Lecture 6 Retinal Correspondence + ARC + Microtropia PDF

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Summary notes for retinal correspondence + ARC + microtropia....

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MICROTROPIA

L6: RETINAL CORRESPONDENCE, ARC AND MICROTROPIA If strabismus develops before 3 years:

If strabismus acquired from 6 years to adult:

- Adaptation to normal development occurs to prevent symptoms - leads to abnormal unconditioned reflexes developing by age 6 years - Usually NO SYMPTOMS

- binocular vision is well established, - symptoms very likely to occur

If strabismus is acquired at 3-6 years: Most likely an accommodative esotropia - symptoms will occur - but the system is unstable and readily breaks down - Adaptations will develop (plastic period) - Symptoms are UNLIKELY to occur.

Anomalous Retinal Correspondence (ARC) Describes a condition in which originally non-corresponding retinal areas of the 2 eyes co-operate to produce a form of binocular single vision. A shift in the spatial localization of the deviating eye occurs to counteract the effect of the ocular deviation.

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Small angle 70” arc

Hence in ARC, the images of the object of regard are given the same visual despite the strabismus. Therefore, there is no diplopia and some very low-grade BV will be present. Px with NRC should have stereoacuity values of 40 sec of arc or greater (lower number better stereoacuity value). Whereas in HARC + microtropia – would be unable to achieve no better than 60 sec or arc because they are using a point at the back of their eye that is not the fovea. Stereopsis is a hyperacuity, which means you have to have excellent VA to have 10 sec of arc and if you’re fixing from a point that is not your fovea, then you will not have that level of VA hence you will not have the ability to have super good stereoacuity values because hyperacuity is just not possible.

• Synoptophore - Compare objective & subjective angles - Look for fusion at (smaller) subjective angle

• 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 (To check whether they are suitable for surgery otherwise, after surgery they will just revert back to their original angle.)

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Easy to elicit, common, Bagolini glasses / Lang’s 2 pen test unstable ARC Convergence with deviation Wirt Randot Stable angle of anomaly on synoptophore Worth’s 4 dot (much harder test, more dissociative test) Prism methods After image test TNO Uncommon, stable ARC

• Eccentric Fixation • Eye does not take up foveal fixation on occlusion

Differential diagnosis of bifoveal binocular single vision and microtropia Bifoveal binocular single vision Microtropia Equal in most cases Unequal Visual acuity Approximately the same in both eyes Anisometropia Refractive error Central Parafoveolar or parafoveal Fixation No manifest deviation Small esotropia or exotropia in most cases Cover/uncover test Normal Affected eye deviates 4∆ ∆ prism test 40 s or better Worse than 40 s Stereoacuity Suppression: Read PDF Notes for more info. • DEPTH – SPISA BAR • EXTENT OF RETINAL AREA (Extent of suppression scotoma)

+ Cross on Bagolini Glasses with a tiny central area missing on either lines + Have reduced vision in one eye usually by more than 1 or more lines.

Summary: • Suppression can transfer from one eye to the other e.g. in ACS (Alternating Convergent Squint) • Probably initiated cortically but may have a retinal component • Eliminates confusion and, if area extends, diplopia In most people with strabismus from an early age, both ARC and suppression are present e.g. in convergent strabismus - form vision suppressed at macula and to nasal retinal point receiving image - localization of object in space is modified across more extensive peripheral area of strabismic eye to avoid diplopia • ARC and suppression are binocular adaptations • Normal correspondence returns and there is no suppression when the fixing eye is occluded and the strabismic eye takes up fixation....