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
• 4^ prism test - L microtropia
• L Microtropia
• L Foveal scotoma
• 4^ moves image into suppression scotoma on retina
• LE doesn’t move, RE doesn’t move
• 4^ moves image toward apex, RE moves to follow image
• LE makes version to follow RE, doesn’t make vergence movement to o/c 4^ as in suppression scotoma
Treatment of microtropia
• Prescribe Full Cycloplegic R - allow 16-18 weeks refractive adaptation
• Aniseikonia is often a problem in high degrees of anisometropia
• Treat underlying amblyopia by occlusion of non-strabismic eye - regular review
• Surgery is not appropriate
Suppression: Investigation mainly concerned with what
• DEPTH
• EXTENT OF RETINAL AREA
Sbisa Bar Method
• Measures Density of Suppression
• The patient fixates on a spot of light
• A filter bar (palest filter) is placed before the non-suppressing eye and the density of the filter is increased until two lights are seen
• This filter is a measure of depth
• Occasionally when the end point is reached fixation swaps to the other eye and suppression swaps over, resulting in no diplopia
• If there is no strabismus use red filters or red/green diplopia goggles
Please note!!
• Most important in gauging the risk of intractable diplopia when considering occlusion in older children
Density of supression
• Suppression not equal deep in all patients.
- some may be readily overcome; in others it is difficult to do so. It is useful and easy to establish how deep the suppression is in a patient
• To make a patient aware of the images perceived by the deviated eye, one must reduce the retinal illuminance of the fixating eye until the patient sees double
• This is best done with a series of red filters of increasing density arranged in the form of a ladder (Spisa Bar)
• The patient fixates a small light source, and the filters are placed in front of the fixating eye. Some patients see double with a single layer; others require three or more layers before they recognize diplopia
•The greater the number of layers needed, the deeper the suppression
measuring area of suppression:
• The patient fixates the spotlight with the dominant eye and prism bar is moved before the suppressing eye - starting at 11
• When the image of the spot has moved off the suppression scotoma the patient reports diplopia
• The difference between the angle of strabismus (from CT) and the prism which moves spot out of scotoma = angular extent of the scotoma
• This can be used to measure vertically as well
Prism Adaptation Test:
• Used in small /moderate convergent angles when considering surgery
• Correct angle with Fresnel prisms split between eyes & reassess angle a few days later
• If demonstrate fusion then termed “responders”
• In ARC will “eat up” prism
• Operate for total angle of prism to produce fusion
Evaluation of suppression and treatment
• The deeper the suppression and greater the extent across the retina - the more difficult it will be to treat
• Factors such as patient’s age, duration of strabismus, co-operation etc. must be taken into account
• When planning surgery in adults or older children you must carry out a measure of the area of suppression or Post Operative Diplopia Test as it is also known
• Only treat amblyopia if the suppression is dense and the patient is not at risk of intractable diplopia
Summary
• suppression can transfer from one eye to the other e.g. in ACS
• 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 eg in convergent strabismus
- form vision suppressed at macula and to nasal retinal point receiving
• 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.
Differential diagnosis between bifoveal single vision vs Microtropia:
Bifoveal BSV
• Visual acuity: Equal in most cases
• Refractive error: Approx the same BE
• Fixation: Central
• Cover/uncover test: No manifest deviation
• 4^ prism test: Normal
• Stereoacuity: 40s or better
Microtropia
• Visual acuity: Unequal
• Refractive error: Anisometropia
• Fixation: Parafoveal
• Cover/uncover test: Small eso or exotropia in most cases
• 4^ prism test: Affected eye deviates
• Stereoacuity: worse than 40s
