HKU E-learning Platform in Clinical Neurosciences

Subtitles in English for this video can be displayed by clicking on  (first button on the bottom right hand corner of the video).

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) [lightbox full=”http://youtu.be/BUf_XrecQvc” title=”Distant Visual Acuity Examination”](see Distant visual acuity video)[/lightbox] 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 [lightbox full=”http://youtu.be/JKH-ICcRPIc” title=”Near Visual Acuity Exam “](see Near Visual Acuity video)[/lightbox].

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.

Technique

1. 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.
2. Measure the acuity of each eye independently with the patient wearing appropriate corrective lenses. If a patient does not have glasses, a pinhole occluder [lightbox full=”http://youtu.be/qwoU7guMbxg” title=”Pinhole “](see Pinhole video)[/lightbox] may be used to obtain a reasonable estimate of the patient’s visual acuity.
3. 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.
4. 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.
5. 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.
6. 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’.

Clinical importance

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.

Different parts of the retina are particularly sensitive to certain hues. The majority of nerve fibres of the anterior visual pathways (optic nerve, optic chiasm and optic tract) serve our central vision, which is particularly sensitive to red and green light. Hence, diseases of the optic nerve (optic neuropathies) will cause red-green colour desaturation.

Technique

• In the specialist out-patient clinic, pseudo-isochromatic color plates like Ishihara charts can be used to both identify and quantify red-green colour desaturation.
• At the bedside, relative colour desaturation can be assessed using a bright red or bright green object. The patient is asked to compare the colour of the object between eyes. In the eye with optic nerve disease, the object will appear less bright. This however only works when there is unilateral disease or bilateral disease with asymmetrical involvement.

Clinical importance

• 10% of men (and 0.4% of women) have x-linked red-green colour blindness.
• Red-green desaturation is disproportionately severe (compared to other visual function parameters) in optic neuritis. The majority of patients with optic neuritis have the retro-bulbar subtype, where the optic nerve head is not inflamed. Hence, optic disc swelling is an uncommon sign in optic neuritis. Early detection of optic neuritis is thus dependent on colour vision testing. In Hong Kong, a common cause of secondary optic neuritis is ethambutol-related toxicity. This drug is an important first line agent in the management of active tuberculosis. Cessation of ethambutol when red-green desaturation is detected prevents permanent visual loss.

Subtitles in English for this video can be displayed by clicking on  (first button on the bottom right hand corner of the video).

The confrontation test is a quick screening test for gross visual field defects that can be done at the bedside. Spectacles should not be worn during the test as they may cause spurious visual field defects.

Technique

1. Seat yourself directly opposite to the patient, at one arm’s length away from each other. Test each eye separately. Ask the patient to cover the non-tested eye, while you cover the eye directly opposite to the patient’s non-tested eye. Therefore your uncovered eye is now directly facing the patient’s tested eye.
2. Ask the patient to focus on either your nose or your uncovered eye for the examination. Focusing on the uncovered eye is preferred to make the patient’s visual field match up with yours as much as possible. Make sure the patient remains focused on your nose or your uncovered eye during the examination.
3. Finger counting test: Hold both your arms out so that your hands are equidistant between you and the patient. Place each hand in 1 of the 4 visual field quadrants. Instruct the patient to count how many fingers you are holding up (do this 2 times for each quadrant). Then repeat this for the last 2 quadrants. This test only works if the patient retains good visual acuity.
4. Moving finger test: For each of the 4 visual field quadrants, start at the peripheral corner and progressively work your way towards the centre until the patient can see your moving finger.
5. Repeat the tests for the other eye, using your contralateral eye.

Clinical importance

The site of the lesion along the visual pathway determines the type of visual field defect seen. In general, because it measures only the outer edge of your visual field, the confrontation test is mainly useful in looking for large visual field defects, such as:

• Bitemporal hemianopia (suprasellar space-occupying lesions)
• Homonymous hemianopia (stroke, space-occupying lesions)
• Homonymous quadranopia (stroke, space-occupying lesions)

Inflammatory, toxic, hereditary and nutritional-deficiency related optic neuropathies tend to give central scotomas or centro-caecal scotomas, which are difficult to pick up on confrontation tests.

Glaucomatous optic neuropathies (glaucoma) commonly begin with nasal visual field defects, but are small and easily missed on confrontation tests.

Common mistakes in examinations

• Forgetting to test each eye separately.
• Forgetting to test the visual fields on the nasal side.
• Failure to notice a nasal visual field defect. It is important to remember that the nasal visual field goes up to 60°.