Binocular Vision Lecture 6 – Fixation Disparity Lecture Notes PDF

Binocular Vision Lecture 6 – Fixation disparity Professor Phillip Buckhurst By the end of the session you will be able to: Describe the theory of fixation disparity Understand how to conduct a fixation disparity test in your routine Interpret the results of the test Consider the...

Binocular Vision Lecture 6 – Fixation disparity Professor Phillip Buckhurst By the end of the session you will be able to: Describe the theory of fixation disparity Understand how to conduct a fixation disparity test in your routine Interpret the results of the test Consider the management implications of the test Haplopia (binocular single vision) F Ideally we would expect that for binocular single vision both visual axis would be perfectly aligned so that the image falls perfectly on each fovea fL fR Panums fusional space Panums F fusional Space However, we know that single vision can still be achieved provided that the axis misalignment is small enough to fall within Panum’s funsional space This misalignment is referred to as Fixational disparity or Retinal slip fL fR Panums fusional area Panums fusional space refers to the amount of misalignment possible in free space Panums fusional area refers to the amount of retinal misalignment possible fovea fovea Panums fusional area Panums fusional area  Panums fusional area FD is likely to occur with uncompensated phoria The amount of prism required to remove FD has traditionally been called ASSOCIATED PHORIA Strictly meaning that there should be no dissociation (disruption of fusion) at all so that the eyes are measured in their active position In practice, some degree of dissociation is essential and the term ALIGNING PRISM has been advocated fovea fovea Panums fusional area Panums fusional area Classification of fixation disparity EXO FD or slip – Visual axes divergent relative to fixation target ESO FD or slip – Visual axes convergent relative to fixation target Hyper or Hypo FD or slip – Visual axes vertically misaligned relative to fixation target Incyclo or Excyclo slip – Visual axes exibiting torsion relative to fixation target Measurement of Aligning prism Conducted following distance refraction Distance and near Mallett units used Subjective test Measure minimum prism that neutralises FD slip Can also measure minimum sphere that neutralises FD  The Mallett Unit (Horizontal slip assessment) Upper nonius target only seen by one eye Central OXO target seen by both eyes O X O (to establish a binocular lock) Lower nonius target seen by the other eye The Mallett unit design The Nonius targets are designed so that they are only seen monocularly – With polarising filters – With Red green filters The central OXO target can be seen binocularly and so forms a binocular fixation target – Ensures associated vision – More natural viewing conditions than a dissociated test Red for distance (accommodative lead) Green for near (accommodative lag) Note: Red Nonius targets are generally used for distance (accommodative lead) and green are generally used for near (accommodative lag) Distance Mallett unit Measuring horizontal FD slip Test conducted with Room lights on O X O Explain the test to your patient Turn Mallett on with OXO horizonal – checking for horizontal FD slip Appropriate distance correction worn (correct PD) Place the polarised filters infront of the eyes and ask the patient Can you see two red lines, one on above and one below the OXO? Are the two lines in line with the center of the X? Note: For this test your Px will be wearing polarised filters (sunglasses) therefore you must have the lights on for the test Possible responses The two Nonius targets are in alignment – The patient sees the two Nonius targets which are in line with the center of the X – No FD slip is present If this response is given then you can move on and check for vertical FD slip O X O Possible responses Only one Nonius target is seen – The patient may not have good enough visual acuity in one eye to resolve the Nonius target OR – The patient is suppressing (more on this later) If this response is given then record the response (no need to check for vertical slip) O X O Possible responses One nonius target has moved – FD slip is present If this response is given then you need to measure the amount of FD slip O X O How to measure horizontal FD slip  Cover RE -- top Nonius The top Nonius target disappears RE sees top target Moves to the right Work out which eye sees which O X O Nonius target Cover LE -- bottom Nonius Cover each eye in turn with an disappears occulder LE sees bottom target When you occlude an eye the nonius target that eye sees should disappear nose L R  The prism moves the Work out which direction to Nonius line in place your prism to allign the direction of its apex the Nonius targets Here the right eye sees the top nodius target – which has moved to the right We need to use a prism to align O X O the nonius target with the X – Need to move to the left nose L R The prism should be placed infront of the relivant eye (base out) Example 2 The bottom target moves to the right Cover RE -- top Nonius disappears RE sees top target O X O Cover LE -- bottom Nonius disappears LE sees bottom target Use prism to align nodius target nose L R Testing for vertical FD slip Set up is the same for the horizontal testing Can you see two red lines, one to the left and one to the right of the OXO? Are the two lines in line with the center of the X? Possible responses The two Nonius targets are in alignment – The patient sees the two Nonius targets which are in line with the center of the X – No FD slip is present If this response is given then the test is complete (provided you have already checked for horizontal fd slip How to measure vertical FD slip  Cover RE -- Right Nonius disappears The right hand Nonius O X O RE sees Right target Moves up Nonius Work out which eye sees which Nonius target Cover LE – left Nonius disappears Cover each eye in turn with an occulder LE sees left Nonius When you occlude an eye the nonius target that eye sees should disappear nose L R Work out which The prism moves the direction to place Nonius line in the direction your prism to allign of its apex the Nonius targets Here the right eye sees the right O X O nodius target – which has moved up We need to use a prism to align the nonius target with the X – Need to move the Nonius target down nose L R The prism should be placed infront of the relivant eye (base UP) Example 2 The right target moves down and the left target moves up Cover RE -- Right Nonius disappears RE sees right target O X O Cover LE -- left Nonius disappears LE sees left target Use prism to align Nodius target Note: prisms change the binocular status nose L R The Near Mallet Unit The Near Mallett Unit Test conducted with the room lights on Explain “This test helps determine whether your eye muscles are causing eye strain when you are reading” Turn Mallett on with OXO horizontal Appropriate near correction worn (correct PD) Demonstrate alignment of strips without polaroid by asking: “Please look at the X in the middle of the OXO” “Do you see two green strips, one above and one below the OXO?” “Are the strips exactly in line with each other and in line with the strips?” Near Mallett Unit Put polaroid visor on Px Point at text that matches Px’s near VA and ask “Please read this paragraph” – this establishes a binocular lock Ask the patient if the two nodus strips are pointing towards the X or if either of the lines have moved Repeat with OXO vertical Check for cyclodeviation by asking “Do the strips appear tilted?” Possible responses The two Nonius targets are in alignment – The patient sees the two Nonius targets which are in line with the center of the X – No FD slip is present If this response is given then you can move on and check for vertical FD slip O X O Possible responses Only one Nonius target is seen – The patient may not have good enough visual acuity in one eye to resolve the Nonius target OR – The patient is suppressing (more on this next later) If this response is given then record the response (no need to check for vertical slip) O X O Possible responses One nonius target has moved – FD slip is present If this response is given then you need to measure the amount of FD slip O X O The top Nonius target Cover LE -- top Nonius disappears Moves to the right LE sees top target Work out which eye sees which Nonius target O X O Cover RE -- bottom Nonius Cover each eye in turn with an occulder disappears When you occlude an eye the nonius RE sees bottom target target that eye sees should disappear nose L R  The prism Work out which direction to moves the place your prism to allign Nonius line in the direction the Nonius targets of its apex Here the left eye sees the top nodius target – which has moved to the right We need to use a prism to align the nonius target with the X O X O – Need to move to the left nose L R The prism should be placed infront of the relivant eye (base IN)  Horizontal Fixation Disparity LE EXO (crossed) FD slip OXO add weakest BI prism or minus sphere for alignment RE LE ESO (uncrossed) FD Slip OXO add weakest base out prism (Can try positive spectacle lenses) RE LE EXO FD or slip in RE add weakest BI prism OXO (can try negative spectacle lenses RE  Vertical fixation Disparity LE OXO R/L (R hyper) FD or slip in both eyes RE add weakest BD RE/BU LE prism for alignment RE OXO L/R (L hyper) FD or slip in both eyes LE add weakest BD LE/BU RE prism for alignment Torsional Fixation Disparity LE RE OXO Incyclo FD or slip in both eyes OXO Excyclo FD or slip in both eye LE RE LE OXO L Incyclo FD or slip RE More information regarding the Mallet unit test Use the minimum prism required in order to correct the FD slip If slip is present in one eye only (only one nonius target moves) then prism should be prescribed to that eye only – This is debatable The slip can indicate if prism or exercises are required to restore stable binocular vision The Mallet unit actually measures the associated phoria – Strictly speaking it does not measure FD slip directly Recording your findings No fixation disparity slip = FD no slip If prism required Record prismatic power = for alignment required and base direction The Sheedy Disparometer The Mallet unit uses polarised targets in fixed positions and a prism was used to correct any misalignment The Sheedy Disparometer has Polarised targets which can move according to a rotating dial If FD is present then the patient moves these targets until they appear to be aligned The distance those targets have moved indicates the amount of fixation disparity Cross-polarised nonius bars for horizontal FD.  The Sheedy Disparometer - No FD If no fixation disparity is present then: The lines will appear aligned to the patient The lines will actually be aligned Actual position of targets Patients perception of where once adjusted by patient the targets are positioned  The Sheedy Disparometer - No FD R Fixation plane L Perception L R  The Sheedy Disparometer - Exo FD If a exo fixation disparity is present then the patient will need to move the lines until they appear alligned: The lines will appear aligned to the patient The lines will actually be misaligned R R L L Actual position of targets Patients perception of where once adjusted by patient the targets are positioned  The Sheedy Disparometer - Exo FD R Fixation plane L Perception L R  The Sheedy Disparometer - Eso FD If a eso fixation disparity is present then the patient will need to move the lines until they appear aligned: The lines will appear aligned to the patient The lines will actually be misaligned R R L L Actual position of targets Patients perception of where once adjusted by patient the targets are positioned  The Sheedy Disparometer - Eso FD R Fixation plane L Perception L R Forced duction fixation disparity curves Not routinely used in UK but has diagnostic value Prisms are added infront of the eye and the Sheedy disparometer is used to measure the FD for each prims Plot FD as function of inducing prism How to create a Fixation disparity curve Example with a patient with no fixation disparity slip No prism has been placed infront of the R eyes Fixation The patient has no plane L FD slip The apparent position of the targets is in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 2 D base IN is Δ introduced R The patient can Fixation compensate for plane L this prism and so no FD slip is found The apparent position of the targets is in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 4 D base IN is Δ introduced R The patient can Fixation still compensate plane L for this prism and so no FD slip is found The apparent position of the targets is still in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 6 D base IN is Δ introduced R The patient can stillFixation compensate for plane L this prism and so no FD slip is found The apparent position of the targets is still in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 8 D base IN is Δ introduced Fixation R The patient can no longer plane L compensate fully for the prism however the targets still lay within Panums fusional area FD slip is found L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 10 D base IN is Δ introduced R The patient can Fixation no longer plane L compensate fully for the prism however the targets still lay within Panums fusional area A greater amount of FD slip is found L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 12 D base IN is Δ introduced R The patient can Fixation no longer plane L compensate for the prism and the patient now has diplopia L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip No prism has been placed R infront of the Fixation eyes plane L The patient has no FD slip The apparent position of the targets is in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 2 D base OUT is Δ introduced R The patient can Fixation compensate for plane L this prism and so no FD slip is found The apparent position of the targets is in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 4ΔD base OUT is R introduced Fixation plane The patient can L still compensate for this prism and so no FD slip is found The apparent position of the targets is still in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 6ΔD base OUT is R introduced Fixation plane The patient can L still compensate for this prism and so no FD slip is found The apparent position of the targets is still in the “correct” place L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 8ΔD base OUT is Fixation R introduced plane L The patient can no longer compensate fully for the prism however the targets still lay within Panums fusional area FD slip is found L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip R 10ΔD base OUT is Fixation plane introduced L The patient can no longer compensate fully for the prism however the targets still lay within Panums fusional area A greater amount of FD slip is found L R How to create a Fixation disparity curve Example with a patient with no fixation disparity slip 12ΔD base OUT Fixation R is introduced plane The patient can L no longer compensate for the prism and the patient now has diplopia L R  How to create a Fixation disparity curve Example with a patient with no fixation disparity slip A graph can then be plotted with the amount of prism used on the x axis and the FD on the y axis Base IN Base OUT Prism 12 10 8 6 4 2 0 2 4 6 8 10 12 FD Dip 4 2 0 0 0 0 0 0 0 2 4 Dip eso eso exo exo  Example of a class 1 curve with no FD slip 6 4 ESO Fixation disparity 2 OUT 0 IN 2 4 EXO 6 12 10 8 6 4 2 0 2 4 6 8 10 12 Prism added  Example of a class 1 curve with a FD slip 6 4 ESO Fixation disparity 2 OUT 0 IN 2 4 EXO 6 12 10 8 6 4 2 0 2 4 6 8 10 12 Prism added Components of fixation disparity curve Y-intercept – fixation disparity X-intercept – Aligning prism (associated phoria) Slope – Indicates ability to adapt to prism induced stress – Flat slope: desirable, good adaptation, Sx unlikely – Steep slope: poor adaptation, Sx likely Type of curve – 4 types  Curve types Type I ESO ESO Type II - 60% population - 25% popn - often no Symptoms - often high SOP BI BO BI BO - no aligning prism! - Frequent Symptoms EXO EXO Type III ESO ESO Type IV - 10% population - 5% popn - high XOP BI BO BI BO - often unstable - no aligning prism! binocularity - frequent Symptoms EXO EXO Further reading Elliott, D.B. (2007) Clinical Procedures in Primary Eye Care. Third Edition. Butterworth Heinemann, Oxford. Benjamin, W.J. (2006) Borish’s Clinical Refraction. Second edition. Philadelphia: WB Saunders. Binocular Vision Lecture 9 – Fixation disparity Dr Phillip Buckhurst