Week 1.07 Associated Phorias Flashcards
What does fixation disparity mean
Fixation disparity refers to a small misalignment of the eyes during binocular vision, where the images from each eye do not fall on exactly corresponding points of the retina
What is exo and eso disparity
Exo disparity - axes slightly divergent (rel to fixation)
ESO disparity - axes slightly convergent (rel to fixation)
Vertical disparity also occurs
What is the difference between phoria and fixation disparity
Phoria: measurement of deviation when fusion is suspended (dissociated conditions)
FD: minute misalignment when fusion is present and active (normal binocular conditions)
What does the term associated phoria mean
Prism named to neutralise FD sometimes termed the associated phoria should
Measuring FD
Usually measured using haploscopic principle
- presenting target or each fovea separately to indicate direction of visual axes - so one target to left and one the right at same time and rest of visual field is a natural scene promotes fusional response
- usually done with polarised light/filters
- but needs fusion lock - central vs peripheral
Mallet unit and FD
A mallet unit is a measurement used in the assessment of fixation disparity. It is a part of a binocular vision test that helps quantify the degree of misalignment (or disparity) between the two eyes when they are aimed at the same visual target
Prism neutralisation in FD
What we do with the mallet unit is find the point where prism is neutralised and FD has gone to zero and that is the associated phoria
Ideal parameters for FD measurement
- Normal visual environment
- Central and peripheral fusion locks
- Enables detection of small disparities
- Monocular markets that are not easily suppressed
- Monocular markers close to central fixation point
- Can detect horizontal and vertical fixation disparities
- Easily understood by px
What should you do if px has an AP?
Should only address by anomaly or a suspected by anomaly if the px has symptoms
Two options: orthotics or prescribe prism
How to treat exo associated phoria
Weakest base in to eliminate slip
Or modify rx in -ve direction (I.e over-minus or under-plus) stimulate accommodation
Don’t rule out oprthoptic exercises
How to treat eso associated phoria
Weakest base out prism to eliminate slip
Or modify rx by adding +ve lens power relax accommodation, unlikely to help at distance but may be possible at near
Don’t rule out orthoptic exercises
How to treat vertical associated phoria
Eliminate any horizontal slip
Check optical centres on specs
Check head postures
Any residual vertical slip use prism
Prism adaptation
If precerive prisms leave in place for 10 minutes
Adaptation may take place
I.e. after 10-20mins, AP which has neutralised has now returned
Don’t prescribe prism if adaptation has taken place