In a population-based screening study in India, patients with newly diagnosed primary open-angle glaucoma were stratified by intraocular pressure (IOP) at presentation. Which is the most common IOP range at the time of initial diagnosis of POAG?

Explanation

## IOP Distribution at POAG Diagnosis in India **Key Point:** In Indian and broader Asian populations, Normal Tension Glaucoma (NTG, IOP ≤21 mmHg) is the most common IOP category at the time of initial POAG diagnosis, accounting for a substantial majority of cases — often cited at 50–70% in Indian population-based studies. ### Epidemiology of IOP at Presentation in India **High-Yield:** Indian population-based studies (Vellore Eye Study, Chennai Glaucoma Study, Andhra Pradesh Eye Disease Study) consistently demonstrate that: - ~50–70% of newly diagnosed POAG patients in India have IOP ≤21 mmHg (NTG) - ~20–30% have IOP in the 22–30 mmHg range - ~5–10% have IOP 31–40 mmHg - <2% present with IOP >40 mmHg This contrasts with Western populations (e.g., Framingham Eye Study, Baltimore Eye Survey) where NTG accounts for ~15–20% of POAG, and IOP 22–30 mmHg is the most common range. ### IOP Categories in POAG — Indian Context | IOP Range | Frequency (India) | Clinical Significance | |-----------|-------------------|----------------------| | **≤21 mmHg** | **50–70%** | **Most common in India/Asia**; NTG; vascular and CSF pressure mechanisms | | **22–30 mmHg** | 20–30% | Mild-to-moderate elevation; most common in Western populations | | **31–40 mmHg** | 5–10% | Moderate-to-high elevation; faster progression risk | | **>40 mmHg** | <2% | Rare; suggests secondary glaucoma or acute angle closure | ### Why NTG Predominates in Indian/Asian Populations 1. **Genetic predisposition:** Asian optic nerve heads may be more susceptible to pressure-independent damage at normal IOP levels. 2. **Vascular factors:** Vasospasm, nocturnal hypotension, and systemic vascular disease contribute to optic nerve ischemia independent of IOP. 3. **CSF pressure:** Lower intracranial pressure increases the translaminar pressure gradient across the lamina cribrosa, promoting axonal damage even at normal IOP. 4. **Screening context:** Population-based screening in India detects a large proportion of NTG cases that would otherwise be missed due to absence of elevated IOP on routine tonometry. **Clinical Pearl:** In India, a patient with glaucomatous optic neuropathy and visual field defects but IOP ≤21 mmHg should NOT be dismissed — NTG is the predominant POAG subtype in this population. Secondary causes (low-tension mimics: normal pressure hydrocephalus, carotid artery disease, sleep apnea) must be excluded before confirming NTG. ### Contrast with Western Data - In Western studies (Framingham, Baltimore), IOP 22–30 mmHg is the most common range (~50–60%), and NTG accounts for only 15–20%. - The question specifically situates the scenario in **India**, making NTG (≤21 mmHg) the correct answer per Indian epidemiological data (Chennai Glaucoma Study, Vellore Eye Study). **Mnemonic: "In India, Normal is Most Common"** - NTG (≤21 mmHg) = most frequent POAG subtype in Indian population-based studies - Elevated IOP POAG = more characteristic of Western populations *Reference: Vijaya L et al., Chennai Glaucoma Study; Ramakrishnan R et al., Vellore Eye Study; Quigley HA & Broman AT, Br J Ophthalmol 2006 (global glaucoma epidemiology).*