Controversy remains as to whether normal-tension glaucoma (NTG) represents a distinct disease entity or whether it is simply POAG developing in individuals with IOP within the statistically normal range. Glaucoma can develop at any level of IOP within the range observed in the general population. Thus, IOP is a continuous risk factor for glaucoma, and any cutoff between “normal” and “abnormal” IOP is arbitrary. Accordingly, many authorities believe the terms normal-tension glaucoma and low-tension glaucoma should be abandoned.
Risk Factors and Clinical Features
As previously emphasized, glaucoma is a multifactorial disease process for which elevated IOP is just 1 of several risk factors. Studies of Japanese populations have found that the proportion of OAG patients with IOPs in the average range is particularly high. Other risk factors may play a greater role in individuals with NTG than in those with POAG with higher IOPs. One hypothesis is that local vascular factors may have a significant role in the development of NTG in these persons. Some studies suggest that patients with NTG have a higher incidence of vasospastic disorders (eg, migraine and Raynaud phenomenon), ischemic vascular disease, autoimmune disease, sleep apnea, systemic hypotension, and coagulopathies than patients with high-tension POAG. However, these findings have not been consistent.
As in POAG, NTG is characteristically bilateral but often asymmetric. In glaucomatous eyes with IOPs that are within the statistically normal range but asymmetric, worse damage typically occurs in the eye with the higher IOP. Optic disc hemorrhage (Fig 1-7) may be more common among patients with NTG than among those with high-tension POAG.
The visual field defects in NTG tend to be more focal, deeper, and closer to fixation, especially with early disease, than those commonly seen with high-tension POAG. Also, a dense paracentral scotoma encroaching near fixation is not an unusual initial finding on the visual field tests of NTG patients. However, these differences may be due to detection bias, given that patients experiencing visual disturbance are more likely to seek care. Further, differences in optic nerve appearance and visual field defects between patients with NTG and those with high-tension POAG have not been uniformly confirmed in studies. Therefore, for any individual patient, there is no characteristic abnormality of the optic nerve or visual field that distinguishes NTG from high-tension POAG.
Figure 7-1 Subtle optic disc hemorrhage (arrow) in a patient with normal-tension glaucoma.
(Courtesy of Wallace L.M. Alward, MD. ©The University of Iowa.)
Cartwright MJ, Anderson DR. Correlation of asymmetric damage with asymmetric intraocular pressure in normal-tension glaucoma (low-tension glaucoma). Arch Ophthalmol. 1988;106(7):898–900.
Sommer A. Ocular hypertension and normal-tension glaucoma: time for banishment and burial. Arch Ophthalmol. 2011;129(6):785–787.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.