Thickness, Phase Retardation, Birefringence, and Reflectance of the Retinal Nerve Fiber Layer in Normal and Glaucomatous Non-Human Primates

Abstract

PURPOSE. We identified candidate optical coherence tomography (OCT) markers for early glaucoma diagnosis. Time variation of retinal nerve fiber layer (RNFL) thickness, phase retardation, birefringence, and reflectance using polarization sensitive optical coherence tomography (PS-OCT) were measured in three non-human primates with induced glaucoma in one eye. We characterized time variation of RNFL thickness, phase retardation, birefringence, and reflectance with elevated intraocular pressure (IOP). METHODS.One eye of each of three non-human primates was laser treated to increase IOP. Each primate was followed for a 30-week period. PS-OCT measurements were recorded at weekly intervals. Reflectance index (RI) is introduced to characterize RNFL reflectance. Associations between elevated IOP and RNFL thickness, phase retardation, birefringence, and reflectance were characterized in seven regions (entire retina, inner and outer rings, and nasal, temporal, superior and inferior quadrants) by linear and non-linear mixed-effects models.RESULTS. Elevated IOP was achieved in three non-human primate eyes with an average increase of 13 mm Hg over the study period. Elevated IOP was associated with decreased RNFL thickness in the nasal region (P ¼ 0.0002), decreased RNFL phase retardation in the superior (P ¼ 0.046) and inferior (P ¼ 0.021) regions, decreased RNFL birefringence in the nasal (P ¼ 0.002) and inferior (P ¼ 0.029) regions, and loss of RNFL reflectance in the outer rings (P ¼ 0.018). When averaged over the entire retinal area, only RNFL reflectance showed a significant decrease (P ¼ 0.028).CONCLUSIONS. Of the measured parameters, decreased RNFL reflectance was the most robust correlate with glaucomatous damage. Candidate cellular mechanisms are considered for decreased RNFL reflectance, including mitochondrial dysfunction and retinal ganglion cell apoptosis. (Invest Ophthalmol Vis Sci. 2012;53:4380-4395) DOI:10.1167/iovs.11-9130 G laucoma is an optic neuropathy characterized by the progressive death of retinal ganglion cells and loss of associated axons in the retinal nerve fiber layer (RNFL). Without early diagnosis and therapeutic intervention, glaucoma eventually results in visual field abnormalities and vision loss. Studies indicate that as many as 40% of the axons may die before the visual field is affected, 1 suggesting an opportunity for the development of diagnostic tools that can prevent visual field loss in conjunction with application of neuroprotective strategies. As axons in the RNFL are lost, an associated RNFL thinning is observed. 2-4Optical coherence tomography (OCT) is an imaging modality that provides high-resolution (2-15 lm) cross-sectional images of the retina at video rate, and can quantify macroscopic changes in the optic nerve head (ONH) and RNFL. Inasmuch as previous studies have demonstrated a link between decreased RNFL thickness measured with OCT and glaucoma, 3 OCT measurements of RNFL thickness have become a routine component of glaucoma screening, diagnosis, and monitoring protocols...

Retinal nerve fiber layer reflectometry must consider directional reflectance

“…Recent studies have shown that a decrease in RNFL reflectance occurs prior to thinning of the RNFL [5][6][7][8][9][10][11], and suggest that direct OCT measurement of RNFL reflectance may provide a powerful new method for the early detection of glaucoma. Although directional reflectance is a major source of variability for RNFL reflectance measurements [24][25][26], none of the in vivo studies took this into account.…”

“…For instance, Optical Coherence Tomography (OCT), which is widely used in clinical diagnosis of glaucoma, measures RNFL thickness by identifying the RNFL as a highly reflective layer under the retinal surface, with change of thickness used as a biomarker for axonal damage [1][2][3][4]. Recent studies of glaucomatous retinas reveal that a decrease of RNFL reflectance occurs prior to thinning of the RNFL [5][6][7][8][9][10][11]. Glaucoma damages retinal ganglion cells (RGC) and their axons [12][13][14][15][16][17][18][19][20].…”

“…Many of these RNFL reflectance studies were performed in vivo in humans and other primates and RNFL reflectance was derived from OCT images [5,6,[8][9][10]. The reflected signal in an OCT image is related to the scattering properties of the RNFL and, in addition, depends on other factors, such as ocular media opacities and the intensity of the light beam that reaches the retina.…”

“…The reflected signal in an OCT image is related to the scattering properties of the RNFL and, in addition, depends on other factors, such as ocular media opacities and the intensity of the light beam that reaches the retina. To compensate the effects of these factors, the ratio of the OCT reflected signal within the RNFL to a reference layer was often calculated to normalize the reflectance of the RNFL alone [5,6,[8][9][10]. Although this normalization may eliminate the effect of other factors on estimations of RNFL reflectance, directional reflectance of the RNFL could be a major source of variability for measured RNFL reflectance [24][25][26], and none of the in vivo studies took this variable into account.…”

Henle fiber layer phase retardation measured with polarization-sensitive optical coherence tomography

“…A similar analysis was followed by Yamanari et al [10]. Kemp et al and Dwelle et al fitted RNFL DPPR data in the Poincaré sphere [11,12]. Using an open air PS-OCT system without single mode fiber and an input state that was circularly polarized, Götzinger et al determined the birefringence induced by the RNFL by fitting DPPR data with a linear regression method [13].…”