Anterior scleral canal geometry in pressurised (IOP 10) and non-pressurised (IOP 0) normal monkey eyes

Aims: To characterise lamina cribrosa and anterior scleral canal wall architecture in pressurised (IOP 10 mm Hg) and non-pressurised ( IOP 0 mm Hg) normal monkey eyes. Methods: Eight normal eyes from eight monkeys were enucleated before sacrifice and the optic nerve heads (ONH) trephined and immersion fixed in glutaraldehyde ( IOP 0). Nine normal eyes from nine monkeys were perfusion fixed in situ with paraformaldehyde at IOP 10 mm Hg ( IOP 10), and the ONHs trephined and stored in glutaraldehyde. Each ONH specimen was embedded in glycol methacrylate and cut into vertical or horizontal, 4 mm thick, serial sections. Within digitised images of every sixth section, anterior laminar position and laminar thickness were measured at nine evenly spaced locations across the scleral canal opening. Additionally, scleral canal diameters at Bruch's membrane (SCD-B) and at the anterior laminar insertion (SCD-ALI) were measured within the 15 middle section images of each vertically sectioned ONH. Results: Anterior laminar position was significantly more anterior ( nearer Bruch's membrane) in the IOP 10 eyes, compared with the IOP 0 eyes ( 116 (+/-95% CI; 2) mm v 184 ( 2) mm, respectively). Also in the IOP 10 eyes, the lamina cribrosa was thinner ( 195 ( 2) mm v 264 ( 2) mm) and the scleral canal diameter was larger ( SCD-B: 1751 (23) mum v 1591 (19) mum; SCD-ALI: 1961 (21) mum v 1717 (17) mum), compared with the IOP 0 eyes. Conclusion: The anterior scleral canal wall is expanded and the lamina cribrosa is thinned and more tautly stretched within pressurised ( perfusion fixed at IOP 10) young monkey eyes, compared with nonpressurised ( immersion fixed at IOP 0) young monkey eyes. The constricted scleral canal and the relaxed and thickened lamina in the non-pressurised eyes may represent phenomena that contribute to optic disc swelling in hypotonous eyes.