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OV2018 binocular vision > fixation disparity > Flashcards

fixation disparity Flashcards

1
Q

what is binocular vision ?

A
  • an image perceived by each eye
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2
Q

what is binocular single vision?

A
  • an image is perceived by RE and LE and sensory fusion occurs where an image from each eye fused to form a common visual percept
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3
Q

what is total dissociation?

A

. there is no stimulus to fuse

  • occlude ( cover test)
  • septum ( Maddox wing)
  • optical distortion ( Maddox rod)
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4
Q

what is partial dissociation?

A
  • there is some stimulus to fuse
  • red and green goggles ( worth’s lights )
  • polarising filters ( Mallet unit)
  • striated lenses ( Bagolini glasses )
  • blurring lens ( binocular balance )
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5
Q

explain heterophoria under binocular conditions?

A
  • alignment of visual axes is maintained using motor fusion
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6
Q

explain heterophoria under dissociated conditions?

A
  • there is no stimulus to fuse
  • visual axes do not remain aligned
  • exophoria; visual axes move outward
  • esophoria; visual axes move inwards
  • heterophoria; visual axes misalign vertically
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7
Q

what is decompensation?

A
  • when the amplitude of the heterophoria approaches the limits of the subjects motor fusion range
  • the subject can no longer maintain binocular single vision
  • heterophoria becomes heterotropia( this can lead to diplopia or suppression)
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8
Q

what is fixation disparity?

A
  • the fusion brings the object within Panum’s fusional area, but not on the horopter
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9
Q

when is the object seen with slight crossed disparity?

A

. slight crossed disparity if in front of the horopter

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10
Q

when is the object seen with slight uncrossed disparity?

A

. slight uncrossed disparity if behind the horopter

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11
Q

when does fixation disparity occur?

A
  • occurs because the patient either over converges or under converges
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12
Q

what is an eso fixation disparity?

A
  • px fixates before cross
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13
Q

what is an exo fixation disparity?

A
  • px fixates behind the cross
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14
Q

how to measure fixation disparity?

A
  • by using partial dissociation
  • some areas seen binocularly ( the parts of the target seen binocularly have cyclopean projection - appear straight ahead)
  • some areas of target seen monocularly ( parts of the target seen monocularly have monocular projection)
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15
Q

what if there is no fixation disparity ?

A
  • monocular projection in same direction as cyclopean
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16
Q

what if there is no fixation disparity ?

A
  • monocular parts of image are projected to the side
17
Q

what are the two instruments you can use to measure fixation disparity?

A

1- disparometer -

  • use polaroid filters where some parts of target are seen monocularly and some parts seen binocularly
  • lines are adjusted to see lines in straight line

2-Wesson card-

  • polaroid filters which partially dissociate the eye
  • displacement of arrow read off on scale
  • 25 or 40 cm viewing scale
18
Q

explain ogle research?

A
  • he tried to measure how the fixation disparity changes when you put stress on ocular motor system
    1 - system was put under exo stress ( measure fixation disparity through introduced base out prism)

2- system was put under exo stress ( measure fixation disparity through introduced base in prism )

  • FD measured in minutes of arc
19
Q

explain the ogle curve?

A
  • the X axis
    . power of introduced prism ( BO or BI prism stress)
    . Y=0=associated phoria
  • the Y axis
    . fixation disparity
    . X=0=fixation disparity in normal viewing

-CS=centre of symmetry

20
Q

what is associated phoria?

A
  • the amount of prism which makes the fixation disparity 0
21
Q

what are the different curve types?

A
  • type 1: normal pattern of response
  • type 2: system under eso stress ( the patient has eso fixation disparity )
  • type 3: system under exo stress( patient has an exo fixation disparity)
  • type 4: unstable binocularity
22
Q

what is the difference between fixation disparity and associated phoria?

A

1- fixation disparity: measured in minutes of arc

  • fixation disparity is misalignment of visual axes durning normal fixation
  • limited in size by dimensions of Panum’s fusional area at the point where visual axes cross

2- associated phoria:

  • prism or lenses that reduce fixation disparity to zero
  • measured in prism dioptres or dioptre sphere
23
Q

what are the methods of assessment of phoria?

A
  1. mallet unit

2. computerised system

24
Q

what is the mallet unit?

A
  • uk’s favoured method of measuring associated phoria

- OXO

25
Q

explain the technique of mallet unit?

A
  1. good lighting
    - partial dissociation: polaroid filters halve lighting
    - all parts of test should been seen
  2. demonstrate target without polaroids
  3. put up polaroids
  4. stabilise accommodation ( ask patient to read a few letter of 6/6 or N5 before asking about misalignment )
  5. fixation
    . look at X
    . comment about position of nonius markers
26
Q

what are the responses you can get from mallet unit?

A
  1. target horizontal, strips vertical
    - assess horizontal associated phoria
    - check which eye sees which strip
    . crossed disparity ( exo FD)
    . uncrossed disparity ( eso FD)
  2. target vertical, strips horizontal
    - assess vertical associated phoria
27
Q

how is torsion relieved in fixation disparity?

A
  • torsion cannot be relieved by prisms or lenses

- torsion may be relieved by using head tilt

28
Q

when to correct phoria?

A
  • correct associated phoria only if symptomatic
  • in symptomatic deviations use results of associated phoria alongside other information
  • vertical deviations : correct associated phoria
  • horizontal deviations : assess in relation to results of other investigations ( FD curve , fusional reserves , accommodative function and previous ocular history)
29
Q

what is the treatment for distance exo?

A
  • base in prisms

- negative lenses

30
Q

what is the treatment for distance eso?

A
  • base out prisms
31
Q

what is the treatment for near exo?

A
  • base in prisms

- negative lenses

32
Q

what is the treatment for near eso?

A
  • base out prisms

- positive lenses

33
Q

what is the treatment for R hyperphoria?

A
  • base down RE

- base up LE

34
Q

what is the treatment for L hyperphoria ?

A
  • base up RE

- base down LE

OV2018 binocular vision (15 decks)

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