 Normal-tension glaucoma (also "NTG", "low-tension glaucoma", "normal-pressure glaucoma") is a condition where the optic nerve is damaged although intraocular pressure (IOP) is in the normal range (12-22mm Hg).Individuals with a family history of NTG, those of Japanese ancestry, those with a history of systemic heart disease, and those with Flammer syndrome are at a higher than average risk of developing NTG.The cause of NTG is unknown.