Fixation disparity Flashcards
What is fixation disparity?
Fixation disparity (FD) is a small misalignment or offset of the visual axes
Visual axes do not intersect precisely…slight under or over convergence
Fixation point not projected onto centre of fovea in both eyes. It occurs without causing diplopia, still have BSV as this is still within Panum’s Fusional Area (PFA)
Fixation disparity is also known as
An ‘associated phoria’
Amount the visual axes drift without dissociation
Exo disparity- axes slightly divergent
Eso disparity- axes slightly convergent
Vertical disparity
Can also have a torsional FD but not measured clinically
Eso fixation disparity is when
there is slight over-convergence of right eye
Stimulation of point nasal to fovea
Inward shift of the horopter
Object lies beyond horopter
Uncrossed retinal disparity
Exo fixation disparity is when
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
BSV and fixation disparity
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.
Fixation disparity norms
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)
10 mins arc = 0.16 degrees = approx. 0.32 PD
60 mins arc = 0.96 degrees = approx. 1.92 PD
Fixation disparity size
‘A common misunderstanding is the belief that FD is comparable to a small angle of anomaly’ (Decker et al., 1975)
Not related to movement on CT so not directly comparable to a microtropia
Is it the stage prior to anomalous BSV? (Crone, 1969)
Orthophoria -> fixation disparity -> heterophoria -> microtropia (Crone, 1969)
Fixation disparity is measured by
Optoms:
Sometimes: 85%
Always: 35%
(Karania and Evans, 2006)
Fixation disparity testing to check
Eyes partially dissociated
Part of target seen by each eye (monocular markers) – are these aligned?
Part target seen by both eyes (binocular/fusion lock)
Nonius Lines
Presented dichoptically (one to each eye)
Eyes are associated (not magnitude of heterophoria)
Measure the offset
Tests for fixation disparity
Mallet Unit
Saladin card
Wesson card
Infrared eye tracker system (De Luca et al, 2009)
Mallet unit for fixation disparity https://www.youtube.com/watch?v=ZFwuXIV66Js
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
Saladin card
Polarisation for monocular markers
Which circle contains lines which are aligned?
Wesson card
Polarisation for monocular markers
One eye sees lines and the other sees an arrow…
which line does the arrow point to?
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 is no fixation disparity
If no FD there are better fusional reserves to cope with any existing heterophoria
E.g. patient with 3 dioptre heterophoria…
If no/little FD = asymptomatic
If larger FD = some may have asthenopic symptoms
Should prisms be prescribed to eliminate FD?
Arguments for prescribing aligning prisms
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 prescribe aligning prisms even if asymptomatic
Prescribing is based on associated, rather than dissociated phoria – more natural viewing condition
- typically optometrist view
Arguments against prescribing aligning prisms
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)
- typically orthoptist view
Behavioural 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.”
Vision therapy had 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’. British Association of Behavioural Optometry (BABO)
Behavioural Optometry/Vision Therapy benefits
Dyslexia
Poor concentration
Maths
Attention deficit disorder (ADD)
Clumsiness
More common in USA but increasing in UK
(Barrett, 2009)
Examples taken from Barrett (2009):
Yoked prisms for binocular disorders
Yoked prisms for postural changes
Yoked prisms to treat pathologic pain
Behavioural Optometry/Vision Therapy
Can improve reading speed, accuracy, improvement to stereopsis.
Vision therapy for amblyopia treatment
Exercises tried and improvement found
https://youtu.be/mPh-1x1lryw
Behavioural Optometry/Vision Therapy IS NOT
funded by NHS
Lack of evidence base
Example:
“daily home practice and regular sessions over zoom. These 45 minute sessions of fortnightly one-to-onetherapywill enable X to learn more reliable brain integration to reduce effort, increase awareness and develop visual and attentional skills. A course of 25 sessions is recommended. Sessions are £50 each.”
NEED TO KNOW
Be able to define Fixation Disparity (FD)
Be aware of how FD is tested
Be aware of the arguments supporting/against prescribing prisms for FD
Be aware of the terms behavioural optometry and vision therapy
