Measurement and Associations of the Optic Nerve Subarachnoid Space in Normal Tension and Primary Open-Angle Glaucoma

Purpose To measure the area of the optic nerve subarachnoid space (ONSASA) in patients with normal tension glaucoma (NTG), primary open-angle glaucoma (POAG), and controls and examine its association with relevant ocular and systemic parameters. Design Cross-sectional study. Methods The study included 40 patients with NTG, 42 with POAG, and 45 healthy controls. B-scan ultrasound was performed binocularly, using a 12.5-MHz linear array probe. The measurement of the optic nerve subarachnoid space (ONSAS) and calculation of the ONSASA using ImageJ 1.51e analysis software was done by 2 experienced observers in a masked manner. Results The ONSASA between 3 and 7 mm behind the globe in NTG (5.15 ± 0.81 mm2) was significantly smaller than that in the POAG (6.24 ± 1.62 mm2, P = .0008) or control (6.40 ± 2.20 mm2; P = .0007) groups. ONSASA in the POAG and control groups were not significantly different (P = .13). ONSASA was significantly associated with mean IOP (P = .0004) and highest IOP (P = .0007). The optic nerve sheath diameter in NTG compared to POAG was significantly different at 3 mm (4.46 ± 0.43 mm vs 4.79 ± 0.40 mm, P = .0007), 5 mm (4.40 ± 0.39 mm vs 4.65 ± 0.47 mm, P = .003), and 7 mm (4.36 ± 0.35 mm vs 4.61 ± 0.30 mm, P = .004) behind the globe. Conclusions The ONSASA is smaller in NTG as compared to normal control. This is compatible with a lower cerebrospinal fluid pressure in the optic nerve in NTG, implying that trans–lamina cribrosa pressure difference might be abnormally higher in the NTG group than in normal controls. To measure the area of the optic nerve subarachnoid space (ONSASA) in patients with normal tension glaucoma (NTG), primary open-angle glaucoma (POAG), and controls and examine its association with relevant ocular and systemic parameters. Cross-sectional study. The study included 40 patients with NTG, 42 with POAG, and 45 healthy controls. B-scan ultrasound was performed binocularly, using a 12.5-MHz linear array probe. The measurement of the optic nerve subarachnoid space (ONSAS) and calculation of the ONSASA using ImageJ 1.51e analysis software was done by 2 experienced observers in a masked manner. The ONSASA between 3 and 7 mm behind the globe in NTG (5.15 ± 0.81 mm2) was significantly smaller than that in the POAG (6.24 ± 1.62 mm2, P = .0008) or control (6.40 ± 2.20 mm2; P = .0007) groups. ONSASA in the POAG and control groups were not significantly different (P = .13). ONSASA was significantly associated with mean IOP (P = .0004) and highest IOP (P = .0007). The optic nerve sheath diameter in NTG compared to POAG was significantly different at 3 mm (4.46 ± 0.43 mm vs 4.79 ± 0.40 mm, P = .0007), 5 mm (4.40 ± 0.39 mm vs 4.65 ± 0.47 mm, P = .003), and 7 mm (4.36 ± 0.35 mm vs 4.61 ± 0.30 mm, P = .004) behind the globe. The ONSASA is smaller in NTG as compared to normal control. This is compatible with a lower cerebrospinal fluid pressure in the optic nerve in NTG, implying that trans–lamina cribrosa pressure difference might be abnormally higher in the NTG group than in normal controls.