**2.5 Normotensive glaucoma**

Normotensive glaucoma presents with the same characteristics as POAG, but it lacks an elevated IOP above the normal physiological range. Speculation concerning the mechanism includes lack of blood flow to the optic nerve, vascular spasm, and multiple mutations [11]. Recently, some investigators have used the marmoset, a small primate, as an experimental glaucoma model. Its advantages include a high reproduction rate and a short time to sexual maturation (12–18 months), as well as their ease of management and breeding [46]. The transgenic marmoset was reported in 2009 as well. Moreover, in 2019 it was documented that aged marmosets showed glaucomatous retinal and brain degeneration as well as thinning of the lamina cribrosa [46]. They did not have accompanying mutations in glaucomaassociated genes nor elevation of IOP, which suggest a normotensive glaucoma phenotype. It was noted that there was increased oxidative stress and reduced brain-derived neurotrophic factor levels, which is neuroprotective for RGCs and also reduced in human glaucoma patients [46].
