what is amblyopia and its prevalence?
- Deficit in visual acuity with no detectable organic cause
- Prevalence 2-3%
– 6-10 million Americans
– 60,000 children per year
– leading cause of monocular vision loss in children - Amblyopia is always associated with a history of an early sensory anomaly
– Strabismus
– Form Deprivation (media opacities)
– Anisometropia, Astigmatism, High hyperopia
What is amblyopia also associated with?
Prevalence of 3.2%
(<20/30 in 3654 adults; Attebo et al., 1998)
50% associated with anisometropia
27% associated with strabismus and anisometropia
19% associated with strabismus
4% associated with visual deprivation
VA in amblyopia
- Reduced visual acuity
– Acuity of 20/40 or worse in the weaker eye
– and/or a minimum acuity difference of 2 lines on the acuity chart between eyes
– Measured with recognition acuity tasks (letters or optotypes)
Errors with snellen in VA testing
- Wide range of errors: miss some large letters & read some smaller ones
- Miscalls are not confusion letters
- Read letters out of order, fewer or more in number
- End-of-row letters more often correct
- Abnormal head position during acuity testing
Visual Acuity Testing in Amblyopia
- Vision indistinct (not helped by pinhole).
- Line of letters has local compressions and expansions. Separation difficulty or crowding effect found mainly in strabs
- Living with the preferred eye patched, even with fairly good acuity in amblyopic eye, is more bothersome to Strab than Aniso.
- Trend toward non-acuity definitions of amblyopia
Spatial uncertainty and distortion with amblyopia
- Not sure where some objects are located in space (uncertainty)
-Strabismics show spatial uncertainty AND distortion
-Anisometropes show only spatial uncertainty
Contour Interaction and Visual Acuity testing
- S-chart: 21 slides that span 20/9 to 20/277 VA
- At each acuity level, the amount of contour interaction is constant
- Visual acuity responses for both eyes of an amblyope tested with S-chart
- The flatter curve for the amblyopic eye reflects the marked variability in acuity responses
Visual Acuity Characteristics in Amblyopia
- Single optotypes underestimate acuity loss compared to chart acuity tests
- Contour interaction
- Crowding effect in normals is exaggerated in amblyopia
Snellen v grating acuity in amblyopia
- Grating acuity underestimates loss of Snellen acuity
- Anisometropia
– reduced Snellen VA is proportional to reduced grating VA - Strabismus
– Snellen VA is affected more than grating VA
Visual Acuity in
Amblyopia between Snellen and vernier
- Grating, vernier and Snellen are linearly related for anisometropes
- Only vernier and Snellen are linearly related for strabismics
- Both grating acuity and Vernier acuity have a strong correlation with optotype acuity
- The loss in Vernier acuity is almost directly proportional to the loss in optotype acuity
– exponent of the power function indicated by the best-fitting line is 1.15 - The loss in grating acuity is on average smaller than the corresponding loss in optotype acuity
– exponent of the best-fitting power function is 0.65, but the correlation value is high
Optotype acuity for strabismics vs strabismic anisometropes
- For all groups, the loss in grating acuity is less than the loss in optotype acuity.
- Strabismics shallower slope than the whole abnormal population; exponent is 0.31.
- Strabismic anisometropes differs in the intercept of the fitted line – on average, the grating acuity of members was roughly 15% lower than for the whole abnormal population, for any given level of optotype acuity, (prevalence of points in the plot that fall below the line).
Relationship between aniso, strab w/ aniso, strab, sporadic strab, eccentric fixators, deprivation amblyopia
- For all groups, the loss in grating acuity is less than the loss in optotype acuity.
- strab aniso and strab deviate from the overall trend
- The relationship between grating and optotype acuity in amblyopes with strabismus is different from the relationship in other amblyopes
Ratio of grating to Vernier acuity
- Development of Vernier and grating acuity
– By 6 mo, ratio 2:1
– By 4 yr, ratio 4:1
– Adult ratio 10:1
Contrast sensitivity functions in amblyopia
- A grating can be detected if it is visible in any part of the field of view it covers
- Details in letters (optotypes) are hard to see due to local
distortions of the image
sensitivity ratio vs spatial frequency
- Marked losses of threshold contrast sensitivity in
amblyopia - Reduction in contrast sensitivity is greatest at high spatial frequencies
- Loss of CSF at high SF increases with severity of amblyopia
- Reduced CSF is a neural loss in foveal function
Types of Fixational eye movements in amblyopia and definition
- Micro tremors
– normal in frequency and amplitude - Fixation
– more unsteady in strabismics than anisometropes - Drifts
– normal amplitude but asymmetric (more nasal drift)
– have error producing then error correcting saccades
– time average position OFF the foveola - Saccades
– size of abnormal saccades approximately proportional to visual acuity (Snellen denominator)
– latency of saccades longer in strabs than anisos
Accommodation in 20/120 amblyopia
Reduced accommodative responses as compared to their dominant eye responses
Abnormalities in Ocular Motility with Amblyopia
- Fixation
- Saccades and pursuits
- OKN
- Accommodation
Does the infant visual system provide
a reasonable model for amblyopia?
- Amblyopic visual system resembles the immature visual system
– Normal function at low spatial frequencies
– Reduced spatial vision: visual acuity, contrast sensitivity and hyperacuity
– Reduced ocular motility; accommodation, fixation, pursuits, saccades and OKN - Many aspects of the abnormal visual function in amblyopia may be understood on the basis of abnormal development due to either arrest or extinction
-Presence of a sensory obstacle (strabismus) arrests the
development of visual acuity
-Acuity stays at level achieved at time of onset of strabismus
-Amblyopia of extinction follows as result of binocular inhibition
-Only amblyopia of extinction could be recovered by treatment
Classification of Amblyopia
- Form (image) deprivation*
–Congenital cataract
–Ptosis
–Corneal opacities
–Tumors
–Occlusion
*unilateral worse than bilateral
Effects of congenital cataracts on contrast sensitivity
- Loss of CS in deprived eye relative to normal
- Loss of CS relative to norms for deprived eye
- Depth of amblyopia in unilateral cataract depends on the following:
– Age at which the cataract began
– Length of time the cataract was present
– Age at which the cataract was removed
– Time between aphakia and optical correction
– Presence of strabismus either before or after the cataract was removed
Classification of Strabismus
–Direction - esotropia v exotropia
–Magnitude
–Laterality - unilateral v. alternating
–Frequency - constant v. intermittent
–Comitance
Infantile Esotropia
- Onset of ET before 6 months (not at birth)
- Prevalence 1-2% of population
– Account for 28-54% of all pts with esotropia - Large deviation
>30 pd up to 120 pd - Often alternating ET
– 40% have amblyopia
Prognosis poor if Tx after 2 yr, fair if before 2 yr
Microtropia
microstrabismus; monofixation syndrome
* Magnitude: 1-9 prism diopters
* Mechanism: unknown
– Often post-surgical ET or Vision therapy; aniso with ET
* Shallow amblyopia; poor/absent stereopsis, central suppression, peripheral fusion
* Prognosis
– poor for bifoveal fixation
– usually stable end stage condition
