Subjective refraction by optom praveen

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mulamoottil eye hospital
kerala

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Subjective refraction by optom praveen

  1. 1. Subjective refraction <br />By <br /> praveen <br />
  2. 2. Subjective refraction technique rely on the patient’s response to obtain the refractive correction that gives the best visual acuity .<br />If all the refractive errors were simply spherical ,subjective refraction will be easy.<br />Determining the astigmatic portion of the correction is more complex .<br />
  3. 3. Different subjective refraction techniques are employed .09323330039 paul<br />
  4. 4. Cross-cylinder technique[jackson cross cylinder ] <br />First step in cross cylinder refraction is adjusting the sphere to yield best visual acuity without accommodation .<br />Place the prescription the patient is wearing ,if any ,into a trial frame or phoropter .<br />Fog the eye to be examined with plus sphere while the patient views a visual acuity chart.<br />Then decrease the fog until best visual acuity is obtained <br />
  5. 5. If no cylindrical correction is present initially ,the cross cylinder may still be used ,placed arbitrarily at 90 0 and 180 0 .<br />To check the presence of astigmatism .<br />If a preferred flip position is found ,cylinder is added with axis parallel to the respective plus or minus axis of the cross cylinder until the two flip choices are equal .<br />
  6. 6. If no preference is found with the cross- cylinder axis at 90 0 and 180 0 ,45 0 and 135 0 should always be checked before assuming that no astigmatism is present .<br />Once any cylinder power is found , axis and power are refined in the usual manner .<br />
  7. 7. Cylinder axis is always refined first .<br />This sequence is necessary because the correct axis can be found in the presence of an in correct power .<br />But the full cylinder power will not be found in the presence of an incorrect axis .<br />
  8. 8. To refine the axis place the cross cylinder with its principal meridians 45 0 away from the principal meridians of the correcting cylinder .<br />Present the patient with alternative flip choices and inquiring which is” blackest and sharpest “.<br />Rotate the axis of the correcting cylinder toward the corresponding plus or minus axis of the cross cylinder .<br />
  9. 9. Low-power cylinder are rotated in large steps ,high powered cylinder are rotated in small steps ,repeat the procedure until the flip choices appear equal .<br />To refine cylinder power ,align the cross – cylinder axis with the principal meridians of the correcting lens .<br />As the examiner changes cylinder power according to the patient’s response .<br />The spherical equivalent of the refractive correction should remain constant . <br />
  10. 10. That is for every 0.50 D of cylinder power change ,the sphere is changed 0.25 D in the opposite direction .<br />If the cylinder power is changed a large amount ,the sphere power should be readjusted for best visual acuity .<br />Continue to refine cylinder power until choices appear “about the same “to the patient ,that will be the correct endpoint .<br />
  11. 11. Remember always to use the proper cross cylinder for the patient’s visual acuity level .<br />For eg :- +- 0.25 D cross cylinder is commonly used with visual acuity levels of 20/30 and better .<br />A high cross cylinder [+- 0.50 D or +- 1.00 D ] should be used with poorer vision .<br />
  12. 12. Following steps are used in cross –cylinder refraction .<br />Adjust sphere to the most plus or least minus that gives best visual acuity .<br />Use test figure one or two lines larger than the patient’s BCVA , because the introduction of the JCC produce blur .<br />If cylinder correction is not already present ,look for astigmatism .<br />
  13. 13. Refine axis first <br />Refine cylinder power .<br />Refine sphere ,cylinder axis ,and cylinder power .<br />
  14. 14. Tricking <br />Avoid confusion with previous choices by giving different number to subsequent choices <br />Which is better ,one or two ,three or four?<br />If the patient persists in always choosing either the first or the second number ,reverse the order .<br />
  15. 15. Clock Dial <br />When the clock dial is presented to the patient under sufficient fog .<br />The examiner’s first job is to determine the axis of the correcting cylinder .<br />This is done by first asking the patient if he or she can see three lines in any or all of the spokes .<br />And then ask to report in which of the spokes the three lines are the sharpest or most distinct .<br />
  16. 16.
  17. 17. Most clock dial charts equipped with numbers similar to those on the face of a clock .<br />Expected response of the patient is that the spoke from 12 to 6 ,from 1 to 7 ,from 2 to 8 o’clock is most distinct .<br />To determine the axis of the correcting cylinder ,the smaller of the two number reported by the is multiplied by 30.<br />
  18. 18. For example ,if the patient reports that the 12 t 6 o’clock spoke is most distinct the examiner would place the axis of the correcting cylinder at 180 degrees.<br />The examiner then begins adding minus cylinder power ,0.25 D at a time .<br />Questioning the patient each time as to the relative sharpness of the lines in the spokes representing the two principal meridians of the eye . <br />
  19. 19. If sufficient minus cylinder power is added ,the horizontal and vertical focal lines will be located in the same plane .<br />Then the patient will report that the 12 to 6 and the 3-to -9 spokes are equally distinct .<br />This procedure is referred to as “collapsing the conoid of sturm “ the horizontal and vertical focal line are replaced by a point image .<br />
  20. 20. Monocular subjective refraction <br />Monocular subjective refraction consists of the following procedure <br />Determining the cylindrical correction under fog. <br />Refining the cylindrical correction without fog .<br />Determining the spherical end point .<br />Following these procedure ,one or more binocular balancing tests are performed .<br />
  21. 21. Determining the cylinder under fog <br />Suitable fogging lenses will have to be put into place before beginning the monocular subjective refraction .<br />The left eye is occluded ,a block of letter is introduced at the 6-m distance .<br />Plus power in front of the right eye is reduced ,0.25 at a time until the patient can read all of the 20 /40 letters .<br />Ensures that the eye is fogged by approximately 1.00 D ,so the entire conoid of sturm is in front of the retina . <br />
  22. 22. The astigmatic chart is presented ,and the axis of the correcting cylinder is determined .<br />If clock dial is used ,the patient is asked first whether three lines can be seen in any or all of the spokes .<br />If he can see then ask the patient to report in which of the spokes the three lines are the most distinct .<br />
  23. 23. Minus cylinder is added to the indicated axis 0.25 D at a time until equality is obtained .<br />Additional minus cylinder power is then added 0.25 D at a time to obtain a reversal .<br />Then the examiner returning to the lowest-power cylinder that brings about equality .<br />
  24. 24. The right is then occluded and the procedure are repeated for the left eye .<br />
  25. 25. Refining the cylinder without fog <br />On completion of the astigmatic chart test .<br />The right eye is defogged ,reducing plus lens power 0.25 D at a time ,until the best visual acuity line [20/20 or 20/15 ] is reads.<br />When the removal of additional plus power [or addition of minus power ]fails to improve visual acuity ,the spherical lens power is returned to the maximum plus or minimum minus power that resulted in the best visual acuity for that eye .<br />
  26. 26. Beginning with the cylinder power and axis found with the astigmatic chart under fog .<br />The practitioner first refines cylinder axis and then cylinder power .<br />
  27. 27. Crossed –cylinder test for Axis <br />
  28. 28. Crossed –cylinder Test for power<br />
  29. 29. Additional cylinder check tests <br />Additional check test for cylinder power .<br />Many patient tended to reject cylinder power in the crossed- cylinder test even though it has been evident both in retinoscopy and in the astigmatic chart test .<br />This occurs ,the patient is asked to watch the smallest readable row of letters .<br />
  30. 30. An additional -0.25 D cylinder is placed in the refractor ,and the patient is asked to report which of the two views appears to be more distinct .<br />If the additional -0.25 D cylinder does not improve the clarity of the letters ,the original cylinder power is left in the refractor .<br />
  31. 31. If the additional cylinder power causes the letters to be more distinct ,the examiner can choose to leave the additional cylindrical power in the refractor.<br />
  32. 32. Additional check test of axis <br />The additional cylinder check test for axis is sometime referred to as bracketing .<br />It is useful mainly when the power of the correcting cylinder is 1.00 D or more .<br />The patient is asked to view a row of 20/20 or 20/15 letter <br />And ask to report when the cylinder lens in the refractor is slowly rotated.<br />
  33. 33. When the blur is reported ,the change in the axis is noted mentally .<br />And the test is repeated with the same instructions .<br />For example : if the original correcting cylinder axis is located at 180 degrees .<br />The patient reports a blur at 15 degrees and blur at 165 degrees ,then the examiner can assume that the orginal axis was correct .<br />
  34. 34. Determining the spherical end point <br />Each eye is fogged to 0.75 or 1.00 D and defogged to best acuity.<br />Maximum plus power of best visual acuity .<br />Examiner should present a block of letters extending from 20/40 to 20/15.<br />We should make a mental note of the patient’s acuity for each eye through the +0.75 or +1.00 D fogging lens prior to defogging .<br />
  35. 35. It is important that accommodation be relaxed in arriving at the monocular end point .<br />For example : +1.00 D fog may blur the right eye to 20/30 ,but it may blur the left eye only to 20/20 .<br />This evidence that the left eye is underplussed [or over minused ] ,and the subjective end point for both eye should be determined again .<br />
  36. 36. Patient instruction <br />To determine the monocular endpoint correct patient instruction are important .<br />The examiner should understand that the patient’s subjective evaluation of the clarity or distinctness of the letters is not the important consideration .<br />The important ,overriding consideration is the ability of the patient to resolve the letters .<br />
  37. 37. As plus power is decreased 0.25 D at a time .<br />When the point has been reached where an additional decrease in plus power of 0.25 D does not make any more letter readable.<br />On the other hand ,if the examiner allows the patient to respond in terms of clarity of the letter or in terms of which lens is preferred .<br />
  38. 38. Many young patient will continue to accommodate with each 0.25 D of reduction in plus ,with the result that the end point will be completely invalid.<br />When minus lens power is added to the point that accommodation is necessary to keep the letters in sharp focus on the retina <br />Many patient will notice that the letter appear to be smaller . <br />
  39. 39. This is purely optical effect ,known as accommodative micropsia .<br />it is right to ask the patient if the letter looks smaller to avoid overminusing the patient .<br />If patient report that the letter are not smaller should not be taken as evidence .<br />
  40. 40. Binocular balance <br />The purpose of binocular balancing tests is not to balance the visual acuity but to balance the state of accommodation of the two eyes .<br />If the corrected visual acuity is same in both eye ,the balancing procedure may consist of comparison of the visual acuity for the two eyes .<br />If the correcting visual acuity is not same in both eyes ,then a method not based on visual acuity must be used .<br />
  41. 41. Procedure used for balancing the state of accommodation for the two eyes are often referred to as equalization tests or as binocular balancing tests .<br />
  42. 42. Balancing with little or no fog <br />On completion of the monocular subjective refraction ,the lenses are left in the refractor .<br />The occluders has to remove from both eyes .<br />The patient’s attention is called to a block of letters at 6 m.<br />Plus lens power is added in front of both eyes until the 20/20 letter are blurred but the 20/25 letter are easily resolved .<br />
  43. 43. This normally requires an increase in plus or a decrease in minus of 0.25 to 0.50 D.<br />Then ask to compare the clarity of the 20/25 letters for the two eyes .<br />Using either prism dissociation or alternative occlusion . <br />
  44. 44. Prism dissociation <br />The examiner place 3 of base –down prism in front of the right eye and 3 of base-up prism in front of the left eye .<br />The patient will see two charts separated –vertically .<br />The upper chart seen by right eye .<br />Patient attention is called to the 20 /25 letters . <br />
  45. 45. Then asked to report whether the letter are more distinct or easier to read in the upper chart or the lower chart .<br />If the equally distinct for the two eyes, the accommodative state of the two eyes is considered to be balanced ,and the test is over .<br />
  46. 46. If the patient reports a difference in clarity of the letters for the two eyes.<br />0.25 D is added in front of the eye with the better vision and the test is repeated .<br />Often the patient fails to report equal clarity for any lens combination .<br />This problem can be solved by instructing the patient to report “which of the lens combination causes the upper and lower charts to be more nearly equal .<br />
  47. 47. Another way of resolving this difficulty is to give better acuity to the dominant eye .<br />Once the patient’s acuity is balanced at 20/25 the patient is defogged binocularly.<br />
  48. 48. Alternate occlusion <br />Patient is instructed to compare alternate views of the chart while the each eye is alternately occluded.<br />Occluder in the refractor can be used for occlusion .<br />Fogging to 20/25 .<br />Adding 0.25 D to the better eye ,are done in the same manner of prism dissociation test .<br />
  49. 49. Check Test for the Binocular End point <br />Once the examiner is satisfied with the binocular balance ,binocular end point can be verified .<br />Patient’s attention is called to the 20 /20 letter .<br />Add +0.25 D sphere to both eye and ask for any difference in the clarity of the letters.<br />
  50. 50. The same question is again asked after adding 0.25 D sphere .<br />Again third +0.25 D sphere is added ,ask of the difference in the clarity .<br />Expected response are that the 20/20 letters will be “slightly blurred “. With the first 0.25 D of plus .<br />“badly blurred “ with the second 0.25D.<br />“blurred out “ with the third 0.25 D .<br />
  51. 51. Bichrome test <br />As a monocular End –point test <br />The bichrome test must be done in an almost completely darkened room .<br />Starting with the result of monocular subjective finding .<br />+0.50D or + 0.75 D of spherical power is placed in front of each eye.<br />The red and green filter is placed on the projector.<br />
  52. 52.
  53. 53. The patient is asked to report which of the letters ,those on the red background or those in the green background –are “sharper blacker ,or more distinct “.<br />The patient is expected to report that the letters on the red background are more distinct than those on the green background .<br />
  54. 54. As the plus power is reduced 0.25 D at a time .<br />At some point the patient should report that the letter on red and green background are equally distinct .<br />As plus power is further reduced ,the patient should say those on the green side are more distinct than the red side .<br />
  55. 55. If the original monocular subjective endpoint was correct ,the patient will typically report that the red letters are more distinct with +0.75,+0.50,+0.25 D fog .<br />When all the fog is removed the letter will appear to be equally distinct on both the red green sides .<br />
  56. 56. Prescribing for children <br />Prescribing visual correction for children often has two goals :<br />Providing a focused retinal image .<br />Achieving the optimal balance between accommodation and convergence .<br />In some cases subjective refraction is impossible because child’s inability to cooperate.<br />The optimal refraction in an infant requires the paralysis of accommodation with complete cycloplegia .<br />
  57. 57. The presence of strabismus may modify normal prescribing guidelines .<br />
  58. 58. Myopia<br />[general guidelines for correction of significant childhood myopia]<br />Cycloplegic refraction are mandatory .<br />Full refractive error ,including cylinder ,should be corrected .young children tolerate cylinder well.<br />Some will under correct myopia ,other may even use bifocals with or without atropine ,on the theory that prolonged accommodation hastens .<br />
  59. 59. Intentional undercorrection of a myopicaesotrope to decrease the angle of deviation is rarely tolerated .<br />Indentional overcorrection of a myopic error can be of some value in controlling an intermittent exodeviation .<br />Parents should be educated about the natural progression of myopia and the need for frequent refraction and possible prescription changes .<br />
  60. 60. Contact lenses may be desirable in older children to avoid the problem of image minification found with high- minus lenses.<br />
  61. 61. Hyperopia<br />When hyperopia and esotropia coexist ,initial management includes full correction of the cycloplegic refractive error .<br />In school age child ,the full refractive correction may cause blurring of distance vision because of the inability to relax accommodation fully .<br />The amount of correction may have to be reduced for the child to accept the glasses.<br />

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