Bv Lec 3 Flashcards
What is eccentric fixation
Condition which a non-foveal retinal point is consistently used for fixation under monocular conditions, even though the fovea is intact and functional
How to classify eccentric fixation in diagnosis
Which eye
Direction of EF on retina e.g nasal
Magnitude e.g degrees or prism away from anatomical fovea
Steady or unsteady
Treatment of eccentric fixation
Voluntary control - use entopic phenomena to project fovea into VF
Pleoptics-bleaching retina outside fovea forcing to use fovea
What is suppression
Lack of perception of normally visible objects in all or parts of vision due to cortical inhibition
-only active under binocular conditions and is involuntary
Treatment for suppression
Vision training
Amblyopia therapy if binocularity can be re-established
What’s normal retinal correspondence
Where both foveas have same visual direction, allowing normal sensory fusion and stereopsis
What’s anomalous retinal correspondence
Both foveas don’t have same visual direction so fovea of one eye corresponded to off fovea point of other eye (pseudofovea/point of anomaly)
Why do people develop sensory adaptations to strabismus
AND
what happens if strab is constant
Strabismus results in diplopia and confusion
So suppression and ARC is used to reduce symptoms
EF is also developed but doesn’t have a purpose
——-
If strab is constant the sensory changes become constant
How do you get “diplopia” from strabismus
Eyes point in different directions so visual axes are misaligned
So images coming into both eyes are landing in non corresponding points
How do you get “confusion” from strabismus
Eyes point differently so foveas are physically pointing at different objects
So images landing on foveas are different
But brain can interpret them as the same visual direction so you see 2 differnt objects on top of each other
Theories of etiology for eccentric fixation
1) Anomalous fixation
- secondary to constant unilateral strabismus in early life that modified the fixation reflex that wasn’t developed properly
2) secondary to amblyopia
- strabismic amblyopia prevents/interrupts development of normal foveal peak in IOV
- so brain uses an eccentric point with similar acuity for fixation
How to clinically DIAGNOSE Eccentric Fixation
Reduced VA
Angle Kappa - monocular corneal reflex test
Visuoscopy - fixation reflex using opthalmoscope and graticule
Entopic phenomenon caused by macular pigment - Haidinger Brushes & Maxwells Spot
Why do we get suppression and what is the cost
Used to eliminate diplopia and confusion
Cost is fusion and stereopsis as simultaneous perception can’t occur so higher levels of BV cannot occur
Properties of strabismus suppression scotomas
Traditionally modelled as D shape
Relative, not absolute scotomas (overcome)
Only in binocular conditions
-in unilateral strab when dom eye fixing
Scotomas depth/size depend on viewing conditions
- more suppression if target central, high SF(small), overlapping, similar
