Fixation Disparity Flashcards
What is Fixation Disparity?
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)
What do people refer to fixation disparity as?
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.
What happens in an Eso Fixation Disparity?
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
What happens in an Exo Fixation Disparity?
- 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
Can a patient with fixation disparity maintain BSV?
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.
What are the norms in fixation disparity?
- 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)
What is 10 minutes of arc in degrees and PD?
10 mins arc = 0.16 degrees = approx. 0.32 PD
What is 60 minutes of arc in degrees and PD?
60 mins arc = 0.96 degrees = approx. 1.92 PD
In Jaschinski et al. (2018) study what % of people had 0 FD?
17%
What do people mix FD with?
‘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
What did Crone (1969) think FD was a stage prior to?
Anomalous BSV
Orthophoria -> fixation disparity -> heterophoria -> microtropia
What did Karania & Evans (2006) find out about FD?
That fixation disparity was sometimes checked by optometrists 85% of the time and always, 35% of the time
How do we test fixation disparity?
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
What tests are used in fixation disparity?
- Mallet Unit
- Saladin Card
- Wesson Card
- Infrared eye tracker system (De Luca et al., 2009)
What does the Mallet Unit test and how does it do this?
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
Look at slides 16 to 23 for images of fixation disparity and test yourself
What does the Saladin card test for and how does it work?
Fixation Disparity
- Polarisation for monocular markers
- Which circle contains lines which are aligned?
Slide 25 for picture
What does the Wesson card test and how does it work?
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
Why does fixation disparity matter?
- 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
What arguments do we have for prescribing aligning prisms in fixation disparity?
- 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
What are the arguments against prescribing aligning prisms in FD?
- 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
Why should we be wary of prisms given by optometrists if we don’t have the prescription? Particularly in new patients
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
What is behaviour optometry/vision therapy?
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.”
What is the goal of vision therapy according to BABO (British Association of Behavioural Optometry)?
“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’.”
What conditions BABO (British Association of Behavioural Optometry) think it can treat?
- Dyslexia
- Poor concentration
- Maths
- Attention deficit disorder (ADD)
- Clumsiness
This is more common in the USA but is increasing in the UK (Barrett, 2009)
What did Barrett (2009) find behavioural optom/vision therapy could be used for?
- 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
