POAG, target pressure and normal-tension disease

Glaucoma represents a leading cause of irreversible blindness worldwide, encompassing a heterogeneous group of progressive optic neuropathies unified by accelerated apoptosis of retinal ganglion cells (RGCs), axonal loss at the lamina cribrosa, and characteristic functional visual field loss. In the European population, Primary Open-Angle Glaucoma (POAG) represents the predominant phenotypic entity, accounting for more than 75% of all glaucoma cases. Managing this chronic, asymptomatic disorder requires a sophisticated understanding of aqueous dynamics, risk stratification, target intraocular pressure (IOP) calculation, and systemic vascular comorbidities.

POAG: Definition, Pathophysiology & Diagnostic Criteria

According to the European Glaucoma Society (EGS) guidelines, Primary Open-Angle Glaucoma is defined as a chronic, progressive, multifactorial optic neuropathy occurring in adult individuals in the absence of identifiable secondary causes, fulfilling three cardinal clinical parameters:

1. Open Anterior Chamber Angle: Gonioscopic visualization of the trabecular meshwork without iridotrabecular contact, peripheral anterior synechiae (PAS), neovascularisation, or heavy pathological pigmentation across at least three quadrants (Shaffer Grade 3 to 4; Spaeth grading displaying a visible scleral spur or ciliary body band).
2. Characteristic Glaucomatous Structural Optic Neuropathy: Thinning or excavation of the neuroretinal rim, vertical elongation of the optic cup, focal notching, cup-to-disc ratio (CDR) asymmetry exceeding 0.2 between fellow eyes, breach of the physiological ISNT rule (Inferior $\ge$ Superior $\ge$ Nasal $\ge$ Temporal neuroretinal rim width), localized retinal nerve fibre layer (RNFL) bundle defects, and disc (Drance) haemorrhages crossing the scleral ring.
3. Corresponding Functional Perimetric Deficits: Visual field defects on Standard Automated Perimetry (SAP) matching the structural topography of RNFL loss, such as paracentral scotomas, nasal steps, and arcuate (Bjerrum) scotomas respecting the horizontal raphe.

Historically, elevated intraocular pressure ($>21$ mmHg) was considered a diagnostic requirement. Under modern EGS definitions, elevated IOP is understood as the most important modifiable causal risk factor, rather than an obligate diagnostic criterion. Approximately 30% to 50% of individuals diagnosed with open-angle glaucoma in European epidemiological surveys manifest baseline untreated IOP within the statistically normal range ($\le 21$ mmHg).

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Risk Assessment in OHT and POAG

Age, IOP, family history, ancestry, myopia and corneal factors influence risk, with different predictive value for development and progression. Ocular-hypertension risk calculators apply to their validated populations and use factors such as age, IOP, CCT, vertical cup-to-disc ratio and field pattern standard deviation. They are not diagnostic tests for a suspect with a different secondary mechanism.

Document untreated pressure where feasible, corneal thickness, angles, disc/RNFL and reliable baseline fields. Preperimetric glaucoma may have convincing structural damage before repeatable field loss. Follow-up depends on lifetime risk, severity and the rate of change, not an annual schedule for every eye.

The Target IOP Concept

Target IOP is an individual estimate of the pressure likely to protect useful vision over the patient's lifetime. Consider damage severity, baseline IOP, measured progression, age/life expectancy, fellow-eye vision, adverse effects and patient preferences. Review the target at monitoring visits. Confirm apparent progression and measurement quality before escalating treatment.

A target from 30 to 12 mmHg represents a 60% reduction, not 40–50%. This is an illustrative calculation, not a universal EGS recommendation. Advanced fixation-threatening disease often needs lower pressures than early disease, but pressure bands alone cannot define success. A field slope of −0.5 dB/year may be important in a young patient or a severely damaged eye. Discuss treatment burden, surgery, adherence and monitoring rather than labelling that rate automatically stable.

See the EGS sixth-edition target-pressure guidance.

Normal-Tension Glaucoma (NTG)

Normal-Tension Glaucoma (NTG) is classified under EGS taxonomy as a phenotypic subset within the open-angle glaucoma spectrum characterized by glaucomatous optic neuropathy, nerve fibre layer atrophy, and visual field loss occurring in the presence of untreated IOP consistently $\le 21$ mmHg across repeated diurnal testing.

Landmark Evidence: The CNTGS Trial

The landmark Collaborative Normal-Tension Glaucoma Study (CNTGS) definitively established that intraocular pressure plays an active role in NTG. The trial evaluated whether lowering IOP alters the natural history of disease in patients with progressive or centrally threatening NTG:

• Patients randomized to a 30% IOP reduction (achieved via topical medications, laser trabeculoplasty, or filtration surgery) exhibited a 12% rate of confirmed visual field progression at 5 years, compared to a 35% rate of progression in untreated controls ($p < 0.001$).
• A critical secondary finding of the CNTGS was that a substantial proportion of untreated NTG patients (65%) did not progress significantly over the 5-year observation window, underscoring the clinical heterogeneity of the condition and the importance of establishing confirmed progression before initiating aggressive surgical therapy.

Distinct Clinical & Morphological Hallmarks of NTG

Compared to high-tension POAG, eyes with NTG present distinct structural and functional characteristics:

• Optic Disc Features: Discs are often larger with thinner, flatter neuroretinal rims; focal deep ischemic notching is common, particularly in the inferotemporal and superotemporal sectors; acquired pits of the optic nerve (APON) occur with higher frequency; and Drance (splinter) disc haemorrhages are observed 3 to 5 times more frequently than in high-tension POAG.
• Perimetric Profile: Scotomas in NTG tend to be closer to fixation, deeper, steeper, and more localized paracentrally, contrasting with the broader arcuate step patterns of high-tension POAG.

Systemic Vascular Risk Factors & Vascular Dysregulation

While IOP lowering remains effective, non-IOP-dependent vascular factors are central to NTG pathogenesis:

• Flammer Syndrome (Primary Vascular Dysregulation): A systemic vascular phenotype characterized by abnormal microvascular autoregulation. Clinical features include cold hands and feet (peripheral vasospasm), low body mass index (BMI), decreased thirst sensation, altered circadian vascular resistance, and heightened sensitivity to cold, physical stress, or medications.
• Vasospastic Disorders: Strongly correlated with classical Raynaud phenomenon and a personal history of migraine headaches (especially with visual aura).
• Nocturnal systemic hypotension: Low perfusion pressure and substantial nocturnal blood-pressure dips are associated with progression in some patients. The commonly used estimate $OPP = \frac{2}{3}MAP - IOP$ is not a direct measurement of optic-nerve perfusion and varies with posture. Review suspected excessive dipping with the treating physician; do not independently stop or reschedule antihypertensives from a fixed diastolic threshold.
• Obstructive sleep apnoea: Intermittent hypoxia and vascular dysregulation are possible contributors. Ask about symptoms and arrange appropriate systemic assessment; prevalence and the strength of associations depend on the studied population.

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Differential Diagnosis of Suspected NTG: Red Flags & Mimics

Because non-glaucomatous optic neuropathies can simulate the cupping and visual field loss of NTG, a meticulous differential workup is mandatory. The clinician must systematically evaluate for alternative or masquerading etiologies: