Fourier-domain optical coherence tomography (FD-OCT) (RTVue-100, Optovue) and time-domain OCT (TD-OCT) (Stratus, Carl Zeiss Meditec) have both showed that the retinal nerve fibre layer (RNFL) is significantly thinner in glaucomatous eyes. However, there were significant differences in RNFL measurements between the two instruments, and they should not be used interchangeably.

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 also was found to be significantly thinner in glaucomatous eyes, making the measurement potentially valuable for assessing patients with glaucoma, said Sunita Radhakrishnan, MD, at the annual meeting of the American Glaucoma Society.

Measuring the macular thickness for assessing glaucoma is not a new idea and has been done using TD-OCT, said Dr Radhakrishnan, research director, Glaucoma Research and Education Group, San Francisco, and an associate at the Glaucoma Centre of San Francisco.

However, previous studies measured the total macular thickness, which is a relatively insensitive parameter compared with measuring the inner retinal layers that are preferentially affected in glaucoma.

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 their retrospective study, they evaluated the correlation between the GCC and RNFL thicknesses in normal eyes, eyes with suspected glaucoma, and glaucomatous eyes by comparing the RNFL thickness measurement with TD-OCT and FD-OCT in the three groups of eyes.

Review of patient charts

The charts of all patients who had undergone scanning with TD-OCT and FD-OCT at the Glaucoma Centre of San Francisco were reviewed. TD-OCT was used to measure the 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 was that the subjects with glaucoma were significantly older than the 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 ageing.

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 was that the thickness of the GCC was well correlated with the thickness of the RNFL among the three study groups.

"Although both OCTs 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."