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Measurement of Fixation Disparity
Mallett Units <ul><li>Most popular method in UK </li></ul><ul><li>Does not measure physical disparity </li></ul><ul><li>Me...
Ideal Parameters <ul><li>Normal visual environment </li></ul><ul><li>Central and peripheral fusion locks </li></ul><ul><li...
Distance Unit
Distance Unit <ul><li>Red markers </li></ul><ul><li>Foveal fixation 10 min arc </li></ul><ul><li>Central lock OXO </li></u...
Near Unit
Near Unit <ul><li>Green markers </li></ul><ul><li>Foveal fixation 18 min arc </li></ul><ul><li>Central lock OXO </li></ul>...
Mallett Unit
Correcting Disparity <ul><li>Use ∆ before eye with slip </li></ul><ul><li>Add enough ∆ to align markers with X </li></ul><...
Treating the Disparity <ul><li>Only correct if patient has symptoms </li></ul><ul><li>Add ∆ before eye with slip </li></ul...
Exo Disparity <ul><li>Weakest BI ∆ to eliminated slip </li></ul><ul><li>Modify Rx in -ve direction </li></ul><ul><li>Ortho...
Eso Disparity <ul><li>Weakest BO ∆ to eliminate slip </li></ul><ul><li>Not possible to alter Rx at distance but may be pos...
Vertical Disparity <ul><li>Eliminate any horizontal slip </li></ul><ul><li>Check optical centres on specs </li></ul><ul><l...
Objective Measure of FD <ul><li>Pickwell and Stockley 1960 </li></ul>
Objective Measure of FD
Summary I <ul><li>Size of phoria not necessarily related to presence of FD </li></ul><ul><li>Size of phoria not always rel...
Management of Heterophoria <ul><li>Oculomotor Measures </li></ul><ul><ul><li>Size of phoria </li></ul></ul><ul><ul><li>Fus...
Normative Data
Fixation Disparity Data <ul><li>Curve type </li></ul><ul><li>Y intercept </li></ul><ul><li>X intercept </li></ul><ul><li>S...
Phoria and FD
Frequency of Curve Type 7 3 IV 10 7 III 25 30 II 58 60 I Near Distance Type
Discriminative Value 89% correct 89% correct 90% correct Opposing Vergence Vergence Recovery Break Range Break Range Oppos...
Treatment <ul><li>Vision Training </li></ul><ul><li>Lenses </li></ul><ul><li>Prisms </li></ul><ul><li>Combination of above...
Summary II <ul><li>FD Useful additional test </li></ul><ul><li>Don’t FD use in isolation </li></ul><ul><li>Too much emphas...
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Mallet Unit

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Mallet Unit

  1. 1. Measurement of Fixation Disparity
  2. 2. Mallett Units <ul><li>Most popular method in UK </li></ul><ul><li>Does not measure physical disparity </li></ul><ul><li>Measures amount of ∆ to reduce disparity to zero </li></ul><ul><li>Used at distance and near </li></ul>
  3. 3. Ideal Parameters <ul><li>Normal visual environment </li></ul><ul><li>Central and peripheral fusion locks </li></ul><ul><li>Detect small disparities </li></ul><ul><li>Monocular markers not easily suppressed </li></ul><ul><li>Monocular markers close to central fixation point </li></ul><ul><li>Detect horizontal and vertical disparities </li></ul><ul><li>Easily understood by patients </li></ul>
  4. 4. Distance Unit
  5. 5. Distance Unit <ul><li>Red markers </li></ul><ul><li>Foveal fixation 10 min arc </li></ul><ul><li>Central lock OXO </li></ul><ul><li>Peripheral lock room features </li></ul><ul><li>Use with distance RX </li></ul>
  6. 6. Near Unit
  7. 7. Near Unit <ul><li>Green markers </li></ul><ul><li>Foveal fixation 18 min arc </li></ul><ul><li>Central lock OXO </li></ul><ul><li>Peripheral lock text on page </li></ul><ul><li>Use with near Rx </li></ul>
  8. 8. Mallett Unit
  9. 9. Correcting Disparity <ul><li>Use ∆ before eye with slip </li></ul><ul><li>Add enough ∆ to align markers with X </li></ul><ul><li>Disparity can be altered using lenses </li></ul>
  10. 10. Treating the Disparity <ul><li>Only correct if patient has symptoms </li></ul><ul><li>Add ∆ before eye with slip </li></ul><ul><li>If ∆ too high split between eyes </li></ul>
  11. 11. Exo Disparity <ul><li>Weakest BI ∆ to eliminated slip </li></ul><ul><li>Modify Rx in -ve direction </li></ul><ul><li>Orthoptic exercises </li></ul>
  12. 12. Eso Disparity <ul><li>Weakest BO ∆ to eliminate slip </li></ul><ul><li>Not possible to alter Rx at distance but may be possible at near </li></ul><ul><li>Orthoptic exercises </li></ul>
  13. 13. Vertical Disparity <ul><li>Eliminate any horizontal slip </li></ul><ul><li>Check optical centres on specs </li></ul><ul><li>Check head position </li></ul><ul><li>Any residual vertical slip use ∆ </li></ul>
  14. 14. Objective Measure of FD <ul><li>Pickwell and Stockley 1960 </li></ul>
  15. 15. Objective Measure of FD
  16. 16. Summary I <ul><li>Size of phoria not necessarily related to presence of FD </li></ul><ul><li>Size of phoria not always related to symptoms </li></ul><ul><li>FD not always related to symptoms </li></ul><ul><li>Amount of ∆ not related to size of FD </li></ul><ul><li>Use in conjunction with other tests of BV </li></ul>
  17. 17. Management of Heterophoria <ul><li>Oculomotor Measures </li></ul><ul><ul><li>Size of phoria </li></ul></ul><ul><ul><li>Fusional reserves (blur, break, recovery) </li></ul></ul><ul><ul><li>AC/A </li></ul></ul><ul><ul><li>Fixation Disparity </li></ul></ul>
  18. 18. Normative Data
  19. 19. Fixation Disparity Data <ul><li>Curve type </li></ul><ul><li>Y intercept </li></ul><ul><li>X intercept </li></ul><ul><li>Slope </li></ul>
  20. 20. Phoria and FD
  21. 21. Frequency of Curve Type 7 3 IV 10 7 III 25 30 II 58 60 I Near Distance Type
  22. 22. Discriminative Value 89% correct 89% correct 90% correct Opposing Vergence Vergence Recovery Break Range Break Range Opposing Vergence Recovery Range Recovery Range X Intercept Opposing Vergence FDC Slope Y Intercept FDC Slope Phoria Sheards Sheards SOP XOP All Subjects
  23. 23. Treatment <ul><li>Vision Training </li></ul><ul><li>Lenses </li></ul><ul><li>Prisms </li></ul><ul><li>Combination of above </li></ul>
  24. 24. Summary II <ul><li>FD Useful additional test </li></ul><ul><li>Don’t FD use in isolation </li></ul><ul><li>Too much emphasis on FD </li></ul><ul><li>Clinical judgement required </li></ul>

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