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Visual Optics 2006/2007

Visual Optics 2006/2007. Chapter 6 Astigmatism & Subjective Refraction. Phoropter JCC. 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. JCC Handle. 105. 75. 120. 60. 135. 45. 150. 30. 165.

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Visual Optics 2006/2007

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

  2. Phoropter JCC

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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)

  10. 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:

  11. Page 6.43 Clinical Subjective Refraction: Focal Line Approach

  12. Clinical Subjective Refraction Focal Line Approach Fig. 6.28 Page 6.43 Take a Compound hyperopic astigmat to demonstrate the method: (works for any astigmat)

  13. Clinical Subjective Refraction Focal Line Approach (CHA) Fig. 6.28 (1) Page 6.43 Plus sphere To maintain control of patient accommodation, the posterior focal line is moved to the retina with sphere before starting the astigmatic correction

  14. The Astigmatic Fan Chart Fig. 6.29 Page 6.45 With a vertical FL on the retina, the patient should see the 90 Fan Chart Line clear. The 180 lines should be most blurred

  15. The Astigmatic Fan Chart Fig. 6.29 Page 6.45 Because the anterior FL is in front of the retina, the clinician has control of patient accommodation. The patient cannot make the horizontal lines clear

  16. Example 6.7 - The "Fan and Block" Method of Refraction This is the most complete form of subjective focal line refraction. Like JCC, it is rarely used as a fully subjective procedure After a difficult retinoscopy, or inconsistent JCC findings, the Fan and Block Method is a good alternative, especially to locate cylinder axis. The Fan and Block Method is most useful for patients with large amounts of astigmatism

  17. Example 6.7 - The "Fan and Block" Method of Refraction Fig. 6.30 Page 6.46 Step 1: BVS (b) Step 2: vision with BVS. Should get ~20/60 Therefore we predict 2 D astigmatism

  18. Example 6.7 - The "Fan and Block" Method of Refraction Fig. 6.30 Page 6.46 Step 2: vision with BVS. Should get ~20/60 Therefore we predict 2 D astigmatism Step 3: Move the posterior FL to the retina (c) With the COLC on the retina of a 2 D astigmat (b), we add +1.00 DS to shift the posterior FL to the retina (c).This moves the COLC 1 D in front

  19. Example 6.7 - The "Fan and Block" Method of Refraction Fig. 6.30 Page 6.46 With the posterior FL on the retina, we now direct the patient to view the Fan Chart

  20. Example 6.7 - The "Fan and Block" Method of Refraction Fig. 6.30 Page 6.46 With the posterior FL on the retina, we now direct the patient to view the Fan Chart

  21. Most Clinicians then move the Posterior FL slightly i.f.o. the Retina Fig. 6.30 Page 6.46 Next step is axis refinement This is done with a Maddox ‘V’ (Locator) in the center of the full Fan and Block Chart

  22. The "Fan and Block" Chart Fig. 6.32 Page 6.49 Maddox V

  23. 22.5 22.5 The "Fan and Block" Chart Fig. 6.32 Page 6.49 Each leg of the ‘V’ subtends 22.5 with the direction the V is pointing The two legs therefore correspond to Fan Chart line orientations 22.5 away from where the ‘V’ is pointing

  24. The "Fan and Block" Chart Fig. 6.32 Page 6.49 Both sides equally blurry (22.5 away from 90)

  25. The "Fan and Block" Chart Fig. 6.32 Page 6.49 When the V points at the retinal FL, both sides appear equally blurry If the retinal FL is vertical, and the V is rotated 22.5 clockwise, the right leg will be parallel to 90, and the left leg will be pointing at 135

  26. The "Fan and Block" Chart Right side clearer (parallel to 90)  rotate in opposite direction to clearer leg The key to axis refinement: rotate the ‘V’ in the opposite direction to its clearer leg until both legs are equally blurry. Just like JCC, the endpoint will be an “equal blur” response

  27. The "Fan and Block" Chart Fig. 6.32 Page 6.49 Right side clearer  rotate toward 90 again

  28. The "Fan and Block" Chart Fig. 6.32 Page 6.49 Both sides equally blurry

  29. We now know the posterior FL is oriented at exactly 90 (V pointing at 90) Next we add minus cylinder to move the anterior (horizontal) FL back toward the retina Negative cylinder (power meridian 90) axis 180 will move the horizontal FL backward. Note that cyl axis is parallel to the anterior focal line

  30. Initially the horizontal “blocks” will be very blurred As we add minus cyl axis 180, the horizontal blocks gradually become clearer. Endpoint = H and V blocks equally clear Fig. 6.32 Page 6.49 Corrector “Block”

  31. The endpoint of the cylinder phase is a point focus 0.50 D in front of the retina. If too much cylinder is added, a new IOS is created with the horizontal FL becoming the posterior FL. If 0.25 D excess cyl is added, the horizontal FL will be 0.25 D in front of the retina and the vertical remains 0.50 D in front A reversal of “block” clarity is the cue that excess cylinder has been added Cyl is dropped back to 2.00 DC (equal clarity of blocks) The final stage is a sphere fog (+0.50 DS; total +1.00 DS) and defog to optimum Visual Acuity) With posterior FL 0.50 D in front of the retina, we will add minus cyl axis 180 in 0.50 D, then 0.25 D increments (Fig 6.30 (e) – (j), Page 6.46). Notice that no sphere adjustment is made throughout the cylinder phase

  32. Fig. 6.34 Page 6.52 The Fan and Block Procedure

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