11. causes and risk factors of amblyopia

What is amblyopia

A condition of diminished visual form sense which is not associated with any
structural abnormality of disease of the media, fundi or visual pathway and
which is not overcome by correction of the refractive error

3 risk factors for amblyopia

Refractive error

• anisometropic

• ametropic

• meridional- astigmatism

stimulus deprivation

Strabismus-tropia

What is the critical period for developing amblyopia

eight years old and is relatively easy to correct until that age by improving the quality of visual input in the affected eye but becomes increasingly resistant to reversal with age

Whe is the most sensitive period for amblyopia

until 2-3 yrs old

what is happening in amblyopic eyes

• changes in visual cortex (area V1 and some in V2) with a loss of bincoular driven cells and neurones that are driven by amblyopic eye

amblyopia can be unilateral or bilateral and caused by one or more factors:

• light deprivation- not enough light on retina

• form deprivation

• abnormal binocular interaction

Reasons against amblyopia treatment

• Binocularly good vision

• May develop abnormal BV

• May not restore normal BV

• Risk of intractable diplopia

• Psychological issues

Reason for treating amblyopia

• If something happens to the good eye then the patient has another eye with at least reasonable VA

• improved BV development

Which eye does Strabismic amblyopia effect?

monocular

When is strabismic amblyopia most likely?

Constant deviation

More likely to occur in esotropia

Why is strabismic amblyopia more likely for esotropia

Exotropia often remains intermittent during childhood

Why is alternating deviation amblyopia less likely

both eyes receive visual stimulus

Which eye does Stimulus deprivation amblyopia effect?

monocular or bilateral

What is Stimulus deprivation amblyopia

Lack of adequate visual stimulus (light and/or form)

What can cause stimulus deprivation amblopia

• May be complete such as ptosis when no light and form enters the eye,

• May be partial such as cataract when light and some form enters the eye.

• bilateral stimulus deprivation may result from congenital nystagmus

What is Anisometropic amblyopia

• Monocular condition

• Difference in the refractive error between the two eyes which ensures that one eye receives better visual input at all distances

• the refractive error may cause a spherical or astigmatic difference between both eyes

What is ametropic amblyopia?

• Occurs bilaterally

• High degree of uncorrected bilateral refractive error

What rx can cause ametropic amblyopia

Normally greater than 6D of hypermetropia (Cannot be compensated for with accommodation),

High myopia

When does Meridional (astigmatic) amblyopia occur

• Occurs monocularly with anisometropic amblyopia

• Occurs binocularly with ametropic amblyopia

what is Meridional (astigmatic) amblyopia

A relatively clear image is formed along the more emmetropic axis, A blurred image is formed along the more ametropic axis

how to diagnose on amblyopia examination

what kind of qs to ask in H&S

• What is the problem?

• Which eye?

• What age did the problem start?

• How long has it been there?

• If strabismus, constant or intermittent?

• Previous treatment in the form of glasses, occlusion therapy or others?

• If treatment, when was this given and why was it discontinued?

Why is log mar chart better than snellen when taking VAs for an amblyopic px

it takes into account the crowding phenomenon

if px has latenet nystagmus how would you adjust taking VAS

use a transculent occluder

What are Glasgow acuity cards

(marketed nowadays as Crowded Keeler logMAR) have been specifically designed to obtain accurate measurements of VA in amblyopia

what is the mean VA of 4-5 yr olds

0.087 (approx 6/7.5) +/- 0.10 for crowded,

-0.010 (approx 6/6) +/- 0.10 for uncrowded

in children with amblyopia the difference in VA crowded and uncrowded is
larger

How can eccentric fixation be grossly tested

corneal reflexes

How would you asses eccentric fixation using corneal reflexes

1. Cover the non-strabismic eye and ask the patient to look at a near pen torch (strabismic eye fixating)

2. If strabismic eye takes up central fixation i.e. the corneal light reflex is central in the pupil then there is no eccentric fixation • If the corneal reflex is not central, there could be eccentric fixation or an angle kappa is present.

3. To check if the lack of central fixation in the strabismic eye is due to angle kappa, cover over the strabismic eye and look at the corneal reflex in the nonstrabismic eye

4. If this is central then the person does not have an angle kappa so the displacement of the corneal reflex in the strabismic eye is due to eccentric fixation

5. If the corneal reflex in the good eye is nasally displaced then there is an angle kappa and the clinician has to determine whether the displacement of the corneal reflex in the same in both eyes

How do you interpret the results from assessing eccentric fixation from corneal reflexes

If it is the same then there is no eccentric fixation

• If it is different then there is eccentric fixation

When is the eccentric fixation test with corneal reflexes used?

young or uncooperative patients

How is stereopsis in children with amblyopia

likely to have stereopsis values bigger than 70"

How can you use ophthalmoscopy to assess eccentric fixation

1. Project the ophthalmoscopic target (visuscope) on the patient's retina

2. Start testing the non-amblyopic eye to check the patient's response

3. The eye that is not being assessed is occluded • Instruct the patient to look straight at the centre of the target

4. The target will be seen by the practitioner at the centre of the fovea (nonamblyopic eye)

5. The repeat the process in the amblyopic eye

6. If the target is seen at the centre of the fovea in the amblyopic eye no eccentric fixation

7. If the target is seen on any other part of the retina (when patient is instructed to look straight to the target) this is eccentric fixation

in SOT where will the eccentric fixation be

nasal

in XOT where will the eccentric fixation be

temporal

If eccentric fixation, how does the location of the eccentric point indicate of VA level

Further from the fovea the worse the VA

why would you dilate the pupil durimg opthalmoscopy

the ophthalmoscope light directed to the foveal area will cause significant pupil constriction

why would you do DO after cycloplegic refration

young patients may accommodate when asking to look at the target and this
will blur the practitioner’s view of the fundus

How can you measure size of suppression

Sbiza bar

What is a sbiza bar?

Graded bar of varying density of red filters

How do you use a Sbiza bar for measuring depth of suppression

1. Placed in front of non-amblyopic/non-deviating eye

2. Patient is requested to view light and asked what colour

3. As we increase the filters we compromise the vision of the non-amblyopic/nondeviating eye

4. Increase density of filters until patient reports two lights or a white light

if a white light is reported it means the non amblyopic eye is no longer the eye fixating, its now the amblyopic eye fixating

if two lights are seen, both eyes are fixating

what would motitly test for

to detect incomitant deviations and muscle palsies

what could Accommodation and convergence test for

Amblyopia is associated with an abnormal accommodative function, which
is clinically detected as reduced amplitude of accommodation and greater
lags of accommodation in the amblyopic eye