asigmatic clock and fan .pdf
This document describes two techniques for measuring astigmatism: the astigmatic clock dial technique and the astigmatic fan and block technique. For the clock dial technique, the patient looks at an astigmatic clock face and identifies the darkest line. Cylindrical lenses are added until all lines appear equally clear. The axis is calculated by multiplying the clock number by 30. For the fan and block technique, a fan chart with radiating lines is used. Cylindrical lenses are added until the blocks appear equally clear. The axis is determined by rotating an arrow until the limbs appear equally blurred. Both techniques involve fogging the eye, adding lenses until lines or blocks appear clear
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asigmatic clock and fan .pdf
- 1. Astigmatic Clock andFan Technique MEDICOACH INTERNATIONAL ACADEMY
- 2. Astigmatic Clock DialAnd Fogging Technique Steps:- Obtain best visual acuity using sphere only in one eye,with other being occluded Fog the eye by putting enough plus sphere before the eye to focus all meridia anterior to the retina Thus the patient relax accommodation and stabilize the refractive error of the eye. Patient is asked to look at the astigmatic dial and identify the darkest and sharpest line.
- 3. Add minuscylinder of progressively increasing power with axis perpendicular to the blackest and sharpest line till all lines appear equally clear. Rule of 30s may be used to calculate the axis of the minus cylinder That is multiply the lower number of the astigmatic dial clock with 30 to find the axis Now all the lines of the astigmatic dial appear equally black which means they are neutralized. But still are not in focus they are blurred since the eye is still fogged. Then switch to a distance vision chart and reduce sphere until the patient achieves maximum clarity. Focus is now on the retina.
- 4. Eg: 1.If a patient’sdarkest line is 3-9 o’clock(180° axis) line,then calculate the axis of the cylindrical lens Ans:3 × 30=90° 2.If a patient’s sharpest line is 6-12 o’clock position then calculate the axis of cylindrical lens Ans:6×30=180°
- 5.
- 6. Astigmatic Fan AndBlock Technique Also known as Maddox V test. Fan and block consist of a series of radiating lines spaced at 10 ° interval and arranged after the manner of the rays of a rising sun around a central panel carrying a V and two sets of mutually perpendicular lines(blocks) Also consist of series of radiating line spaced @ 15°,30° interval,but 10° spacing gives better accuracy. But to avoid confusing wider spacing is prefere6by many. The V and the block simultaneously can be rotated through 180°
- 8. Steps:- Obtain bestvisual acuity using sphere only Add positive sphere equal to half of the estimated amount of astigmatism in order to bring the eye into a state of simple myopic asigmatism. Refer the patient to the fan chart and ask which line or group of lines apper clearest and darkest. The blackest lines should blur,but if not,more positive sphere should be added until they do .
- 9. Direct theattention to the Maddox arrow and rotate it away from its blacket limb until both limbs appear equally blurred. This gives the axis of the astigmatism,should note that patient’s head is upright Directing attention now to the blocks,add negative cylinder at the appropriate axis until the second becomes as clear as the first Both block should blur equally,but if the blackest lines change over the astigmatism has been over corrected.
- 10. That isask the patient which block is clear,right or left If both are equally clear or blur the cylindrical power is correct If the patient says right is more clear that means patient need more cylindeical power so we should increase the power If the patient says left is clear the power is stronger so we should decrease cylindrical power . Continue the same until patient says both block are equally clear If right block is clear- Increase the power If left block is clear - Decrease the power
- 11. If theoriginally darker block again become darker ,the original sphere was wrong and must ne rechecked. Return to the letter chart and determine the sphere giving best visual acuity The cylindrical element remaining as jsut determined. If no lines appear blacker than the other there may be no astigmatism present
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