Testing sensory visual function

1. Testing sensory visual function

2. 2 types:1) psychophysical tests2) electrophysical tests

3. Psycho-physical tests: 1- visual acuity 2-contrast sesitivity 3-amsler grid 4-light brightness comparison test 5-photostress test 6- Dark adaptation 7-color vision test

4. Electro-physical tests • Electroretinography • Electrooculography • Visual evoked potential

5. Visual acuity

6. the most common clinical measurement of visual function • is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc • VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied • 6/6 vision" the numerator refers to the distance in meter from which a person can reliably distinguish a pair of objects. The denominator is the distance from which an 'average' person would be able to distinguish —the distance at which their separation angle is 1 arc minute.

7. Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or Tumbling E.

8. Contrastsensitivity .

9. Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background . • determined by the difference in the color and brightness of the object and other objects within the same field of view. • Can detect very eary visual dysfunction, even when snellen VA is normal • Example: Pelli-Robson chart

10. Amsler grid

11. a grid of horizontal and vertical lines used to monitor a person's central 20 visual field. • Mainly for screening &monitoring macular disease. • The original Amsler grid was black and white. A color version with a blue and yellow grid is more sensitive and can be used to test for a wide variety of visual pathway abnormalities, including those associated with the retina, the optic nerve, and the pituitary gland. • There are 7 charts ,each consisting of 10cm squar.

12. An Amsler grid, as seen by a person with normal vision.

13. An Amsler grid, as it might be viewed by a person with age related macular degeneration.

14. Macular disease = wavy lines • Optic nerve disease = missing or faint lines but not distorted

15. Dark adaptation • the ability of the visual system to adjust to various levels of darkness and light. • Tested by dark adaptometry. • Maily useful in patients with nyctalopia.

16. Photo-stress test • Gross test of dark adaptation. • Used to differentiate visual loss caused by macular from that caused by an optic nerve lesion. • Normal recovery time = 15-30 seconds • Delay in visual recovery indicates macular pathology (e.g CME ,CSR)

17. Light brightness test • To test optic nerve function by comparing light brightness in one eye with the fellow. • In optic neuropathy , light is less bright in the affected eye.

18. Color vision • ability to perceive differences between colors . • color blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish. • Tests: • Ishihara: most often used to diagnose red-green color deficiencies.

19. Hardy-rand-rittler: more sensitive than ishihara , it detect all 3 congenital defects. • Farnsworth-Munsell 100-hue : most sensitive. Example of an Ishihara color test plate

20. Electro-retinography

21. measures the electrical responses of various cell types in the retina. Electrodes are usually placed on the cornea and the skin near the eyewith a reference electrode on forehead. Potential between 2 electrodes is amplified & displayed. Normal ERG is biphasic ,an a-wave (initial negative deflection) followed by a b-wave (positive deflection). The leading edge of the a-wave is produced by the photoreceptors, while the remainder of the wave is produced by a mixture of cells including photoreceptors, bipolar, amacrine, and Muller cells.

22. If a flash ERG is performed on a dark-adapted eye, the response is primarily from the rod system and if a flash ERGs performed on a light adapted eye will reflect the activity of the cone system . The multifocal ERG is used to record separate responses for different retinal locations. Clinically used mainly by ophthalmologists and optometrists, the electroretinogram (ERG) is used for the diagnosis of various retinal disease ,examples: Retinitis pigmentosa Leber's congenital amaurosis Cone dystrophy Diabetic retinopathy Retinal vain acclusion..etc

23. Electrooculography

24. a technique for measuring the resting potential of the retina. • The main applications are in ophthalmologicaldiagnosis and in recording eye movements. • Unlike the electroretinogram, the EOG does not represent the response to individual visual stimuli. • The EOG is used to assess the function of the pigment epithelium.

25. During dark adaptation, the resting potential decreases slightly and reaches a minimum ("dark trough") after several minutes. When the light is switched on, a substantial increase of the resting potential occurs ("light peak"), which drops off after a few minutes when the retina adapts to the light. The ratio of the (light peak divided by dark trough) is known as the Arden ratio.

26. Evoked potential

27. VEP is a recording of electrical activity of the visual cortex after retinal stimulation. Main indications: Monitoring visual function in babies. Optic neuropathy Monitor macular pathway function