Measuring the visual acuity of each eye is the first step of the neuro-ophthalmic examination. It should be carried out on every patient presenting with an ocular symptom. Distant visual acuity (at 6 metres or 20 feet) (see Distant visual acuity video) is the measurement of choice in an ophthalmology specialist outpatient clinic. However, near visual acuity (at 35 cm or 14 inches) is a more practical measurement that can be performed at the bedside (see Near Visual Acuity video).
The most common method of assessment is using a Snellen chart, which comprises of random letters, arranged in rows, decreasing in size in each row.
- The room is kept as well lit as possible to maximize the contrast between the black letters and the white background on a Snellen chart.
- Measure the acuity of each eye independently with the patient wearing appropriate corrective lenses. If a patient does not have glasses, a pinhole occluder (see Pinhole video) may be used to obtain a reasonable estimate of the patient’s visual acuity.
- The Snellen chart is read from left to right, from top to bottom till the patient can no longer read any further. Allow the patient to guess if he/she is not sure.
- At the side of each row is a fraction, with the numerator (top number) indicating the distance at which the chart is being read. The denominator (bottom number) indicates the distance at which a person with normal vision would be able to read that particular row. Hence, the numerator stays constant, whilst the denominator increases as we go down the chart.
- As a rule of thumb, if the patient can read more than ½ of the row, then the reading is recorded as the fraction corresponding to that row. If the patient can read less than ½ of the row, then the reading is recorded as the fraction corresponding to the previous row.
- If the patient is unable to read the letters on the top row of a Snellen chart, then try counting fingers. If the patient is unable to see this, then try hand movements. If the patient is unable to see this either, then try light perception (with room lights off). If the patient sees nothing at all, then the visual acuity is recorded as ‘no light perception’.
The measurement of visual acuity has medico-legal implications:
- Driving: The transport department of Hong Kong requires a visual acuity of at least 20/40 in one eye to be eligible to carry a driving license. To work as a commercial driver, both eyes are required to have a visual acuity of at least 20/40.
- Disability Allowance and Compensation: Visual acuity is the most important parameter in determining the severity of visual impairment and disability. Often a medical practitioner will be required to determine a patient’s level of visual disability to determine whether he/she qualifies for disability allowance. Also, visual acuity is used to determine the level of compensation given from work-related injuries.
Hence, an accurate and repeatable visual acuity measurement is a key skill for every medical practitioner.
Common mistakes in examinations
- Forgetting the correct testing distance
- Forgetting to test each eye separately
- Forgetting to switch off the room light when testing for light perception
- Calculation error during converting visual acuity as a fraction to a decimal number. It is important to remember that the visual acuity can be reported as a fraction without conversion.