Entoptic Phenomena of Retinal Origin

1. Entoptic Phenomena of Retinal Origin Page 2.36

2. Retinal Adaptation • Retinal receptors (actually receptive fields) require constantly changing stimulus • Attempted steady fixation  eyes in constant motion: • small amplitude, rapid tremor • larger drifts • correcting saccades • Result  images always moving over retina • “Receptor” stimulation therefore constantly changing: • minimizes retinal adaptation • prevents image fading

3. The Troxler Effect

4. The Troxler Effect • Retinal receptive fields smallest at the fovea  1:1 correspondence cone  ganglion cell • Receptive fields much larger in peripheral retina  many cones feeding into a single ganglion cell

5. High cone density Central Retina Peripheral Retina Thick ganglion cell layer

6. The Troxler Effect • Larger receptive fields in peripheral retina  will see retinal adaptation here first with very steady fixation

7. View center of pattern with one eye for ~ 30 seconds Fig 2.23, Page 2.36

8. Retinal Adaptation • “Switches off” our perception of pre-retinal vessels (cast very distinct shadow on retina) under normal conditions

9. Entoptic Perception of Retinal Vessels Page 2.37

10. Purkinje Tree • Can overcome normal adaptation that prevents us seeing shadows of retinal vessels: • shine light into eye at a very oblique angle • keep light moving to prevent adaptation • See shadows of retinal vessels entoptically • Projected into visual field based on inverted receptor map

11. Purkinje Tree See projection of larger vessels with penlight

12. Purkinje Tree - Optimum Conditions Directional light shone into eye (point source at first focus)  perceive retinal vessels down to capillary level Avascular fovea visible (“no branches”). Can “map” foveal diameter by entoptic perimetry

13. No branches Avascular Fovea

14. LIGHT

15. Luminous Darting Points - Yellow Dancing Spots • View a uniform white background  see yellow spots moving in short, arcuate patterns in paracentral visual field • Optimum Conditions: view uniform, bright, blue background  dark spots against blue background • Origin: leukocytes in pre-retinal capillaries • Erythrocytes absorb blue  create a dark background • Leukocytes transmit blue  light interruptions on dark background Page 2.37

16. Blue Field Entoptoscope - Optimized Viewing Conditions • Diagnostic device to detect/monitor retinal pre-occlusive and occlusive conditions • Brightly back-illuminated blue filter viewed through magnifying eyepiece • Field divided into quadrants by large cross-hair “reticle” • Can “perceive” avascular fovea with steady fixation  area that “spots” never enter Page 2.38

17. No spots in central region (avascular fovea) Blue Field Entoptoscope See many spots Follow short, arcuate paths Diagnosis: Looking for difference in number and speed of motion between quadrants (and eyes)

18. Phosphenes Page 2.38

19. Light is “projected” into the opposite visual field Stimulates underlying receptors Press on the globe of the eye Phosphenes: Proof of the Inverted Retinal Map N

20. Phosphenes: Proof of the Inverted Retinal Map N

21. Phosphenes: Proof of the Inverted Retinal Map • Phosphenes produce the same inverted retinal map as light shone into the eye. • Obtaining the same projection without light proves that the retina is an inverted map of the visual field N

22. Phosphenes - Flashes of Light • Clinically significant may indicate posterior vitreous detachment or retinal detachment

23. Inner limiting membrane Vitreous membrane VITREOUS RETINA Photoreceptor Layer RPE OLM Bruch’s Membrane CHOROID SCLERA Posterior Segment Anatomy

24. Posterior Vitreous Detachment Page 2.38

25. Posterior Vitreous Detachment • Vitreous loosely attached to retina by vitreous membrane (stronger at ora seratta and optic disc). • With age vitreous gel shrinks and becomes more “fluid” (loss of hyaluronic acid support for collagen) • Shrinkage creates traction  may lead to posterior vitreous detachment • Rare in patients younger than 50

26. Posterior Vitreous Detachment Vitreous pulls away from the retina at the posterior pole

27. Posterior Vitreous Detachment - Symptoms • Patient sees vertically oriented “lightning” streaks in temporal visual field (nasal retina)  Moore’s Lightning Streaks • At the same time, a cluster of floaters appear at the posterior pole (tend to resolve with time)

28. Posterior Vitreous Detachment - Symptoms • Lightning Streaks probably due to detached vitreous bumping into retina during eye movements (vitreous ballotment) • Floaters due to vitreous degeneration and possibly minor vitreous hemorrhage. Usually resolves with time as vitreous detachment expands to periphery • Risk of retinal tears or detachment, but this only occurs in a small percentage of cases  as the vitreous detaches from the underlying retina, traction can produce a retinal tear

29. Liquefied vitreous enters sub-retinal space  enlarging detachment Detaching vitreous creates retinal tear Retinal Tear with Vitreous Detachment

30. Retinal Detachment Page 2.39

31. Retinal Detachment • Retinal detachment is a clinical emergency • Signs (all more likely with a detachment near the fovea): • Photopsia (flashes of light)  entoptic • Scotomas (visual field defects) • Floaters (possibly large)  entoptic • Metamorphopsia (distortion of central vision)

32. Types of Retinal Detachment • Rhegmatogeneous Retinal Detachment - due to a retinal break (liquefied vitreous enters space between RPE and sensory retina  detachment) • Tractional Retinal Detachment - due to vitreous traction on the underlying retina • Serous (Exudative) Retinal Detachment -due to fluid accumulation beneath the sensory retina without a retinal break

33. Vitreous membrane Types of Retinal Detachment Inner limiting membrane Tractional VITREOUS Break RPE Serous Rhegmatogeneous CHOROID Bruch’s Membrane SCLERA

34. Blue Arcs of the Retina NO clinical significance Page 2.39

35. Blue Arcs of the Retina Due to secondary electrical activity in the retina One neuron firing stimulates adjacent neurons all the way back along the arcuate nerve fiber bundle to the optic disc Seen best when fixation target parallel to arcuate nerve fiber bundle in stimulated retinal region

36. Fovea Optic Disc Fixating temporal edge of vertical rectangle (orange) Blue Arcs Fig 2.24, Page 2.40

37. Disc projects to temporal VF Viewing temporal edge of rectangle with OS Blue Arcs Nasal Field Temporal Field N Disc

38. Fovea Optic Disc Blue Spike - Target Horizontal Fig 2.25, Page 2.40 Fixating nasal edge of horizontal rectangle (orange)

39. Entoptic Phenomena of Macular Origin Page 2.41

40. Plane Polarization

41. Plane of vibration Direction of propagation Plane Polarization: Rope Analogy Zero energy in horizontal plane

42. Plane Polarization: Rope Analogy Snapping a taut rope vertically at the free end causes a vertically oscillating wave to propagate horizontally along thelength of the rope. This is analogous to plane polarized light Some crystals (e.g. tourmaline) freely transmit light along one crystal axis and totally extinguish light along a perpendicular axis  emit plane polarized light from unpolarized incident light. Many are also dichroic (absorbing some s more than others)

43. Vertically Polarized Light • Vertical plane of vibration (maximum energy) • Horizontal plane of extinction (zero energy) zero energy

44. Vertically Polarized Light • Polarizing sunglasses are plane polarizers with vertical transmission axis (cut out horizontally polarized light) • Most reflected glare (e.g. from a lake surface) is horizontally polarized

45. Vertically Polarized Light • Plane polarized light “looks” no different from unpolarized light under normal conditions • Need an “analyzer” (second polarizer) to detect polarization of incident light • Crossing two polarizers (one with vertical transmission axis; second with horizontal transmission axis) results in total extinction of light

46. Crossed Polarizers Knowing the transmission axis of one polarizer (the analyzer) we can rotate the second polarizer until it is “crossed” with the analyzer. This locates the transmission axis of the second polarizer. • Polarizing sunglasses are plane polarizers with vertical transmission axis (cut out horizontally polarized light) • Most reflected glare (e.g. from a lake surface) is horizontally polarized • Plane polarized light “looks” no different than unpolarized light under normal conditions • Need an “analyzer” (second polarizer) to detect polarization of incident light • Crossing two polarizers (one with vertical transmission axis; second with horizontal transmission axis) results in total extinction of light

47. Entoptic Phenomena of Macular Origin • Haidinger’s Brushes

48. Haidinger’s Brushes - the Eye’s Analyzer Human macula contains an analyzer that (under specific viewing conditions) can entoptically differentiate the transmission and extinction axes of P-state light  transmits differently Macular analyzer has polarizing properties and is dichroic (selectively absorbing blue light)

49. Macula lutea(yellow spot)

50. Macular Pigment - Xanthophyll Macular pigment xanthophyll creates a “yellow” filter for light under normal conditions  blue cones appear to turn up their sensitivity to compensate