What are the WHO categories of vision impairment and blindness?
WHO grades distance vision by the presenting visual acuity in the better eye (acuity with the person's usual glasses if any), using six categories (updated 2019). Surveys report the better-seeing eye; moderate and severe impairment together are called MSVI.
| Category | Name | Acuity worse than | Acuity equal to or better than |
|---|---|---|---|
| 0 | No impairment | — | 6/12 |
| 1 | Mild impairment | 6/12 | 6/18 |
| 2 | Moderate impairment | 6/18 | 6/60 |
| 3 | Severe impairment | 6/60 | 3/60 |
| 4 | Blindness | 3/60 | 1/60 |
| 5 | Blindness | 1/60 | Light perception |
| 6 | Blindness | No light perception | — |
How did the ICD-10 classification define blindness, including the visual field?
ICD-9 (1975) introduced an international five-category system, with categories 1–2 as low vision and 3–5 as blindness. The 2006 ICD-10 revision replaced 'low vision' with two categories of visual impairment and kept the blindness cut-off:
| Category | Presenting acuity, better eye |
|---|---|
| 1 — moderate visual impairment | Worse than 20/70 up to 20/200 (about 6/18 to 6/60) |
| 2 — severe visual impairment | Worse than 20/200 up to 20/400 (about 6/60 to 3/60) |
| 3, 4, 5 — blindness | Worse than 20/400 (3/60), or visual field of no more than 10° radius around central fixation in the better eye |
- Presenting vision replaced best-corrected vision in the 2006 revision, so uncorrected refractive errors count towards impairment and blindness.
- Field of vision — the WHO/ICD-10 definition counts a field of 10° or less as blindness; India's NPCB used 20° or less.
- Indian history — the ICMR multicentre survey of 1971–74 reported total blindness (acuity worse than 20/400 in the better eye with spectacles), economic blindness (worse than 20/200 in the better eye) and one-eye blindness (worse than 20/400 in one eye and better than 20/200 in the other). 'Manifest blindness' (worse than 20/1200) was also used in India.
- Older terms still seen in textbooks: economic blindness (inability to do work for which sight is essential), social and legal blindness — their numerical criteria differ between countries.
How does India's NPCB definition of blindness differ from WHO's?
The 2017 review in the Indian Journal of Ophthalmology describes the long-standing NPCB definition as presenting distance visual acuity worse than 6/60 (20/200) in the better eye, or a field limited to under 20° from the point of fixation. The WHO ICD-10 threshold is worse than 3/60 (20/400) or a field of 10° or less.
| Feature | NPCB (traditional) | WHO ICD-10 / 2019 |
|---|---|---|
| Acuity cut-off | Worse than 6/60 (20/200) | Worse than 3/60 (20/400) |
| Visual field | Under 20° | 10° or less |
| Type of vision | Presenting distance acuity | Presenting acuity (since 2006) |
| WHO severe visual impairment (6/60 to 3/60) | Counted as blind | Not blind — severe impairment |
| Prevalence in India (2017 review) | About 1.0% | About 0.67% |
Because the NPCB included WHO's 'severe impairment' group, India's blindness was overstated against international figures: the review compared about 12 million blind and 50 million visually impaired (NPCB definition) with about 8 million blind and 54 million visually impaired (WHO). It recommended aligning the NPCB definition with WHO. The national survey of 2015–19 reported blindness using the WHO threshold (presenting acuity worse than 3/60) and visual impairment as worse than 6/12, and the programme was renamed NPCBVI in 2017 (next section).
What are the causes of blindness in India?
The national blindness and visual impairment survey 2015–19 (RAAB version 6, persons aged 50 years and above, 31 districts) is the most recent nationwide data. Blindness (presenting acuity worse than 3/60) was 1.99% and visual impairment (worse than 6/12) was 26.68%. Blindness was strongly associated with age 80 or above and illiteracy, and not with gender or place of residence.
| Outcome | Principal causes (share) |
|---|---|
| Blindness | Cataract 66.2%; corneal opacity 8.2%; cataract surgical complications 7.2%; posterior segment disorders 5.9%; glaucoma 5.5% |
| Visual impairment | Untreated cataract 48.01% then uncorrected refractive error 41.53% |
| Avoidable share | 92.9% of blindness and 97.4% of visual impairment were from avoidable causes |
What are the global causes of vision impairment, and how do children differ?
WHO estimates that at least 2.2 billion people have a near or distance vision impairment, and in at least 1 billion it could have been prevented or has yet to be addressed. Among these 1 billion, distance impairment or blindness is mainly due to cataract (94 million), refractive error (88.4 million), age-related macular degeneration (8 million), glaucoma (7.7 million) and diabetic retinopathy (3.9 million); presbyopia (826 million) is the main cause of near impairment.
- Leading causes overall: refractive errors, cataract, diabetic retinopathy, glaucoma and age-related macular degeneration.
- Low- and middle-income countries have more impairment from unoperated cataract; high-income countries have more glaucoma and macular degeneration.
- Children — congenital cataract leads in low-income countries and retinopathy of prematurity in middle-income countries; uncorrected refractive error is a leading cause in all countries.
- Eyeglasses and cataract surgery are among the most cost-effective health interventions, yet about 1 in 2 people who need cataract surgery cannot access it.
- Preventable causes include infection, trauma, unsafe traditional medicines, perinatal disease and nutrition-related disease — see xerophthalmia.
How is blindness measured in community surveys (RAAB)?
The Rapid Assessment of Avoidable Blindness (RAAB) is a population-based cross-sectional survey of people aged 50 years and above, the group in which most blindness occurs. The India 2015–19 survey used RAAB version 6 and weighted the 31 surveyed districts to estimate national figures.
- Presenting visual acuity and pinhole acuity are recorded for each eye; pinhole acuity acts as a surrogate for best-corrected acuity, so it separates refractive causes from others.
- The lens is examined with a torchlight and, where there is no clear anterior segment cause of vision loss, a dilated fundus examination follows.
- The ophthalmologist records the principal cause for each eye with acuity worse than 6/12; the cause for the person is taken from the better eye.
- Only one diagnosis is recorded per eye. If two conditions coexist, the more avoidable or treatable one is chosen — which is why cataract and refractive error head the lists.
- Survey results are reported as blindness (worse than 3/60), severe, moderate and mild visual impairment, and the avoidable fraction.
What is the NPCBVI and what does it aim to do?
- The National Programme for Control of Blindness (NPCB) was launched by the Ministry of Health in 1976, described as the first programme in the world to tackle blindness; it began as a 100% centrally sponsored scheme.
- From 2017 the programme was strengthened to cover all kinds of visual impairment and renamed the National Programme for Control of Blindness and Visual Impairment (NPCBVI).
- Its stated target is to reduce the prevalence of avoidable blindness to 0.25% by 2025.
- India performs more than 6 million cataract surgeries per year (figure quoted in the 2017 review), and the NPCBVI funded the 2015–19 national survey.
- The programme also motivated ophthalmologists to promote eye donation and set up eye banks; see also national health programmes.
How is visual acuity read on a Snellen chart?
A Snellen fraction gives the test distance over the distance at which a normal eye can read the line: 6/60 means the patient reads at 6 metres what a normal eye reads at 60 metres. Acuity is the smallest line read correctly, and below the top line patients may be tested by counting fingers, hand movements and light perception.
