Orthoptics I - BSV 2

What are the 3 grades of BSV?

simultaneous perception
fusion
stereopsis

What is sensory fusion?

combining information from each eye to create a single image
- requires images to be similar
- requires corresponding retinal points to be stimulated

What is motor fusion?

ability to maintain sensory fusion through a range of vergence movements
- keeping the image single "under stress"
- impacts squint management

What is stereopsis?

relative depth of objects on the basis of fixation disparity
- greater the horizontal disparity = greater stereopic effect

What is crossed disparity?

Objects positioned closer than the horopter and their representations on retinas are farther apart than the foveas
- images appear to "stick out"
- image IN FRONT of the horopter

What is uncrossed disparity?

A type of binocular disparity produced by an object that is farther away than the horopter
- images look "behind"

What are some monocular cues to stereopsis?

motion parallax
light and shade
aerial perspective
linear perspective
size of known objects
relative size and position

When does stereopsis develop?

abrupt onset and development at 3 months up until 18 months
critical period overlaps that for vision

How do we test sensory fusion?

bagolini glasses or worth's lights

How do we test motor fusion?

fusional amplitudes
- horizontal and vertical
free space measurements via prisms
or synoptophore

How do we test stereopsis?

stereoacuity - quantitative or qualitative

Describe prismatic displacement

1D of prism causes displacement of light of 1cm at 1m
- this causes eye vergence to maintain foveal fixation

What is Herrings Law of Equal Innervation?

CNS provides equal, simultaneous innervation to muscles of both eye for a particular direction
- whatever happens to one muscle happens to the contralateral partner

What is stereoacuity a measure of?

minimum resolvable binocular disparity necessary for stereopsis
- quantified in seconds (") of arc: angular measurement used as the horopter is circular

Name a quantitative stereotest

random dot

What is abnormal retinal correspondence?

a binocular condition in which there is a change in visual projections such that the fovea of the fixing eye has a common visual direction with an area other than the fovea on the deviating eye
- the pairing of all retinal elements is similiarly changed
- a sensory adaptation to strabismus which facilitates BSV

What is small-angle strabismus?

a condition favourable for ARC development
stable angle as extrafoveal fixation point used
esotropia most likely

Advantages of ARC

next best thing to BSV
allows for BSV and stereopsis when manifest squint is present
prevents diplopia/suppression

Disadvantages of ARC

reduced quality compared to BSV
- normal BSV may not be achievable
- treatment of ARC is contradicted

What is harmonious abnormal retinal correspondence?

point in the strabismic eye corresponding to the fovea of the fixing eye is set at the angle of deviation

What is unharmonious abnormal retinal correspondence (UARC)?

point in the strabismic eye corresponding to the fovea of the fixing eye is set between the angle of deviation and the fovea

How do we test ARC?

same as BSV