Binocular Vision Lecture 6: Fixation Disparity
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Questions and Answers

What is the term used for the amount of retinal misalignment that can occur while still achieving single vision?

  • Panum's Fusional Area (correct)
  • Fixational Disparity
  • Haplopia
  • Panum's Fusional Space

    • Which type of fixation disparity occurs when the visual axes are convergent relative to the fixation target?

  • Hyper FD
  • Eso FD (correct)
  • Incyclo slip
  • Exo FD

    • What is the main purpose of the Mallett Unit in measuring fixation disparity?

  • To evaluate visual acuity
  • To measure the minimum prism that neutralizes the FD slip (correct)
  • To establish the patient's depth perception
  • To assess monocular vision

    • Which factor is key to ensuring an associated vision during the fixation disparity test?

    <p>Having a binocular fixation target</p> Signup and view all the answers

    What does the term 'Associated Phoria' specifically refer to in the context of fixation disparity testing?

    <p>A condition without any dissociation at all</p> Signup and view all the answers

    In which scenario would Panum's Fusional Space be exceeded?

    <p>The axes misalignment exceeds the limit of comfortable fusion</p> Signup and view all the answers

    What is the main visual target used for assessing horizontal fixation disparity in the Mallett Unit?

    <p>The OXO target</p> Signup and view all the answers

    What would be the likely classification of a fixation disparity where visual axes are vertically misaligned?

    <p>Hyper FD</p> Signup and view all the answers

    Which visual condition is defined as binocular single vision under ideally aligned visual axes?

    <p>Haplopia</p> Signup and view all the answers

    Which of the following best describes the method for measuring horizontal fixation disparity?

    <p>Keeping room lights on and using appropriate distance correction</p> Signup and view all the answers

    What indicates that the two Nonius targets are correctly aligned?

    <p>No FD slip is present</p> Signup and view all the answers

    What could be the reason for a patient only seeing one Nonius target?

    <p>The patient may not have adequate visual acuity in one eye</p> Signup and view all the answers

    If an FD slip is present, what is the next step to take?

    <p>Measure the amount of FD slip</p> Signup and view all the answers

    How should the occluder be used during the test?

    <p>Cover each eye alternately to observe target disappearance</p> Signup and view all the answers

    What is the purpose of placing a prism in the alignment process?

    <p>To align the Nonius targets with the apex of the prism</p> Signup and view all the answers

    During vertical FD slip testing, what should the patient be asked to identify?

    <p>Two red lines, one to the left and one to the right</p> Signup and view all the answers

    What happens if the bottom Nonius target is perceived to move to the right?

    <p>A correction to the left base of the prism is required</p> Signup and view all the answers

    Which condition could prevent good response during Nonius target testing?

    <p>Suppression in one eye</p> Signup and view all the answers

    What does an 'alignment' of the Nonius targets specifically signify?

    <p>The targets are correctly positioned with respect to the X</p> Signup and view all the answers

    What should be observed if the top Nonius target disappears when the right eye is covered?

    <p>The right eye is suppressing the target</p> Signup and view all the answers

    What does the Sheedy Disparometer indicate when no fixation disparity is present?

    <p>The lines will appear aligned to the patient.</p> Signup and view all the answers

    What happens when an exo fixation disparity is present?

    <p>The patient perceives the lines as misaligned.</p> Signup and view all the answers

    What is indicated by a steep slope in a fixation disparity curve?

    <p>Poor adaptation to prism-induced stress.</p> Signup and view all the answers

    When a prism of 8 D base IN is introduced, what is expected if the patient can no longer compensate?

    <p>The patient will experience diplopia.</p> Signup and view all the answers

    What is the significance of the Y-intercept in a fixation disparity curve?

    <p>It indicates the fixation disparity.</p> Signup and view all the answers

    What happens when 12 D base OUT is introduced and the patient can no longer compensate?

    <p>The patient develops diplopia.</p> Signup and view all the answers

    Which of the following statements is true regarding patients with eso fixation disparity?

    <p>They will align the lines that appear correctly to them.</p> Signup and view all the answers

    What characterizes Type I curves in fixation disparity classification?

    <p>They are usually associated with low symptoms.</p> Signup and view all the answers

    What is Panum's fusional area related to in the context of fixation disparity?

    <p>The ability to fuse images when prisms are introduced.</p> Signup and view all the answers

    Which of the following describes the actual position of targets when a patient has no fixation disparity slip?

    <p>They coincide with where the patient perceives them.</p> Signup and view all the answers

    What is typically observed when 6 D base OUT is introduced in a patient still able to compensate?

    <p>There would be no fixation disparity slip.</p> Signup and view all the answers

    What does a flat slope on a fixation disparity curve indicate?

    <p>Good adaptation to prism-induced stress.</p> Signup and view all the answers

    When creating a fixation disparity curve, what is plotted on the X-axis?

    <p>Aligning prism used.</p> Signup and view all the answers

    What does it indicate if the patient sees two Nonius targets in line with the center of the X?

    <p>No fixation disparity (FD) slip is present.</p> Signup and view all the answers

    Which movement of the Nonius target would suggest that the measurement of vertical FD slip is necessary?

    <p>One Nonius target has moved relative to the other.</p> Signup and view all the answers

    What should be done if the patient reports that only one Nonius target is visible?

    <p>Record the response without further testing.</p> Signup and view all the answers

    In the context of horizontal FD slip, which prism should be added for an exo deviation?

    <p>Weakest base in prism or minus sphere.</p> Signup and view all the answers

    What is the primary purpose of the Near Mallett Unit test?

    <p>To determine eye muscle-related strain during reading.</p> Signup and view all the answers

    When measuring vertical FD slip, how should the prism be placed if the left eye sees the top Nonius target which has moved to the right?

    <p>Base in front of the right eye.</p> Signup and view all the answers

    What adjustment is needed when there is a right hyper deviation in fixation disparity?

    <p>Add weakest base down prism for right eye.</p> Signup and view all the answers

    What signifies the presence of fixation disparity during testing with the Near Mallett Unit?

    <p>One Nonius target appears to move relative to the other.</p> Signup and view all the answers

    Which adjustment would be appropriate for correcting torsional fixation disparity that shows an excyclo slip?

    <p>Add weakest base out prism.</p> Signup and view all the answers

    How should findings be recorded if no fixation disparity slip is detected?

    <p>Note 'FD no slip'.</p> Signup and view all the answers

    What is essential for the successful execution of tests using the Sheedy Disparometer?

    <p>The polarised targets should be adjustable for alignment.</p> Signup and view all the answers

    What indicates that the Mallet unit primarily measures the associated phoria rather than directly measuring FD slip?

    <p>It uses fixed, polarised targets.</p> Signup and view all the answers

    What is the appropriate action if a prism is required for horizontal fixation disparity?

    <p>Specify prism orientation and power.</p> Signup and view all the answers

    Which protocol improves the accuracy of the results during the Near Mallett Unit test?

    <p>The patient should wear a polaroid visor.</p> Signup and view all the answers

    Study Notes

    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

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    Related Documents

    Binocular Vision Lecture 6 – Fixation Disparity Lecture Notes PDF

    Description

    Explore the theory of fixation disparity and learn how to conduct and interpret fixation disparity tests. The quiz covers key concepts such as Panum's fusional space and management implications related to binocular vision. Understand the importance of visual axis alignment for achieving single vision.

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