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Entoptic Phenomena of Retinal Origin

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Title: 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 ?
    11 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
Avascular Fovea
14
(No Transcript)
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
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
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
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) ?
    Moores 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
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
Types of Retinal Detachment
Inner limiting membrane
Tractional
VITREOUS
Break
RPE
Serous
Rhegmatogeneous
CHOROID
Bruchs 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
Blue Arcs
Fig 2.24, Page 2.40
37
Blue Arcs
Nasal Field
Temporal Field
N
Disc
38
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 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 the length 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
  • Haidingers Brushes

48
Haidingers Brushes - the Eyes 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
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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
51
Neuroglial Fibers (Muller Cells)
52
Muller Cell Fibers in Macular Region
Inner retinal layers sloped away from center in
foveal region ? supporting fibers on Muller cells
form a radial pattern
53
Radial Analyzer
Xanthophyll tends to associate with Muller cell
fibers ? yellow radial analyzer
54
Appearance of Haidingers Brushes
  • Haidingers brushes appear optimal when viewing a
    rotating plane polarizer through a blue filter

55
Appearance of Haidingers Brushes under optimum
conditions
Fig 2.26, Page 2.41
56
Mechanism of Haidingers Brushes
  • Xanthophyll pigment aligned with radial Muller
    fiber arrangement in macular region ? analyzer
    radial
  • Polarized blue light affected differently
    depending on state of polarization
  • in transmission axis ? free transmission?
    effectively overcomes yellow filter ? greater
    transmission of blue
  • light perpendicular to transmission axis ? fully
    absorbed by xanthophyll pigment ? blue ? blue
    dark (dark)

57
Mechanism of Haidingers Brushes
  • Remember under normal conditions, polarized
    light does not look any different from
    unpolarized light
  • Need the analyzer to see the state of
    polarization of light

58
Haidingers Brushes Dichroic RA Theory
WHITE LIGHT
BLUE LIGHT
Fig 2.28, Page 2.43
59
Haidingers Brushes Dichroic RA Theory
If we did not have the radial analyzer, what
would we see? Blue light
WHITE LIGHT
BLUE LIGHT
If we did not have xanthophyll at the macula,
what would we see? Blue light
If we did not have the rotating polarizer, what
would we see? Blue light
Fig 2.27, Page 2.41
60
Haidingers Brushes - Clinical Applications
  • Diagnosis of macular edema - even small amount of
    macular edema (hard to see with BIO) disrupts
    Muller cell fiber radial analyzer? Haidingers
    brushes not seen
  • Detection of eccentric fixation (strabismus
    patient) ? when fixating eccentrically, do not
    see Haidingers Brushes
  • Training central fixation seeing Haidingers
    Brushes ? tells patient when they are centrally
    fixating (feedback)

61
Recap Key Objectives
  • The most clinically important entoptic phenomena
    are phosphenes (possible retinal detachment),
    entoptic haloes (possible corneal edema), and
    macular entoptic phenomena (Haidingers brushes
    macular integrity, central fixation training for
    eccentric fixation
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