Differentiating NTG vs POAG

NTG and POAG belong to the same spectrum of progressive IOP-sensitive optic neuropathies. Roibeárd Ó hÉineacháin reports from the 9th World Glaucoma E-Congress

While there are at present no sets of clinical parameters or risk factors for differentiating normaltension glaucoma (NTG) from primary open-angle glaucoma (POAG), advances in imaging technology and genetic analysis may provide a more clinically useful classification of glaucoma subtypes, according to Jeffrey Liebmann MD.

“Normal-tension glaucoma and primary open-angle glaucoma are part of the same clinical spectrum of optic neuropathies, characterised by progressive structural and functional damage to the optic nerve with pressure-dependent and pressure-independent risk factors,” Dr Liebmann said.

He noted the classical definition for NTG is open-angle glaucoma with an IOP of 21mmHg or below. However, IOP measurements can be unreliable due to factors such as variations in corneal thickness and hysteresis, as well as human and instrument error. Furthermore, tonometers only measure transcorneal pressure. True IOP can only be obtained via intracameral cannulation.

“Goldmann tonometry is our reference standard, but not a ‘gold standard’ and certainly not ground truth. We all know that there is no perfect tonometer, and an inaccurate measurement cannot define different diseases,” Dr Liebmann said.

Research has identified many IOP-independent risk factors for NTG and POAG, including age, myopia, and factors related to perfusion and retinal ganglion cell health. But no sets of risk factors yet identified are specific to either NTG or POAG, he noted.

Research has also revealed many subtypes of glaucomatous degeneration of the optic nerve, such as focal ischaemic, senile sclerotic, and myopic with generalised enlargement. But none are exclusive to either NTG or POAG. NTG is also indistinguishable from POAG by the appearance of the visual field.

However, new imaging technologies may allow a finer distinction between different glaucoma subtypes. Clinicians can now examine the microvasculature of a patient’s retina and the optic nerve and intracellular organelles, such as the mitochondria. Research is also providing an improved understanding of tissue metabolism and oxygenation and the role that it plays in POAG’s pathophysiology.

The greatest advances in glaucoma diagnostics may come from human genetics. Already, research conducted at centres around the world has identified links between different glaucoma subtypes and genetic variants. The discoveries include links between variants of the myocilin gene and juvenile glaucoma and between variants in the optineurin gene and some types of NTG. Research has also identified many other genes and groups of genes associated with POAG.

Jeffrey Liebmann MD is Professor of Ophthalmology, Glaucoma Service Director, and Vice Chair for the Department of Ophthalmology at Columbia University Medical Center, New York, USA jml2314@cumc.columbia.edu

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