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Editors Selection IGR 18-4
Clinical Forms of Glaucoma: ON Subarachnoid Space in Normal Pressure Glaucoma
Comment by Hanspeter Killer on:
75531 Measurement and Associations of the Optic Nerve Subarachnoid Space in Normal Tension and Primary Open-Angle Glaucoma, Liu H; Yang D; Ma T et al., American Journal of Ophthalmology, 2018; 186: 128-137
Liu et al. present an interesting and well researched paper on the optic nerve sheath diameter (ONSD) in 40 patients with normal tension glaucoma, 42 patients with primary open-angle glaucoma (POAG) and a control group of 45 healthy subjects.
Applying B-scan ultrasound they measured the smallest ONSD in the NTG group. Taking the ONSD as a surrogate for intracranial pressure (ICP) they concluded that NTG patients must have the lowest ICP and therefore the largest translaminar pressure gradient (TLP) gradient.
These findings contrast the measurements reported in two studies performed in Caucasian patients.1,2 The study of Pinto also applied B-scan ultrasound and did not find a significant difference between the ONSD of 46 NTG patients, 61 POAG patients and 42 healthy controls. Jaggi et al. measured the ONSD with computed tomography in 17 patients with NTG and compared them to 17 age and sex matched controls without optic nerve and intracranial disease.2 This study found significantly larger ONSD in the NTG group.
There are several possibilities that could explain the differences in these studies:
- There might be a discrepancy between the methods. Giger et al.3 compared the results of ONSD measurement between CT, MRI and ultrasound. They found a high correlation between CT and MRI while comparability between ultrasound and computed tomography or magnetic resonance tomography seemed to be less reliable.
- The ONSD is dependent on the position of the globe and therefore is variable.4
- A genetic difference in the two populations. This might affect the sheath compliance and the optic canal diameter.5
- Jaggi et al. included mostly late stage NTG patients that developed optic nerve sheath compartmentalization as demonstrated by cisternography. Compartmentation might have led to an accumulation of lytic peptides such as metalloproteinases that could have changed the sheath compliance.
- The anatomy of the orbit of Asians might be different compared to Caucasian. This could influence the pressure in the orbit around the optic nerve. A higher intraorbital pressure could affect the optic nerve sheath diameter.
References
- Abegão Pinto L, Vandewalle E, Pronk A, Stalmans I. Intraocular pressure correlates with optic nerve sheath diameter in patients with normal tension glaucoma. Graefes Arch Clin Exp Ophthalmol. 2012;250(7):1075-1080.
- Jaggi GP, Miller NR, Flammer J, Weinreb RN, Remonda L, Killer HE. Optic nerve sheath diameter in normal-tension glaucoma patients. Br J Ophthalmol. 2012;96(1):53-56.
- Giger-Tobler C, Eisenack J, Holzmann D, et al. Measurement of Optic Nerve Sheath Diameter: Differences between Methods? A Pilot Study. Kling Monbl Augenheilkd. 2015;232(4):467-470.
- Gans MS, Byrne SF, Glaser JS. Standardized A-scan echography in optic nerve disease. Arch Ophthalmol. 1987;105(9):1232-1236.
- Pircher A, Montali M, Berberat J, Remonda L, Killer HE. The optic canal: A bottleneck for cerebrospinal fluid dynamics in normal-tension glaucoma? Front Neurol. 2017;23;8:47.
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