Week 1.10 Concomitant Strabismus Flashcards
Why does every px needs an assessment of their BV
- binocular coordination and cooperation contribute to visual function and comfort
- helps explain current symptoms or past visual development
- provides indication of px cranial health
What is the average refraction of a newborn
+2.00DS (+/-2D) refraction changes as they get older
VA~ 6/120
Stereopsis >600sec
Poor accommodation accuracy and vergence control
When does stereopsis develop
BV not inborn, stereopsis develops during first year after birth
Series of conditioned reflexes develop in the first 3 years of life (critical period) to produce binocular input to cortical cells
These reflex become established by about 5 to 6 years, or later for some aspects (plastic period) and hence become unconditioned reflexes
Will never get to normal BV if did not have normal BV at end of critical period
Sensory and motor fusion
Sensory – ability to simultaneously appreciate two similar images, one with each eye, and to interpret them as one
Motor – ability to maintain a single image by correctly aligning the two visual axes during eye movements
Sensory fusion + motor fusion = BSV
BSV= binocular single vision
What are the grades of binocular vision?
- Monocular vision
- Simultaneous macular perception
- (Sensory) fusion
- Stereopsis
What is simultaneous macular perception
When signals transmitted from the two eyes to the visual cortex are perceived at the same time I.e. the ability to see two dissimilar objects simultaneously
What is the definition of strabismus
A deviation of the visual axes that cannot be overcome (controlled) by motor fusion. Constant vs intermittent
What does concomitant mean
Angele of deviation is the same in all directions of gaze
Also knows as: manifest deviation, heterophic, squint
What are the two main symptoms for binocular single vision with a person who has a strabismus
Diplopia - two uncrossed images
and confusion - crossed images one top of each other
What happen if a px develops strabismus before 3 years
- Sensory adaptations from normal development occurs to prevent symptoms
- leads to abnormal unconditioned reflexes developing by age of 6yrs (often ARC)
- usually no symptoms
What happens if strabismus is acquired from 6yrs onwards
- normal BV has been established
- now at the end of plastic period so..
- symptoms very likely to occur
If strabismus acquired between 3/6yrs?
- symptoms initially occur
- but system is unstable and readily breaks down
- adaptations will develop (still in plastic period)
- and again symptoms are unlikely to be reported
How do you work out if diplopia is monocular or binocular?
Monocular is less of a concern however binocular often related to cranial nerves
How to work out?
- cover each eye - diplopia remains
- if cover RE, LE still dip then there is monocular dip
- then cover LE, RE also dip which means both eyes have monocular dip and there’s 4 images going on
- if diplopia optical (due to large uncorrected astigmatism or cataract) - pinhole in dip disappears
- if diplopia remains with pinhole then cause within visual cortex
- cover each eye - binocular disappears then binocular diplopia, next test is CT, expect to see manifest dev
What are the binocular sensory adaptations?
Global suppression
Abnormal/ anomalous retinal correspondence (ARC)
Monocular sensory adaptations
- eccentric fixation
- amblyopia
What’s the main aim of sensory adaptations
To prevent diplopia and confusion
Which binocular sensory adaptations usually occur in larger strabismus compared to smaller strabismus
- In smaller strabismus <25prismD: both suppression and ARC often occur
- In larger strabismus >25 prism D: suppression dominates
If the BV anomaly is well adapted (deep) what do we do
Don’t really have to do much if no symptoms
What are three reasons to intervene/treat BV anomaly
- causing problems
- likely to deteriorate if left untreated
- if anomaly is sign of ocular or systemic,mic pathology
Most will be referring to orthoptists in these cases
What is global suppression
Is a binocular sensory adaptations usually that occurs when the px is using both eyes in the prescence of a strabismus to avoid confusion and diplopia
Is a cortical adaptive phenomena which prevents images in the strabismic eye from reaching a conscious level
An obstacle in trying to restore normal sensorimotor function
Retinal rivalry
When different images are presented to normally corresponding point, the strongest image or dominant eye predominates
E.g. F L —> E
In strab eye corresponding point constantly receive diff img
How is suppression an elimination of retinal rivalry
Suppression is an elimination of retinal rivalry over the area from fovea to point receiving image (nasal in convergent strabismus)
- I.e the dominant eye ‘takes over’ for this area of the VF
- This loss of form in the deviated eye prevents confusion
Suppression is an anomaly of BV although it can also occur without strabismus in amblyopia (lazy eye reduction in VA of one eye)
What is normal retinal convergence?
Fovea’s of two eyes are corresponding neural points in cortex - all other points also corresponding
Normals can tolerate up to 2 degrees of visual axis misalignment
What is abnormal retinal correspondence (ARC)?
A binocular sensory adaptations occurs when the two fovea’s do not correspond to the visual cortex and instead a non fovea’s point in the deviated eye is associated with the fovea of the fixating eye in the cortex. Allows person to receive single image despite eyes being misaligned
Basically like rewiring of the eyes
What is harmonious ARC (HARC)
The angle of rewiring matches the angle of strabismus
Most common type of ARC
Perfect rewiring to give binocular fixation
What is unharmonius ARC (UARC)
Very rare
The retired point does not match the angle of strabismus
Can be considered strab on top of strab
Imperfect rewiring —> would still have diplopia
Ideal conditions for development of ARC
- Strabismus before 6 years old (especially before 30 months)
- Delay in treatment
- Small constant strabismus angle
- Esotropia>exotropia
- Unilateral>alternating
Deep adaptations are well established so
Should be avoiding symptoms week
Also more difficult to treat/remove
Strabismic px will manifest a choice of three sensory adaptations…
Diplopia - no sensory adaptations
Global suppression - switching off an eye
ARC - rewiring
Methods of investigation to differentiate suppression from ARC
Bagolini lenses
Modified mallet unit
Worths four dot test
Bagolini strained lenses
Used to differentiate suppression from ARC
Trail case lens with grooves. Creates streak of glare 90 degrees to those striations
At any viewing distance
Ask PX if they see streaks of light
Lenses position is to create 2 streaks 45 and 135
If one streak = global suppression
If two streaks = HARC
If two lights and lines = NRC
2 lines + strab = ARC
How do we measure the depth of suppression
Use neutral density filter bar
In the case when you see ONE streak in Bagolini
Place ND filter bar in front of fixating eye
- increase filter until second line is seen
- I.e. strabismic eye switches back on
Can be used to measure depth of supp or arc - depending on what eye put filter bar in front of.
If LE suppressed out par in front of RE so LE can switch back on
Read units of the bar
How can you test the depth of ARC
In the case where you see 2 streaks in bagolini
Placer NDF filter in front of strabismic eye
- increase filter until ARC adaptation breaks down
- line may disappear I.e be suppressed
- or may get diplopia - 2 lights
Modified mallet unit OXO
Maintains a stimulus to binocular fixation
Distance unit at 1.5m viewing distance
Large OXO on newer mallet units
In line with x top and bottom = HARC
Only 1 line at bottom = supp
Two oxo side by side one with line under one with top = NRC + diplopia
OXOOXO - line top and bottom of x = UARC
Measure depth with filter bar slightly to bagolini
What’s the issue with the modified mallet unit?
Slight problem = reduced illumination due to Polaroid filters is greater dissociation than occurs with bagolini - may miss some ARC
What is the worth’s four dot test?
Px wears red and green goggles
1 red, 2 green and 1 white dots
Only. Check for gross stereopsis and can produce many false positive/negative results
Red filter on right eye
Green filter on left
Px sees 4 lights = normal fusion
If px sees 2 red lights = left suppression
If pc sees 3 green lights = right suppression
If px sees 2 red lights and 3 green lights = diplopia
Green and red lights alternating = alternating suppression
What are the potential problems with the worths four dot test
It disrupts normal binocular difference due to the different colours producing rivalry between 2 eyes
Decrease in light level as well
A suppression area may fall within the centre of the test and miss all the dots
What is the definition of eccentric fixation?
When a non foveal point is used for fixation under monocular viewing conditions
Common feature of amblyopia
Usually only occurs in testing room as px usual fixates with other eye
When do you test for eccentric fixation
Only look for it when you have reduced VA
If the angle of EF matches angle of deviation what do you see on the CT
No movement is detected on cover test
How do we investigate eccentric fixation
- project target on retina
- visible by both px and prac
- px is asked to loom at centre of target and position of fovea is noted
- position is then recorded in diagram and also record - steady/unsteady, usually EF is slightly nasal in SOT
- can calibrate using the size of the optic disc
In amblyopia what is the rough guide for how much reduced the VA is
In amblyopia – reduced VA by one snellen line per 0.5 degree of eccentricity (very rough guide)
What are the other ways of investigating EF
- past pointing
- after-image
- perimeter - blind spot, ambler grid
- corneal reflex
- entropic phenomenon
How do we investigate EF with past pointing
Related to egocentric localisation
- good eye first - checks normal ability amid increases confidence
- occlude amblyopia eye, hold pen 25cm from px and as px to touch pen with tip of their finger
- repeat with normal eye occluded
If finger goes few cms to side then past pointing demonstrated
- do not repeat as px can adapt
- result indicates fixation does not coincide with the centre of localisation
How we investigate EF with corneal reflex
- compare reflex position in each eye in turn with the other eye occluded
- relative displacement of the reflex by 1mm= approx 11 degrees or 20PD
- eccentricity is not usually this great however making EF difficult to detect by this method
How can we investigate eccentric fixation with perimetry?
Normal subjects blind spots = same angular distance from fixation in both eyes - depends on rx
Should be 15 degrees +/-3degrees
Plot blind spots carefully in both eyes and compare positions
Degree of eccentricity can be measured by the difference in angular distance of blind spot in each eye
Required exceptional px cooperation and steady fixation
How can we investigate eccentric fixation with amsler grid
Amblyopia may have a small foveal scotoma which shows up as disturbance on amulet
Occurs centrally if central localisation
Eccentricity if EF
Not very convincing test
How do we investigate eccentric fixation using the after image test
After images are transferred to normally corresponding points in the eye
E.g. photography flashgun is masked to provide a very bright strip of light
Occlude amblyopia eye and px fixates the centre of the strip
Flash then produces a central after image
How do we record EF
- centricity of fixation (central vs eccentric) - if eccentric record whether it is temp nasal inf or sup
- magnitude - size
- quality of fixation (steady/unsteady/wandering)
- pattern of fixation (drifts,saccades, nystagmus)
- percent foveation
- Directional bias (nasal, temporal)
- subjective localisation of primary visual direction
What is the treatment for eccentric fixation?
- in amblyopia have to encourage foveal fixation - patch
- direct occlusion alone may improve fixation but often a slight eccentricity remains
- after image transfer - use to locate foveal fixation
- pleoptic treatment desensitises eccentrically fixing area
Established EF hard to remove
What is a microtropia?
Manifest deviation
Definition: a misalignment of the eyes with an angle of deviation so small (less than 10 prism dioptres) that is usually assumed to also be fully adapted - (to further improve binocularity)
What types of microtropia are there
Primary microtropia:
- unknown aetiology
- maybe genetic element
Secondary micrtropia:
- residual deviation after optical or surgical treatment of larger strab
- more prevalent
When does microtropia usually occur
Happens early in life - don’t find till later as infant unreliable in tests
Frequently present between 2-3yrs
May find on routine check as VA is slightly low - may even appear as phoria on CT
Made evident by crowding phenomenon
Exo is rare usually eso
Microtropia with identity
Angle of EF = habitual angle of deviation
- no movement when dominant eye is covered on CT
- need very subtle tests to diagnose
Microtropia without identity
Angle of EF not equal to angle of deviation
- small movement when dominant eye is covered on CT Made evident
- may not actually have EF but usually inaccurate
What does microtropia feel like for px
Usually asymptomatic
And often no movement seen on CT
so how can we find out of something is wrong?
Visual acuity: microtropia
Presence of amblyopia in one eye is usually the first clue to microtropia
- likely to still get 6/6 to 6/12
What is crowding phenomenon
Crowding phenomenon is present with amblyopia and microtropia
Letters may be missed due to central (suppression) scotoma - they may be doing more well on the letters on the outside/edges over a few lines
Usually also have ~1.50D of anisometropia - but can have equal rx in each eye
What is the 4 dioptre prism test
- 4 base out placed before dominant eye
- img moves across retina and the eye moves to take up fixation
- non dom eye moves latrerally in same direction as its not fixating - versional movement is seen
- prism is then removed and a recovery versional movement is seen
- prism then placed before amblyopia eye
- image moved across retina within suppression are: no movement of either eye
What confirms the presence of microtropia
Amblyopia + no CT movement + positive 4prism dioptre test = microtropia
To be able to describe somebody as microtropia:
- small angle <10PD
- amblyopia
- eccentric fixation
- HAC on bagolini or modified mallet unit
How do you manage a microtropia
- refractive error correction - esp in high anisometropia
- aniseikonia may be a problem - contact lens correction may help
- refer to treat underlying amblyopia by occlusion of non squinting eye (if px <6yrs old)
What treatment options are not appropriate for microtropia
Orthoptics not app
Surgery not app
In px >6yrs - correct refractive error otherwise do not treat the mt
Most cases correction of refractive error if necessary is only profitable action
What treatment options are not appropriate for microtropia
Orthoptics not app
Surgery not app
In px >6yrs - correct refractive error otherwise do not treat the mt
Most cases correction of refractive error if necessary is only profitable action
