Amblyopia

Question 1

What is Amblyopia?

Answer 1

• A reduction in vision in one or both eyes, persistent after correction of refractive error
• Absence of retinal pathology or any disease of the afferent visual pathway
• Most common cause of vision loss in children, interruption of normal visual development

Question 2

Mechanism of Amblyopia

Answer 2

• Deprivation of form vision
  
  • Can be partial or complete
  • Complete - No image/stimulus reaches fovea
  • Partial - Degraded imaging reaching fovea
• Abnormal binocular interaction
  
  • Incomplete images fall on the retina
  • Images are incompatible
  • Eyes compete for control over cortical connections during development period - better eye gains control

Question 3

Signs of Amblyopia

Answer 3

• Reduced Snellen and grating acuity
• Loss of contrast sensitivity
• Shape distortion
• Motion deficits
• Crowding effect

Question 4

Visual Function in Amblyopia

Answer 4

• Light sense - ability to distinguish light and dark (rods)
• Form sense - ability to distinguish between spatially separate visual stimulii, ability to discern size and shape of objects - positon/orientation (rods and cones)
• Colour sense - distinguish between light of different wavelengths (cones)
• Motion sense - ability to detect movement of images across retina (visual cortex)

Question 5

Periods of Visual Development _ Critical Period

Answer 5

• A few months old, deprivation causes damage
• Period with active neural plasticity (ability of the neural system to undergo change)
• Period where deprivation impacts visual development and amblyopia can develop
• Amblyopia can only develop within this time
• Earlier the onset of deficit and the longer the period of deprivation - worse the outcome

Question 6

Periods of Visual Development - Sensitive Period

Answer 6

• Deprivation in teenage yrs, amblyopia less likely to occur but improvement possible
• Improvement is possible during this time
• The younger the px the quicker the response to treatment
• Less common after 8 yrs of age

Question 7

Effect of Strabismus

Answer 7

• Abnormal visual cortex may be responsible for loss of binocular stereoscopic vision
• Alternating strabismus results in equal number of cells for right and left and virtually no binocularly driven cells

Question 8

Investigations for Amblyopia

Answer 8

• H&S - FH of childhood eye problems!
• Ocular examination - assess media and fundus
• Full cycloplegic refraction
• VA Assessment - age and ability, appropriate test selection
• Cover test - is there a manifest deviation? What is the fixation preference/pattern
• Contrast sensitivity - very useful, amblyopes can have reduced contrast sensitivity at higher spatial frequencies in comparison to non-amblyopes
• Uniocular fixation
• 4^ test

Question 9

Assessment of Uniocular Fixation

Answer 9

• Assess the point of the retina that the px is using for fixation when the other eye is occluded
• Using visuscope or ophthalmoscope

Question 10

Assessment of Uniocular Fixation - Method

Answer 10

• Dim room lights
• Ask px to fixate at distance
• Occlude “fellow normal eye”
• Line up instrument
• Get px to fix on centre of light
• Assess where the “bright” reflex is positioned

Question 11

Uniocular Fixation - Findings

Answer 11

• Central fixation - object on fovea
• Eccentric fixation
  
  • Uniocular condition
  • Fixation is by a point which isn’t the fovea
  • This point is the principle visual direction
  • The degree of eccentric fixation is defined by the distance between the fovea in degrees
  • The further from the fovea - worse the level of VA
  • Estimated line reduces by 1 line per 0.5 degrees of eccentricity
• Wandering fixation - uniocular condition, fovea has lost its fixation superiority and no single area of the retina is used for fixation

Question 12

Strabismic Amblyopia

Answer 12

• Result of constant or near-constant childhood strabismus in one eye
• Mostly esotropias as many exotropia’s are intermittent in childhood

Question 13

Strabismic Amblyopia - Clinical Characteristics

Answer 13

• Reduced vision in one eye
• Strabismus found on CT - usually not freely alternating
• No pathology detected on ocular examination
• Risk is 4x greater if one parent has strabismus

Question 14

Anisometropic Amblyopia

Answer 14

• Significant anisometropia present (At least 1D difference)
• Clearer vision in one eye for all distances
• Can be mostly spherical or mostly astigmatic
• Hypermetropia - most common
• Meridional (astigmatism) - Oblique astigmatism: more likely myopic
• Myopia - Can be avoided if one eye clear for distance and one eye clear for near

Question 15

Stimulus Deprivation Amblyopia

Answer 15

• Stimulus form vision deprivation amblyopia
• One or both eyes
• Little or no light enters the eye

Question 16

Stimulus Deprivation Cataract - Congenital Cataract

Answer 16

• Most common
• Ptosis
• Haemangioma
• Vitreous Opacity e.g. bleeding
• Corneal Scar

Question 17

Meridional Amblyopia

Answer 17

• Moderate-high degree of uncorrected astigmatism
• Can be bilateral or unilateral
• More significant risk in oblique astigmatism

Question 18

Ametropic Amblyopia

Answer 18

• Likely bilateral
• High degree of bilateral refractive error goes uncorrected during critical period
• Blurred vision in BE at all distances
• Typically, a result of high bilateral hypermetropia 6D or greater (cannot be compensated using accommodation)

Question 19

Organic Amblyopia - Reversible

Answer 19

• Toxic amblyopia - not always reversible

Question 20

Organic Amblyopia - Irreversible

Answer 20

• Not able to be treated, no lesion
• Nystagmus
• Albinism (usually associated with nystagmus)

Question 21

Toxic Amblyopia

Answer 21

• Painless, progressive, bilateral vision loss
• Dyschromatopsia
• May also be referred to as “toxic optic neuropathy”

Question 22

Toxic Amblyopia - Nutritional Amblyopia

Answer 22

• Vitamin B12 deficiency
• Seen in px’s with extreme diets - reports in px’s with ASD
• May see complete/incomplete recovery with improved diet/vitamin intake

Question 23

Toxic Amblyopia - Other Causes

Answer 23

• Alcohol - may be associated with B12 deficiency
• Tobacco
• Antimalarials (e.g. Chloroquine)
• Anticancer treatments (e.g. Vincristine)

Question 24

Investigations for Amblyopia

Question 25

Management of Amblyopia - Refractive Adaptation

Answer 25

- Px's are prescribed their full refractive correction for FT wear - Correction of refractive error results in resolution of anisometropic amblyopia in 1/3 of 3-7 yr olds - Refraction adaptation mostly complete by 18 weeks - Improvement can continue for up to 30 weeks

Question 26

Management of Amblyopia - Occlusion Treatment

Answer 26

- Occlude the non-amblyopic eye to encourage used of amblyopic eye which stimulates visual development

Question 27

Types of Occlusion Treatment

Answer 27

- Total occlusion - excludes light and form vision (e.g. sticky patch or fabric patch) - Total occlusion - excluded form vision, allows some light passage e.g., blenderm tape) - Partial occlusion - some form vision but reduces acuity (e.g. Bangerter foils)

Question 28

How many hrs for Occlusion Treatment

Answer 28

- Moderate amblyopia (0.300-0.600) - 2-6 hrs when combined with 1 hr of NV activities in children under 7 yrs - Begin with 2 hrs and if no significant improvement, increase to 6 hrs - Severe amblyopia - FT occlusion (all waking hrs, or all but one) - Part time occlusion - set hrs per day - In px's with strabismic, anisometropic and combined - results of 6 hrs of occlusion, FT in children ages 3-7 yrs