Normal-Tension Glaucoma

Definition & Epidemiology

Normal-tension glaucoma (NTG) is glaucomatous optic neuropathy and visual field loss occurring with IOP consistently within the statistically normal range: below 21 mmHg on diurnal testing. All the structural and functional hallmarks of glaucoma are present, cupping and characteristic field loss included. Only the pressure number fails to fit the typical POAG picture.

Prevalence varies markedly by population. The Tajimi Study, a Japanese population-based survey, found NTG accounted for the majority of open-angle glaucoma cases identified, a substantially higher share than most Western cohorts report. Several studies have also found a female predominance among NTG patients specifically, a pattern less pronounced in high-tension forms.

Vascular Dysregulation as the Leading Theory

Since IOP itself isn't elevated, NTG pathophysiology leans more heavily on vascular explanations than mechanical ones. Migraine with vasospasm is the most classically associated vascular condition; Raynaud phenomenon and other vasospastic disorders show a similar association, together supporting systemic vascular dysregulation as a contributor to optic nerve ischemia independent of IOP. Nocturnal systemic hypotension compounds the problem. Excessive nocturnal blood pressure dips, sometimes driven by bedtime antihypertensive dosing, can critically reduce optic nerve head perfusion pressure and cause ischemic injury despite normal daytime IOP readings. A prior episode of hemorrhagic shock has also been linked to subsequent NTG, reinforcing the ischemic component of the disease.

A separate structural theory centers on the translaminar pressure gradient: some NTG patients have lower cerebrospinal fluid pressure in the optic nerve subarachnoid space, widening the IOP-minus-CSF gradient across the lamina cribrosa and potentially contributing to damage independent of IOP. A third, autoimmune theory rests on elevated circulating autoantibodies against optic nerve antigens, glycosaminoglycans, and heat shock proteins, reported in a subset of NTG patients.

Clinical Features & Workup

Compared with POAG, NTG visual field defects tend to sit closer to fixation and run steeper, which can threaten central vision earlier despite the milder pressure profile. Disc hemorrhages occur more characteristically at the inferotemporal disc margin in NTG than in POAG and mark ongoing progression. NTG is fundamentally a diagnosis of exclusion, so other causes of optic neuropathy must be ruled out with MRI of the brain and orbits. This matters most when a field defect respects the vertical rather than the horizontal meridian, a pattern covered in differentiating glaucomatous from neurologic field loss, which points toward a compressive lesion rather than true NTG.

Treatment: Lower Pressure Anyway

The Collaborative Normal-Tension Glaucoma Study (CNTGS) randomized NTG patients to treatment versus observation. A 30% reduction in IOP from baseline significantly slowed visual field progression compared with untreated controls, establishing the case for treating NTG even though pressures start in the "normal" range. The Low-Pressure Glaucoma Treatment Study, run separately, linked systemic arterial hypotension to worse progression. In practice, a patient who keeps progressing despite hitting that 30% IOP reduction should get ambulatory 24-hour blood pressure monitoring to check for excessive nocturnal dipping, a reversible contributor fixed by adjusting antihypertensive medication timing rather than escalating glaucoma therapy further.