Vergence adaptation Flashcards
How to maintain stable BSV
Peripheral fusion (motor fusion)
Vergence Adaptation
Vergence adaptation is
The slow buildup of tonic vergence innervation to
recalibrate the extraocular muscle tonus and
re-align the visual axis maintaining BSV
when a prism is placed over one or both eyes.
Spencer and Firth (2007)
Vergence adaptation is also known as…
Tonic vergence, phoria adaptation and prism adaptation (not fresnel prism adaptation)
Vergence adaptation is not
NOT a measure of fusional reserves
(the same fusional reserves are measurable before and after adaptation)
Vergence adaptation is
a 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; prism over eye= 0
Vergence adaptation and BSV
Most comfortable BSV at baseline
Also, sustained cyclovergence – leads to torsional phoria adaptation back to baseline
Mechanism of vergence adaptation - 2 steps
Place prism over one eye to induce a deviation
Stage 1 of vergence adaptation
Immediate temporary stage
Rapid adaptation
Regain fusion
Sethi and Henson (1984)
Stage 2 of vergence adaptation
More permanent stage
Slow change back to original phoria
Helps to maintain that position as prolonged viewing through prism causes excessive vergence effort
Otherwise patient risks becoming symptomatic – asthenopic symptoms if stage 1 occurs only
Sethi and Henson 1984
Vergence adaptation affects the muscles as…
vergence adaptation and level of fast fusional vergence provide 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
Guyton (2006)
Mechanism of muscle changes
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
Hard to see changes in muscle remodelling
If patient has 2D X pre adaptation what will they post adaptation if 2D BI is introduced
2D X
this is because vergence adaptation accounts for this
What happens to the muscle tonus when the prism is removed?
It slowly dissipates
EOM tonus builds up to realign visual axis
Longstanding vergence adaptation may require hours/days to fully dissipate.
If patient is NAD pre adaptation what will they post adaptation if 2D BI is introduced
NAD
Clinical importance of vergence adaptation
Slow dissipation of vergence adaptation can mask a large phoria.
Vergence adaptation has a memory.
Measure phorias before vergence amplitudes (PFR)
Prism introduces
vergence amplitude
Vergence adaptation symptoms
Small phoria with poor vergence adaptation can result in asthenopic symptoms
Large phoria with good vergence adaptation may be asymptomatic.
Study- compared 3 groups with decompensated phoria, compensated phoria (no diff in size of phoria) and no phoria FINDINGS
Poor vergence adaptation can cause stress in the vergence system leading to asthenopic symptoms.
Vergence adaptation may account for…
High incidence of Orthophoria/small angle heterophoria.
Need for prolonged dissociation to reveal maximum deviation in heterophoria
Vergence adaptation can mask a larger deviation
Reason for needing diagnostic occlusion in these patients
What does prolonged dissociation do
reveal maximum deviation in heterophoria
What does diagnostic occlusion do
Diagnostic occlusion breaks down phoria adaptation to reset binocular misalignment toward its uncompensated value.
Vergence adaptation can account for
Comfortable BSV being maintained despite:
Anisometropic corrections
Incorrect decentration of spectacle lenses
Post Kestenbaum procedures for nystagmus also (non prism induced vergence adaptation)
Vergence adaptation can 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
- where VA is poor in one eye, can happen after BT
How to test this phenomenon
Measure heterophoria
Apply prisms - any size but not too large
Allow binocular viewing
Repeat measurements at regular intervals to see if there a change in deviation
Avoid disruption to fusion so maddox rod ideal – partial dissociation and quicker, esp if use tangent scale
Adaptation time
The change in heterophoria response over time
6∆ BO prism – 3 mins (exophoria)
6∆ BI prism – 2 mins (esophoria) Henson and North (1980)
-Prismatically induced exophoria with BO
Prismatically induced esophoria with BI
Quicker to adapt to BI or BO
BI
Adaptation time
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
-5 young adult participants
Dissipation process
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)
Dissociation- findings Rosenfield et al 1997
16 asymptomatic normal individuals
9 subjects ● - small increase in deviation following 5 mins of dissociation
7 subjects □ - significant increase in exophoria following 30 mins of dissociation
Decay of slow fusional vergence took approximately 25 mins.
Further finding showed no significant increase in deviation from 25 mins to 3 hours.
Clinical application of dissociation
More accurate assessment of heterophoria obtained after 25 mins of dissociation.
Presence of vergence adaptation can be assessed after 5 mins of dissociation.
Visual acuity
Sensory deviation can occur due to reduced VA.
Deviation goes back to baseline when VA resolves due to vergence adaptation.
Do we get full adaptation if VA is reduced
Artificially reduced VA in one eye (<0.7) affects the ability to adapt.
6Δ BO up to 0.7 logMAR - vergence adaptation unaffected
6Δ BO worse than 0.7 logMAR - some
participants had incomplete vergence adaptation
Suggests VA affects the ability to adapt
Davis and Firth 2006
Vergence adaptation affects which test
AC/A ratio?
Prism Fusion Range?
Stereopsis?
Vergence adaptation affects on AC/A ratio
We estimate the AC/A ratio after just a few seconds of dissociation.
Stimulus AC/A
Rosenfield et al (2000) compared values of stimulus AC/A measured before and after a 1‐hr period of monocular occlusion in normals.
Sustained occlusion produced significant change in near heterophoria in 10 out of the 21 subjects, but no significant change in stimulus AC/A.
Response AC/A
Rainey (2000) found no change to response AC/A following adaptation to 6∆ BO and BI prism in 8 subjects.
Response AC/A v stimulus AC/A
RespOnse AC/A generally higher than stimulus AC/A ratio
Prism fusion fusion range affects on vergence adaptation
Similar BI fusional
vergences before and
after adaptation of
2 min, 5 min and
10 min to a 10Δ
BO prism.
Tuff et al (2000)
Compared BI rangesbefore and after BO testing
No significant difference in break point whichever tested first…
but reduced BI recovery point if BO tested first.
BO range tested stimulatedvergenceadaptation, and produced a reduction in the subsequently measured BI recovery value.
Clinical application of PFR
Recommend test direction most appropriate for deviation first (compensatory range first)
Eso: test BI first
Exo: test BO first
Vertical phoria adaptation
Henson and North (1980) found similar PFR findings with 2∆ vertical prism.
Kono et al (1988) found a significant correlation between the vertical PFR and gain in phoria adaptation to 3∆ prism
Better you are at phoria adaptation, correlated with better vertical fusion range.
Prism bar v single prism
Better PFR range with prism bar due to vergence adaptation
Stereopsis affects on vergence adaptation
Stereopsis significantly reduces with increasing prism with all 4 tests.
Davis and Bibi (2013)
Inducing a deviation with
BO prisms negatively
affected near stereoacuity.
As adaptation occurred,
level of stereoacuity increased back to baseline measurement.
Spencer and Firth (2007)
As adaptation occurs
stereopsis improves
When divergence is induced with a prism
stereo reduces
Associated symptoms with deficient vergence adaptations
For individuals with a deficient slow vergence mechanism, the fast fusional controller maintains the vergence response resulting in visual/asthenopic symptoms.
North and Henson (1981)
15 patients with abnormal BSV and/or asthenopia
majority lacked/had deficient adaptation system to BI/BO prisms
Excessive demand on fast component results in asthenopia in individuals with deficient vergence adaptation.
E.g. near X, with poor positive fusional vergence and poor vergence adaptation mechanism - prolonged near work results in symptoms such as blurred vision, diplopia, headache etc
(Scheiman and Wick, 2002)
Training vergence adaptation
An improvement in degree of vertical adaptation to a 2∆ prism has been shown in normals.
Patel and Firth (2003)
Residual deviation was measured after 1-min period of adaptation to a 2Δ vertical prism.
Measurement repeated 10 times, with
5-min rest period between each trial.
Two weeks of orthoptic exercises led to greater and faster phoria adaptation to a 12∆ BO prism in normal subjects
Thiagarajan et al (2010)
Symptomatic phoria association
Vergence adaptation may be reduced or absent
Some subjects can be improved with treatment
Those that do not improve may remain symptomatic
E.g. No improvement in CI – perhaps they have poor vergence adaptation, so test this in clinic?
North and Henson (1992)
CI patients can be given
vergence and convergence exercises
Convergence insufficecny vergence findings
Less vergence adaptation in CI compared to normals
Demonstrating normalisation of vergence adaptation during orthoptic exercises treatment
Sreenivasan and Bobier (2015)
Phoria adaptation
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
Nilsson et al (2011)
Implications of 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
Qs for revision
Should strabismus surgery be planned according to the measured phoria found or the deviation produced by the induced vergence adaptation?
Can some patients become used to high levels of vergence adaptation and become uncomfortable post-op when strabismus surgery removed the need for vergence adaptation?
Are symptomatic phorias due to abnormal vergence adaptation?
Does prism adaptation test* and prolonged monocular occlusion uncover the effects of vergence adaptation to the same extent?
Patient has a small exophoria that slowly builds on dissociation to 40 prism diopters. What is happening?
Prolonged dissociated often increases the measured deviation by breaking down phoria adaptation to reset binocular misalignment toward its uncompensated value.
Diagnostic occlusion is needed in these patients
Vergence adaptation accounts for
high prevalence of orthophoria/small angle heterophoria
Diagnostic occlusion is useful to
reveal maximum angle in heterophoria
Patients with small phobias may
have asthenopia symptoms if poor vergence adaptation
Test direction of PFR is
most appropriate for deviation first (compensatory range first)
To know…
Be able to describe proposed mechanisms for vergence adaptation
List reasons why vergence adaptation enables comfortable BSV
Outline the effect of vergence adaptation on clinical measures
Be able to describe clinical indications of lack of vergence adaptation
