BVP - Strabismus Assessment I - Week 11

Question 1

Define strabismus.

Answer 1

A misalignment of one visual axis from fixation in the presence of full opportunity to fuse

Question 2

Define phoria.

Answer 2

Misalignment of one visual axid from fixation when the opportunity to fuse is removed, but accurate alignment when fusion is allowed

Question 3

Define sensory fusion.

Answer 3

Ability of the eyes to contribute to the binocular percept

Question 4

What is first degree fusion? Explain.

Answer 4

Simultaenous perception - being aware of an input into each eye that is different, such as Maddox rod and torch

Question 5

What is second degree fusion? Explain.

Answer 5

Superimposition - being aware of an input into each eye that is similar and in the same position such as a phoria card

Question 6

What is third degree fusion? Explain.

Answer 6

Stereopsis - being aware of depth due to stimulation of disparate receptions

Question 7

Clinically, what is the common way to measure sensory fusion?

Answer 7

Stereopsis - third degree fusion

Question 8

Define motor fusion. What is it measured with?

Answer 8

The ability to maintain motor alignment to achieve sensory fusion
Measured with prism in free space

Question 9

Strabismus develops due to an imbalance between what two factors?

Answer 9

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)

Question 10

List three factors which increase the demand on fusion.

Answer 10

High refractive error
Abnormal innervation
Eye muscle disturbance

Question 11

What high refractive error will especially cause strabismus and what kind?

Answer 11

High hyperopia causing esotropia

Question 12

List 4 factors which decrease the quality of fusion.

Answer 12

Congenital lack of fusion
Reduced acuity in one eye
Peripheral retinal disease
Nystagmus

Question 13

List 3 things that can reduce acuity in one eye leading to strabismus.

Answer 13

Anisometropia
Amblyopia
Dracula dystrophy/disease

Question 14

If there is esotropia in the immediate family, what is the chance of esotropia in a patient?

Answer 14

17%

Question 15

What proportion of esotropia presentations are pseudostrabmisbus?

Answer 15

50%

Question 16

Define pseudostrabismus. What is it caused by? Explain.

Answer 16

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

Question 17

List the 6 components of assessing strabismus.

Answer 17

Detection of strabismus
Direction of strabismus
Magnitude of strabismus
Laterality (which fixates/deviates)
Comitancy (change of magnitude with gaze direction)
Distance near incomitance

Question 18

List the four ways of describing a strabismus and explain them.

Answer 18

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

Question 19

List three tests to detecting a strabismus.

Answer 19

General inspection
Hirschberg test
Cover test

Question 20

Describe the hirschberg test and how strabismus/no strabismus appears.

Answer 20

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

Question 21

What is the most sensitive test for strabismus?

Answer 21

Cover test

Question 22

What forms the basis of the cover test? Explain in terms of the macula.

Answer 22

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

Question 23

What can a unilateral cover test tell you (3)?

Answer 23

If a phoria or tropia is present, and its direction

Question 24

What test gives the least precise estimation of prism dioptre for a tropia?

Answer 24

Hirschberg estimation

Question 25

Describe hirschberg estimation for prism dioptre.

Answer 25

1mm displacement of reflex is 22^D strabismus

Question 26

Can the hirschberg test detect intermittent tropia?

Answer 26

No

Question 27

Can the hirschberg test detect phoria?

Answer 27

No

Question 28

What is the estimation of strabismus using the following landmarks?
Pupil margin
Mid-iris
Limbus

Answer 28

Pupil margin - 15^D
Mid-iris - 30^D
Limbus - 45^D

Question 29

Describe the krimsky test and its accuracy range.

Answer 29

Like the hirschberg test but uses a prism bar to move the tropic eye to estimate prism
Accurate to 10^D

Question 30

What is the most accurate objective tool for strabismus?

Answer 30

Neutralising cover test with prism

Question 31

Can subjective tools like maddox rod be used for strabismus? Is it more accurate?

Answer 31

It can but is only usable in a minority of adults due to sensory adaptations such as suppression

Question 32

Does the alternate cover test diagnose phoria from tropia?

Answer 32

No

Question 33

Does alternate cover test diagnose laterality?

Answer 33

No

Question 34

What prism is used with what tropia (list for each)?

Answer 34

Base out for esotropia
Base in for exotropia
Base down for hypertropia
Bade up for hypotropia

Question 35

Which test is used to assess laterality of the strabismus?

Answer 35

Unilateral cover test

Question 36

With a unilateral cover test, describe how to distinguish unilateral strabismus from alternating strabismus.

Answer 36

Unilateral strabismus - one eye is always used for fixation

Alternating strabismus - either eye will freely fixate

Question 37

Describe forced alternation.

Answer 37

The strabismic eye fixates at times on cover test

Question 38

What is VA like in patients with alternating strabismus? Explain why.

Answer 38

Equal acuity in each eye because fixation can be held with either eye - each macula has equal sensory superiority

Question 39

What is VA like in patients with unilateral strabismus?

Answer 39

They will often, but not always, have amblyopia in the strabismic eye