Orthoptics I - Introduction to latent and manifest squint

What is a squint/strabismus?

an ocular deviation

What is a manifest and latent squint?

manifest squints are visible with both eyes open, all the time
latent squints visible after dissociation/fusion interruption

What are the general classifications of strabismus?

eso = turn in
exo = turn out
hyper = up
hypo = down

Describe the muscular theory of strabismus

1840s
mechanical malfunction - as it can be mechanically corrected
- maldeveloped muscle
- muscle absence
- facial abnormality

Describe Donders theory

1894
suggests a defect in accommodation/convergence linkage due to association between refractive error and A/C

Descrive Worth's theory

1903
failure of fusion
- as not all longsighted people have an eso

Describe neurological causes

disruption to nerve supply
damage to convergence/divergence control centres

What other theories are there for strabismus?

psychological causes
genetics
visual changes

Which theory is correct?

none!
different theories suit different squints

Describe how hetereophorias come about

constant sensory and motor fusion maintains eyes in a straight position - when this is interrupted the eye may deviate to a position of rest
- would return to a straight position when fusion returns

What is orthophoria?

state of no deviation when sensory fusion is interrupted

How much heterophoria is considered normal?

1-2D of esophoria
1-4D of exophoria
vertical heterophoria much more likely to be symptomatic

How can a heterophoria decompensate to a heterotropia?

if the angle of deviation is not adequately compensated by motor fusion, or if there is some disruption to fusion

What is a heterotropia?

a condition in which one visual axis is not directed towards the fixation point
- images of the eye is not corresponding so there is no fusion
- either a diplopic or a suppression response
OR if the tropia is small = ARC

Describe a diplopic heterotropia response

either heteronymous (crossed) or homonymous (uncrossed) pathological diplopia

What is a confusion response to heterotropia

brain may attempt to merge the two disparate images into one
- overlapping image but no separation
- results from the stimulation of corresponding retinal points which rise to superimposition

What is a suppression response to heterotropia

adaptation seen mostly in children
could be central/foveal or peripheral suppression

What type of diplopia is caused by an esotropia?

homonymous/uncrossed - image on the SAME side as the deviating eye

What type of diplopia is caused by an exotropia?

heteronymous/crossed = image on the opposite side

How do we assess strabismus?

assess head and lid position to start
corneal reflections
cover/uncover test
alternating cover test

What observations do we make from corneal reflections?

symmetrical?
central. nasal, temporal etc
makes no indication to latent squint

What is angle kappa?

angle between pupillary axis and visual axis
- the pupillary axis is slightly inferonasal to the fovea
- nasally displaced reflection = +K
- temporally displaced reflection = -K

How can angle kappa impact strabismus?

changes can alter the appearance of heterotropias
large +K can cause pesudoexotropia or mask the extent of a true exotropia and vice versa for a -K

What are the limitations to observing CR's?

no latent squint detectable
uncontrolled accommodation
donders theory

What is a cover/uncover test?

measures heterotropias at near and distance
cover one eye whilst looking for movement in the other eye

What is an alternating cover test?

assess heterophorias (or the latent aspect of a tropia)
near and distance
cover an eye, look at the other, swap occluder and look at the newly uncovered eye
check recovery when removing occluder for the final time

Interpret:

CT c gls N mod LET c diplopia
D sm LET c diplopia
s gls N sm E c delayed rec
D min E c gd rec

with glasses, at near moderate left esotropia, smaller at distance, both uncrossed diplopic
without glasses small esophoria at near with poor recovery, smaller esophoria at distance with good recovery