Heterophoria vs Heterotropia

• Heterophoria:

  • The deviation of the visual axes when fusion is disrupted (under artificial conditions NOT naturally, such as cover test)​
  • ​About the balance b/w eyes​ rather than a defect of the right or left eye - caused by binocular vision (CANNOT HAVE A TROPIA AS A RESULT)
  • Present in most patients - exophoria common in myopes

  *Eye moves outwards when cover taken off= esophoria

  *Eye moves inwards when cover taken off= exophoria

  *Eye moves upwards when cover taken off= hypophoria

  *Eye moves downwards when cover taken off= hyperphoria
• Heterotropia (Strabismus):

  • deviation of visual axes that is manifest even when strong fusional  stimuli are present (under natural conditions) - abnormality / absence of true binocular  vision​
  • Only one eye images the object of interest onto the  fovea - other eye permanently points in another direction (peripheral retina)
  • When one eye turns, two different images are sent to the brain - brain learns to ignore and sees only the image from the straight/better eye.

  ! only affects 3% of the population
• Heterophoria (Strabismus):

  • In children only = abnormal retinal correspondence (aligns the fovea of ‘good’ eye w/ a non-foveal point in strabismic eye) and amblyopia (lazy eye)
  • In adults = diplopia b/c brain is already trained to receive images from both eyes + cannot ignore the image from the turned eye

  *Eye moves outwards during cover of alternate eye = esotropia

  *Eye moves inwards = exotropia

  *Eye moves upwards = hypotropia

  *Eye moves downwards = hypertropia

  ! px cannot have a phoria if tropia present - normal BV is impossible, single vision only
• What is Binocular Vision?

  • The ability to maintain visual focus on an  object with both eyes, creating a single  visual image, dependent on:

  *Motor functions (6 extraocular muscles enable eye to move)​

  *Sensory input (muscles are supplied by 3 cranial nerves which tell eyes how to move)​

  *Anatomical structure (ensures eyes move in coordination)

  • Fusion is only possible if both eyes can see​ the same objects - similar images are falling  onto the two retinas.​
• The Cover Test:

  • Place occluder before one eye for 2–3 seconds (enough to disrupt fusion), transfer it quickly to the other eye

  *px must not view target binocularly at any time

  • Heterotropia = on covering the fixating eye  the deviated eye moves to take up fixation

  *only move onto next step if no tropia

  • Heterophoria = no movement on covering either eye (but  movement on eye being uncovered)​

  !Tropia only specific to one eye, phoria specific to both eyes

  !Distance fixating target = 1 line above V/A or spotlight for poor vision

  !Near fixating target = budgie stick letters or spotlight
• Heterophoria Subjective Tests:

  • Objective tests = cover/uncover test

  *insensitive to small eye movements below 2▲

  • Subjective tests will usually be performed in an eye exam only if the cover test indicates a large phoria/poor fusional recovery /significant change in refractive correction (and therefore a  possible change in phoria)​

  *Maddox rod and wing usually done after corrective refraction found, with Rx in place

  *Recovery speed = how quickly eye recovers when it is uncovered (quickly means that the phoria is well compensated and unlikely to cause symptoms)
• Normal Levels of Heterophoria:

  • Orthophoria = none
  • Distance = 2 PRISM DIOPTRES of orthophoria​ (none)

  *this could mean 1 prism D in each direction

  *not affected by age - stays constant

  • Near in a youth = 1±2 PRISM DIOPTRES exophoria

  *due to eyes having to converge when reading 

  • Near exophoria increases with age = 1prism dioptre/decade​

  *near phoria is more common than distance phoria - as we read at near, eyes converge, pupils constrict and lens accommodates​ (ability to accommodate decreases with age) 

  • Vertical phorias = 0.5 PRISM DIOPTRES

  ! can still cause symptoms even if normal
• Differences b/w Subjective and Objective tests:

  • Abnormal retinal correspondence (ARC) is a sensory adaptation to  strabismus and aligns the fovea of the ‘good’ eye with a non-foveal point in  the strabismic eye – very rare, occurs in childhood strabismus ​

  *eyes have adjusted so that fovea in strabismic eye is not in the right place – it has moved to help them

  *px may experience no diplopia and 6/6 VA as a result

  • In a patient with strabismus and ARC, a subjective test such as the Maddox  or von Graefe can indicate a different deviation from that found by the cover  test​
• Other Methods of Measuring Heterophoria:

  • Thorington test​
  • Von Graefe technique (prism dissociation)​
  • Synaptophore​