Chronic or primary open angle glaucoma

In the ophthalmology literature and in clinical practice, idiopathic open-angle glaucomas tend to be classified into chronic or primary open-angle glaucoma (POAG) and normal-tension glaucoma (NTG) or low-tension glaucoma (LTG). These two subtypes are defined on the basis of intraocular pressure. For example, a patient with open-angle glaucoma and with an untreated intraocular pressure below 21 mmHg (confirmed with multiple IOP measurements at different times of the day) is considered to have NTG. However, it is not clear whether the use of this arbitrarily selected parameter has somehow successfully identified two different disease entities.