A PRACTICAL APPROACH TO STRABISMUS

     John Grigg
     Senior Lecturer
     Department of Ophthalmology, University of Sydney
     johng@eye.usyd.edu.au
 
Oculomotor System
 
  • What is the Function?
 
Oculomotor System
 
  • Function

    - to maintain the object of interest on the fovea

 
Oculomotor System
 
  • What are the components?
 
Oculomotor System - Components
 
  • Extra-ocular muscles
  • cranial nerves III, IV, VI
  • brainstem nuclei
  • brainstem gaze centres (horizontal and vertical)
  • cortical movement centres
  • vestibulo-ocular system
  • sensory input (visual pathways)
 
Binocular Vision - Aim is Single Vision
 
  • Motor fusion
    - align eyes to allow sensory fusion
  • Sensory fusion
    - unification of visual excitations from corresponding retinal images into a single visual image
 
Common Visual Direction
 

    Hold pen top about 15cm in front of face then line up pen top with x in red bar with right eye and then x in green circle with left eye. Then fix on pen top and all 3 objects will line up.

 
Definition of Strabismus
     
  • When the two visual axes are not directed towards the object of attention
 
Strabismus Definitions
     
  • Version
    - movement of both eyes in the one direction
  • Vergence
    - movement of the two eyes not in a parallel manner
  • Fusion
    - image formed on each retina is perceived as one image
Strabismus Definitions
     
  • Heterophoria
    - Latent strabismus, tendency for the eyes to turn away from the fusing position
    - esophoria (EP), exophoria (XP)
  • Heterotropia
    - Manifest strabismus, one eye is not directed towards the object of attention
    - esotropia (ET), exotropia (XT)
Information required for exam
     
  • bifoveal fusion
  • fusional vergences
  • nature and degree of deviations
  • Measurements
    - if tropia
    - characteristics of suppression scotoma
Essential Information to enable accurate interpretation of ocular alignment
     
  • cycloplegic retinoscopy
  • refractive error corrected
  • accommodative target distant and near
Strabismus examination
 
  • Factors to be controlled

    - Fixation
    - Accommodation
    - Fusional vergences

 

  

Near Fixation
 

 
Strabismus examination
 
  • Suspect

    - if abnormal head posture
    - if closing one eye

 

  

 
Principle Actions of the Extraocular Muscles
 
 
Cover / Uncover Test
 
  • A cover is held over the right eye for several seconds so that the right eye can assume its normal position of rest independent of binocular influences
 
Right Exotropia
 
 
Cover / uncover test to diagnose RXP
 
     

 
Prism cover test for right exotropia
 
     

  • 10 dioptre prism  

     

     

  • 20 dioptre prism  

     

     

  • When squint neutralised no movement on cover
 

  

 
Hirschberg Method
 
 
Grading lateral rotation of the right eye
 
 
Sensory system tests
 
  • Inestimable importance in squint
  • Valid Inferences
    - age of onset
    - type of squint
    - refraction correction
    - motor examination
 
Strabismus
 
  • Aetiology
    - Congenital
    - Paralytic
    - Secondary
    - unilateral visual disturbance
       • eg cataract, retinoblastoma
    - ocular muscle or orbital tissue disorders
       • eg trauma, thyroid eye disease
    - Accommodative
    - Consecutive
 
Strabismus
 
  • Aim of treatment
  • good vision in each eye
  • no cosmetic defect
 
Infant Examination
 
  • test quality of vision
  • refraction and examine fundus use cycloplegia
 
Sensory / Suppression Inference
 
  • early onset tropia
    - binocular suppression scotoma
  • early onset exotropia
    - temporal hemiretinal suppression scotoma
  • early onset esotropia
    - regional nasal hemiretinal suppression
 
Suppression Patterns
 
 
Strabismus Treatment
 
     

  • Glasses  

  • Occlusion  

  • (exercises)  

  • Surgery
 

  
Neuronal Basis of Amblyopia
 
  • Development of normal vision in each eye depends on each eye experiencing sharply focused images
  • During the critical period of early life this is genetically programmed
 
Critical Period
 
  • Is in early childhood

    - most sensitive 3 months to 3 1/2 years
    - less sensitive 3 1/2 years to 10 years

 
Environmental Influence on Neural Connections
 
  • Plasticity in developing geniculocortical connections "Critical Period " - Blakemore
  • in developing anatomical structure, visual path present in utero wave of maturation sweeps from eye to visual cortex correct formation of connections depends on
    - precisely timed interactions between axons and their targets
    - competition between axons
    - cell death
    - axon withdrawal
    - trophic interactions
    - other mechanisms
  • lay down basic "maps" of visual field before birth
 
Critical Period
 
  • synaptic plasticity continues after birth on basis of information from visual environment permitting cortical neurones to refine their processing capacities (synapses)
  • plasticity dependent on input from noradrenergic neurones and subject to pharmacological manipulation
  • sensitivity to deprivation
    - maximum 6 - 9 months
    - continuing to 8 years
    - critical period
        • 4 months to 3 1/2 years and to 8 - 10 years
 
Causes of Amblyopia
 
    Unilateral Bilateral
    manifest strabismus nystagmus
    anisometropia uncorrected hypermetropia
    visual deprivation visual deprivation