Central Vision. Visual Acuity.
CENTRAL VISION.
An eye's best vision; used for reading and discriminating fine detail and colour → Results from stimulation of the fovea and the macular area, where majority of photoreceptors are concentrated.
VISUAL ACUITY
Visual acuity is an objective measure of what an individual can see. Measurement of visual acuity (VA) is the fundamental measure in ophthalmology and should be done prior to instilling any eye drops. It tests the form sense. The most common visual acuity tests are Snellen’s test and LogMar test.
Snellen’s Test
Top line is 6/60
6/12 - at this level of vision, many social activities are possible (UK driving test standard)
6/6 - 'normal' eyesight, also known as '20/20 vision'
- Snellen vision is measured at 6m.
- The Snellen chart has letters, but there are also versions with the ‘illiterate E’ and numbers. The 6/6 line is ‘normal’ vision. Patients can often read the lower lines, 6/5 or 6/4, which is better than normal.
- The numerator at the top describes the distance the patient is from the Snellen chart; 6/6 means the patient is at 6 m
- Important points to keep in mind:
- Test each eye separately.
- Assess VA before dilating the pupil and before shining a bright light into the eye.
- Test with a pinhole to detect a refractive error
Recording of Visual Acuity for Distance Vision
- Each eye is tested separately. A normal person can read all the lines, i.e. up to 6 m line. The normal visual acuity is = 6/6.
- When the patient can only read the 18m line, his distant vision is defective = 6/18.
- When the patient cannot read the largest letter, he is asked to walk slowly towards the chart. If he can read the top most letter at 5, 4, 3 or 2 m, his visual acuity = 5/60, 4/60, 3/60, 2/60 respectively.
- If the patient is unable to see the top letter when close to it, he is asked to count the surgeon’s fingers held at 1 m against a dark background. If he can count the fingers, the visual acuity = 1/60.
- If he can count fingers only at 50 cm, the visual acuity = counting fingers at 50 cm.
- If he cannot count fingers, the surgeon’s hand is moved in front of the eyes close to face. If he can appreciate the movements, the visual acuity = hand movements.