Give examples of binocular and monocular sensory adaptations and how they’re revealed
Bino: HARC and/or suppression
Mono: amblyopia/eccentric fixation revealed by testing strabismic eye
Give 4 types of organic amblyopia
Photoreceptor dysfunction
Toxic (lead)
Nutritional (tobacco/alcohol toxicity ~ VitB deficiency)
Idiopathic
Explain psychogenic amblyopia
conversion disorder brought on by anxiety/stress, common in younger adults
suddenly onset of vision/hearing loss
Explain functional amblyopia and its causes
visual loss due to abnormal vision development (<3.6%), no organic lesions but abnormal cortex
good eye dominates cortex leaving suppressed eye only able to contribute to low spatial frequency detection in monocular sites
suppression of poorer eye in binocular condition becomes monocular
Explain the input and disparity tuning of binocular cells
80% of V1 cortical cells are binocular allowing fusion driven by input from 2 corresponding retinal points
1/2 of here respond to either zero, crossed or uncrossed disparity
higher proportion of these disparity sensitive cells are in the higher visual cortex (V2)
Give 3 ways of measuring VA
Grating acuity: smallest separation between adjacent high contrast bars
Resolution/Recognition: identify single letters (angular acuity) or line of letters (morphoscopic acuity)
Vernier acuity: detect if objects are aligned - hyper acuity with threshold measured in seconds
Explain crowding
deleterious effect of adjacent contours on visual discrimination
1/2 the angle of eccentricity over a very wide area in peripheral retina
What are the differences in strabismics vs anisometropia for
- Vernier Acuity
- Pelli Robson Contrast Sensitivity
- Edge Contrast Threshold
against optotype (Snellen) acuity?
Strabismic amblyopes have loss of 2 log units but strabismic anisometropes have 1 log unit loss VA
Vernier: anisometropes worse than expected, strabismics better
PRCS: contrast threshold 16% better (lower) than whole group
Edge: anisometropes 28% higher (worse), strabismic anisometropes 26% lower (better)
Give 7 visual function deficits of amblyopes
Spatial distortions (especially strabismic)
Oculomotor abnormalities
Reduced AoA
Eccentric Fixation
Reduced contrast sensitivity
Reduced reading speed
Compromised fine motor skills
Explain eccentric fixation and the orientation for eso/exotropia
use of non-foveal point by strabismic eye as good eye is occluded, unlike ARC it’s a monocular adaptation to strabismus
esotropia: non-foveal fixation is nasal
exotropia: non-foveal fixation is temporal
on CT we get the impression the squint angle is smaller
How is eccentric fixation detected?
Focus opthalmoscope graticule on macular region, px with fellow eye occluded looks at graticule centre
Check good eye 1st for exaggerated reflex. Normal foveae will be centred but off-centre in EF
Can calibrate Welch-Allen (white light) or Keeler at 57cm
What are the pros & cons of detecteding EF?
Pros: objective, possible in children
Cons: bright, invasive, pupil constricts, not all px has visible reflex, large uninteresting target
What are the key reasons for treating amblyopia?
restore morphoscopic VA/match VAs in each eye for binocularity and occupational requirements
3x greater risk of serious vision loss in fellow eye
Give 5 disadvantages of occlusion therapy
Intractable diplopia
Manifest phoria
Poor Cosmesis/Bullying
Skin irritation/Conjuctival + Lid infections
VF loss issues
Summarise the treatment protocol for amblyopia
FT specs for 12-18wks
Follow-up every 6-8wks until = good VA
Re-refract/consider occlusion/penalisation if no VA gain for 12wks
Which factors can affect the choice of surgery?
deviation size/muscle affected
HV components (A/V patterns)
BV status/torsional elements
Surgery history
What are the 3 types of surgery for strabismus?
Weakening
Strengthening
Transposition
What are the diagnostic and therapeutic uses of Botulinum Toxin?
Diagnostic: assessing post-op BSV potential and risk of intractable diplopia
Therapeutic: restore fusion, infantile ESOTERIC, cosmetic strabismus
