Week 7 - Retinal correspondence, Arc and Microtropia Flashcards
BV reflexes are not:
• BV is not inborn
• Conditioned reflexes develop in the first three years of life to produce binocular input to the cortical cells leading to the development of binocularly driven cells in the cortex
• These reflexes become firmly established by about 5 to 6 years (the plastic period) and hence become unconditioned reflexes
Difference between strabismus before 3; or after 6 to adult
• if strabismus develops before three years
- adaptation to normal development occurs to prevent symptoms
- leads to abnormal unconditioned reflexes developing by age 6 years.
- Usually NO SYMPTOMS
• if strabismus acquired from 6 years to adult:
- binocular vision is well established,
- symptoms very likely to occur
Strabismus acquired between 3-6:
• if strabismus is acquired at 3-6 years:
- symptoms will occur
- but the system is unstable and readily breaks down
- adaptations will develop (plastic period)
- usually NO symptoms
Two ways BV is embarrassed:
• Diplopia
• Confusion
How does visual system cope with strabismus?
• In young patients sensory adaptations overcome diplopia and confusion.
• These binocular sensory adaptations are:
- suppression
- abnormal (anomalous) retinal correspondence (ARC).
REVIEW LECTURE’s SCHEMATICS SHOWING CROSSINGS/UNCROSSINGS
Anomalous retinal Correspondence:
• Anomalous Retinal Correspondence (ARC) describes a condition in which originally non-corresponding retinal areas of the two eyes cooperate to produce a form of binocular single vision
• A shift in the spatial localisation of the deviating eye occurs to counteract the effect of the ocular deviation
Types of Arc:
• HARMONIOUS (HARC) - if the angle of anomaly = angle of strabismus
• UNHARMONIOUS (UN-HARC) - if angle of anomaly is greater than zero but less than the angle of deviation
Explain ARC:
• present in the retinal area receiving the peripheral image in the strabismic eye
• convergence of nerve fibres from here means there is no longer point to point correspondence ie LOSS OF RESOLUTION
• in effect produces enlarged’ pseudo-Panums’ area centred on the point receiving the image, which corresponds with the fovea of the other eye: HARMONIOUS ARC (90% of strabismus)
• hence in ARC the images of the object of regard are given the same visual direction despite the strabismus
• therefore there is no diplopia and some very low grade BV will be present
Is Abnormal retinal correspondence binocular or monocular?
Abnormal Retinal Correspondence is a BINOCULAR condition and disappears when fixing monocularly
“Classic” ARC Characteristics
• Occurs in long-standing deviations
• Small angled deviation less than 20^
• Microtropia less than 10^
• Usually convergent
• Only mild amblyopia
• Rare in exotropia
• Provides useful BSV in manifest strabismus
• May revert to original angle after surgery
ARC investigation:
• Binocular tests possible in manifest strabismus
- Bagolini Glasses - cross response in manifest strab.
- Lang s 2 Pen Test - binoc. more accurate than monoc.
- Worth’ sLights 4 lights
- Stereotests possible but not >70”arc
• 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
Implications of ARC:
• Generally an advantage provided cosmetically good
• Explains unexpected clinical findings
• May cause poor surgical results if not assessed correctly
- Paradoxical/ incongruous (in which the projection of the images does not tally with the type of strabismus or the angle of deviation
- Intractable diplopia
- Reversion to original angle
Clinical features of Microtropia:
• Small angle <10^ with ARC
• Common
• Stable
• Anisometropia (usually 1.50DS or more)
• Always mildly amblyopic (one line at least)
• Central suppression
• Good but not perfect BSV
• Strong motor fusion
• Differs from classic ARC because associated with eccentric fixation
• Not possible to treat but may prevent successful orthoptic exercises
Classifications of Microtropia:
• Classic microtropia without identity - small manifest angle
• Microtropia with identity - ARC & eccentric fixation at same retinal point, no movement cover test.
- diagnosed with fixation ophthalmoscope
• Microtropia associated with larger latent component (increases on alternate CT or if one eye covered longer than usual)
• Microtropia associated intermittent esotropia
- e.g. fully accom (A primary microtropia which becomes decompensated particularly between 1-3 years as a result of an accommodative element or superimposed phoria)
• Divergent microtropia
• Secondary - follows optical or surgical correction
Classic microtropia without identity
1.Small manifest deviation
2.Less than 10^
3.Mostly Esotropia but can be exotropia
Classic microtropia without identity with Latent Component
1.Manifest deviation will increase on continued dissociation
2.Must measure manifest component with simulated Prism cover test
3.Measure Latent Component with full Prism cover test
Microtropia with Identity
1.ARC & eccentric fixation at same retinal point
2.No movement on cover test.
3.Only diagnosed with fixation ophthalmoscope and 4^ prism test
Microtropia associated with other intermittent esotropia e.g. fully accom
- Anisometropic Fully Accommodative Esotropia
2.Small manifest esotropia when hypermetropia corrected
3.ARC instead of NRC
Investigation of microtropia
• Visual acuity
• Crowding
• Fixation: ophthalmoscope, visuscope
• Cover test may or may not be strabismic movement
• 4 Dioptre Prism Test
• Bagolini Lens with or without suppression gap
• Amsler chart - scotoma may be seen
• AMBLYOPIA + NO CT MOVEMENT + POSITIVE 4A TEST = MICROTROPIA with identity
Microtropia Investigation differences: With identity vs against identity
With Identity
• No movement on CT
• Harmonious ARC
• Subjective angle = 0
• Absolute eccentric fixation
• Angle of anomaly = angle of eccentricity
• Visuscopy - stable parafoveal fixation
Without identity
•Movement seen on CT
May have:
•Central fixation with ARC
•Unharmonious ARC
•Central fixation with NRC, central suppression and peripheral fusion - extended Panum’s area
•Visuscopy - unstable
Ophthalmoscopic Methods of Viewing
Fixation
• A target is projected and focussed onto the retina and is seen by both the Px and the practitioner
• Px is asked to look at the centre of the target and the position of the fovea is noted
• Position is then recorded in diagram - also record if steady/unsteady
- usually EF is slightly nasal in SOT
- can calibrate using the size of the optic disc in the graticule Disc = 5 deg × 7 deg
NB accommodation is usually induced using this method - change focus or cycloplegia
Viscuscope:
• This is a modified ophthalmoscope that projects a fixation target on the fundus. The eye not tested is to be occluded. The examiner projects the fixation mark close to the fovea and the patient is asked to look directly at the asterisk.
• The position of the fixation target on the fundus is noted.
• 4^ prism test - normal bifoveal fixation
• Bifoveal fixation
• 4^ prism moves image to apex
• LE moves nasally to follow image
• RE makes version movement
•Diplopia appreciated
• RE makes vergence
• movement to o/c 4^ prism and maintain BSV