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
Occurs without causing diplopia, still have BSV
As still within Panums Fusional Area (PFA)
what is fixation disparity also referred to
Some refer to it 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
what is an eso fixation disparity
Visual axes not meeting at target
Inward shift of the horopter
Object lies beyond horopter
Uncrossed retinal disparity
As long as the magnitude of
the FD is not larger than the
width of PFA the patient can
maintain BSV
(If exo disparity…horopter shift beyond object…crossed retinal disparity)
what is the size of a 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)
10 mins arc = 0.16 degrees = approx. 0.32 PD
60 mins arc = 0.96 degrees = approx. 1.92 PD
Only 2/12 (17%) have zero FD (Jaschinski et al, 2018)
what is the size of a fixation disparity
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)
how is fixation disparity measured
Mallet Unit
Saladin card
Wesson card
Disparometer – no longer available
Infrared eye tracker system
how is fixation disparity measured
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 heterophoria)
Measure the offset
what is a mallet unit
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
Lines aligned? Yes = no FD; No = FD
and give prismatic power till aligned
Rather than magnitude of FD the tests measure the strength of prism required to eliminate the FD
= Aligning prism
Can turn to measure vertical FD
why does fixation disparity matter
Large FD can be associated with asthenopic symptoms
Stress on the binocular system
Associated with reduced stereopsis
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 eliminated fd
Typically optom view – measured by some for all patients (35% all; 85% sometimes (Karania and Evans, 2006))
Large FD results in symptomatic phorias (Karania and Evans, 2006)
Some prescribe aligning prisms even if asymptomatic
Prisms correct FD as well as the heterophoria
Magnitude of FD links to level of stereo and fusional reserves
Prescribing based on associated, rather than dissociated phoria – more natural viewing condition
should prisms be prescribed to eliminated fd
Typically optom view – measured by some for all patients (35% all; 85% sometimes (Karania and Evans, 2006))
Large FD results in symptomatic phorias (Karania and Evans, 2006)
Some prescribe aligning prisms even if asymptomatic
Prisms correct FD as well as the heterophoria
Magnitude of FD links to level of stereo and fusional reserves
Prescribing based on associated, rather than dissociated phoria – more natural viewing condition
what are the arguments against prescribing aligning prisms
Orthoptist’s view
Poor literature evidence (Chetty et al 2007)
Associated phoria very variable (Kommerell et al 2015)
Reliability of tests and studies
Tests not always repeatable from one test to another (Jackson et al 2007)
Mallet unit – instruction set influences results (Karania and Evans 2006)
FD should match direction of heterophoria but not always the case (many have paradoxical FD)
when should you prescribe prisms
Underlying problems causing symptoms? e.g. large heterophoria, CI?
Only prescribe prisms if a patient is symptomatic!
Are they truly symptomatic?
Karania and 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
Cost
Watch out for pt’s with prisms prescribed by their optom…
what is behavioural optometry
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 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 are benefits of behavioural optometry
British Association of Behavioural Optometry (BABO):
Benefits many conditions such as:
Dyslexia
Poor concentration
Maths
Attention deficit disorder (ADD)
Clumsiness
More common in USA but increasing in UK
what are examples of behavioural optmetry
Yoked prisms for binocular disorders
Yoked prisms for postural changes
Peripheral awareness training for intermittent ET
Syntonic (balance) phototherapy to bring visual system into balance
Yoked prisms to treat pathologic pain
Convergence insufficiency (CITT)
what is vergence adaptation
Normal phenomenon of BSV
Induce a horizontal deviation in a person with BSV (usually by prisms), the new deviation will reduce back to the baseline deviation
Even if the baseline deviation is zero
Most comfortable BSV at baseline
Also, sustained cyclovergence – leads to torsional phoria adaptation back to baseline
define vergence adaptation
Vergence adaptation refers to the process by which the eyes adjust their convergence (inward turning) or divergence (outward turning) abilities over time to maintain single binocular vision. This adaptation occurs in response to changes in the visual environment, such as the distance or position of an object being viewed.
When an object is viewed at a closer distance, the eyes must converge to maintain binocular vision. Over time, the eyes will adapt to the new convergence demand and become more efficient at maintaining clear binocular vision at that distance. Conversely, when an object is viewed at a farther distance, the eyes must diverge to maintain binocular vision, and the eyes will adapt to this demand as well.
Vergence adaptation is an important aspect of binocular vision and is necessary for maintaining clear, comfortable vision at different distances. When the eyes are properly aligned, the visual images from each eye are fused together in the brain to create a single, three-dimensional image. However, when the eyes are misaligned due to a vergence disorder, such as convergence insufficiency or convergence excess, binocular vision can be disrupted and lead to symptoms such as eyestrain, headaches, and double vision.
Vergence adaptation can be tested through various clinical tests, such as prism adaptation or the measurement of fusional vergence ranges.
what are the symptoms of vergence adaptation
Small phoria with poor vergence adaptation can result in asthenopic symptoms whilst large phoria with good vergence adaptation may be asymptomatic
Compared 3 groups: decomp phoria, comp phoria (no diff in size phoria), no phoria
6BO prism over RE for 10 mins (measurements every min)
Decomp phoria group had deficient vergence adaptation
Comp phoria group was slower to adapt than ‘normals’ but did adapt to a similar level
Concluded poor vergence adaptation can cause stress in the vergence system leading to asthenopic symptoms
what does vergence adaptation account for
Need for prolonged dissociation to reveal maximum deviation in heterophoria
Vergence adaptation is masking a larger deviation
Reason for needing diagnostic occlusion
Comfortable BSV being maintained despite
Anisometropic corrections (imbalance phoria)
Decentration of spectacle lenses (adaptation)
Post Kestenbaum procedures (non prism induced vergence adaptation for any discrepancy)
what does vergence adaptation account for
Sensory deviations ‘disappearing’ on regaining good VA and BSV with prisms as vergence adaptation kicks in
Apparent increase in deviation with increase in prism prescription
describe the mechanisms of mergence adaptation
Place prism over one eye to induce a deviation
Eye realigned by the fast fusional vergence system (phasic)
This then inputs to the slow fusional vergence system (tonic) which adapts to the fusional demand
what are the 2 stages of adaptation
Immediate temporary stage
Rapid adaptation
Regain fusion
More permanent stage
Slow change to original phoria
Helps to maintain that position as prolonged viewing through prism causes excessive vergence effort
Otherwise patient risks becoming symptomatic if stage 1 only
what muscle changes are present in mergence adaptation
It is proposed that the vergence adaptation and level of fast fusional vergence provides the primary input to EOM length adaptation
It eventually affects muscle lengths as they will also adapt to this new position
Permanent changes to compensate for this disparity/deviation to achieve good comfortable BSV
why do the muscles change in mergence adaptation
Guyton (2006) proposes changes to muscle length due to gain or loss of sarcomeres
Kushner (2010) suggests that remodelling of muscle contractile activity may occur rather than change to muscle structure
Permanent changes so may not be able to tell they ever had a different deviation
how to assess phenomena
Measure heterophoria
Apply prisms – any size but not too large
Allow binocular viewing - 2 stages
Repeat measurements at regular intervals – is
there a change in deviation?
Avoid disruption to fusion so maddox rod ideal – partial dissociation and quicker, esp if use tangent scale
how long does it take to adapt to vergence adaptation
The change in heterophoria response over time
Exo (6∆ BO prism) – 3 mins
Eso (6∆ BI prism) – 2 mins
Begins 1 second after viewing
Occurs more quickly for base in than base out
Although others report the opposite (Tuff et al 2000)
On removal of prism, return to baseline may take several hours
Greater in children and may reduce in >65yrs
what is dissipation
Removal of prism
Time taken for effect on angle from
start to finish
In open loop conditions effect may
last longer (Gabor patch - retinal disparity response only)
how is convergence adaptation related to convergence insufficecy
Less vergence adaptation in CI compared to normals
what are the associations between esophorias and convergence insufficency
Average phoria adaptation in response to 6∆ BO and BI
Better vergence adaptation in controls
Than in those with esophoria
Those not adapting well may be the ones that go on to be symptomatic
what are the indications for treatment
Aids comfortable BSV
Dissociation on CT/monoc occlusion reveals unadapted angles
Removing obstacle for fusion (e.g. reduced VA) could allow vergence adaptation to occur to realign the eyes and regain fusion
orthoptic treatment may aid ability to adapt –
