Your doctor will scrutinize your refraction, so why not learn to do it yourself first? Following are some strategies for analyzing your refractive findings: 1. A one diopter change in Rx should equal 3 lines of improvement on the Snellen eye chart. Compare the spherical equivalent of the WRx and the MRx. A spherical equivalent is a way to express a refractive error in a simpler spherical format for comparative purposes. Learn to do spherical equivalents in your head to compare prescriptions.
Here's how: Take half of the cylinder, add it to the sphere and drop the cylinder and axis. For example: If the prescription is -1.00 +1.00 x 180 the patient's spherical equivalent is -0.50.
If the patient had a one diopter change in spherical equivalent but only one line of improvement in VA, the refraction doesn't make sense. Recheck the refraction, the VA or both.
Here are some refracting rules of thumb:
1. Always give the least amount of minus sphere or most amount of plus sphere that maximizes VA.
2. Give the least amount of cylinder that provides the best VA.
3. If there is a change in axis, ask the patient to compare their VA at the new axis and VA at the wearing axis. If the patient doesn't notice an improvement in VA at the new axis, keep the patient's RX at their wearing axis.
4. The more cyl the patient has, the more precise the axis must be.
5. If the patient refracts to 0.50 D cyl or less, take away the cyl and ask the patient if they notice any difference. If they say it looks the same with and without the cyl, take it away.
6. The more ADD power, the closer the patient will hold things to read. If the patient is tall with long arms, they will likely need less ADD power. If they are short in stature, they likely will require more ADD power.
7. Give the patient the least amount of ADD that enables them to read the smaller print on the near card at their preferred reading distance.
8. The average reading distance is 14"-16". if the patient prefers to read much closer than 14" or much farther than 16"measure and note that reading distance on the chart.