9. Fixation disparity

Question 1

What is fixation disparity?

Answer 1

A small misalignment of the eyes when focusing on a specific point in space.
It is subtle enough that it doesn’t disrupt the perception of a single image as the images are in panum’s fusional area so the brain can still fuse the two slightly misaligned images into one.

Question 2

By how much is the misalignment of the eyes in fixation disparity?

Answer 2

Each eye’s gaze is slightly different only by less than 6min arc.

Question 3

What is the difference between pathological and physiological diplopia?

Answer 3

Physiological = Visual axis alligned

Pathological = Visual axis misalligned

Question 4

What is the difference between heterophoria and Fixation disparity?

Answer 4

Heterophoria = occurs during dissociation

Fixation disparity = occurs during binocular vision

Question 5

Does fixation disparity involve pathological diplopia or physiological diplopia?

Answer 5

Pathological diplopia

Question 6

What is the level of vergence innervation (How much vergence is needed) determined by?

Answer 6

By the direction and size of fixation disparity?

Question 7

What are some examples of large fusional demands?

Answer 7

1. Near vision work
2. Low illumination
3. Switching between near and far focus

Question 8

For eso deviation where does the image land and where is it projected?

Answer 8

Image lands on nasal retina,
Projected (seen) temporally.

Question 9

For exo deviation where does the image land and where is it projected?

Answer 9

Image lands on temporal retina. Projected (seen) nasally.

Question 10

What are the two methods to measure fixation disparity?

Answer 10

1. Measure the fixation disparity funtion -
   in minutes of arc
   or with sheedy disparometer
2. Determine the vergence change (prism) that reduces the fixation disparity to zero -
   in prism dioptres
   using mallett unit, zeiss polatest.

Question 11

Using the mallet unit, what is the method to measure fixation disparity?

Answer 11

1. Patient wears cross polaroid filter glasses
2. Instruct Px to tell you whether the two strips are in alignment (If not, establish which one is misaligned and in which direction)
3. Use prism bar to correct misalignment and record prism strength.
4. Test horizontal and vertical
5. Test at near and far.

Question 12

What is the problem with the sheedy disparometer?

Answer 12

Uses peripheral fusional lock only.

This is unnatural conditions as real world conditions offer both central and peripheral stimuli.

Also panum’s area is larger in periphery so would have a greater tolerance to misalignment resulting in larger fixation disparity measure.

Question 13

What is the relationship between fixation disparity and heterophoria?

Answer 13

Could be related but Weak correlation.

Px with large phoria may not have Fixation disparity
Whilst a Px with small phoria could have significant Fixation disparity.

Question 14

If there is a large fixation disparity what is the result of this?

Answer 14

Can produce symptoms and indicate decompensated phoria.