What do we mean by normal-tension glaucoma?

Ophthalmologists do not agree on whether normal-tension glaucoma(NTG) is the same as primary-open angle glaucoma (POAG). AngeloP. Tanna, MD, director of the Glaucoma Service, Feinberg Schoolof Medicine, Northwestern University, Chicago, approached thisdebate by offering a detailed definition of NTG.

Glaucoma is a group of diseases with a shared final commonpathway of optic neuropathy characterized by progressive retinalganglion cell loss, optic disc excavation, and correspondingvision loss. According to Dr. Tanna, NTG is POAG that isaggressive in nature and occurs in the absence of a known historyof statistically abnormal IOP. In POAG, however, IOP is acontinuous risk factor for development of disease.

To describe NTG in terms of "normal" or "abnormal" IOP isawkward, because the generally accepted cut-off point of 21 mm Hgis an arbitrary limit based on the mean plus two standarddeviations found in large population studies, Dr. Tanna said.

As an example, he questioned the logic of classifying a patientwho develops glaucoma at an IOP of 20 mm Hg as having a differentdisease than someone who develops glaucoma with an IOP of 22 mmHg. Many individuals with IOP over 21 mm Hg do not developglaucomatous optic neuropathy, regardless of their cornealthickness, even when followed for a number of years.

The optic nerve changes in response to disease processes in alimited number of ways, such as excavation and swelling.

"It's naive to believe that only one mechanism can lead to opticdisc excavation," Dr. Tanna said. "More likely, many differentmolecular and cellular abnormalities conspire to result in whatwe call glaucomatous optic disc neuropathy. It's safe to assumethat at lower [IOPs], these pressure-independent types ofmechanisms tend to predominate, whereas at higher [IOPs],pressure-dependent mechanisms probably tend to predominate."