Entoptic Phenomena of Retinal Origin

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Entoptic Phenomena of Retinal Origin
Page 2.36
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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

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The Troxler Effect
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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

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High cone density
Central Retina
Peripheral Retina
Thick ganglion cell layer
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The Troxler Effect

• Larger receptive fields in peripheral retina ?
  will see retinal adaptation here first with very
  steady fixation

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View center of pattern with one eye for 30
seconds
Fig 2.23, Page 2.36
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Retinal Adaptation

• Switches off our perception of pre-retinal
  vessels (cast very distinct shadow on retina)
  under normal conditions

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Entoptic Perception of Retinal Vessels
Page 2.37
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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

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Purkinje Tree
See projection of larger vessels with penlight
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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
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Avascular Fovea
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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
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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
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Blue Field Entoptoscope
See many spots Follow short, arcuate paths
Diagnosis Looking for difference in number and
speed of motion between quadrants (and eyes)
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Phosphenes
Page 2.38
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Phosphenes Proof of the Inverted Retinal Map
N
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Phosphenes Proof of the Inverted Retinal Map
N
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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
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Phosphenes - Flashes of Light

• Clinically significant ? may indicate posterior
  vitreous detachment or retinal detachment

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Posterior Segment Anatomy
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Posterior Vitreous Detachment
Page 2.38
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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

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Posterior Vitreous Detachment
Vitreous pulls away from the retina at the
posterior pole
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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)

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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

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Retinal Tear with Vitreous Detachment
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Retinal Detachment
Page 2.39
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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)

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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

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Types of Retinal Detachment
Inner limiting membrane
Tractional
VITREOUS
Break
RPE
Serous
Rhegmatogeneous
CHOROID
Bruchs Membrane
SCLERA
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Blue Arcs of the Retina
NO clinical significance
Page 2.39
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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

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Blue Arcs
Fig 2.24, Page 2.40
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Blue Arcs
Nasal Field
Temporal Field
N
Disc
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Blue Spike - Target Horizontal
Fig 2.25, Page 2.40
Fixating nasal edge of horizontal rectangle
(orange)
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Entoptic Phenomena of Macular Origin
Page 2.41
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Plane Polarization
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Plane Polarization Rope Analogy
Zero energy in horizontal plane
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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)

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Vertically Polarized Light

• Vertical plane of vibration (maximum energy)
• Horizontal plane of extinction (zero energy)

zero energy
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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

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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

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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

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Entoptic Phenomena of Macular Origin

• Haidingers Brushes

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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)

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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
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Neuroglial Fibers (Muller Cells)
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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
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Radial Analyzer
Xanthophyll tends to associate with Muller cell
fibers ? yellow radial analyzer
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Appearance of Haidingers Brushes

• Haidingers brushes appear optimal when viewing a
  rotating plane polarizer through a blue filter

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Appearance of Haidingers Brushes under optimum
conditions
Fig 2.26, Page 2.41
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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)

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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

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Haidingers Brushes Dichroic RA Theory
WHITE LIGHT
BLUE LIGHT
Fig 2.28, Page 2.43
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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
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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)

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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