During any examination in an OSCE, it is important to understand the pathology and reasoning behind each sign and symptom you elicit. This applies even if the patient is 'normal'. This article explains how to perform a visual acuity assessment and the key findings you should look for, showing you what each step means and what conditions it may indicate.
Visual acuity is a measure of the eye's ability to resolve fine detail. It reflects the function of three parts of the visual system. The first is the central retina: the retina is the light-sensitive layer at the back of the eye, and its centre is the macula, with the fovea at the middle, which gives the sharpest vision. The second is the optical media, the clear structures that light passes through: the cornea (the clear front window of the eye), the lens and the vitreous (the clear jelly behind the lens). The third is the visual pathway as far as the visual cortex. A formal acuity measurement is one of the most important pieces of information in ophthalmology (the medical specialty of the eye). It both quantifies the patient's vision and helps localise (pinpoint) where in the visual system a problem lies.
Distance Visual Acuity (Snellen Chart)
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) . Then ask them to read down the chart to the smallest line (the lowest one on the chart) 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 (the top number) is the distance the patient is from the chart in metres (usually 6). The denominator (the bottom number) is the number printed beside the smallest line the patient could read. This number is 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).
Colour Vision (Ishihara Plates)
Colour vision can be assessed using Ishihara plates. Each plate displays a circle made up of coloured dots, within which dots of a contrasting colour form a number or shape. The figure is easily seen by people with normal colour vision but is difficult or impossible to identify for those with a red-green colour deficiency.
Image - An Ishihara test plate. A person with normal colour vision sees the embedded number clearly, whereas someone with a red-green deficiency may be unable to distinguish it
Public Domain source by Shinobu Ishihara [Public domain]
To perform the test, ensure good lighting and test each eye separately with the patient wearing any usual reading correction. The first plate is a test plate that does not assess colour vision – it can be read by everyone, including those who are colour blind, and confirms that the patient has adequate contrast sensitivity and acuity to undertake the test. If the patient can read the test plate, work through the remaining plates in turn, asking them to identify the number on each, and document the number of plates read correctly out of the total (for example 13/13).
Acquired colour vision defects (those that develop later in life) are clinically significant. A reduction in colour vision, particularly the loss of red saturation (red desaturation), is an early and sensitive sign of optic nerve disease such as optic neuritis (inflammation of the optic nerve), which is strongly associated with multiple sclerosis (a condition in which the immune system damages nerves in the brain and spinal cord). Inherited red-green colour deficiency, by contrast, is common. It affects up to about 1 in 12 men and about 1 in 200 women, because of its X-linked inheritance (the gene is carried on the X chromosome, so men are affected far more often than women). It is usually a lifelong, non-progressive finding rather than a sign of acute disease.
Completing the Examination
Thank the patient and wash your hands.
Summarise your findings, clearly documenting the acuity for each eye (and both together), whether correction or a pinhole was used, the near vision result and the colour vision score.
To complete the assessment, suggest performing a full examination of the eyes and vision, including assessment of the visual fields, pupillary reflexes and eye movements. When testing the pupils, look in particular for a relative afferent pupillary defect. On the swinging torch test, both pupils dilate when the light is moved from the normal eye to the affected eye. This points to optic nerve disease or severe retinal disease. If the patient describes distortion of central vision, an Amsler grid is a useful bedside test. The patient looks steadily at a central dot and reports any wavy, blurred or missing squares. This can reveal early macular pathology such as age-related macular degeneration.
Image - A normal fundus (the back of the eye) as seen on fundoscopy (looking at the back of the eye with an ophthalmoscope, a handheld instrument with a light and lenses). Reduced acuity that does not improve with a pinhole should prompt a careful look at the optic disc (where the optic nerve leaves the eye), macula and retinal vessels for a structural cause
Image credit: SimpleMed original
You should also offer fundoscopy to examine the optic disc, macula and retina, and where appropriate a slit-lamp examination (using a microscope that magnifies the front of the eye) and measurement of intraocular pressure (the pressure inside the eye). Consider relevant bedside tests that may explain reduced vision. In particular, a blood pressure reading may point to hypertensive retinopathy (retinal damage from high blood pressure). A capillary blood glucose (a finger-prick sugar test) or HbA1c (glycated haemoglobin, a blood test that reflects average blood glucose over the previous 2 to 3 months) may point to diabetic retinopathy. In a patient with diabetes, also check that they are attending NHS diabetic eye screening, which uses retinal photography and is offered every 1 or 2 years from the age of 12. Any patient with new, unexplained or acute visual loss should be discussed with the ophthalmology team, as timely referral can be sight-saving.
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