Visual Assessment

Visual Assessment is 7% of the exam. It covers distance and near acuity, pinhole testing, and measuring acuity in patients of every age and ability. Visual acuity is the vital sign of ophthalmology — measured before anything else is done to the eye: before drops, before pressure, before bright lights. In a chemical splash, irrigation comes first, before acuity (see the General Medical Knowledge guide).

Snellen notation — 20/40 means the patient reads at 20 feet what a standard eye reads at 40. The top number is the testing distance; the bottom number is the distance at which a standard eye can just read that line. At the same test distance, a bigger bottom number means poorer vision, so 20/15 beats the 20/20 standard; the fraction is never a percentage of normal sight.

Standard distance — 20 feet. Metric charts use 6 meters: 6/6 = 20/20, and 6/12 = 20/40.

How a letter is built — an optotype is any test figure: a letter, number, or symbol. A 20/20 optotype spans 5 minutes of arc at 20 feet, with 1-minute detail, drawn on a five-by-five grid; a 20/40 letter is the same design at twice the size.

Choosing the chart — match the optotype to what the patient can do:

ETDRS / logMAR charts — five letters per line, uniform geometric steps: the standard for research and low vision, where 'lines lost' must mean the same thing everywhere on the chart.

The same routine every visit lets any reader compare results:

Right eye first — OD (right), then OS (left), then OU (both), by convention; charting a result under the wrong eye is a serious error.

Occlusion done right — a solid occluder or cupped palm resting on the orbital rim. Spread fingers invite peeking; pressure on the globe blurs the covered eye for its own turn. Leaning to see around the occluder suggests peeking. Squinting narrows the lids into a slit that works like a pinhole and flatters the result, so ask the patient to open the eye normally.

Recording — record each eye separately, with correction (cc) and without (sc), always right eye first, partial lines with plus-minus notation (20/40+2). 'VA sc OD 20/60' means the right eye read 20/60 without correction. All of the 20/40 line plus two letters of the 20/30 line is 20/40+2; the 20/30 line with one letter missed is 20/30−1. Offices set their own rule for how many misses a line can have and still count; apply it every time.

What was actually tested — habitual correction, newest Rx, or unaided? The chart must say (cc/sc), because the same eye can honestly produce three different numbers. The routine record uses the correction the patient actually wears (glasses or contacts), plus uncorrected acuity where protocol requires.

Consistent conditions — chart luminance, room lighting, and testing distance must match visit to visit, or trend lines mean nothing. Short lanes use calibrated mirrors or distance-scaled charts.

Near acuity — tested at 40 cm (16 in) with the near card and the patient's reading correction; recorded in Jaeger, point, or reduced Snellen. A presbyope without their add will test falsely poor — note missing correction.

Near technique — light the card well and keep it at the stated distance; a card that drifts closer makes the print look bigger, so near acuity reads better than it really is. Test each eye, then both, per protocol, and record the notation, the distance, and the correction worn. Jaeger numbers (J1, J2, and so on) rise as print gets bigger, point sizes are the type sizes used in printing, and reduced Snellen gives equivalents such as '20/20 near'.

Timing matters — acuity (and pupils) come before drops. After dilation, accommodation is impaired and near acuity reads falsely poor; retest another day if the number matters.

Pinhole — if acuity improves through a pinhole, uncorrected refractive error is the likely cause; no improvement suggests media opacity or retinal/neural disease. Check it whenever acuity is reduced. The tiny opening shrinks the blur circle that refractive error creates.

Pinhole technique — keep the fellow eye covered and hold the pinhole before the tested eye, usually over the current glasses, so it shows what they leave uncorrected. Record it as its own entry (PH 20/25, or PHNI for no improvement). Improvement suggests a refraction may help; the Refractometry guide uses it as a target for the final lenses and explains when glasses fall short.

Glare testing — a cataract patient can read 20/25 in a dark lane and be disabled in sunlight. Brightness acuity testing documents the real-world loss and often justifies surgery. Glare, contrast, and color testing are in the Supplemental Testing guide.

When the chart can't be read — walk the patient closer and record the true distance (5/400), then descend the scale in order: count fingers with distance (CF @ 3 ft), hand motion (HM), light perception with projection, light perception (LP), no light perception (NLP).

The top number is the real distance — the 400 letter read at 5 feet is 5/400, never 20/400.

Converting to a 20-foot equivalent — to compare a closer result with standard notation, multiply both numbers by 20 divided by the test distance. At 10 feet the factor is 2, so 10/200 equals 20/400 and 10/400 equals 20/800; at 5 feet the factor is 4, so 5/200 equals 20/800 and 5/400 equals 20/1600. A patient who must stand closer sees less than the letter's label implies, so the bottom number always grows. The equivalent is for comparison: the chart entry still shows the distance actually used, and 5/400 is never written as 20/400, because its equivalent is 20/1600.

Below count fingers — hand motion and light perception are used only when the patient cannot count fingers; CF always carries its distance, so visits can be compared.

Testing with light — cover the fellow eye completely, since a bright light can be seen around a loose cover, and record NLP only after the brightest available light has been tried.

Sudden loss is urgent — any sudden drop in vision, at any acuity level, goes to the physician at once (see the History and Documentation guide).

Preverbal infants — fix-and-follow per eye with a small toy or light, recorded as F&F or CSM (central, steady, maintained), plus how strongly the child objects to covering each eye. Central: the fovea does the fixing, judged by the corneal light reflex; steady: no wandering or jerking while the target is held still, then moved slowly; maintained: the eye keeps fixing when the other eye is uncovered. Fussing only when one eye is covered suggests the eye left uncovered sees less.

Preliterate children — Lea symbols, HOTV with a matching card, Allen pictures. The child points to the match; speed and play win cooperation. Teach the game with both eyes open, then test each eye; an adhesive patch stops a young child from peeking around a handheld occluder.

Amblyopia and crowding — amblyopic eyes read isolated letters better than full lines. Test with lines or crowding bars, or the deficit hides. Neighboring figures make the task harder for an amblyopic eye (the crowding phenomenon); crowding bars are lines drawn close around a single figure to recreate that effect. Amblyopia itself is in the Ocular Motility Testing guide.

Latent nystagmus — full occlusion makes it worse; fog the fellow eye with a high plus lens rather than covering it, so both eyes stay open.

Head turns in nystagmus — with any nystagmus, a head turn that steadies the eyes is not peeking: allow it, note it, and also record both eyes together, which often read better than either eye alone.

Adults who don't read English letters — tumbling E or Landolt C: the patient signals the direction of the opening, preserving a true optotype measurement.

Limited English, speech, or cognition — for limited English, a qualified medical interpreter explains the task (see the History and Documentation guide). For limited speech or cognition, use the simplest task that works, such as pointing to a matching figure or showing which way the E points. A companion never answers for the patient; demonstrate, keep instructions short, and chart the test used and how reliable the answers seemed.

Inconsistent performance — when claimed vision doesn't match observed behavior, chart objective observations neutrally and inform the physician. Sorting out functional vision loss is the doctor's job; accusations are nobody's.

Be ready to read a Snellen fraction and its metric equivalent, describe a 20/20 letter, record each eye with cc, sc, and partial lines, occlude correctly, give the near distance and notations, interpret a pinhole result, step down to CF, HM, LP, and NLP, choose a test for an infant, a preliterate child, or an adult who cannot read, explain crowding and fogging, and handle inconsistent results without accusation.