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Strabismus > Vergence adaptation > Flashcards

Vergence adaptation Flashcards

1
Q

How to maintain stable BSV

A

Peripheral fusion (motor fusion)
Vergence Adaptation

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2
Q

Vergence adaptation is

A

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)

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3
Q

Vergence adaptation is also known as…

A

Tonic vergence, phoria adaptation and prism adaptation (not fresnel prism adaptation)

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4
Q

Vergence adaptation is not

A

NOT a measure of fusional reserves
(the same fusional reserves are measurable before and after adaptation)

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5
Q

Vergence adaptation is

A

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

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6
Q

Vergence adaptation and BSV

A

Most comfortable BSV at baseline
Also, sustained cyclovergence – leads to torsional phoria adaptation back to baseline

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7
Q

Mechanism of vergence adaptation - 2 steps

A

Place prism over one eye to induce a deviation

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8
Q

Stage 1 of vergence adaptation

A

Immediate temporary stage
Rapid adaptation
Regain fusion
Sethi and Henson (1984)

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9
Q

Stage 2 of vergence adaptation

A

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

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10
Q

Vergence adaptation affects the muscles as…

A

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)

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11
Q

Mechanism of muscle changes

A

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

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12
Q

If patient has 2D X pre adaptation what will they post adaptation if 2D BI is introduced

A

2D X
this is because vergence adaptation accounts for this

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13
Q

What happens to the muscle tonus when the prism is removed?

A

It slowly dissipates
EOM tonus builds up to realign visual axis

Longstanding vergence adaptation may require hours/days to fully dissipate.

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14
Q

If patient is NAD pre adaptation what will they post adaptation if 2D BI is introduced

A

NAD

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15
Q

Clinical importance of vergence adaptation

A

Slow dissipation of vergence adaptation can mask a large phoria.
Vergence adaptation has a memory.
Measure phorias before vergence amplitudes (PFR)

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16
Q

Prism introduces

A

vergence amplitude

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17
Q

Vergence adaptation symptoms

A

Small phoria with poor vergence adaptation can result in asthenopic symptoms

Large phoria with good vergence adaptation may be asymptomatic.

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18
Q

Study- compared 3 groups with decompensated phoria, compensated phoria (no diff in size of phoria) and no phoria FINDINGS

A

Poor vergence adaptation can cause stress in the vergence system leading to asthenopic symptoms.

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19
Q

Vergence adaptation may account for…

A

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

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20
Q

What does prolonged dissociation do

A

reveal maximum deviation in heterophoria

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21
Q

What does diagnostic occlusion do

A

Diagnostic occlusion breaks down phoria adaptation to reset binocular misalignment toward its uncompensated value.

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22
Q

Vergence adaptation can account for

A

Comfortable BSV being maintained despite:

Anisometropic corrections
Incorrect decentration of spectacle lenses
Post Kestenbaum procedures for nystagmus also (non prism induced vergence adaptation)

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23
Q

Vergence adaptation can account for

A

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
24
Q

How to test this phenomenon

A

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

25
Q

Adaptation time

A

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

26
Q

Quicker to adapt to BI or BO

A

BI

27
Q

Adaptation time

A

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

28
Q

Dissipation process

A

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)

29
Q

Dissociation- findings Rosenfield et al 1997

A

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.

30
Q

Clinical application of dissociation

A

More accurate assessment of heterophoria obtained after 25 mins of dissociation.
Presence of vergence adaptation can be assessed after 5 mins of dissociation.

31
Q

Visual acuity

A

Sensory deviation can occur due to reduced VA.

Deviation goes back to baseline when VA resolves due to vergence adaptation.

32
Q

Do we get full adaptation if VA is reduced

A

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

33
Q

Vergence adaptation affects which test

A

AC/A ratio?
Prism Fusion Range?
Stereopsis?

34
Q

Vergence adaptation affects on AC/A ratio

A

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.

35
Q

Response AC/A v stimulus AC/A

A

RespOnse AC/A generally higher than stimulus AC/A ratio

36
Q

Prism fusion fusion range affects on vergence adaptation

A

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.

37
Q

Clinical application of PFR

A

Recommend test direction most appropriate for deviation first (compensatory range first)
Eso: test BI first
Exo: test BO first

38
Q

Vertical phoria adaptation

A

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.

39
Q

Prism bar v single prism

A

Better PFR range with prism bar due to vergence adaptation

40
Q

Stereopsis affects on vergence adaptation

A

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)

41
Q

As adaptation occurs

A

stereopsis improves

42
Q

When divergence is induced with a prism

A

stereo reduces

43
Q

Associated symptoms with deficient vergence adaptations

A

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)

44
Q

Training vergence adaptation

A

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)

45
Q

Symptomatic phoria association

A

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)

46
Q

CI patients can be given

A

vergence and convergence exercises

47
Q

Convergence insufficecny vergence findings

A

Less vergence adaptation in CI compared to normals

Demonstrating normalisation of vergence adaptation during orthoptic exercises treatment
Sreenivasan and Bobier (2015)

48
Q

Phoria adaptation

A

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)

49
Q

Implications of treatment

A

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

50
Q

Qs for revision

A

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?

51
Q

Patient has a small exophoria that slowly builds on dissociation to 40 prism diopters. What is happening?

A

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

52
Q

Vergence adaptation accounts for

A

high prevalence of orthophoria/small angle heterophoria

53
Q

Diagnostic occlusion is useful to

A

reveal maximum angle in heterophoria

54
Q

Patients with small phobias may

A

have asthenopia symptoms if poor vergence adaptation

55
Q

Test direction of PFR is

A

most appropriate for deviation first (compensatory range first)

56
Q

To know…

A

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

Strabismus (46 decks)

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