Visual Acuity

The familiar Snellen chart has one large letter at the top, which is designed to be just visible to a normal-sighted person at 60 m. The chart is viewed from a distance of 6 m. If a patient is just able to see this large letter, the vision is recorded as 6/60. Below the large letter are rows of smaller

Table 3.1. m

History

Age

Ophthalmic:

Subnormal vision

Duration. Difference

between eyes

Disturbances of vision

Distortion, haloes,

floaters, flashing

lights, momentary

losses of vision -

field defects

Pain/discomfort

Increase/decrease

Discharge

Change in

appearance -

discolouration

Change in lacrimation

Swelling/mass

Diplopia

Displacement

General medical:

Diabetes/

hypertension/

COAD/dysthyroid/

connective tissue

disease

Drugs

FH social/

occupational

Examination

VA: distance/near (with and

without glasses)

Colour vision

Visual fields

Orbit

Proptosis/

enophthalmos

Ocular movements -

Eyelids and lacrimal

conjugate and convergence

apparatus

Pupils

Intraocular pressure

Position of eyes

Conjunctiva, cornea

AC

Iris

Media - lens/vitreous

Fundus - retina/choroid,

optic disc

Special investigations

Fluorescein angiography

Radiological and ultrasound

Haematological/biochemical

Bacteriological/immunological

Diagnosis

Anatomical

E.g., cataract

Aetiological

E.g., diabetes

Figure 3.1. The Snellen chart.B3

letters, decreasing in size down to the bottom. The size of letter normally visible to a normal-sighted person at 6 m is usually on the second-to-bottom line. Patients reading this line are said to have a vision of 6/6. If a patient cannot read the top letter, he is taken nearer to the chart. If the top letter becomes visible at 3 m, the acuity is recorded as 3/60. If the letter is still not visible, the patient is asked if he can count fingers (recorded as "CF") and, failing this, if he can see hand movements ("HM"). Finally, if even hand movements are not seen, the ability to see a light is tested ("PL").

Figure 3.2. The Stycar test.CQ

Young children and illiterates can be asked to do the "E" test, in which they must orient a large wooden letter "E" so that it is the same way up as an indicated letter "E" on a chart. Perhaps better than this is the Stycar test (Figure 3.2), in which the child is asked to point at the letter on a card that is the same as the one held up at 6 m. Other ways of measuring visual acuity are discussed in Chapter 17.

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