Fixation Disparity

Question 1

What is Fixation Disparity?

Answer 1

Fixation disparity (FD) is a small misalignment or offset of the visual axes

The visual axes don’t intersect precisely but there’s a light under or over-convergence

The fixation point is not projected onto the centre of the fovea in both eyes but you’d still get BSV as still occurring within Panum’s Fusional Area (PFA)

Question 2

What do people refer to fixation disparity as?

Answer 2

An ‘associated phoria’

The amount of visual axes drift without dissociation and can be Exo, Eso or vertically. Torsional also exists but isn’t clinically measured.

Question 3

What happens in an Eso Fixation Disparity?

Answer 3

Slight over-convergence meaning that the stimulation point is nasal to the fovea

Inward shift of the horopter with object lying beyond the horopter causing an uncrossed retinal disparity

Question 4

What happens in an Exo Fixation Disparity?

Answer 4

• There is slight under-convergence
• Stimulation of a point temporal to fovea
• Outward shift of the horopter
• Object lies in front of horopter
• Crossed retinal disparity

Question 5

Can a patient with fixation disparity maintain BSV?

Answer 5

Yes

As long as the magnitude of the Fixation Disparity is not larger than the width of PFA (Panum’s Fusional area) the patient can maintain BSV.

This is because the disparate retinal points are within Panum’s fusional area.

Question 6

What are the norms in fixation disparity?

Answer 6

• Typically less than 10 minutes of arc (Elliott, 2013) (p167)
• Mean -6.6 mins of arc (exo)
• Range +20 (eso) to -60 (exo) mins arc (Jaschinski et al, 2010)

Question 7

What is 10 minutes of arc in degrees and PD?

Answer 7

10 mins arc = 0.16 degrees = approx. 0.32 PD

Question 8

What is 60 minutes of arc in degrees and PD?

Answer 8

60 mins arc = 0.96 degrees = approx. 1.92 PD

Question 9

In Jaschinski et al. (2018) study what % of people had 0 FD?

Answer 9

17%

Question 10

What do people mix FD with?

Answer 10

‘A common misunderstanding is the belief that FD is comparable to a small angle of anomaly’ (Decker et al., 1975)
BUT it’s not related to movement on CT so cannot be compared directly to a microtropia

Question 11

What did Crone (1969) think FD was a stage prior to?

Answer 11

Anomalous BSV

Orthophoria -> fixation disparity -> heterophoria -> microtropia

Question 12

What did Karania & Evans (2006) find out about FD?

Answer 12

That fixation disparity was sometimes checked by optometrists 85% of the time and always, 35% of the time

Question 13

How do we test fixation disparity?

Answer 13

Eyes partially dissociated
- Part of target seen by each eye (monocular markers) – are these aligned?
- Part target seen by both eyes (binocular/fusion lock)

OR

Nonius Lines
- Presented dichoptically (one to each eye)
- Eyes are associated (not magnitude of heterophoria)
- Measure the offset

Question 14

What tests are used in fixation disparity?

Answer 14

• Mallet Unit
• Saladin Card
• Wesson Card
• Infrared eye tracker system (De Luca et al., 2009)

Question 15

What does the Mallet Unit test and how does it do this?

Answer 15

Fixation Disparity

• Near or distance (FD can differ)
• Polarised filter glasses
• Fusion lock = OXO (Eyes are assoc as viewing similar images which aid sensory fusion)
• One red line seen by RE, other by LE
• Give prismatic power until aligned
• Rather than magnitude of FD the tests measure the strength of prism required to eliminate the FD =
• Aligning prism
• Can turn test to measure vertical FD

Question 16

Look at slides 16 to 23 for images of fixation disparity and test yourself

Question 17

What does the Saladin card test for and how does it work?

Answer 17

Fixation Disparity

• Polarisation for monocular markers
• Which circle contains lines which are aligned?

Slide 25 for picture

Question 18

What does the Wesson card test and how does it work?

Answer 18

Fixation Disparity

• Polarisation for monocular markers
• One eye sees lines and the other sees an arrow
• Which line does the arrow point to?

Slide 26 for picture

Question 19

Why does fixation disparity matter?

Answer 19

• Large FD can be associated with asthenopic symptoms
• Can cause stress on the binocular system
• Associated with reduced stereopsis
• If there’s no FD there are better fusional reserves to manage existing heterophorias

Question 20

What arguments do we have for prescribing aligning prisms in fixation disparity?

Answer 20

• Large FD results in symptomatic phorias (Karania and Evans, 2006)
• Prisms correct FD as well as the heterophoria
• Magnitude of FD links to level of stereo and fusional reserves
• Some optoms prescribe aligning prisms even if asymptomatic
• Prescribing is based on associated, rather than dissociated phoria – more natural viewing condition

Question 21

What are the arguments against prescribing aligning prisms in FD?

Answer 21

• Poor literature evidence (Chetty et al., 2007)
• Associated phoria very variable (Kommerell et al., 2015)
• Reliability of tests and studies questionable
• Tests not always repeatable from one test to another (Jackson et al., 2007)
• Mallet unit – instruction set influences results (Karania and Evan, 2006)
• FD should match direction of heterophoria but not always the case (many have paradoxical FD)
• Are there underlying problems causing issues like CI or a large heterophoria?
• Are they symptomatic?
• Costly
• Karania & Evans (2006) used a symptom and history questionnaire to identify symptomatic patients… Frequently rub eyes, holds books too close/far, tilts head when reading/writing, poor concentration

Question 22

Why should we be wary of prisms given by optometrists if we don’t have the prescription? Particularly in new patients

Answer 22

Optoms may prescribe for fixation disparity so we need to know the total prism before we can do any additional work or give any incorporated prisms

Question 23

What is behaviour optometry/vision therapy?

Answer 23

British Association of Behavioural Optometry (BABO)

“Standard optometry looks atwhat letters you can seeon a chart….whereas behavioural optometry takes things a step further and looks atthe way your brain interprets what you are seeing.”

Question 24

What is the goal of vision therapy according to BABO (British Association of Behavioural Optometry)?

Answer 24

“Vision therapy has an end result of…a reliable visual system which correctly interprets visual and visual-spatial data and enables good integration of this skill with other body senses’.”

Question 25

What conditions BABO (British Association of Behavioural Optometry) think it can treat?

Answer 25

• Dyslexia
• Poor concentration
• Maths
• Attention deficit disorder (ADD)
• Clumsiness

This is more common in the USA but is increasing in the UK (Barrett, 2009)

Question 26

What did Barrett (2009) find behavioural optom/vision therapy could be used for?

Answer 26

• Yoked prisms (paired prisms to shift everything in one direction) for binocular disorders
• Yoked prisms for postural changes
• Yoked prisms to treat pathologic pain
• Peripheral awareness training for intermittent ET
• Syntonic (balance) phototherapy to bring visual system into balance