Normal tension glaucoma: risk factors pertaining to a sick eye in a sick body

Since its original description by von Graefe in 1857, normal tension glaucoma (NTG) has remained a diagnostic and therapeutic challenge for ophthalmologists. NTG is a clinical condition associated with a pathologically excavated optic disc and characteristic glaucomatous visual field loss. Apart from the intraocular pressure (IOP) being in the statistically ‘normal’ range, the clinical course of the disease shares similarities with that of primary open angle glaucoma (POAG). Some authors have questioned whether NTG merely represents an artificial distinction from POAG by the IOP or represents a subset of disease along a continuum of which patients show a different clinical course as far as patterns of optic disc damage and quantity and quality of visual field loss are concerned, possibly as a result of different mechanisms of damage. The current literature appears to support the hypothesis that NTG and POAG represent a continuum of OAGs, in which a certain level of IOP is the predominant causative risk factor in POAG, while factors in addition to IOP are more important in NTG. There is considerable overlap between the 2 conditions. Additionally, within the population of patients with NTG, there are subsets in which blood flow and other factors may assume relative importance.