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 mm(2)) was significantly smaller than that in the POAG (6.24 +/- 1.62 mm(2), P = .0008) or control (6.40 +/- 2.20 mm(2); P = .0007) groups. ONSASA in the POAG and control groups were not significantly different (P = .13). ONSASA was significantly associated with mean lOP (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. (C) 2017 Elsevier Inc. All rights reserved.)