Week 1.07 Assesing Control Of Deviation Flashcards
What is the fusion reflex
Eye movements are controlled by the fusion reflex
Fusion reflex = brains ability to coordinate both eyes so that they move together and focus on the same point. When this reflex is active, the eyes are controlled to move in a coordinated way.
However, when the fusion reflex is “ suspended” (I.e. No longer working) the eyes with a late deviation will shift to a different position.
What are the three main types of eye movements?
Ductions: movements of one eye alone (like moving that eye left or right)
Versions: movements where both eyes move in the same direction together (e.g. Both eyes moving to the right). Angle between visual axis parallel and maintained.
Vergences: movements where the eyes move in opposite direction (e.g. One eye moves left while the other moves right such as when focusing on a near object.) angle between visual access changes.
Different types of ductions
Adduction - to the nose (nasal)
Abduction - from the nose (looking temporally)
Elevation and depression
Versions
Look at week 1.07 assessing control for the picture (basically just eyes looking in diff gazes and the names)
Can be voluntary or involuntary
Assessed during ocular motility test
Vergence eye movements
Movement of two eyes in opp directions ‘ disjunctive movements’
Angle between visual axis changes
Vergences are slow and tonic which slower than versions
Fusion eye movements are vergence eye movements
Purpose of vergence movements is to bring their eyes into proper alignment and maintain this alignment
Converge and diverge
What is fusional vergence?
If a phoria exists, compensatory vergence impulses adjust the activity of the extraocular muscles to ensure proper relative positioning of the eye. Think of it as an overdraft but for eyes
Involuntary
Other types of vergence eye movements
Tonic
Accommodative
Proximal
Fusional
Tonic vergence eye movements
Moves eyes back from their anatomical (dead) divergent up and out position and brings them back in
May be slightly divergent or convergent
Accommodative convergence and what is the near traid of responses
Ciliary muscle constricts (accommodation increases positive power of lens), pupil constricts and eye will converge
Near triad of responses (accom, convergence, pupil constriction)
What is meant by AC/A ratio
Amount of convergence per dioptre of accommodation is called “AC/A ratio”
How much convergence per D of accommodation e.g. 6:1 —> 6prism diopters of convergence for every 1D of accommodation
AC = accommodative convergence
Average AC/A ratio 3.5prism diopters/1D
AC/A ratio that too high or too low can cause problems
Proximal convergence
Convergence that arises due to awareness of nearness of a target e.g. binoculars
- This awareness gives rise to reflex convergence
- Revealed under conditions of optical infinity: object is near, but no accommodation required
- Difference between distance phoria and phoria under optical infinity viewing conditions – at optical infinity person still knows it is near even though stimulus to accommodation is very low
- Arises because target is near
- Amount of proximal convergence increases with the nearness of the target. (Pc may be around 2-4 prism dioptres)
- E.g. looking through binoculars ppl may think near so converge but still looking at object at infinity
For distance viewing what Vergences do we use
Tonic vergence and fusional vergence (if both eyes opened, suspended when one eye closed)
We don’t use accom vergence or proximal vergence
Look at notes for the diagram
What Vergences do we use for near viewing
Tonic vergence
Accom vergence
Proximal vergence
Fusional vergence - removed when cover one eye closed
See notes for diagram to help explain
What 5 things do we use to measure the control of the deviation
- Case history
- Cover test (recovery)
- Measuring vergence amplitudes
- Measuring associated phoria (fixation disparity)
- Change in stereopsis
How can we use case history to measure the control of deviation?
Px with a decompensated heterophoria is likely to have symptoms from:
- eye strain
- headaches
- blurred vision
- diplopia
Need to differentiate whether caused by decompensation or other causes e.g. time of onset, relationship to visual tasks
How do we use cover test to measure the control of deviation
CT recovery movement indicates degree of control as well as type and magnitude of deviation
Manifest deviation = no control
Intermittent deviation = poor (occasional control)
Decompensated phoria – slow jerky or hesitant recovery movements
Change in stereopsis
Poor stereopsis CAN indicate decompensation (if there has been a CHANGE in their stereopsis compared to what their best is and if they are capable of better and its changed to poor)
Measuring the total vergence amplitudes
How much of this ability is being used to CONTROL the misalignment
For example if the max amount of weight u can carry is 100kg and u carry 6kg then its within enough of your tolerance and you can control it fine
Many things can be assumes to have good control if only using 33% to 50% of total available
To investigate a fusional response you need to…
…move image out of Panama area
Positive and negative fusional reserves
Positive fusional reserves: how much can eyes turn in to maintain single vision (convergence)
Negative fusional reserves: how much can the eye turn out to maintain single vision (divergence)
Measure at dist and near
So 4 measures in total - dist conv, dist div, near conv, near div
Record base in or base out rather than +ve or -ve
In order to measure u use prism bar to either force the eyes to converge or diverge
Base out (apex in) - force eyes to converge
Base in (apex out) - force eyes to diverge
Positive fusional vergence amplitudes
Base out prism power slowly increase until fusion breaks down
Amount of BO that just produces diplopia = base out fusional vergence amplitude
Negative fusional vergence amplitudes
Base in prism power slowly increases until fusion breaks down
Amount of base in prism that just produces diplopia = base in fusional vergence amplitudes
How to measure positive and negative FVA
- Measure horizontal before vertical fusional amplitudes
- First measure the FVA that opposes phoria - (if XOP measure base out before base in)
- Wear distance rx
- Isolate single letter slightly >VA of poorer eye
- Prism bar on one eye “tell me as soon as it blurs or doubles” “make it clear”
Watch for eye movement with each change
3 measures blur, break, recovery
Increase to blur and break point (doubled) then reduce to obtain recovery point (single)
Vertical fusional vergence amplitudes
Measured with base up or down
Vertical fusional vergence amplitudes are very small
No blur point only break and recovery ( not linked to accommodation so no blur point)
Change prism power slowly
Measured twice with prism base up over RE and the prism base down over RE
Vertical FVAs not often measured in clinics
Px may report blurring before doubling why?
At some level of convergence cannot relax accom at the same time —> choose to:
- relax accom and convergence = clear and double
- or
- maintain convergence and stop trying to relax accommodation = single and blurred
What is amplitude of motor fusion
Maximum amount of prism power that you can put in and still maintain single vision
Amplitude of motor fusion = fusional vergence (I.e. phoria) + opposing fusional regency amplitude
What are you measuring at the blur point?
- if measuring base out vergence amplitudes px needs to converge to eliminate disparity but then also relax accom to keep target clear - positive relative vergence
- if measuring base in vergence px needs to diverge to eliminate disparity but then also increase accommodation to keep target clear - negative relative vergence
Prism dioptre meaning
The deviation of 1cm at a viewing distance of 1m
What are you measuring at the break point
Prism disparity that cannot be overcome I.e diplopia
What are you measuring at the recovery point?
Prism disparity at which point vergence system realises that can correct it
What does a large difference between the break and recovery mean
Lazy vergence system
What is Percival’s criterion
States that if the demand line (phoria position) lies within the zone of conduct which is the middle third of total range then binocular vision is fulfilled.
In other simpler words —> if phoria is within the middle third of the total vergence range binocular vision is fulfilled
“Middle third technique”
Need to know base in and base out FVA and add them up and divide by 3
Sheards criterion
To establish whether px is going to experience discomfort in BV
States that amount of phoria should be less than half the opposing fusional vergence blur in reserve
If px has 10 prism diopters of exophoria, the positive vergence reserves base out should be at least 20 prism diopters to satisfy this criterion
How to measure vergence facility
- Choose app target - letter on chart or circle
- Select appropriate ‘prism flippers’
- Start the clock and introduce prism
- When px says ‘clear and single’ rotate
- When px says ‘clear and single’ again that’s 1 cycle
Count no of cycles in 1 minute
Prism flippers may be issued for treatment
