13985898
| Question | Answer |
| Define visual acuity | The ability of the eye to discriminate fine detail |
| Method used to determine VA, including recognition, detection, resolution and vernier acuity | Detection: threshold of vision in normal eye ~ 5-14 quanta of light * measured using ark adaptometry or luminance increment threshold Discrimination: ability of visual system to distinguish an object from its background * using a linear target against a background (0.5-30 secs) Resolution: ability to detect gratings or checkerboard patterns (mainly in research) (30-40 sec) Recognition: ability to detect letters, words, orientation of gap in Landolt C (1 min) Vernier acuity: hyperacuity - discrimination of small displacements of target (2-10 secs) |
| Describe in detail how distance and near VA are assessed and recorded clinically | Distance VA * assessed at standard distance of 6m - to ensure accommodation is relaxed * Charts typically have letters lines in progression of size - with each line designated by distance at which overall letter size should subtend 5 min and the detains width subtending 1 min * Recording: D'/D where D' is the viewing distance (standard 6m) and D is the distance at which the stroke width of smallest letters read subtends 1 min of arc, +/- the number of letters they can or can't read * Clinical procedure: being by measuring monocularly first (occluding one eye), Px is asked to read smallest letter on the chart - which is then recorded. If Px is unable to read 1/60, hand movements and light perception is recorded. Then binocular is measured. Near VA * Do not push Px to threshold - good enough if they can read in real world * paragraphs are used ranging from N5 (6/9)-N20 * Clinical procedure: Get Px to indicate their preferred working distance - measure it. Px should read the smallest paragraph. * Recording: N.. @ working distance (with/without habitual Rx) |
| Different clinical test available to assess VA for different populations | Snellen & logMAR - for literate Illiterate E or Landolt C - for non-English speaking population Numbers or pictures - for children Gratings - for babies |
| Explain the design principle of various vision charts | Snellen * Capital letters decreasing in size down the line (not uniformly spaced or size) * designed by be viewed at 6m/20feet * used for distance VA LogMAR - distance * Capital letters - specific letters (British standard - almost equally legible - style) * same amount of letters per line, with uniform spacing and decrease in size (0.1 log units) LogMAR - near chart * contains words of different sizes per line * words don't make sense - unable to guess words - more equivalent * font: Times New Roman * only lower case letters * viewing distance: 25/40cm Near chart: paragraph * range of text that is normally encounter in real life * working distance: ~40cm (specified by patient) * Style: lower case letters |
| Recognise normal and abnormal visual acuity values | Normal range for ages 18-24: * mean: 6/4.5 or -0.13 (SD 0.06) Normal range for ages >75: * mean: 6/6 +1 or -0.2 (SD 0.05) Abnormal: decrease of greater than or equal to 5 letters. |
| Main factors and conditions that may affect VA | 1. Photoreceptor spacing (receptor theory - based on diameter and separation of foveal cones) * suggest that it limits resolution to 49 sec of arc 2. Optics (aberrations and diffraction) * wave theory - based on wave-like nature of light * suggests that it limits resolution to 47 sec of arc 3. pupil size - as pupil size increases, VA gets worse due to aberrations * max: 2-2.4mm for optimal 4. illumination - VA increases with greater illumination (but only up to certain point) |
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