Let’s suppose you have just finished your subjective refraction…
OD: -0.50 – 0.75 x 025, Distance VA: 20/20
OS: -2.75 – 3.00 x 165, Distance VA: 20/20
Would you prescribe the following manifest SRx as your final spectacle prescription for your patient?
Of course NOT, since your patient would probably complain of headaches due to unequal image sizes!
From this patient’s manifest SRx, you know that he/she has significant anisometropia due the large difference in both sphere and cylinder powers in the manifest SRx.
How do you modify this patient’s manifest SRx so that the patient has comfortable vision?
1) Using the concept of spherical equivalence to balance the circle of least confusion.
– If you remove -0.50 D cylinder power, then add -0.25 D of sphere power to the phoropter.
– Put up the 20/80 to 20/40 lines/VA chart.
– As you modify the cylinder power, ask the patient when there’s a large change in the clarity of chart
Ex: -2.75 – 3.00 x 165 –> Remove -0.50 D cyl power and add -0.25 D sphere power
Ex: -3.00 – 2.50 x 165 –> -3.25 – 2.00 x 165 –> -3.50 – 1.50 x 165 –> -3.75 – 1.00 x 165
2) After reducing the cylinder power, then have your patient wear a trial frame with the tentative reduced cylinder power SRx, and have your patient walk outside of your exam room, and view other people and objects in the real world:
3) OD: -0.50 – 0.75 x 025
OS: -3.75 – 1.00 x 165
4) Since there’s still significant amount of anisometropia, we need to reduce the spherical power of the left eye’s SRx
– Add +1.00 D (over-refract) to the OS until patient notices a difference in comfortable vision.
5) Final SRx:
OD: -0.50 – 0.75 x 025
OS: -1.75 – 1.00 x 165
Be sure to educate your patient regarding adaptation period for new SRx, and to return to clinic if difficulty in adaptation persists.
Don’t be one of the future eye care practitioners that forgets to balance an anisometropic patient’s SRx.
I can guarantee you that your anisometropic patient will be happy and grateful that you took the extra time to determine his/her MOST comfortable vision. Finally, this is an excellent example of when the patient’s 20/20 vision is not as important as comfortable vision.