Visual Acuity OSCE Assessment

1. Introduction
2. General Inspection
3. Distance Visual Acuity (Snellen Chart)
4. Reduced Visual Acuity
5. Pinhole Test
6. Near Visual Acuity
7. Colour Vision (Ishihara Plates)
8. Completing the Examination
9. Quiz

Gather the necessary equipment before you begin: a Snellen chart (or a 3 metre logMAR chart) positioned at the correct distance, an occluder to cover each eye, a pinhole occluder, a near vision reading chart, a set of Ishihara plates and a pen torch. Having everything to hand makes the assessment slick and avoids breaking the flow of the examination.

Wash your hands and don personal protective equipment if appropriate.

Introduce yourself to the patient and ensure to mention your grade e.g. 3rd year medical student/junior doctor/consultant.

Confirm the patient's details taking 3 points of identification usually; full name, date of birth and NHS/hospital number.

Obtain consent for the examination, ensuring to explain what the assessment will entail in patient-friendly language – that you would like to test how well they can see using a chart of letters.

Ask whether the patient is experiencing any pain or visual symptoms before you begin.

Position the patient seated at the correct testing distance from the chart (usually 6 metres), and ensure the room is well lit.

Critically, ask whether the patient normally wears distance glasses or contact lenses. Visual acuity should be tested with the patient's usual distance correction worn, as the purpose of the test is to measure their best corrected vision rather than an uncorrected refractive error. If the patient has forgotten their glasses, you can still obtain a meaningful result by testing through a pinhole, which largely corrects for refractive error.

Before formally testing, take a moment for a general inspection of the patient and the bedside. Look for visual aids such as glasses or a magnifier, and note whether the patient uses a white cane or is accompanied by a guide dog, both of which suggest significant pre-existing visual impairment.

Observe the patient's behaviour and how they orientate to you and to objects in the room. A patient who turns their head to one side to look at you may be relying on an area of preserved peripheral vision to compensate for central loss, while obvious difficulty navigating the room suggests poor overall vision.

Note any obvious ocular abnormalities such as a squint (strabismus), ptosis (drooping eyelid), a cloudy cornea, or a red eye, as these can all affect acuity. Finally, assess whether the patient appears comfortable or is in distress – acute painful visual loss is a red flag requiring urgent senior review.

Distance visual acuity is most commonly measured using a Snellen chart, which displays rows of letters that decrease in size as you move down the chart. The chart is designed so that each row corresponds to the distance at which a person with normal vision should be able to read those letters.

Position the patient 6 metres from the chart. If space is limited, many clinics use a mirror to create an effective 6 metre path within a smaller room.

Ask the patient to cover one eye (traditionally with an occluder or the palm of their hand, taking care not to press on the eye) and to read down the chart, identifying the lowest line they are able to read. Always test each eye separately, conventionally starting with the right eye, and then test both eyes together.

Image - A standard Snellen chart. The letters become progressively smaller down the chart, with each row labelled by the distance at which a normally-sighted person can read it

Creative commons source by Jeff Dahl [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

Visual acuity is recorded as a fraction. The numerator is the distance the patient is from the chart in metres (usually 6), and the denominator is the number printed beside the lowest line the patient could read – this represents the distance at which a normally-sighted person could read that same line.

For example, an acuity of 6/6 means the patient can read at 6 metres what a normally-sighted person reads at 6 metres – this is normal vision (equivalent to 20/20 in imperial units). An acuity of 6/60 means the patient can only read at 6 metres what a normally-sighted person could read from 60 metres, indicating significantly reduced vision. The larger the denominator, the worse the vision.

When recording acuity, you should note how many letters were missed on the last line read. If the patient reads the 6/6 line but gets two letters wrong, this is recorded as 6/6 (-2). If the patient misses more than two letters on a line, you should record the line above as their acuity. Always document whether vision was unaided (UA), with glasses, or with a pinhole (PH).

If the patient is unable to read even the top line of the Snellen chart at 6 metres (the large single letter, corresponding to 6/60), you should work through a series of steps to quantify the remaining vision. Each step assesses a grosser level of visual function than the last.

First, reduce the distance. Walk the patient closer to 3 metres and ask them to read the top line again; if successful this is recorded as 3/60. If still unsuccessful, move to 1 metre (recorded as 1/60). Reducing the distance increases the angular size of the letter on the retina, so a patient with poor but not absent vision may be able to resolve it closer up.

If the patient still cannot read the top line at 1 metre, assess progressively cruder visual function in the following order:

• Counting Fingers (CF) – hold up a number of fingers about 1 metre from the patient and ask them to count them. This tests whether the patient can resolve large, high-contrast shapes.
• Hand Movements (HM) – if they cannot count fingers, wave your hand and ask whether they can detect the movement. This tests for the most basic perception of motion.
• Perception of Light (PL) – if they cannot detect hand movements, shine a pen torch towards the eye and ask whether they can see the light. A patient who can perceive light is recorded as PL; one who cannot is recorded as No Perception of Light (NPL), which indicates complete blindness in that eye.

This stepwise grading is clinically important because it distinguishes, for example, a patient with a dense cataract (who typically retains light perception and may even perceive its direction) from a patient with complete retinal or optic nerve failure (who has no perception of light at all).

Preview the Visual Acuity OSCE Assessment quiz