Fixation Disparity Flashcards
When does vergence occur?
fixation changes from 1 depth plane to another causing divergence/convergence
Explain fixation disparity and give 3 other terms for it
deviation from intended state of vergence when both eyes open ~ stimulates vergence system to maintain level of innervation
retinal slip, vergence error/disparity
Explain the differences between ideal BV, FD and diplopia with esotropia
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
What’s the difference between physiological/pathological diplopia?
physiological: visual axes aligned (normal BSV) images in front /behind horopter
pathological: visual axes misaligned
What’s the difference between FD and heterophoria?
FD: no dissociation, normal viewing with BV (fusion active)
Phoria: dissociation present so fusion disrupted when 1 eye covered
How is fixation disparity measured?
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)
Why are FD tests flawed?
real world conditions offer central & peripheral stimuli
Panum’s area larger in periphery so peripheral stimuli only has greater tolerance to misalignment ~ larger FD
How are FD curves created? Explain the difference between them and Mallet units
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
Explain manifest strabismus and the different adaptations/consequences to it?
one eye deviated ~ amblyopia/suppresion, poor fusion/stereopsis development, EF, ARC
Explain the findings of monocular deprivation studies
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
Which 2 ways can binocular cells develop?
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
Explain amblyopia and its causes
VA reduced even with Rx/no pathological obstacles (cataracts)
develops during critical period when neural plasticity renders vis. system vulnerable to abnormal experience
Describe 6 types of amblyopia
-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)
How is amblyopia treated?
occlusion therapy (eye patch) - strabismic eye works to reduce inhibition from dominant eye ~ more cells input for strabismic eye
Explain suppression with 2 types and examples
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
Explain diplopia/confusion in a strabismic px
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
Explain 4 suppression tests
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)
Why is the Sbisa bar test important?
measures density (amblyopia/occlusion therapy)
density <7 (low suppression so risk of intractable diplopia)
reduce time wearing eye patch
What is eccentric fixation and the difference between absolute/non-absolute?
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
Explain the signs of eccentric fixation and how it can be diagnosed
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
Explain abnormal retinal correspondence
1 eye fovea misaligns with strabismic fovea, aligns with pseudo fovea
ARC: sensory adaptation to manifest facilitative strabismus for suboptimal BSV
