Glaucoma Screening
On this page
Direct answer
A large share of glaucoma in India — most studies suggest more than half of primary open-angle cases — is undiagnosed at any given time, because the disease is silent until advanced; screening therefore means structured case-finding, not a single pressure reading. Intraocular pressure alone fails as a screen: roughly half of open-angle glaucoma eyes present at or below 21 mmHg at a check, and pressures vary diurnally. The effective triad is tonometry plus optic disc assessment plus angle estimation: cup-disc ratio of 0.5 or more (or inter-eye asymmetry of 0.2 and above), notching, disc haemorrhage or ISNT-rule violation raises the flag; van Herick grading under one-quarter corneal thickness identifies the occludable angles at risk of closure, disproportionately important in Asian eyes. Risk stratification drives testing — age over 40 (earlier with family history), first-degree relatives, myopia, diabetes, steroid use, pseudoexfoliation — with 24-2 fields and OCT completing the referral package.
What you must remember
- Epidemiology in one line: open-angle glaucoma prevalence in Indians over 40 is commonly reported around 2-4 per cent, mostly undetected; angle closure contributes a higher share of blindness than in European populations.
- IOP's limits: normal range 10-21 mmHg, but about half of glaucomatous eyes screen within it; diurnal fluctuation of 3-5 mmHg means a single reading excludes nothing.
- Disc red flags: vertical cup-disc ratio 0.5 or more, inter-eye asymmetry 0.2 or more, rim notching violating the ISNT pattern, disc haemorrhage, peripapillary atrophy.
- Angle closure screen: van Herick under one-quarter corneal thickness (grade 1-2) earns gonioscopy — essential before dilatation in hyperopic, elderly and Asian eyes; short axial length and shallow anterior chamber are the anatomical markers.
- Risk register: age above 40, first-degree family history (multiplying risk several-fold), myopia, diabetes, long-term steroids (including over-the-counter combinations), pseudoexfoliation, prior ocular trauma.
- Field testing: 24-2 threshold perimetry — early defects are arcuate scotomas, nasal steps and paracentral points; a single unreliable field proves nothing (learning effect), and defects must be repeatable and match the disc.
- OCT's role: retinal nerve fibre layer and ganglion cell analyses quantify structural loss (average RNFL about 90-100 micrometres normally) — a referral-strengthening measurement, never a stand-alone diagnosis.
- Ocular hypertension logic: elevated pressure with healthy disc and fields is a risk state, not glaucoma; untreated conversion runs a few per cent per year in trial cohorts, modified by corneal thickness.
A case-finding pathway at the refraction desk
A 48-year-old attends for reading glasses. Opportunistic screen because she is over 40: tonometry 24 mmHg right, 25 left; pachymetry 560 micrometres, thickness inflating the reading modestly; van Herick grade 3, angles open. Fundus through dilated (post-van Herick) pupils: right cup-disc 0.7 with inferior rim thinning, left 0.5 — asymmetry 0.2. History: father blind from "eye pressure". The package assembles — family history, borderline pressures on thick corneas, asymmetric suspicious discs — and referral follows with a 24-2 field and OCT RNFL. The field returns a superior arcuate defect matching the inferior rim loss: glaucoma, now a diagnosis rather than a suspicion.
Contrast the false alarm: a 55-year-old myope at −6.00 D, pressure 19, large physiologically cupped discs at 0.6 with healthy rims, full fields, OCT flags red on a database built of non-myopic eyes. This is red disease — large discs, stretched nerves — watched rather than treated. Screening is a decision pipeline: measure, but adjudicate with asymmetry, anatomy and family history before writing the referral.
What the viva examiner hunts
The one-liner that half of open-angle glaucoma screens under 21 mmHg — IOP is a risk factor, not a diagnostic test. The disc-signs list (0.5/0.2 rule, ISNT violation, haemorrhage) against the physiological large disc. The van Herick-before-dilation discipline, with angle closure's Asian burden. The ocular-hypertension-versus-glaucoma distinction, corneal thickness correcting the reading and modifying risk. And field-testing honesty: a screener who refers on one unreliable field chokes the system; one who normalises away a reproducible nasal step blinds a patient. Programme credit: opportunistic screening of every over-40 refraction, with NPCB&VI-linked vision centres as the referral spine.
Frequently asked questions
Why is intraocular pressure alone a poor screening test?
Because roughly half of open-angle glaucomatous eyes screen at or below 21 mmHg and pressures swing 3-5 mmHg diurnally — a normal single reading excludes nothing.
Which disc findings raise glaucoma suspicion?
Cup-disc ratio 0.5 or more, inter-eye asymmetry 0.2 or more, superior or inferior rim notching violating ISNT, disc haemorrhages and peripapillary atrophy.
What van Herick grade triggers gonioscopy referral?
Grade 1-2 (peripheral chamber depth under one-quarter corneal thickness), indicating an occludable angle — resolved before any pupillary dilatation.
How does central corneal thickness confound screening?
Thick corneas over-read applanation pressure and thin corneas under-read while carrying higher glaucoma risk — pachymetry contextualises every borderline tonometry value.
What distinguishes ocular hypertension from glaucoma?
Pressure above 21 mmHg with healthy discs and full fields — a risk state converting at a few per cent per year in trial cohorts, managed by risk profile rather than immediate treatment.