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

Abstract

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 7mm behind the globe in NTG (5.15 ± 0.81mm2) was significantly smaller than that in the POAG (6.24 ± 1.62mm2, P= .0008) or control (6.40 ± 2.20mm2; 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 3mm (4.46 ± 0.43mm vs 4.79 ± 0.40mm, P= .0007), 5mm (4.40 ± 0.39mm vs 4.65 ± 0.47mm, P= .003), and 7mm (4.36 ± 0.35mm vs 4.61 ± 0.30mm, 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.