The pediatric ophthalmologist has to perform this skill routinely, but what about the rest of us? This skill comes in handy. How about that little old lady with a spine problem who just can't straighten up behind the phoropter? How about that gentleman with a permanent head tilt? You can get accurate retinoscopy readings on anyone, without a phoropter, and the procedure is relatively easy once you understand it.
When retinoscopy is performed with trial lenses, using a cylinder is not the most efficient (fastest) method to determine the astigmatic correction. The optical (power) cross works nicely for this task. With this procedure, only spherical lenses are needed. The steps, assuming there is astigmatism, are as follows:
1. Neutralize the most minus meridian. You will be holding up spherical lenses only in front of the eye. Just as with a phoropter, add minus power if necessary until all meridians have "with" motion. Then add more plus power until one of the meridians is neutralized. Write down the power of the spherical lens that was used to reach neutrality. Also write down the meridian that is being streaked. By "meridian", I mean the axis that you are sweeping, not the axis of the streak.
2. Neutralize the meridian 90 degrees away from the one that was just neutralized. This meridian will still have with motion, so you will be using a more plus (or less minus) lens to arrive at neutrality. Write down the power of the spherical lens that was used to reach neutrality. Write down the number (axis) of the meridian that was scoped; this will be 90 degrees from the first meridian.
3. Use the optical cross to compute the refractive error.
Example: Suppose the first meridian to neutralize is at 90 degrees. This means that your streak is aligned with 180 and you are moving the streak along the 90 degree meridian. Suppose that it neutralizes with a -4.50 sphere held up in front of the eye. Now you will streak the 180 degree meridian and suppose it neutralizes with a -3.00 sphere. Your notes are as follows:
-4.50 at 90 -3.00 at 180
You will draw an optical cross as follows:
Now we will convert the optical cross to a prescription:
We will start with the vertical meridian and write down -4.50 as our sphere power. On the number line we travel from -4.50 to -3.00, which is a distance of +1.50, which we write down as our cylinder power. The axis is the same as the meridian that corresponds with our starting sphere power, which is 90 in this case. This results in:
-4.50 +1.50 x 90 or -3.00 -1.50 x 180
You will also need to subtract the working distance from the sphere power. The two most common working distances are 26 inches (subtract 1.50) and 20 inches (subtract 2.00). If using 26 inches, your retinoscopy result will be:
-600 +150 x 90 or -4.50 - 1.50 x 180