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B. Fortune, G. A. Cull, C. F. Burgoyne; Retinal Nerve Fiber Layer Birefringence Declines Prior to Thickness After Onset of Experimental Glaucoma or Optic Nerve Transection in Non-Human Primates. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3761. doi: https://doi.org/.
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Retinal nerve fiber layer (RNFL) birefringence is thought to be due to cytoskeletal structures such as microtubules and neurofilaments. The purpose of this longitudinal study was to compare peripapillary RNFL birefringence with RNFL thickness (RNFLT) after onset of experimental glaucoma (EG) or optic nerve transection (ONT) in non-human primate eyes.
RNFL birefringence was measured by scanning laser polarimetry (SLP, GDxVCC, Carl Zeiss Meditec, Inc) and RNFLT by spectral-domain optical coherence tomography (OCT, Spectralis, Heidelberg Engineering, GmbH) in both eyes at baseline and weekly after unilateral laser treatments to induce chronic intraocular pressure (IOP) elevation (N=4), or after unilateral ONT (N=4) in anesthetized rhesus macaque. All measurements were made 1.12 mm from the center of the optic disc. In 4 separate eyes, the effects of acute IOP elevation on RNFLT were evaluated by comparing measurements after 30 min of IOP=10 mmHg with those after 60 min of IOP=45 mmHg.
To date, IOP elevation has occurred in 2 of the 4 EG eyes (32±12 mmHg above fellow control eyes) and ONT surgery in 2 of the 4 ONT eyes. Results are presented in Fig1 as percent change from baseline. For reference, the average coefficient of variation among fellow control eyes was 5.2% for birefringence and 2.7% for RNFLT. For both EG and ONT, the internal structure of the prelaminar and peripapillary RNFL appeared less organized as compared with OCT images obtained at baseline and in control eyes. There was no change in RNFLT after acute IOP elevation (+0.6±0.09%, p=0.15).
RNFL birefringence declined prior to and more rapidly than RNFL thickness after onset of EG and after ONT. OCT revealed abnormal RNFL internal structure and reflectivity. These results suggest that internal axonal structural elements become abnormal prior to substantial changes in axonal caliber and/or loss. Acute IOP elevation does not directly affect RNFLT, suggesting that changes observed during early EG represent axonal thinning or loss due to cumulative effects of EG, rather than direct compression by elevated IOP.
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