MICROTROPIA - ESO OR EXO

Question 1

What is Panum’s fusional space ?

Answer 1

• If retinal elements stimulated are almost corresponding, the object will still be seen singly as long as it lies within Panum’s fusional space.
• Small area around the horopter where single vision is still present.
• Space is narrower centrally than peripherally.
• Diplopia outside the horopter and Panum’s fusional space.
• Corresponding part to Panum’s fusional space on the retina is Panum’s fusional area.

Question 2

What is ARC?

Answer 2

• Adaptation to manifest strabismus – allows BSV (lower quality) – normally ET
• Fovea of one eye has same visual direction as extrafoveal area of the other eye (pseudo-fovea)
• Fovea of deviating eye will project as temporal retina
• Harmonious: Subjective angle < objective angle (Subjective angle = 0 : they think their eye is straight, move lion into cage)
• Unharmonious (click): Subjective angle < objective angle (Subjective angle > 0 : Know that eye is not straight, would move arm slightly but not as much as us)

Question 3

What is microtropia?

Answer 3

• Small angle unilateral strabismus with BSV
• Manifest component no greater than 10^
• Most common form of ABSV

Question 4

Characteristics of microtropia?

Answer 4

• Without/without identity (With identity = NO movement seen on cover test , without identity = movement seen on cover test)
• Anisometropia (+ ± cyl) !!!
• Amblyopia – VA worse with affected eye
• Small manifest deviation (spct = 10^ or less, pct maybe slightly larger)
• Heterophoria – large underlying latent component (SPCT to measure)
• Central suppression Scotoma in affected eye – using pannums fusional space (otherwise would have dip) therefore Parafoveal Fixation or Eccentric Fixation
• ABSV = Reduced or absent stereopsis + reduced Motor fusion
• May be primary or residual
• May be associated with convergence excess/fully accommodation esotropia
• Majority ET (vertical rare) (Cob et al (2002) 37 cases of microtropia: 3 micro-exotropia + 34 micro-esotropia)

Question 5

Investigations in microtropia?

Answer 5

• Refraction = Hypermetropia/Anisometropia
• Fundus and Media = healthy
• Case history
• VA = amblyopia
• CT = Small tropia with latent component or latent only
• PCT - <10^ / > 10^
• SPCT- simultaneous = < 10 ^
• 4^ - abnormal in affected eye
• Fixation – parafoveal = visuscope or Polaroid 4 dot test
• Sensory test: bagolini gls or worths lights – will see gap in cross
• PFR = normal or reduced - ABSV
• Stereopsis = normal or reduced - ABSV

Question 6

What does a microtropia with identity compared to without identity have?

Answer 6

• 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
• Central fixation with ARC
• Unharmonious ARC
• Central fixation with NRC, central suppression and peripheral fusion – extended Panum’s area
• Visuscopy - unstable

Question 7

What are Lang’s classifications of microtropias?

Answer 7

• Primary- Microtropia initial defect
• Primary decompensated - Microtropia has increased in size
  o Uncorrected +
  o High AC/A
• Secondary - Originally large angle tropia - Angle reduced to microtropia
  o Surgery
  o Exercises
  o Optical treatment

Question 8

Management of microtropia?

Answer 8

• No active treatment for microstrabismus unless other strabismus coexists
• Next best thing to bifoveal BSV
• Aim to maintain best possible VA
  1. Refractive error
  2. Treat amblyopia (Ansons and Davis – generall equal VA doesn’t occur post treatment, Check fixation at intervals, Sbisa bar)
  3. Treat associated strabismus
  4. Occlude fellow eye – has been shown to promote recovery to normal foveation in some patients (henshall + rowe 1999)

Question 9

What is significance of microtropia diagnosis?

Answer 9

• Explains reduced VA
• May alter prognosis for improvement in VA
• Reduces level of BSV achievable
• Anomalous BSV with small angle good prognosis for long term stability
• Surgical outcome may be revised
• Longterm – reading ability speed (Stifter et al. 2005) slightly reduced compared to control -