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Editors Selection IGR 15-4

Anatomical Structures: Optic disc morphology and location of scotoma

Comment by Chris Leung on:

50899 Characteristics of optic disc morphology in glaucoma patients with parafoveal scotoma compared to peripheral scotoma, Jung KI; Park HY; Park CK, Investigative Ophthalmology and Visual Science, 2012; 53: 4813-4820

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Parafoveal scotoma and nasal step are common patterns of visual field defects in glaucoma. With a higher density of retinal ganglion cells in the central/paracentral retina, Kyoung and colleagues hypothesized that eyes with paracentral scotoma would have more structural damage in the optic nerve head compared with eyes with nasal step. They compared the degree of optic disc and RNFL damage, measured by CSLO (HRT, Heidelberg Engineering, Heidelberg, Germany) and OCT (Cirrus HD-OCT, Carl Zeiss Meditec, Dublin, CA), respectively, in 35 normal-tension glaucoma patients with parafoveal scotoma and 35 with nasal step. Both groups had comparable MD and PSD. The authors showed that the parafoveal scotoma group had significantly narrower neuroretinal rim in the inferotemporal sector than the nasal step group and concluded that patients with parafoveal scotoma had more structural abnormality in the optic disc and thus, these patients should be examined carefully.

The study is well conducted, although I would interpret the results slightly differently. In the study, ~80% of the parafoveal scotoma were superior and ~80% of the nasal steps were inferior. Therefore, it is not surprising that the paracentral scotoma group had narrower inferotemporal rim. While the superotemporal rim would be expected to be narrower in eyes with nasal step, the HRT measurements were not significantly different between the groups. Notably, the superotemporal RNFL was significantly thinner in eyes with nasal step and the inferotemporal RNFL was significantly thinner in eyes with parafoveal scotoma. The average RNFL thickness did not differ between the groups. Based on the OCT finding, the degree of glaucomatous damage was similar between parafoveal scotoma and nasal step. Why the superotemporal neuroretinal rim was not narrower in eyes with nasal step? Firstly, RNFL loss may precede rim narrowing. In fact, it has been shown in a number of studies that RNFL measured by OCT has a superior diagnostic performance to detect glaucomatous damage compared with neuroretinal rim measured by HRT. Secondly, HRT may not be sensitive enough to detect small neuroretinal rim change. Most spectral-domain OCT devices provide both RNFL and neuroretinal rim measurements. Comparing RNFL and rim measurements using the same device would eliminate biases consequential to differences between instruments.

Perhaps, the optic discs of patients with nasal step should be examined even more carefully as their neuroretinal rim changes may be subtle. OCT RNFL imaging is informative to evaluate the degree of retinal ganglion cell damage.

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