFL measurement is sometimes difficult to obtain, for example, in [eyes with high myopia and] extremely tilted discs."
Sunita Radhakrishnan, MD
Dr. Radhakrishnan has no proprietary interest in the subject.