Intravitreal colchicine causes decreased RNFL birefringence without altering RNFL thickness

To test the hypothesis that longitudinal differences between retinal nerve fiber layer (RNFL) birefringence, measured by scanning laser polarimetry (SLP), and RNFL thickness, measured by optical coherence tomography (OCT), are informative about the state of axonal degeneration.

Colchicine was injected into the vitreous cavity of one eye in each of six vervet monkeys (Chlorocebus sabaeus; estimated vitreal concentration: 1 mM, n = 3; 2 mM, n = 1; 10 mM, n = 2); an equivalent volume (approximately 0.1 mL) of sterile saline was injected into fellow control eyes. RNFL birefringence was measured by SLP before injection and every 10 minutes after injection for 2 hours. RNFL thickness was measured by OCT before injection and 2 hours later. After isolating each retina, biopsy specimens were obtained from the inferotemporal arcade region, approximately 2 mm from the center of the optic disc, using a 2-mm trephine and were processed for transmission electron microscopy (TEM). Retinas were then flat-mounted and stained with an antibody against polymerized beta-III-tubulin.

RNFL birefringence measured by SLP decreased over time in all six colchicine-injected eyes, appearing to reach a plateau of -20% +/- 7% (P < 0.0001) approximately 100 minutes after injection. There were no significant differences between quadrants (P = 0.44) and no apparent dose effect (P = 0.87). The change in vehicle-injected control eyes was -3% +/- 3% (P = 0.06; NS). The change in RNFL thickness measured by OCT was +1% +/- 4% (P = 0.81; NS) in colchicine-injected eyes and +6% +/- 6% (P = 0.13; NS) in control eyes. There was no evidence of macular edema by fundus biomicroscopy, stereo fundus photography, or OCT. TEM revealed disorganization of microtubules, swelling of mitochondria, and blurred axonal membrane borders in colchicine-injected eyes. Flat-mounted retinas stained with an antibody against polymerized beta-III-tubulin showed only a mild reduction of peripapillary stain intensity in the colchicine-injected eyes compared with controls.

Intravitreal injection of colchicine caused microtubule disruption within the axons of the RNFL in nonhuman primate eyes. This was manifest as a reduction of RNFL birefringence, without alteration of RNFL thickness, suggesting that such discrepancies can be informative about the status of axonal degeneration.