Subjective Refraction Duochrome PDF

SUBJECTIVE REFRACTION: PART 2: DUOCHROME OTMS2613 N RAWAT OUTCOMES FOR THE UNIT: Describe the Duochrome theory Describe the history of the Duochrome technique Describe the purpose of the Duochrome technique Explain the setup of the Duochrome technique Correctly perf...

SUBJECTIVE REFRACTION: PART 2: DUOCHROME OTMS2613 N RAWAT OUTCOMES FOR THE UNIT: Describe the Duochrome theory Describe the history of the Duochrome technique Describe the purpose of the Duochrome technique Explain the setup of the Duochrome technique Correctly perform the Duochrome technique using the phoropter and trial frame State when the Duochrome technique is contraindicated List common errors encountered during the Duochrome technique Using diagrams, explain the effect of lenses on the clarity of the target Explain how other factors can influence the result of the Duochrome technique  Most optical systems, including the human eye, display axial chromatic aberrations Because of the chromatic aberration of the eye, the shorter wavelengths are focused in front of the longer wavelengths Duochrome Rabbetts estimated that when yellow light is focused on the retina, a patient would have excellent Visual acuity Theory If this wavelength is used as a reference, green light would then be focused in front of the retina and red light behind the retina. Red and green light are used in the Duochrome test because their dioptric distance of their focal points is equivalent to 0,50DS.  EMMETROPE or FULLY CORRECTED AMETROPE (PSEUDO-EMMETROPE) Patients should see targets equally on both the red and green background (EQUALITY). This would mean that yellow is focussed on the retina (best visual acuity). If you were performing the Duochrome test AFTER balancing, this could be a quick test used to determine if the final Rx is overminussed or underplussed. Patient reports that red is clearer, add minus lenses until EQUALITY. Patient reports that green is clearer, add plus lenses until EQUALITY.  E.g. 2. -0,75 -0,50 -0,25 plano +0,25 +0,50 +0,75 +1,00 +1,25 +1,50 +1,75 +2,00 Patient reports: G G G G G EQUAL EQUAL EQUAL R R R R To reach an endpoint of “1 into green”, how should we change the prescription if the patient reports: a. The letters on the red side are clearer b. The letters on both sides appear equal c. The letters on the green side are clearer  RECAP When Duochrome is performed DURING Subjective refraction, the endpoint for Duochrome is NOT EQUALITY. The endpoint is to leave the patient “1 into Green”. If patient is seeing the letters on the If patient is seeing the letters on If patient is seeing the letters on the RED side clearer, we will need to use both sides as being EQUALLY CLEAR, GREEN side clearer, we will need to MINUS lenses to DIVERGE the rays we will need to use MINUS lenses to use PLUS lenses to CONVERGE the so that green is closer to the retina DIVERGE the rays so that green is rays so that we can determine “1 closer to the retina into green” Purpose ◦ This procedure is performed to fine-tune the spherical correction of a patient ◦ This procedure, when performed before JCC, places the Circle of least confusion (COLC) on the retina ◦ This procedure, when performed after balancing, can be check to determine if patient has been overminused/underplussed Equipment required Phoropter or Trial frame+Trial case Spectrum or Projector Cannot be performed with only a Visual acuity chart  Setup Patient is seated with Rx Room lights off, door closed Target size: line above best VA (following on from Best sphere) (causes pupil dilation and (after Best sphere) prevents veiling glare) If blurry, try two lines above best VA Purpose and instruction to the patient ◦ “Ignoring the color of the backgrounds, are the letters TO or PL clearer and sharper?” ◦ Other notes/textbooks may include the use of the word “darker”. ◦ This can be misleading, as “darker”/”donker” can sometimes be misunderstood as unclear Procedure Letters on the red side are clearer: Add -0,25 sphere and repeat Q. Continue until 1st green response Letters on the green side are clearer: Add +0,25 sphere and repeat Q. Continue until Equal or Reversal. Then set back, “1 into green” Letters are equally clear: Add -0,25 sphere and repeat Q. Continue until 1st green response Procedure As soon as you complete the technique, Should there have been a change of ≥ 0,50DS switch the room lights back on → remember to check Visual acuity. If the VA has worsened, you can disregard the test If the VA remained the same, but more minus was added, consider that you may have overminused/underplussed the patient during Duochrome and consider disregarding the test  “1 into Green” means the patient is slightly overminused and therefore, this will stimulate accommodation and the patient will place their Circle of Least Confusion (COLC) on the retina themselves This serves as the starting point for JCC Why “1 into Other reasons: Under plussing slightly works well because we refract at 6m or less Green”? There is a tendency for the blue end of spectrum to be more focused at closer working distances (in our consulting rooms) which would generally be at equalisation if testing at true infinity. A patient wanting to use specs for night driving only the eye become slightly more myopic at night so a slight over minus may work well (for non –presbyopic patients), Patients with small pupils and/or high astigmatism (line foci are widely separated) and have a larger depth of focus Decreased stimulation to accommodate Tips and Tricks ◦ Take note of the VA before starting Duochrome ◦ If the VA is worse than 6/12, Duochrome is contraindicated for this eye ◦ Why? ◦ Difference of 0,50D between red and green ◦ Should all patients be left “1 into Green” during Duochrome? ◦ Yes: as a starting point for JCC ◦ Even patients >55 years of age: Yes ◦ Even pseudoaphakes: Yes Tips and Tricks ◦ Can be performed for color defective patients ◦ However, protans will find the red side to be more dull ◦ Other factors can affect the technique ◦ Yellowing of the lens due to nuclear sclerotic cataract can cause patients to prefer red ◦ Patient is overaccommodating can cause px to choose red ◦ Sometimes regardless of spherical prescription, patient prefers once color over another  Performing the technique even though it is contraindicated Poor instructions Common Errors Overminussing the patient during Duochrome Not checking VA after a change of ≥0,50DS