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Visual Optics 2006/2007

Visual Optics 2006/2007. Chapter 6 Astigmatism & Subjective Refraction.

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Visual Optics 2006/2007

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  1. Visual Optics 2006/2007 Chapter 6 Astigmatism & Subjective Refraction

  2. During Jackson Cross Cylinder (JCC) cylinder power refinement with a 0.25 D JCC, the JCC axes are aligned with the patient’s ocular principal meridians. What change occurs in the size of the Interval of Sturm between “first” and “second” views during cylinder power refinement? • 0.25 D • 0.50 D • 1.00 D • 1.50 D

  3. Refining Cylinder Power – JCC Power Meridians Shown Fig. 6.15 Page 6.24 JCC axes parallel and perpendicular to TCA for power refinement (b) 0.50 D IOS (c) 1.50 D IOS

  4. You are conducting the JCC part of a refraction with a sphere and a negative cyl axis 90 in the phoropter. You show the patient “first” and “second” views of a circular target. As a result of their response, “second is clearer”, you rotate the phoropter negative cyl axis to 80. What was the orientation of the JCC negative axis for the “second” view? • 45 • 90 • 135 • 180

  5. Trial Cyl axis 90 45 If the OD rotates the axis to 80, the resultant cyl for axis 90 plus JCC must have been rotated toward 80

  6. Axis Shift for JCC with Handle Parallel to Trial Cylinder Axis Table 6.2 Page 6.32 Axis Shift (degrees) Small trial cyl: large axis shift Large trial cyl: small axis shift

  7. During (JCC) cylinder axis refinement, a patient (correctly) responds to “first” and “second” presentations by saying that both views are unclear, but there is no difference between them. What does this indicate? • You are still a long way from the correct cylinder axis • The patient is confused and should be given a five minute break • The patient has no astigmatism • You have located the correct cylinder axis

  8. You have partially corrected an astigmatic patient’s refractive error and the COLC is on the patient’s retina. Under which of the following conditions will the introduction of a Jackson Cross Cylinder move the COLC off the retina? • during cylinder axis refinement • during cylinder power refinement • any time the JCC axes are not oriented either parallel/perpendicular to or 45 oblique to the patient’s ocular principal meridians • the JCC cannot change the location of the COLC

  9. Refining Cylinder Power – JCC Power Meridians Shown Fig. 6.15 Page 6.24 Patient prefers the view with the negative JCC power meridian parallel to the trial cylinder power meridian (both 90) The clinician thinks of this as negative JCC axis 180; trial cyl axis 180. Patient prefers minus on minus; therefore add more minus cylinder power. Change the 3.00 DC axis 180 to e.g. 3.50 DC axis 180. Compensate??

  10. Refining Cylinder Power – JCC Power Meridians Shown Fig. 6.15 Page 6.24 Change sphere from 2.50 DS to 2.25 DS First shrinks the IOS to zero, so this view is preferred Clinician again guided by negative JCC axis on negative TCA; therefore add more minus Next combination would be 2.00 DS to 4.00 DC First and second would produce “equally bad” response. Try 4.25 DC, then 3.75 DC.

  11. Refining Cylinder Power Page 6.25 When changing cyl from 4.00 DC to 4.25 DC, theoretically compensate with +0.125 DS. Smallest phoropter increment is 0.25 DS. So, what do we do? With COLC on retina, adding 0.25 DC moves the COLC 0.125 D behind the retina. This is fine, because the patient can accommodate the0.125 D to return the COLC to the retina Adding +0.25 DS would move the COLC slightly in front, giving the patient no way to return the COLC to the retina

  12. Final Sphere Determination Page 6.25 • Same procedure as for BVS verification • Fog the patient +1 D. Change phoropter sphere from 2.00 DS to 1.00 DS. This should make the px 1 D myopic and drop vision to 20/60 • Defog in negative 0.25 DS steps • Expected vision: • 1.00 DS (+1.00 D fog) 20/60 • 1.25 DS (+0.75 D fog) 20/40 • 1.50 DS (+0.50 D fog) 20/30 • 1.75 DS (+0.25 D fog) 20/25 or better • 2.00 DS (Zero fog) 20/20 or better (optimum visual acuity) • Patient must be able to read MORE letters down the chart to give them each 0.25 DS defog • No improvement means no more minus

  13. Page 6.26 Example 6.5 Jackson Cross Cylinder Procedure as “Seen” by the Clinician

  14. Dioptric separation of FLs = 5 D Vision with BVS: also a guide to Residual Astigmatism; but DOES not include effect of axis error 3.5 DS Page 6.27 Based on table 6.1, page 6.8

  15. Recap: Valid JCC ViewingTargets Fig 6.17, Page 6.27 A successful JCC procedure consistently maintains the COLC on the retina.  the most appropriate fixation target for the patient is a circular target Why? Because a circular target cannot give the patient any preference for a focal line on the retina over the COLC

  16. Spherical Equivalents in Partial Astigmatic Correction Page 6.42 • Some patients can only tolerate partial astigmatic correction (e.g. due to excessive spatial distortion with full correction) • When prescribing partial correction  patient has residual astigmatism, complete with IOS • Partial correction must place COLC on retina •  equivalent sphere to partial astigmatic correction must equal BVS (equivalent sphere to full correction)

  17. Spherical Equivalents in Partial Astigmatic Correction Page 6.42 Equivalent sphere to partial astigmatic correction must equal BVS (equivalent sphere to full correction) Example: full correction = 2.00 4.00 axis 180 Patient can only tolerate ()2.50 of cylinder Effectively, we have REMOVED 1.50 DC from the full correction To compensate for the 1.50 DC REMOVED, add 0.75 DS to Rx sphere Original correction = 2.00 4.00 axis 180 New partial correction = 2.75 2.50 axis 180 Equivalent sphere to partial correction:

  18. Negative JCC Axis JCC Handle 105 75 120 60 135 45 150 30 Positive JCC Axis 165 15 15 165 30 45 60 75 105

  19. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 TCA • If we are refining axis, where is the TCA? • 180 • 90 • 45 • 135

  20. Resultant of obliquely crossed TCA and JCC negative axis JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 TCA • If the patient prefers this view, what do we do next? •  cyl power 0.5 D •  cyl power 0.5 D • change TCA to 125 • change TCA to 145

  21. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 TCA • If we are refining cyl power, where is the TCA? • 90 or 180 • 45 or 135 TCA OR

  22. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 • If TCA is 180 and the patient prefers this view, what do we do next? •  cyl power 0.5 D •  cyl power 0.5 D • change TCA to 125 • change TCA to 145 TCA

  23. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 TCA • If TCA is 90 and the patient prefers this view, what do we do next? •  cyl power 0.5 D •  cyl power 0.5 D • change TCA to 125 • change TCA to 145

  24. Rationale of “Power Search” (Full Subjective JCC) Pp 6.29, 30 Lab step 4 p. 5 • Vision with BVS provides estimate of amount of astigmatism • e.g. vision with BVS = 20/60  predict 2.0 D astigmatism (Table 6.1) • If our prediction is correct, we will end up with XX DS 2.00 DC axis  • What our power search is doing is giving us an approximate value or “range” for  • The most efficient method for 2.0 D predicted astigmatism is to insert a sphere-compensated cyl that falls short of 2 D, so the JCC can make up the difference ( 0.25 D JCC  +0.25 DS 0.50 DC axis ) • So, for our 2 D “predicted” astigmat, we add 1.50 DC axis 180 (totally arbitrary starting axis) to the BVS. We compensate by changing sphere +0.75 DS from BVS power

  25. Rationale of “Power Search” (Full Subjective JCC) Pp 6.29, 30 Lab step 4 p. 5 • So, for our 2 D “predicted” astigmat, we add 1.50 DC axis 180 (totally arbitrary starting axis) to the BVS. We compensate by changing sphere +0.75 DS from BVS power • We set the JCC axes 90/180. If 180 just happens to be the patient’s axis, when the negative JCC axis is at 180:1.50 DC axis 180 0.50 DC axis 180 (JCC)  2.00 DC axis 180 • The patient is fully corrected, so they “accept” minus on minus • If they “reject” the extra 0.50 DC from the JCC (prefer plus JCC axis 180), they want less than 1.50 DC axis 180, so 180 is unlikely to be the axis • A reject, means try again at 45. Another reject means try 90. Reject means try 135 • Neutral at any of 180, 45, 90, 135 means we may be close to the axis. • Two neutrals, or a neutral and accept 45 apart suggests an in between axis

  26. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 TCA This would be a “Power Search” accept with TCA set at 90, IF the patient prefers this view

  27. JCC Handle 105 75 120 60 135 45 150 30 165 15 15 165 30 45 60 75 105 This would be a “Power Search” reject with TCA set at 180, IF the patient prefers this view TCA

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