Fixation Disparity

Question 1

When does vergence occur?

Answer 1

fixation changes from 1 depth plane to another causing divergence/convergence

Question 2

Explain fixation disparity and give 3 other terms for it

Answer 2

deviation from intended state of vergence when both eyes open ~ stimulates vergence system to maintain level of innervation

retinal slip, vergence error/disparity

Question 3

Explain the differences between ideal BV, FD and diplopia with esotropia

Answer 3

BV: visual axes aligned exactly on R/L fovea
FD: visual axes slightly misaligned (images on non RCPs inside Panum’s FA
Diplopia: visual axes misalign (LE converge) images on non RCPs outside Panum’s FA

Question 4

What’s the difference between physiological/pathological diplopia?

Answer 4

physiological: visual axes aligned (normal BSV) images in front /behind horopter

pathological: visual axes misaligned

Question 5

What’s the difference between FD and heterophoria?

Answer 5

FD: no dissociation, normal viewing with BV (fusion active)
Phoria: dissociation present so fusion disrupted when 1 eye covered

Question 6

How is fixation disparity measured?

Answer 6

Peripheral/Central lock fusion stimuli seen with both eyes

Sheedy Disparometer (mins of arc): (non-clinical) uses peripheral lock (actual FD) but unnatural conditions

Mallet/Zeiss Polatest: peripheral/central lock (OXO) and 2 red nonius strips over polaroid glasses (measures prism needed to correct FD)

Question 7

Why are FD tests flawed?

Answer 7

real world conditions offer central & peripheral stimuli

Panum’s area larger in periphery so peripheral stimuli only has greater tolerance to misalignment ~ larger FD

Question 8

How are FD curves created? Explain the difference between them and Mallet units

Answer 8

plots degree of FD against prism power from sheedy disparometer ~ gives dynamic measure of px FD changes with increased demand (during day)

Mallet units provide snapshot of px FD

Question 9

Explain manifest strabismus and the different adaptations/consequences to it?

Answer 9

one eye deviated ~ amblyopia/suppresion, poor fusion/stereopsis development, EF, ARC

Question 10

Explain the findings of monocular deprivation studies

Answer 10

examines effect of lack of vision/strabismus (cat, monkeys) patch/prism induces strabismus/deviation

in deviated eye ~ lack of lamina structure/smaller LGN cells ~ binocular cell reduction
dominant non-strabismic eye in V1 has prominent effect

Question 11

Which 2 ways can binocular cells develop?

Answer 11

normal: 2 eyes compete for binocular cell input (usually balanced so 80% complete)

imbalanced input disrupts BC development due to uncorrected refractive error or dissimilar images

Question 12

Explain amblyopia and its causes

Answer 12

VA reduced even with Rx/no pathological obstacles (cataracts)

develops during critical period when neural plasticity renders vis. system vulnerable to abnormal experience

Question 13

Describe 6 types of amblyopia

Answer 13

-strabismic (focus loss for non-dominant eye)
-anisometropic (large refraction difference between eyes so one has an advantage)
-ametropic (high error in BE ~ bilateral)
-meridonial (clear image on emmetropic axis/blur along ametropic axis)
-stimulus deprivation (form vision loss ~ obstacle e.g. cataract, ptosis)
-idiopathic (normal BSV)

Question 14

How is amblyopia treated?

Answer 14

occlusion therapy (eye patch) - strabismic eye works to reduce inhibition from dominant eye ~ more cells input for strabismic eye

Question 15

Explain suppression with 2 types and examples

Answer 15

brain inhibits unwanted stimuli - unusual above age 11

physiological (lack of awareness) ~ binocular rivalry (2 dissimilar images into 2 eyes but only 1 perceived, no fusion)

pathological (constant/intermittent strabismus) ~ can overcome diplopia/confusion/incompatible images

Question 16

Explain diplopia/confusion in a strabismic px

Answer 16

px has diplopia so strabismic eye fovea is stimulated by other objects in environment

corrresponding points project to different points in space creating image overlap

suppression inhibits this and diplopia

Question 17

Explain 4 suppression tests

Answer 17

TNO test plate: plate IV, px given RG specs and report no. circles seen (3 is normal BSV, 2 is suppression)

Bagolini lenses: faint 45/135 degree striations over px rx, views spotlight (if suppressed 1 line seen, if not crossover of 2 seen)

Worths 4 light: RG glasses (normal BSV is 4 lights, suppression reports 2R or 3G, diplopia reports 5)

Sbisa bar: strabismic px only, red filter on dominant eye, colour intensity slowly increased (white spot becomes red/pink)

Question 18

Why is the Sbisa bar test important?

Answer 18

measures density (amblyopia/occlusion therapy)

density <7 (low suppression so risk of intractable diplopia)

reduce time wearing eye patch

Question 19

What is eccentric fixation and the difference between absolute/non-absolute?

Answer 19

monocular condition due to strabismus where object fixation fixates off fovea (px using different part of retina to fixate ~ constant deviation not picked up on in early stages)

Absolute: no movement in cover tests (EF angle = deviation angle)

Non-absolute: eso eye slightly turns in (EF angle < deviation) during cover but not completely to fovea

Question 20

Explain the signs of eccentric fixation and how it can be diagnosed

Answer 20

px with history of early onset squint without treatment

associated with amblyopia/microtropia

anisometropia, macular/RCP abnormalities, reduced VA in deviating eye

CT/Ophthalmoscope graticule ~ parafoveolar/parafoveal/peripheral

Question 21

Explain abnormal retinal correspondence

Answer 21

1 eye fovea misaligns with strabismic fovea, aligns with pseudo fovea

ARC: sensory adaptation to manifest facilitative strabismus for suboptimal BSV