fixation disparity

Question 1

what is binocular vision ?

Answer 1

• an image perceived by each eye

Question 2

what is binocular single vision?

Answer 2

• 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

Question 3

what is total dissociation?

Answer 3

. there is no stimulus to fuse

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

Question 4

what is partial dissociation?

Answer 4

• 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 )

Question 5

explain heterophoria under binocular conditions?

Answer 5

• alignment of visual axes is maintained using motor fusion

Question 6

explain heterophoria under dissociated conditions?

Answer 6

• 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

Question 7

what is decompensation?

Answer 7

• 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)

Question 8

what is fixation disparity?

Answer 8

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

Question 9

when is the object seen with slight crossed disparity?

Answer 9

. slight crossed disparity if in front of the horopter

Question 10

when is the object seen with slight uncrossed disparity?

Answer 10

. slight uncrossed disparity if behind the horopter

Question 11

when does fixation disparity occur?

Answer 11

• occurs because the patient either over converges or under converges

Question 12

what is an eso fixation disparity?

Answer 12

• px fixates before cross

Question 13

what is an exo fixation disparity?

Answer 13

• px fixates behind the cross

Question 14

how to measure fixation disparity?

Answer 14

• 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)

Question 15

what if there is no fixation disparity ?

Answer 15

• monocular projection in same direction as cyclopean

Question 16

what if there is no fixation disparity ?

Answer 16

• monocular parts of image are projected to the side

Question 17

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

Answer 17

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

Question 18

explain ogle research?

Answer 18

• 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

Question 19

explain the ogle curve?

Answer 19

• 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

Question 20

what is associated phoria?

Answer 20

• the amount of prism which makes the fixation disparity 0

Question 21

what are the different curve types?

Answer 21

• 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

Question 22

what is the difference between fixation disparity and associated phoria?

Answer 22

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

Question 23

what are the methods of assessment of phoria?

Answer 23

1. mallet unit

2. computerised system

Question 24

what is the mallet unit?

Answer 24

• uk’s favoured method of measuring associated phoria

- OXO

Question 25

explain the technique of mallet unit?

Answer 25

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

Question 26

what are the responses you can get from mallet unit?

Answer 26

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

Question 27

how is torsion relieved in fixation disparity?

Answer 27

- torsion cannot be relieved by prisms or lenses | - torsion may be relieved by using head tilt

Question 28

when to correct phoria?

Answer 28

- 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)

Question 29

what is the treatment for distance exo?

Answer 29

- base in prisms | - negative lenses

Question 30

what is the treatment for distance eso?

Answer 30

- base out prisms

Question 31

what is the treatment for near exo?

Answer 31

- base in prisms | - negative lenses

Question 32

what is the treatment for near eso?

Answer 32

- base out prisms | - positive lenses

Question 33

what is the treatment for R hyperphoria?

Answer 33

- base down RE | - base up LE

Question 34

what is the treatment for L hyperphoria ?

Answer 34

- base up RE | - base down LE