SECOND PRIZE - COS Awards for Excellence in Ophthalmic Research - Papers

Authors: David J. Mathew¹, Izhar Livne-Bar², Jeremy Sivak². ¹Toronto Western Hospital, Krembil Research Institute, ²Krembil Research Institute.

Author Disclosure Block:D.J. Mathew: None. I. Livne-Bar: None. J. Sivak: None

Abstract Body:

Purpose: To create a chronic ocular hypertension rat model without an initial intraocular pressure (IOP) spike
Study Design:
Methods: Six-week-old Long Evans rats were anesthetized using intraperitoneal Ketamine-Xylazine cocktail. A Nylon 8-0 suture on a tapered needle was passed subconjunctivally 1.5 mm posterior to the limbus. The suture was passed all around using 5-6 subconjunctival passes and tied off using a slipknot anchored with 3-5 simple throws. Care was taken to do a snug suturing, without inducing an IOP spike. Control eyes were loosely sutured. Follow-up included weekly IOP measurements with the Tonolab rebound tonometer (iCare, Helsinki, Finland), monthly optical coherence tomography scans of the angle and circumpapillary retinal nerve fiber layer (cpRNFL), and vision assessment using OptoMotry (CerebralMechanics, Lethbride, Alberta). Eyes were collected after 12 weeks of elevated IOP for cryosectioning and confocal microscopy. RBPMS was used to stain the cytoplasm of retinal ganglion cells for quantification of cell survival. Quantification was performed on 250 μm of retina on both sides of the optic nerve insertion.
Results: The mean baseline IOP (mean±SE) for the control and snugly sutured eyes were 10.3±0.2 and 10.5±0.3 mmHg, respectively (p=0.46). The IOP immediately after suturing were 9.1±0.1 (range, 9-10) and 9.6±0.2 (range, 9-10), respectively (p=0.06). IOP increased above 20 mmHg 3-5 weeks post-suturing. After 12 weeks of elevated IOP in the snugly sutured eyes, the IOP measurements were 17.4±1.6 and 27.8±0.8 mmHg in the control and sutured groups, respectively (p<0.001). The number of RBPMS positive cells per 100 μm was 4.22±0.06 for control eyes and 3.43±0.06 for sutured eyes (p<0.05). Vision at baseline was 0.708±0.009 and 0.683±0.009 cycles/deg (p=0.09), respectively, and after 12 weeks of elevated IOP, 0.647±0.010 and 0.489±0.008 cycles/deg (p<0.001), respectively. The average cpRNFL thickness was 43.0±0.4 and 43.6±0.8 at baseline (p=0.51), 41.9±0.2 and 39.3±1.1 at 8 weeks of elevated IOP (p=0.04) and 41.9±0.6 and 35.0±1.1 at 12 weeks of elevated IOP (p<0.001). Angles remained open throughout follow-up. Sections were negative for CD68 (microglia marker), F4/80 (macrophage marker) and SOD2 (superoxide dismutase 2, oxidative stress marker).
Conclusions: One of the major limiting factors of glaucoma research is the lack of an inducible glaucoma model that mimics clinical glaucoma. The snug circumlimbal suture model produces chronic mildly elevated IOP in rats, accompanied by loss of retinal ganglion cells and no evidence of inflammatory cell infiltration. The advantages with this model include the absence of a pathological initial spike in IOP, no intraocular entry or inflammation, and induction of a gradual increase in IOP, similar to clinical glaucoma. The gradual IOP increase may be attributed to connective tissue remodeling around the sutures, leading to tightening; however, this needs further study.