Week 2 - Amblyopia Flashcards
What is amblyopia?
• 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.
Mechanisms of amblyopia
- 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
What does amblyopia look like for the Px?
• Reduced Snellen and grating acuity
• Loss of contrast sensitivity
• Shape distortion
• Motion deficits
• Crowding effect
What four senses are needed for visual function?
- Light sense
- Form sense
- Colour sense
- Motion sense
Describe 1. light sense and 2. form sense
- Light sense
• Most primitive
• Ability to distinguish light and dark
• Rods - Form sense
• Ability to distinguish between spatially
separate visual stimuli
• Ability to discern size and shape of objects
• Position and orientation
• Rods and cones
• Most acute at fovea
Describe 1. Colour sense and 2. Motion sense
- Colour sense
• Distinguish between light of different wavelengths
• Cones - Motion sense
• Ability to detect movement of images across retina
• Visual cortex
What are the periods of visual development?
• Critical Period: A few months old - approximately 5 years old. Deprivation causes damage
• Sensitive Period: Time of deprivation- teenager years*
Amblyopia less likely to occur but improvement possible
*Some evidence in adult cases too
Describe critical period
• 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 + the longer the period of deprivation= worse the outcome.
Describe sensitive period
• Improvement is possible during this time.
• The younger the patient= the quicker the response to treatment.
• Less common after 8 years of age.
strabismus and the brain?
• Abnormally high proportion of monocular cells the visual cortex where there should be binocular cells.
•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.
• Acuity responses in convergent eyes of monkeys reported to be reduced at the retinal ganglion cell (RGC) layer & lateral geniculate nucleus (LGN).
Classifications of Amblyopia:
• Functional
Improvement after treatment is expected
- Strabismic
- Anisometropic
- Stimulus Deprivation
- Meridional
- Ametropic
• No lesion
May be reversible or irreversible
- Organic:
- Toxic
Strabismic amblyopia:
Result of constant or near-constant childhood strabismus in one eye.
Mostly esotropias as many exotropia’s are intermittent in childhood.
Clinical Characteristics
•Reduced vision in one eye
• Strabismus found on CT- usually not freely alternating
• No pathology detected on ocular examination
• Occurs in 5-8% of general population
• The risk is 4x greater if one parent has strabismus
• 65% of patients impacted have a close relative with strabismus
Anisometropic amblopia:
• 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 clear for near
Stimulus deprivation amblyopia:
• Stimulus form vision deprivation amblyopia
• One or both eyes
• Little or no light enters the eye.
• Congenital Cataract- most common
• Ptosis
• Haemangioma
• Vitreous Opacity e.g., bleeding
• Corneal Scar
Meridional Amblyopia:
•Moderate-high degree of uncorrected astigmatism
•Can be unilateral or bilateral
•More significant risk in oblique astigmatism
Ametropic Amblyopia
•Likely bilateral
• High degree of bilateral refractive error goes uncorrected during critical period
•Blurred vision in both eyes at all distances.
• Typically, a result of high bilateral hypermetropia 6D or greater (Cannot be compensated using accommodation)
Organic Amblyopia: types
• Reversible
- Toxic Amblyopia (not always reversible)
• Irreversible
Not able to be treated. No lesion
- Nystagmus
- Albinism (usually associated with nystagmus)
Toxic Amblopia:
• Painless, progressive, bilateral vision loss
• Dyschromatopsia
• May also be referred to as “toxic optic neuropathy”
• Nutrional Amblyopia
- Vitamin B12 deficiency
- Seen in patients with extreme diets- reports in patients with ASD
- May see complete/incomplete recovery with improved diet/vitamin intake
• Other common causes
- Alcohol- may be associated with B12 deficiency
- Tobacco
- Antimalarials e.g., Chloroquine
- Anticancer treatments e.g., Vincristine
Investigations for amblopia:
Case History- Family history of childhood eye problems!
• Ocular Examination- Assess media and fundus
• Full Cycloplegic Refraction
• Visual Acuity Assessment: age and ability appropriate test selection
• Cover Test- is there a manifest deviation? What is the fixation preference/pattern?
• Contrast Sensitivity: Amblyopes can have reduced contrast sensitivity at higher spatial frequencies in comparison to non-amblyopes
• Uniocular fixation
• 4^ Test
Assessment of Uniocular Fixation
• Assess the point of the retina that the patient is using for fixation when the other eye is occluded
• Using a visuscope or ophthalmoscope
Method:
• Dim room lights
• Ask patient to fix at distance
• Occlude “fellow normal eye”
• Line up instrument
• Get patient to fix on centre of light
• Assess where the “bright” reflex is positioned
What can be the 3 findings when assessing the uniocular fixation?
- Central fixation
- Eccentric fixation
- Wandering fixation
Central fixation and wandering fixation:
Central Fixation
o Object on fovea
Wandering fixation
o Uniocular condition
o Fovea has lost its fixation superiority, and no single area of the retina is used for fixation
Eccentric Fixation
o Uniocular condition. Fixation is by a point which isnt the fovea.
o This point is the principle visual direction.
o The degree of eccentric fixation is defined by the distance between the fovea in degrees.
o The further from the fovea= worse the level of VA
o Estimated line reduces by 1 line per 0.5 degrees of eccentricity
Management of Amblyopia
• Patients are prescribed their full refractive correction for full time wear.
• Correction of refractive error results in resolution of anisometropic amblyopia in 1/3 of 3–7 year olds
• Resolution of amblyopia in 32% of patients with strabismic and combined strabismic+ anisometropic amblyopia. Better results in strabismic only versus combined
• Refraction adaptation mostly complete by 18 weeks
• 90% have resolution by 18 weeks of refractive adaptation
• Improvement can continue for up to 30 weeks
