Retinal Correspondence and ARC Flashcards
Whenn is binocularity produced and when are the reflexes firmly established?
- BV not inborn
- Series of conditioned reflexes develop in 1st 3 yrs of life to produce binocular input to cortical cells leading to development of binocularly driven cells in cortex
- These reflexes become firmly established by ~5-6yrs (plastic period) & hence become unconditioned reflexes
o Critical period – within 1st 5yrs of life
Earlier something happens to binocular system, more detrimental that is to binocular system, poorer prognosis is for eyes to work together
o Plastic period – part of critical/sensitive period
Develop squint at 4 yrs – had 4yrs of developing binocularity – cells etc
Binocularly driven cells at 9mths of age – when everything starts to work together
What happens when a strabismus develops before 3 yrs? Acquired from 6yrs to adult? Acquired from 3-6 yrs?
- If strabismus develops before 3yrs:
o Adaptation to normal development occurs to prevent sxs
o Leads to abnormal unconditioned reflexes developing by age 6yrs
o Usually NO SYMPTOMS - If strabismus acquired from 6yrs to adult:
o BV is well established
o Symptoms very likely to occur - If strabismus acquired from 3-6yrs:
o Symptoms will occur
o But system is unstable & readily breaks down
o Adaptations will develop (plastic period)
o Symptoms are unlikely to occur
o Tend to be accomm eso
Because child not going through emmetropisation process fully – either too much hyperopia then over accommodating at nursery & then get eso
What are the 2 ways in which BV is embarassed?
Diplopia and confusion
How does the visual system cope?
- In young pxs sensory adaptations overcome diplopia & confusion
- These binocular sensory adaptations are:
* Suppression
* Abnormal (anomalous) retinal correspondence (ARC)
May still have binocular driven cells that lie dormant
Can investigate to see if potential to be binocular
Sometimes suppression does not kick in then get ARC
Most squints have suppression but few have ARC
Striving to get ARC – cannot ever get infantile esotropia to develop NRC
Stimulating corresponding retinal points is how we know where things are in space – if normally connected ->e.g. on nasal retina and temporal retina on other eye – know its on a specific side
What is suppression and what is important about it?
Suppression is cortical inhibition of image coming from deviating eye – not happening at level of retina
Only way px can cope with dip – happens quickly
Anything falling within suppression area can be ignored
* Want to know how wide the suppression area is & want to know how dense the suppression – how easily can we overcome that and give px diplopia
* Don’t want to deliberately overcome suppression
* To overcome suppression – can be by over-patching someone – if haven’t measured it properly
* Forcing px to use the suppressing eye – size of area gets smaller and smaller and they start seeing two images
* Use prisms to move image across retina – to see how big the suppressed area is
What happens when an image moves out of the suppression area?
- Moved out of suppressed area for the px
- So they will get diplopia
- When moving prisms in front of eye – imagine this happening at retinal level – even though suppression at cortical level
- Do this before giving surgery – symptom free but cosmetically unhappy
- Before surgery – want to know size of squint
- If measures 40^ can you fix it and make it straight
- If make it a little exo and over correct them will we give them diplopia – in this px the answer is yes
- Need to know if they could be referred on for surgery – on a lot of pxs
- Any px who is keen to talk about squint surgery – send them to HES – don’t just say unsure if we can
What is anomalous retinal correspondence (ARC)?
- ARC describes condition in which originally non-corresponding retinal areas of the 2 eyes co-operate to produce a form of BSV
- A shift in spatial localisation of deviating eye occurs to counteract effect of ocular deviation
- Can have situ where px is normally wired, intermittent distance XOT, at near eyes are straight. Look in distance and RE diverges. Immediately click in suppression in distance but go back to binocular at near
o Can switch between the two - ARC better than not having it good as having normal
- Right esotropia
- New point – p = pseudofovea – new point that they are fixing from
- P acting as if it is the fovea
- If think of this in microtropia – left fovea now connected with P, used to be connected with fovea of other eye – this upsets the balance as that’s not how wer meant to be and upsets how brain can use the 2 eyes
Describe harmonious ARC?
Harmonious (HARC) – if angle of anomaly = angle of strabismus
o If everything in harmony – e.g 8^ esotropia – when measure on PCT its 8^ - if want subjective response and ask you to join the two prisms will
o Subjective = objective measure – using new foveal point
o Everything working together
Describe unharmonious ARC?
Unharmonious (UN-HARC) – if angle of anomaly is >0 but <than angle of deviation
o Angle is different
o Angle measured objectively is less than subjective response
o Objective 8^, subjective is 4/5^
o Not as good but different response
Describe anomalous/abnormal retinal correspondence?
- Present in retinal area receiving peripheral image in strabismic eye – depending on retinal point is
- Convergence of nerve fibres from here means there is no longer point to point correspondence i.e. LOSS OF RESOLUTION
- In effect produces enlarged ‘pseudo-Panums’ area centred on point receiving the image, which corresponds with fovea of other eye HARMONIOUS ARC (90% of strabismus)
o Microtropia – tends to be anisometropic
o If making new fovea – not on fovea – slightly extra macular
o Best VA that point can get to is 6/12,6/9
o No longer using the best point
o Abnormal RC, abnormal BV
o Lost ability to have really sharp stereo - Hence in ARC the images of object of regard are given same visual direction despite the strabismus
- Therefore there is no diplopia and some very low grade BV will be present
Fovea gives best VA – hard for new fovea to work with old fovea which has highest quality
What is important to note about abnormal retinal correspondence?
Abnormal retinal correspondence is a BINOCULAR condition and disappears when fixing monocularly
Want to know where they are fixing – visuscope to cover one eye & looking which part of retina px is fixing from
What are the classic ARC characteristics?
- Occurs in long-standing deviations
- Small angled deviation less than 20^
- Microtropia less than 10^
o Microtropia have central suppression scotoma – they are unique in this sense - Usually convergent
- Only mild amblyopia
o Mostly anisometropic – if just fixing next to point off fovea then cannot be 6/5 by definition
o May have fully accomm SOT w/ NRC – but if anisometropic then sometimes when put gls on can control to a microtropia - Rare in exotropia
- Provides useful BSV in manifest strabismus
- May revert to original angle after surgery
o Clinically significant 20^ SOT & ARC – if straighten eyes – brain may not like having this fixed and may revert back to having 20^
What are the investigations into ARC?
- Binocular tests possible in manifest strabismus
o Bagolini glasses – cross response in manifest strab
o Lang’s 2 pen test – binoc more accurate than monoc
o Worth’s lights – 4 lights
o Stereotests possible but not >70” arc
Bifoveal fixation – 55” of arc on stereo – lower the number the better it is - If ARC then will never get better than 70” – fact that have stereopsis is good though but never as good as NRC
- Synoptophore
o Compare objective & subjective angles
o Look for fusion at (smaller) subjective angles - Subjective analysis of diplopia does not agree with objective angle e.g. diplopia joined at 10^ but objective angle 25^ (angle of anomaly)
- Prism adaptation test
o Measure px – put Fresnel on of angle that they are measuring
o Once put this on – can they stay straight or does it increase? – do they want to keep using the new fovea that they have
o Optoms wouldn’t use in practice
What are the implications of ARC?
- Generally an advantage provided cosmetically good
o Having squint not ideal but having some binocularity with it e.g. ARC is better than nothing - Explains unexpected clinical findings
- May cause poor surgical results if not assessed correctly
o Paradoxical/incongruous – in which projection of images does not tally with type of strabismus or angle of deviation
Paradoxical – deliberately move px to fixate a point they have not formed a connection with
o Intractable diplopia
o Reversion to original angle
