Microtropia

Question 1

What is microtropia?

Answer 1

• Small angle <10^ with ARC
• Common
• Stable
• Anisometropia – usually 1.50DS or more – of at least 1.00D difference
  o Blur at fovea of one eye – focus with clarity of eye – minute start to develop blur at back of fovea – they either ignore the image or they move slightly where the blur is not noticed (and then get a microtropia)
• Always mildly amblyopic – one line at least (will not be 4 or 5 lines difference but 1 eye 6/5 & other 6/9 for e.g.)
• Central suppression
• Good but not perfect BSV
  o Will not get 40” but may get 70”
• Strong motor fusion
  o See cross on bagolini – will be able to maintain this cross on prisms & movement?
• Differs from classic ARC because associated with eccentric fixation
  o They are always fixing from new fovea they found
• Not possible to treat but may prevent successful orthoptic exercises
  o Have to accept that they will never have equal VA
  o If anisometropic & don’t do all investigations – could patch for ages & nothing happens

Question 2

What is the classification of microtropia?

Answer 2

• Classic microtropia w/o identity – small manifest angle
• Microtropia w/ identity – ARC & eccentric fixation at same retinal point, so no movement on cover test
  o Only diagnosed with fixation ophthalmoscope
• Microtropia associated w/ larger latent component
  o Increases on alternate CT or if one eye covered longer than usual (AKA monofixational syndrome)
• Microtropia associate w/ other intermittent esotropia e.g. fully accomm
  o A primary microtropia which becomes decompensated particularly between 1-3yrs as result of accomm element or superimposed phoria
• Divergent microtropia
• Secondary – follows optical or surgical correction

Question 3

What are the features of classic microtropia WITHOUTidentity?

Answer 3

• 1. Small manifest deviation
     o Do CT – right SOT small angled – cover LE and RE moves to take up fixation – only happen if they are WITHOUT IDENTITY
     o They diagnose themselves on CT – micro strabismus as tiny flick of movement
     o If told it is very small and there is a manifest movement – being told they have microtropia
     o They have amblyopia
• 2. Less than 10^
• 3. Mostly esotropia but can be exotropia

Question 4

What are the features of classic microtropia WITHOUT identity with latent component

Answer 4

• 1. Manifest deviation will increase on continued dissociation
     o More start to do alternate CT – start to elicit bigger angle
     o Either have manifest or latent – unless microtropia – only a microtropia can have latent component too
• 2. Must measure manifest component with simulated PCT
     o Rather than fully dissociating px then bringing prisms up – bring prism & cover test up at same time – only measuring small manifest
• 3. Measure latent component with full PCT

Question 5

What are the features of microtropia WITH identity?

Answer 5

• 1. ARC & eccentric fixation at same retinal point
• 2. No movement on cover test
     o No manifest deviation
     o New fovea – P point – they are looking with new fovea – straight in pxs mind – not functioning normally though as not looking bifoveal – slightly amblyopic, anisometropic – but NO MOVEMENT ON CT
• 3. Only diagnosed with fixation ophthalmoscope and 4^ prism test
     o Only way to diagnose to see where they are fixing – 4^ used to look for central suppression scotoma

Question 6

What are the features of microtropia associated with other intermittent esotropia e.g. full accomm?

Answer 6

• 1. Anisometropic Fully Accomm Esotropia
• 2. Small manifest esotropia when hyperopia corrected
     o They have small microtropia with glasses on
     o They MUST be anisometropic
• 3. ARC instead of NRC
• Rare

Question 7

What are the investigations into microtropia?

Answer 7

• Visual acuity
  o Are they amblyopic or not?
• Crowding
  o Px sees outside two letters on a line but not the middle letters
  o CHART MUST BE LOGMAR – not picked up on Snellen
• Fixation: ophthalmoscope, visuscope
• Cover test may or may not be strabismic movement
• 4^ Prism test – MUST be done on anyone with microtropia
• Bagolini lens with or with suppression gap – px don’t often report gap
• Amsler chart – scotoma may be seen – if scotoma is central (often small)

Question 8

What three factors together make you suspect microtropia WITH identity?

Answer 8

Amblyopia + No CT movement + Positive 4^ test

Question 9

What are the features of microtropia WITH identity?

Answer 9

• No movement on CT
• Harmonious ARC
• Subjective angle = 0
• Absolute eccentric fixation
• Angle of anomaly = angle of eccentricity
• Visuscopy – stable parafoveal fixation

Question 10

What are the features of microtropia WITHOUT identity?

Answer 10

• Movement seen on CT – manifest component MUST be <10^ for it to microtropia - small flick on CT seen
• May have:
  o Central fixation with ARC
  o Unharmonious ARC
  o Central fixation with NRC, central suppression and peripheral fusion – extended Panum’s area
• Visuscopy – unstable

Question 11

Describe ophthalmoscope methods of viewing fixation?

Answer 11

• Target is projected & focussed onto retina & is seen by both px & practitioner
  o Use graticule setting on ophthalmoscope
  o Put green filter on – to find fluorescence of fovea
  o Fixate on centre circle of graticule
• Px is asked to look at centre of target & position of fovea is noted
  o Should see twinkly light of fovea in centre of middle circle if fixing with fovea
• MUST BE DONE MONOCULARLY
• Position is then recorded in diagram – also record if steady/unsteady
  o Usually EF is slightly nasal in SOT
  o Can calibrate using size of optic disc in graticule Disc = 5 deg x 7 deg
• NB: accomm is usually induced using this method – change focus or cycloplegia
• Do visuscope/fixation ophthalmoscope on 50^ esotropia – dense amblyopia – when cover eye need to know where they are fixing – does the fixation point match the 6/60 vision – the angle of squint stays same but can move fixation point

Question 12

Describe eccentric fixation in microtropia?

Answer 12

• Microtropia with identity
• When cover LE, RE wont move as fixing from new fovea – eye is straight as far as px is concerned
• When use fixation ophthalmoscope – will see they are not fixing centrally
• Need all these tests to give complete diagnosis
• Do 4^ prism test – determines if someone is bifoveal or not
• Must be done on any suspect microtropia
• Central suppression – between old fovea and new fovea
• 4^ prism test – don’t see movement on CT – need to know if there is a suppression area

Question 13

Describe 4^ in normal binocular single vision?

Answer 13

• If conjugate gaze – when LE moves in – RE moves out – but then since diplopia – RE then turns in to regain binocularity
• Normal – adduction of eye under prism – abduction of other eye

Question 14

Describe 4^ in central suppression?

Answer 14

• LE doesn’t know anything has happened as it is falling on suppressed retina – brain doesn’t know image has moved
• Microtropia in LE
• But if prism is put in front of RE – RE adducts – the LE abducts due to conjugate gaze – but the LE doesn’t come back in as no incentive to move back in due to microtropia – not wired up normally
• Left microtropia with identity
• Movement ignored if prism in front of LE

Question 15

What is the treatment for microtropia?

Answer 15

• Prescribe full cycloplegic Rx – allow 16-18wks refractive adaptation (before starting occlusion)
• Aniseikonia is often a problem in high degrees of anisometropia
  o But pxs wont have massive difference
• Treat underlying amblyopia by occlusion of non-strabismic eye – regular review
  o If still amblyopia after spectacle correction – then occlusion and know they will never have equal vision – would accept 1 line difference between the eyes
• Surgery is not appropriate

Question 16

What is investigation of suppression trying to determine? What does 4^ prism find?

Answer 16

• Investigation of is mainly concerned with determining – depth and extent of retinal area
• Finding small central suppression with 4^ prism

Question 17

What is the Sbisa Bar method?

Answer 17

• Measures density of suppression
  o How deep is it and can we overcome it (don’t want to)
• Px fixates spot of light
  o Ask them to tell you what colour the light is – they see one light because of suppression
  o Ask them to tell you to see the white light on one side and red light on other side – white light from squinting eye – you have given them diplopia
• A filter bar (palest filter) is placed before non-suppressing (fixing) eye & density of filter is increased until 2 lights are seen
• 17 filters – pale pink to really dark red (almost black)
  o If by filter 5 they see a white light and pink light – then worried as they could have diplopia in free space
  o If they can get down to filter 17 then this is better and can keep patching as unlikely to give them dip
• This filter is a measure of depth
• Occasionally when end point is reached fixation swaps to the other eye & suppression swaps over, resulting in no diplopia
• If there is no strabismus use red filters or red/green diplopia goggles
• NOTE:
  o Most important in gauging risk of intractable diplopia when considering occlusion in older children

Question 18

When do you always need to measure suppression?

Answer 18

Anytime px is over 5yrs and squinting then need to measure their suppression

Question 19

Describe density of suppression?

Answer 19

• Suppression is not equally deep in all pxs
  o In some it may be readily overcome; in others it is difficult to do so
  o It is useful & easy to establish how deep the suppression is in a px
• To make a px aware of images perceived by deviated eye, one must reduce retinal illuminance of the fixating eye until the px sees double
• This is best done with a series of red filters of increasing density arranged in form of a ladder (Spisa Bar)
• The px fixates a small light source, & filters are placed in front of fixating eye
  o Some pxs see double w/ a single layer; other require 3 or more layers before they recognise diplopia
• The greater the number of layers needed, the deeper the suppression

Question 20

Describe area of suppression?

Answer 20

• Px fixates spotlight with dominant eye & prism bar is moved before the suppressing eye – starting at 1^
• When image of spot has moved off the suppression scotoma the px reports diplopia
• The difference between the angle of strabismus (from CT) & the prism which moves spot out of scotoma = angular extent of the scotoma
• This can be used to measure vertically as well

Question 21

Describe rea of suppression in 40^ esotropia?

Answer 21

• Px has 40^ SOT && they want squint surgery
• Start at 0 on prism bar – how many of lights or budgies can px see
• Single because of suppression not because they are fusing
• See 2 at 25^ - that’s how much suppression they have
• Only correct how much suppression they have not full angle
• If make them straight and correct whole 40^ then will get diplopia as do not have suppression the whole angle of deviation
• Post-op diplopia test – done on everyone >6yrs who is a candidate for squint surgery

Question 22

What is the evaluation of suppression?

Answer 22

• The deeper the suppression & greater the extent across the retina – the more difficult it will be to treat
• Factors such as px’s age, duration of strabismus, co-operation etc must be taken into account
• Only treat amblyopia if suppression is dense & px is not at risk of intractable diplopia
• When planning surgery in adults or older children you must carry out a measure of area of suppression or Post Operative Diplopia Tests
• NB: suppression is an adaptation to prevent diplopia & confusion

Question 23

What is important to note about suppression?

Answer 23

If appreciate diplopia and under 8 yrs then will have ability to suppress
Do not want to remove that as will get symptoms
If do not have suppression will either have NRC or Arc
Suppression is there all time
Alternating suppression in alternating SOT - Px can switch between the two
Convergence SOT – suppress at near as squinting – but wired to have NRC in distance

Question 24

What is physiological suppression vs pathological suppression?

Answer 24

Physiological suppression – everyone suppress periphery so not overwhelmed
Pathological suppression – suppress one eye to stop diplopia