Effect of Peripapillary Vitreous Opacity on Retinal Nerve Fiber Layer Thickness Measurement Using Optical Coherence Tomography

Optical coherence tomography (OCT) is a widely used technique for the measurement of retinal nerve fiber layer (RNFL) thickness. It emits a light from the light source to the retina or reference mirror and measures RNFL thickness by detecting the different reflectivities of retinal structures.¹ Therefore, any media opacity in the cornea, lens, or vitreous body can affect OCT measurement. However, little is known about the effect of vitreous opacity on RNFL thickness measurement. Vitreous opacity associated with age-related posterior vitreous detachment is a commonly found abnormality.^2,3 In aged eyes with posterior vitreous detachment, a vitreous opacity can float around the optic disc and can affect peripapillary RNFL measurement. We report 3 cases of peripapillary vitreous opacities with or without RNFL thickness change in eyes with ocular hypertension or glaucoma.

The RNFL thickness measurements were performed by Cirrus high-definition OCT (Carl Zeiss Meditec) in patients with ocular hypertension (case 1, Figure 1) and normal-tension glaucoma (case 2, Figure 2; case 3, Figure 3). After the first OCT examination, subsequent repeated examinations were performed within an interval of several minutes in each case.

In case 1, a peripapillary vitreous opacity was detected outside the OCT scan circle in a deviation map and a thickness map at the first examination (Figure 1A). At the second examination, the vitreous opacity had moved to the adjacent area, where the scan circle met the opacity (Figure 1B). At the same time, RNFL thickness at the 5-o’clock sector decreased from 78 to 58 μm compared with that of the first measurement. At the third examination, the vitreous opacity was positioned exactly on the nasal side of the optic nerve head (Figure 1C). Accordingly, the optic disc center automatically identified by Cirrus high-definition OCT was displaced inferior-nasally in the deviation map and the disc margin was changed (the disc margin in Figure 1C was different from that inFigure 1A and B in the thickness map). Therefore, the RNFL thickness of the superior-temporal area (10- to 12-o’clock sectors) increased and that of the inferior-nasal area (4- to 7-o’clock sectors) decreased compared with those of the first examination.

In case 2, a peripapillary vitreous opacity was found outside the scan circle at the first examination (Figure 2A), whereas it crossed the scan circle at the second examination (Figure 2B). The RNFL thicknesses of the 6- and 7-o’clock sectors decreased from 119 to 102 μm and 132 to 47 μm, respectively, compared with those of the first measurement (Figure 2B). At the third examination, although the vitreous opacity crossed the scan circle (Figure 2C), the RNFL thickness change was not remarkable.

In case 3, there was no vitreous opacity floating around the optic disc at the first examination (Figure 3A). However, although it was not associated with a substantial RNFL thickness change, a peripapillary vitreous opacity crossing the scan circle was newly found at the second examination (Figure 3B). There was no signal strength change in all cases.

According to our cases, vitreous opacities floating around the optic disc could affect RNFL thickness measurement with Cirrus high-definition OCT. The opacities could influence the RNFL thickness values (1) when the peripapillary vitreous opacities crossed the scan circle, (2) when they were presented as red color codes in the RNFL deviation map, and (3) when they were demonstrated as black spots in the RNFL thickness map. Although the peripapillary vitreous opacities crossed the scan circle, the RNFL thickness values did not change remarkably when they were not presented as red color codes in the RNFL deviation map and were not visible in the RNFL thickness map. In addition, vitreous opacity could cause an error in the determination of the optic nerve head margin, resulting in a disc center displacement. A decentered OCT scan, described previously,⁴ could change RNFL thickness characteristics.

In patients with a localized RNFL defect, progressive RNFL thinning mainly occurs in a localized pattern rather than as a change in global RNFL thickness.⁵ Our cases demonstrate that a small peripapillary vitreous opacity can influence the measurement of RNFL thickness in a localized area. Therefore, the decreased RNFL measurement in a specific area caused by a small vitreous opacity may be misinterpreted as progressive localized RNFL deterioration in that area. We believe that a careful fundus examination should be done to exclude the possibility of the presence of a floating vitreous opacity near the optic nerve head when an unexpected change in the RNFL thickness is found. In addition, repeated OCT measurement may also be helpful to differentiate an RNFL thickness change associated with a peripapillary vitreous opacity from the true progressive change.

Correspondence: Dr Kim, Department of Ophthalmology, Korea University Guro Hospital, 97 Gurodonggil, Guro-gu, Seoul 152-703, Korea (yongykim@korea.ac.kr).