Vision Assessment
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Direct answer
A vision assessment starts with distance visual acuity on a Snellen chart at six metres, recorded eye by eye as a fraction — the distance at which the patient reads over the distance at which a normal eye reads the same line, so 6/6 is normal and 6/60 means only the top line. Near vision, a pinhole test, colour vision, pupil reactions, tonometry and ophthalmoscopy then complete the basic ophthalmic work-up. When chart vision is lost, acuity is graded downwards as counting fingers, hand movements, perception of light with projection, or no perception of light. No ocular diagnosis should be written before acuity is documented.
What you must remember
- Snellen fraction: 6/60 read at 6 m; an E-chart or Landolt broken ring is used for illiterates; preverbal children are judged by fixation behaviour (central, steady, maintained) or preferential looking with Teller acuity cards.
- The pinhole test is the bedside sieve: improvement by two or more lines points to refractive error or early media opacity; no improvement argues for organic retinal or neural disease.
- Near vision uses the N notation read at 25 to 35 cm; N6 is normal adult reading acuity.
- Legal blindness in India (Rights of Persons with Disabilities Act, 2016) follows the WHO definition: best-corrected acuity below 3/60 in the better eye, or a visual field constricted to less than 10 degrees.
- Colour vision is screened with Ishihara pseudoisochromatic plates and graded with the Farnsworth D-15 or 100-hue test — relevant to railway, defence and pilot recruitment medicals.
- Intraocular pressure by Goldmann applanation tonometry is the gold standard; the normal range is 10 to 21 mmHg. Schiotz tonometry is indentation-based and position-dependent.
- The swinging torchlight test for a relative afferent pupillary defect is the most useful localising sign in unilateral visual loss — it points to optic nerve or severe retinal disease.
- Confrontation fields screen the periphery; automated static perimetry is required to chart and follow glaucomatous and neurological field defects.
How to work through a case of reduced vision
Take a 30-year-old who reports blurred vision in the right eye for two weeks. Record acuity with the left eye properly occluded — an assistant's palm is not occlusion — and suppose the right eye reads 6/36 while the left reads 6/6. The next act is the pinhole: if the right eye jumps to 6/9 through the pinhole, the problem is optical, and retinoscopy followed by subjective refraction will usually produce a myopic or astigmatic correction that restores 6/6, ending the work-up. If the pinhole changes nothing, the pathway turns organic. Look for a relative afferent pupillary defect on swinging torchlight, which would place the lesion at the optic nerve or a dense retinal lesion. Stain the cornea with fluorescein, examine the anterior segment on the slit lamp, and then dilate and view the fundus. A pale swollen disc sends the patient for neuroimaging; a branch retinal vein occlusion or macular lesion explains the drop. The sequence — acuity, pinhole, pupils, media, fundus — resolves almost every "vision reduced" presentation into refractive, media, retinal or neural, and each branch has its own next test.
Where candidates slip
Two errors recur in practical examinations. The first is testing vision with the untested eye merely closed by the patient, who peeps; every FMGE viva examiner has caught a "6/60" eye that read 6/18 once the occluder was used properly. The second is quoting acuity without the correction status — always record both unaided and best-corrected vision, because a diagnosis of "low vision" or "blindness" in India rests on the best-corrected figure. Remembering that counting fingers is still measurable vision, not blindness, also saves marks in disability certification questions.
Frequently asked questions
Why is the pinhole test performed during vision assessment?
A 1 to 1.5 mm pinhole neutralises most refractive errors, so improved acuity through it indicates an optical cause; unchanged acuity suggests retinal, optic nerve or dense media pathology.
What acuity defines blindness under Indian law?
The RPwD Act 2016 adopts the WHO criterion of best-corrected vision below 3/60 in the better eye, or a visual field under 10 degrees, for certifiable blindness.
Which chart is used for an illiterate patient?
The tumbling E chart or the Landolt broken-ring chart, where the patient indicates direction instead of reading letters; young children are assessed by fixation and following behaviour.
How is intraocular pressure measured in the outpatient clinic?
Goldmann applanation tonometry attached to the slit lamp is standard; non-contact air-puff tonometry screens, and Schiotz indentation tonometry is a portable alternative.
What does a relative afferent pupillary defect indicate?
Paradoxical dilatation of the affected pupil on swinging the light to it, indicating asymmetric optic nerve or severe retinal disease — classically optic neuritis or advanced glaucoma.