Motor Fusion Flashcards
What does it mean when eyes converge?
They move inwards
What does it mean when eyes diverge?
They move outwards
What is motor fusion?
Ability to align maintain sensory fusion over a range of vergence movements
The stronger your binocular single vision the ______ your motor fusion?
Higher
How can motor fusion be measured?
By measuring: -Prism fusion range -20 dioptre base out (also known as the Prism Reflex Test) -Can be measure via a Synoptophere
Why is motor fusion important?
Your patients prognosis is based on the presence or absense of fusion. If your patients have good motor fusion (i.e it is present) then prognosis is good. There is much hope that normal BSV can be restored thus this becomes your aim. If your patient has bad motor fusion (i.e. it is absent), there is a poor prognosis. BSV cannot be restored completely thus your aim of treatment is to ensure eyes are cosmetically acceptable.
How do we test Motor fusion (i.e. what’s the principle)?
You measure fusional vergence to test motor fusion
How may textbooks refer to also refer to fusional vergence?
Fusional Reserves
Fusional Amplitudes
Prism Fusion Range
Prism Vergences
What happens to an image if you put a prism in front of an eye?
The image is displaced in the direction of the apex. If you only put the prism infront of one eye then the px shall experience double.
What would happen if you put a prism in front of a px with good motor fusion?
They would move their eyes to avoid seeing double. With bad motor fusion this obviously wouldn’t happen.
When you place a prism in front of a normal eye/ an eye unaffected by that prism yet which direction will the pupil move in?
It will move towards the direction of the apex
How can the prism fusion test be carried out subjectively and objectively?
Subjectively - the px would tell you when they see double.
Objectively- you look out for eye movements i.e. that the px’s eye didn’t move in the direction of the apex thus they didn’t overcome the prism.
Can the Prism fusion test be carried out objectively at both near and far?
Yes
What does it mean if you can overcome more prism to avoid diplopia?
The more prism you can overcome to avoid diplopia:
The higher your amplitude/reserve motor fusion
The better your BSV (team work between eyes)
The less likely you are to decompensate
What does it mean if you can overcome more prism to avoid diplopia?
The more prism you can overcome to avoid diplopia:
The higher your amplitude/reserve motor fusion
The better your BSV (team work between eyes)
The less likely you are to decompensate
If you put base Out prism in front of your eyes what will happen?
They will cOnverge thus positive fusion range occurs.
If you put base In prism in front of your eyes what will happen?
They will dIverge thus negative fusion range shall occur.
What does a base up prism in front of the right eye measure?
Infravergence (i.e. to look down as the base is up)
What does a base down prism in front of the right eye measure?
Supravergence (i.e. to look up at the base is down)
What’s the biggest mistake students make with the Prism fusion range test?
They confuse it with the prism cover test.
In the prism fusion range test why do we get the target to look at a 6/60 target rather than the line above their worst VA?
Because at the line above their worst VA, letters become very small thus they may miss the double vision occurring thus we like to use a big letter at 6/60.
How do you carry out a prism fusion range test?
Patient sits head erect Looks at 6/60 Prism bar placed before one eye (either eye)
Strength of prism bar is increased slowly
Continue increasing strength until patient reports diplopia ALWAYS Ask them if they can make the image single again and keep going if they can Continue to increase prism
Note maximum amount of prism BEFORE diplopia is recorded - This is recorded as fusional amplitude.
Once fusion is lost decrease strength of prism until patient can regain fusion
Can carry this out at near (1/3m & 6m ) working distance and with or without glasses
When you converge what happens?
You accommodate and pupils undergo miosis.
How may accommodation affect the prism fusion range test and so what should you do?
—During test patient may complain of blur
—Normally when vergence changes accommodation changes by a linked amount
- Convergence increases, accommodation increases
- Divergence increases, accommodation decreases
—Note the blur point —Continue onwards until they report diplopia
What are break point and fusional amplitude?
Two terms that literally describe the same thing.
If you were strapped for time which prism fusion range test would you carry out (i.e. would it be at a distance or near , would it be with base up, base down , base in or base out prism)?
You would do it at whatever is most interesting e.g. if you had a right exophoric patient who presents at a distance.
You know they have no problem diverging so you would want to test their convergence thus carry out a base out prism test and do it at a distance.
What are our normal prism fusion range values? (You need to know these off by heart)
The table values or higher means the px is all good.
Values lower than the table ones mean the px isn’t good.
What is recovery in the prism fusion test?
The process of getting the patient to see one single image back after you have surpassed their break point.
Recovery should always be 4 dioptres less than the break point. Record this as part of the test.
What do you have to remember when doing a vertical prism fusion range test ( i.e. your changing your base up and base down)?
If you carry out both a base up and base down test then you do it in front of the same eye (NEVER SWITCH EYES -because you have essentially just repeated the test).
How can analysis of the prism fusion range test be done?
Compare to normal values - this one is easiest and best.
Apply Percival’s logic - Percival states that convergent and divergent fusional reserves should be balanced, one should not be less than half of the other - this ones a bit shitty though.
Apply Sheard’s logic -Sheard states opposing fusional reserve to blur point should be twice degree of phoria - requires lots of calculations cba.
When is the 20 dioptre Base Out test used and what is it?
It is used:
- For Young infants
- Adults with learning difficulties
- When there’s only a Short of time in practice
- When you Don’t have a prism bar
It’s essentially a one step screening test.
How do you carry out a 20 Dioptre base out test?
Single base-out prism placed before one eye & then the other (ALWAYS ON BOTH EYES)
Child fixates toy/picture at 33cm
You have to Work quickly Useful in proving presence of binocular single vision in:
- Suspected pseudostrabimus
- Child with inconclusive cover test
If a 20Δ base-out prism is not overcome encourage patient to fuse images
Important to note recovery movement as prism is removed, speed, accuracy If px does not overcome try 15 or 10 diopter prism
What is the 4 dioptre Base Out test used to check for?
Not mainly used to check for motor fusion but rather for macula suppression- microtropia.
Or to clarify whether the px has central supression or macula supression.
How is a four dioptre Base Out prism test carried out?
Patient fixates letter target at distance or near Fixate a tiny/letter target Consider bringing prism down from above Prism placed before right eye, image displaced outside macula area
Both eyes make a versional movement towards apex of prism to left Left eye then converges to re-establish foveal fixation
If you were testing for a micro-esotropia what direction prism would you use?
Base Out (4 dioptre prism)
If you were testing for a micro-exotropia what direction prism would you use?
Base In (4 dioptre prism)
In a prism fusion test if the eye that is covered by the prism behaves correctly but the uncovered eye doesn’t move what does the patient have?
Px has suppression of the uncovered eye.
Can a synoptophore be used to investigate motor fusion?
Yes.
