fixation disparity Flashcards
what is binocular vision ?
- an image perceived by each eye
what is binocular single vision?
- 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
what is total dissociation?
. there is no stimulus to fuse
- occlude ( cover test)
- septum ( Maddox wing)
- optical distortion ( Maddox rod)
what is partial dissociation?
- 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 )
explain heterophoria under binocular conditions?
- alignment of visual axes is maintained using motor fusion
explain heterophoria under dissociated conditions?
- 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
what is decompensation?
- 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)
what is fixation disparity?
- the fusion brings the object within Panum’s fusional area, but not on the horopter
when is the object seen with slight crossed disparity?
. slight crossed disparity if in front of the horopter
when is the object seen with slight uncrossed disparity?
. slight uncrossed disparity if behind the horopter
when does fixation disparity occur?
- occurs because the patient either over converges or under converges
what is an eso fixation disparity?
- px fixates before cross
what is an exo fixation disparity?
- px fixates behind the cross
how to measure fixation disparity?
- 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)
what if there is no fixation disparity ?
- monocular projection in same direction as cyclopean
what if there is no fixation disparity ?
- monocular parts of image are projected to the side
what are the two instruments you can use to measure fixation disparity?
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
explain ogle research?
- 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
explain the ogle curve?
- 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
what is associated phoria?
- the amount of prism which makes the fixation disparity 0
what are the different curve types?
- 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
what is the difference between fixation disparity and associated phoria?
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
what are the methods of assessment of phoria?
- mallet unit
2. computerised system
what is the mallet unit?
- uk’s favoured method of measuring associated phoria
- OXO
explain the technique of mallet unit?
- good lighting
- partial dissociation: polaroid filters halve lighting
- all parts of test should been seen - demonstrate target without polaroids
- put up polaroids
- stabilise accommodation ( ask patient to read a few letter of 6/6 or N5 before asking about misalignment )
- fixation
. look at X
. comment about position of nonius markers
what are the responses you can get from mallet unit?
- target horizontal, strips vertical
- assess horizontal associated phoria
- check which eye sees which strip
. crossed disparity ( exo FD)
. uncrossed disparity ( eso FD) - target vertical, strips horizontal
- assess vertical associated phoria
how is torsion relieved in fixation disparity?
- torsion cannot be relieved by prisms or lenses
- torsion may be relieved by using head tilt
when to correct phoria?
- 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)
what is the treatment for distance exo?
- base in prisms
- negative lenses
what is the treatment for distance eso?
- base out prisms
what is the treatment for near exo?
- base in prisms
- negative lenses
what is the treatment for near eso?
- base out prisms
- positive lenses
what is the treatment for R hyperphoria?
- base down RE
- base up LE
what is the treatment for L hyperphoria ?
- base up RE
- base down LE
