Binocular Vision Lecture 6: Fixation Disparity

Binocular Vision - Lecture 6 - Fixation Disparity

• Students will be able to describe the theory of fixation disparity, conduct a fixation disparity test, interpret results, and consider management implications.

Haplopia (Binocular Single Vision)

• Ideally, visual axes are perfectly aligned, placing the image perfectly on each fovea.

Panum's Fusional Space

• Single vision can be achieved with some axis misalignment, as long as it falls within Panum's fusional space.
• This misalignment is called fixational disparity or retinal slip.

Panum's Fusional Area

• Panum's fusional space refers to the amount of misalignment possible in free space, while Panum's fusional area refers to the amount of retinal misalignment possible.

Panum's Fusional Area and FD Measurement

• Fixation Disparity (FD) is often associated with uncompensated phoria.
• The amount of prism required to remove FD is previously termed as associated phoria, while Aligning Prism is now recommended in practice.

Classification of Fixation Disparity

• EXO FD/slip: Visual axes are divergent relative to the fixation target.
• ESO FD/slip: Visual axes are convergent relative to the fixation target.
• Hyper/Hypo FD/slip: Visual axes are vertically misaligned relative to the fixation target.
• Incyclo/Excyclo slip: Visual axes exhibit torsion relative to the fixation target.

Measurement of Aligning Prism

• Tests are conducted following distance refraction.
• Distance and near Mallett units are used in a subjective test.
• Minimum prism that neutralizes FD slip is measured.
• Minimum sphere that neutralizes FD may also be measured.

Mallett Unit (Horizontal Slip Assessment)

• The upper nonius target is visible to only one eye.
• The lower nonius target is visible to the other eye.
• A central OXO target is seen by both eyes to establish binocular lock.

Mallett Unit Design

• Nonius targets are seen monocularly with polarizing/red-green filters.
• The central OXO target is seen binocularly, ensuring associated vision and more natural viewing conditions.
• Red targets are for distance, and green targets are for near.

Distance Mallett Unit - Measuring Horizontal FD Slip

• The test is performed with room lights on.
• The patient needs to be educated on the test.
• A correct wearing of distance correction and appropriate PD is required.
• The patient is asked if they see two red lines (one above, one below the OXO), and whether the lines are aligned with the center of the X.

Possible Responses (Distance Mallett)

• Alignment: Nonius targets are aligned with the center of the X; no FD slip.
• Suppression/Poor Acuity: Only one Nonius target is visible, likely due to visual acuity or suppression.
• Slip: One Nonius target is shifted; FD slip is present.

Testing for Vertical FD Slip

• Similar setup to the horizontal test.
• The patient is asked if they see two red lines on either side of the OXO, and if they are aligned with the center of the X.

Possible Responses (Vertical Mallett)

• Alignment: Nonius targets are aligned with the center of the X; no FD slip.
• Suppression/Poor Acuity: Only one Nonius target is visible, likely due to visual acuity or suppression.
• Slip: One Nonius target is shifted; FD slip is present.

How to Measure Horizontal FD Slip

• Cover each eye in turn.
• One eye will reveal the target; the other will disappear.
• Determine the eye which sees the target and which direction it's shifted for.

How to Measure Vertical FD Slip

• Similar methodology to horizontal, but focus on vertical movement of the targets.

The Near Mallett Unit

• The test is conducted with room lights on.
• The patient is educated on what they need to do.
• Correct PD and near correction are required.
• The patient is shown the X in the middle of the OXO and asked to describe the green lines, one above and one below the OXO.
• The patient is asked if the two lines are in alignment.

Possible Responses (Near Mallett)

• Alignment: Nonius targets are aligned with the center of the X; no FD slip.
• Suppression/Poor Acuity: Only one Nonius target is visible, likely due to visual acuity or suppression.
• Slip: One Nonius target is shifted; FD slip is present.

The Sheedy Disparometer

• Uses polarized targets in fixed positions and prisms to correct any misalignments.
• The targets can move according to a rotating dial.
• If FD is present, the patient adjusts targets until alignment.
• The degree of movement indicates the amount of fixation disparity.

The Sheedy Disparometer - No FD

• No fixation disparity is present; lines appear aligned.
• Lines actually are aligned.

The Sheedy Disparometer - Exo FD

• Exo fixation disparity is present; patient moves lines until they appear aligned.
• Lines are still misaligned.

The Sheedy Disparometer - Eso FD

• Eso fixation disparity is present; patient moves lines until they appear aligned.
• Lines are still misaligned.

Forced Duction Fixation Disparity Curves

• Prisms are added in front of the eye; Sheedy Disparometer is used to measure FD for each prism.
• FD is plotted against the inducing prism.

How to Create a Fixation Disparity Curve (No FD Slip)

• Prism is placed in front of the eye.
• The target positions are correct.

How to Create a Fixation Disparity Curve (FD Slip)

• Prisms are introduced.
• Patient can no longer compensate for prism.
• Targets lay outside Panum's fusional area.
• FD slip is found.

Components of Fixation Disparity Curve

• Y-intercept = fixation disparity
• X-intercept = aligning prism (associated phoria)
• Slope = indicates the ability to adapt to prism induced stress, which is ideal.
• Flat slope = good adaptation.
• Steep slope = poor adaptation.

Curve Types

• There are 4 curve types, based on aspects like population percentage, symptoms, and binocularity.

Further Reading

• Elliott, D.B. (2007) Clinical Procedures in Primary Eye Care
• Benjamin, W.J. (2006) Borish's Clinical Refraction