Vergence Adaptation Flashcards
How do we maintain stable binocular vision?
- Peripheral Fusion (motor fusion)
- Vergence Adaptation
How do we define Vergence Adaptation?
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 & Firth, 2007)
Phenomenon of normal BSV
What is Vergence Adaptation also known as?
- Tonic Vergence
- Phoria adaptation
- Prism adaptation (alternate meaning, not Fresnel pre-op prism adaptation PAT)
What did Ogle et al (1967) say about vergence adaptation?
Is NOT a measure of fusional reserves
(the same fusional reserves are measurable before and after adaptation)
What happens in vergence adaptation?
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
What is the mechanism behind vergence adaptation?
Schor (1979)
Place prism over one eye to induce a deviation
Stage 1: Fast phasic response
- Eye realigned by the fast fusional vergence system
- Quick saccadic realignment
- Stage 1 is eliminated by alternate occlusion
Stage 2: Slow tonic response
- This then inputs to the slow fusional vergence system (tonic) which adapts to the fusional demand
What is Stage 1 of Vergence Adaptation?
Fast phasic response
- Immediate temporary stage
- Rapid adaptation
- Regain fusion
Sethi & Henson (1984)
What is Stage 2 of vergence adaptation?
Slow tonic response
- 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 if stage 1 occurs only
Sethi & Henson (1984)
What did Guyton (2006) think the vergence adaptation permanently causes?
Muscle Changes
It is proposed that 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
What possible mechanism did Guyton (2006) think caused vergence adaptation?
Guyton (2006) proposes changes to muscle length due to gain or loss of sarcomeres
What possible mechanism did Kushner (2010) think caused vergence adaptation?
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
If someone was 2BI pre-adaptation, after a prism is introduced what would their post-adaptation measurement be?
2BI
What happens to the muscle tonus when the prism is removed?
EOM tonus builds up to realign visual axis under the prism and slowly dissipates once it is removed
Why does vergence adaptation matter?
Vergence adaptation has a memory.
Slow dissipation of vergence adaptation can mask a large phoria.
Clinical application:
Measure phorias before vergence amplitudes (PFR)!
What symptoms come with vergence adaptation?
Small phoria with poor vergence adaptation can result in asthenopic symptoms, whilst large phorias with good vergence adaptation may be asymptomatic.
What did Przekoracka-Krawczyk et al (2019) find about the symptoms of vergence adaptation?
Compared 3 groups: decompensating phoria, compensated phoria (no diff in size phoria), no phoria
Vergence adapted them with a 6BO prism over RE for 10 minutes (measurements taken once per minute)
Found:
- Decomp phoria group had deficient vergence adaptation
- Comp phoria group was slower to adapt than ‘normals’ but did adapt to a similar level
Conclusion:
Poor vergence adaptation can cause stress in the vergence system leading to asthenopic symptoms.
What might vergence adaptation account for?
High incidence of Orthophoria/small angle heterophoria.
How does vergence adaptation show the need for prolonged dissociation?
Need for prolonged dissociation to reveal maximum deviation in heterophoria
- Vergence adaptation can mask a larger deviation
- Reason for needing diagnostic occlusion
Vergence adaptation may account for comfortable BSV being maintained despite what?
- Anisometropic corrections
- Incorrect decentration of spectacle lenses
- Post Kestenbaum procedures (non prism induced vergence adaptation)
What might vergence adaptation account for?
Sensory deviations ‘disappearing’ on regaining good VA and BSV with prisms as vergence adaptation kicks in (same with BT sometimes, so those with normal BSV function might do better with BT without surgery cosmetically due to vergence adaptation)
Apparent increase in deviation with increase in prism prescription
How do we test for vergence adaptation?
- Measure heterophoria
- Apply prisms - any size but not too large (1 - 12PD)
- Allow binocular viewing for approx 5 minutes
- 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
What did Henson & North (1980) find about heterophoria response over time with vergence adaptation?
The change in heterophoria response over time
6∆ BO prism – 3 mins (exophoria)
6∆ BI prism – 2 mins (esophoria)
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
How long does dissipation occur in vergence adaptation (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.
What is the clinical application of knowing about dissipation and vergence adaptation?
- More accurate assessment of heterophoria obtained after 25 mins of dissociation.
- Presence of vergence adaptation can be assessed after 5 mins of dissociation
Why might sensory deviation go back to baseline when VA resolves?
Sensory deviation can occur due to reduced VA.
Deviation goes back to baseline when VA resolves due to vergence adaptation.
What did Davis and Firth (2006) find out about VA and vergence adaptation?
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
How might Stimulus AC/A ratio be affected by vergence adaptation?
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.
How might Response AC/A ratio be affected by vergence adaptation?
Rainey (2000) found no change to response AC/A following adaptation to 6∆ BO and BI prism in 8 subjects.
How might Prism Fusion Range be affected by 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)
BUT Rosenfield (1995) compared BI ranges before and after testing BO ranges and found 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.
What is the clinical application of knowing about Prism Fusion Range & vergence adaptation?
Recommend test direction most appropriate for deviation first (compensatory range first)
How might vergence adaptation affect vertical phoria adaptation?
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
How might vergence adaptation affect stereopsis?
Davis & Bibi (2013) - Stereopsis significantly reduces with increasing prism with all 4 tests (Frisby, Randot, FD2 and Distance Randot).
What did Spencer & Firth (2007) find about stereopsis & vergence adaptation?
Inducing a deviation with
BO prisms negatively
affected near stereoacuity.
As adaptation occurred,
level of stereoacuity increased back to baseline measurement.
What happens if a patient gets stuck in phase 1 of vergence adaptation?
For individuals with a deficient slow vergence mechanism (phase 2), the fast fusional controller (phase 1) 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
What results in asthenopia in individuals with deficient vergence adaptation?
Excessive demand on fast component (phase 1) 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 & Wick (2002)
Can we train vergence adaptation?
Thiagarajan et al (2010)
Two weeks of orthoptic exercises led to greater and faster phoria adaptation to a 12∆ BO prism in normal subjects
Why might someone be a symptomatic phoria?
North & Henson (1992)
- Vergence adaptation may be reduced or absent
- Some subjects can be improved with treatment
- Those that do not improve may remain symptomatic (such as those in CI)
Do those with Convergence Insufficiency have more or less vergence adaptation?
Sreenivasan and Bobier (2014)
Less vergence adaptation in CI compared to normals
How is vergence adaptation in those with convergence excess esophoria compared to controls?
Nilsson et al. (2011)
Average phoria adaptation in response to 6∆ BO and BI
Better vergence adaptation in controls than in those with convergence excess esophoria
Those not adapting well may be the ones that go on to be symptomatic
What are the implications for treatment relating to vergence adaptation?
- 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 – should we test in patients?
What are some take home points about vergence adaptation?
- Vergence adaption likely accounts for high prevalence of orthophoria/small angle heterophoria
- Diagnostic occlusion is useful to reveal maximum angle in heterophoria
- Patients with small phorias may have asthenopia symptoms if poor vergence adaptation
- Test direction PFR most appropriate for deviation first (compensatory range first)
