8. Vergence Flashcards
(34 cards)
How can people learn to dissociate accommodation and convergence?
- Positive relative convergence/negative relative accommodation
- Negative relative convergence/positive relative accommodation.
Explain Positive relative convergence/negative relative accommodation
When convergence is produced in excess of accommodation.
How much convergence can be induced before image blurs (due to over accommodation)
What can positive relative convergence/negative relative accommodation be used to treat?
Useful to treat exo deviations
Describe method for positive relative convergence/negative relative accommodation using sphere lenses.
- Plus lenses (decreases accommodation)
- View 6/9 near letter
- Increase positive lenses in 0.25DS steps.
- Patient reports when blurred
- Lenses relax accommodation, so vergence must be increased to keep eyes aligned. Blur point reached when accommodation is unable to relax further.
Describe method for positive relative convergence/negative relative accommodation using prisms
- Use base OUT prism
- Base OUT prisms induce convergence, so accommodation is relaxed
- Blur point in prism fusion reserves (PFR) indicates maximum relative convergence (too much accommodation)
Describe method for positive relative convergence/negative relative accommodation using stereograms
- A card with image is held by Px.
- Images can be : Cats, Buckets, Circles.
- Px looks at a pencil that is front of the card.
- So eyes converge on the pencil, but also accommodate to see the image on card.
- Aim to achieve 3 clear cats/buckets/circles.
Pen proximal = Positive relative convergence (exo)
Explain Negative relative convergence/Positive relative accommodation
Convergence is relaxed in relation to accommodation.
How much convergence can be relaxed before image blurs.
What can negative relative convergence/positive relative accommodation be used to treat?
Useful to treat eso deviations
Describe method for negative relative convergence/positive relative accommodation using sphere lenses.
- Minus lenses (Stimulates accommodation)
- View 6/9 near letter
- Increase minus lenses in 0.25DS steps.
- Patient reports when blurred
- Lenses stimulate accommodation, so vergence must be relaxed to keep eyes aligned. Blur point reached when accommodation is unable to increase further.
Describe method for negative relative convergence/positive relative accommodation using prisms
- Use base IN prism
- Base IN prisms reduce convergence, so accommodation is increased
- Blur point in prism fusion reserves (PFR) indicates maximum relative convergence (Accommodation can’t be increased further)
Describe method for negative relative convergence/positive relative accommodation using stereograms.
- A card with image is held by Px.
- Images can be : Cats, Buckets, Circles.
- Px looks at a pencil that is behind the card.
- So eyes converge less on the pencil, but also accommodate to see the image on card.
- Aim to achieve 3 clear cats/buckets/circles.
Pen distal = negative relative convergence (eso)
What is fusional vergence?
It’s a type of vergence used to correct any small misalignments between the eyes, ensuring the brain can “fuse” the images seen by each eye into a single, clear image.
What is the importance of fusional vergence?
Important to maintain BV in presence of phoria.
If fusional vergence is weak what symptoms can this lead to?
Eye strain
Headaches
Diplopia
Blurred vision
Lack of stereopsis
If fusion reserves are low, this can lead to decompensation of a phoria.
How is fusional vergence measured?
Using the prism fusion range
Describe the method for the prism fusion range
- Px needs to have BSV (so can’t perfom on those with strabismus or who can’t fuse)
- Px wears their refractive correction
- Px fixates on accommodative target (6/6 letter) and prism introduced in front of one eye
- Prism is increased until diplopia experience (break point)
- Decrease the prism until BSV is regained (recovery point)
- Fusion taken as the prism value before break point
- repeat for both near and distance
- repeat for Base OUT (Positive) and Base IN (Negative).
What are the results to record for the prism fusion range? And explain them.
Blur, break and recovery points.
Blur point is the limit of positive/negative relative convergence. When can no longer accommodate
Break point is the limit of motor fusion. Loss of bifoveal fixation resulting in diplopia
Recovery point is where motor fusion can restore bifoveal fixation.
What is the normal results for Positive prism fusion range? (Base OUT)
Near: 35/40
Distance: 14
What is the normal results for Negative prism fusion range? (Base IN)
Near: 14
Distance: 4/6
What is the normal results for verticle prism fusion range? (Base UP/DOWN)
Near: 3
Distance: 3
What is tonus?
Continuous, low level contraction of muscles at rest.
How can tonus affect prism fusion range?
If you do Base OUT first, fusional vergence movements would make small inward movements everytime prism increased. The medial recti muscle becomes a bit tighter.
So if Base IN is done next, which is already the weakest range, it is going to be harder to make the movements as we have tonus of medial recti muscles. Resulting in a further weaker range for Base IN
What should be done to avoid the effects of Tonus?
Measure prism fusion range for the most likely weakest first.
How to interpret results to check reserves?
- Compare positive and negative reserves= If positive low then possible exo deviation.
- Compare the amplitudes with normal
- Sheards criterion = Fusional reserve in opposite direction to phoria should be twice the size of phoria
- Compare break and recovery points =
Normal difference 4-6. If difference is large, this indicates that Px finds it difficult to restore fusion once it is broken down - poorly developed fusion reflex.
