BVP - Strabismus Assessment I - Week 11 Flashcards
Define strabismus.
A misalignment of one visual axis from fixation in the presence of full opportunity to fuse
Define phoria.
Misalignment of one visual axid from fixation when the opportunity to fuse is removed, but accurate alignment when fusion is allowed
Define sensory fusion.
Ability of the eyes to contribute to the binocular percept
What is first degree fusion? Explain.
Simultaenous perception - being aware of an input into each eye that is different, such as Maddox rod and torch
What is second degree fusion? Explain.
Superimposition - being aware of an input into each eye that is similar and in the same position such as a phoria card
What is third degree fusion? Explain.
Stereopsis - being aware of depth due to stimulation of disparate receptions
Clinically, what is the common way to measure sensory fusion?
Stereopsis - third degree fusion
Define motor fusion. What is it measured with?
The ability to maintain motor alignment to achieve sensory fusion
Measured with prism in free space
Strabismus develops due to an imbalance between what two factors?
Factors which increase the demands on fusion (if this side is heavier, strabismus will develop)
Factors which improve the quality of fusion (if this sime is heavier, no strabismus)
List three factors which increase the demand on fusion.
High refractive error
Abnormal innervation
Eye muscle disturbance
What high refractive error will especially cause strabismus and what kind?
High hyperopia causing esotropia
List 4 factors which decrease the quality of fusion.
Congenital lack of fusion
Reduced acuity in one eye
Peripheral retinal disease
Nystagmus
List 3 things that can reduce acuity in one eye leading to strabismus.
Anisometropia
Amblyopia
Dracula dystrophy/disease
If there is esotropia in the immediate family, what is the chance of esotropia in a patient?
17%
What proportion of esotropia presentations are pseudostrabmisbus?
50%
Define pseudostrabismus. What is it caused by? Explain.
Some young children appear to have esotropia on casual inspection but are straight with cover test
Caused by epicanthal folds
Nasal sclera is less visible than the temporal sclera in each eye, but nasal sclera is the same in each eye and not in strabismus
List the 6 components of assessing strabismus.
Detection of strabismus
Direction of strabismus
Magnitude of strabismus
Laterality (which fixates/deviates)
Comitancy (change of magnitude with gaze direction)
Distance near incomitance
List the four ways of describing a strabismus and explain them.
By constancy
-intermittant strabismus is present only some of the time
By direction
-exo/eso/hyper/hypo
By laterality
-which eye is turned, and does it alternate
By comitancy
-comitant strabismus has the same magnitude in all directions
List three tests to detecting a strabismus.
General inspection
Hirschberg test
Cover test
Describe the hirschberg test and how strabismus/no strabismus appears.
Shine a pentorch between the patients eyes from 50cm away
Observe the corneal reflex
If there is no strabismus, the reflexes will be symmetric and ~0.5mm nasal to the pupil centre
In esotropia, the fixating eye will be normal, but the reflex on the tropic eye will be displaced
What is the most sensitive test for strabismus?
Cover test
What forms the basis of the cover test? Explain in terms of the macula.
Macula has sensory and motor superiority, so when a deviated eye is uncovered, and non-deviated eye is covered, the deviated eye will move to fixate
What can a unilateral cover test tell you (3)?
If a phoria or tropia is present, and its direction
What test gives the least precise estimation of prism dioptre for a tropia?
Hirschberg estimation
Describe hirschberg estimation for prism dioptre.
1mm displacement of reflex is 22^D strabismus
Can the hirschberg test detect intermittent tropia?
No
Can the hirschberg test detect phoria?
No
What is the estimation of strabismus using the following landmarks?
Pupil margin
Mid-iris
Limbus
Pupil margin - 15^D
Mid-iris - 30^D
Limbus - 45^D
Describe the krimsky test and its accuracy range.
Like the hirschberg test but uses a prism bar to move the tropic eye to estimate prism
Accurate to 10^D
What is the most accurate objective tool for strabismus?
Neutralising cover test with prism
Can subjective tools like maddox rod be used for strabismus? Is it more accurate?
It can but is only usable in a minority of adults due to sensory adaptations such as suppression
Does the alternate cover test diagnose phoria from tropia?
No
Does alternate cover test diagnose laterality?
No
What prism is used with what tropia (list for each)?
Base out for esotropia
Base in for exotropia
Base down for hypertropia
Bade up for hypotropia
Which test is used to assess laterality of the strabismus?
Unilateral cover test
With a unilateral cover test, describe how to distinguish unilateral strabismus from alternating strabismus.
Unilateral strabismus - one eye is always used for fixation
Alternating strabismus - either eye will freely fixate
Describe forced alternation.
The strabismic eye fixates at times on cover test
What is VA like in patients with alternating strabismus? Explain why.
Equal acuity in each eye because fixation can be held with either eye - each macula has equal sensory superiority
What is VA like in patients with unilateral strabismus?
They will often, but not always, have amblyopia in the strabismic eye
