|The duochrome balance test is based on chromatic aberration, with red light having a longer wavelength than green light. – PHOTO COURTESY OF MARCO.
The eye works like a camera. Parallel light rays travel through the cornea where they are refracted (bent). The light rays then pass through the lens where they are refracted again and ultimately come to a focal point on the retina. If those light rays do not come to a focal on the retina, then the eye has imperfect optics. Imperfect optics are a result of the following conditions: eye too long, eye too short, curvature of the cornea, and curvature of the crystalline lens. If any of these conditions exist, then the result is the eye having a refractive error.
FINDING THE STARTING POINT
Prior to starting the initial refraction, you will need to decide what prescription to start from. If the patient had a refraction performed at the prior visit, start with that prescription. The starting point can also be obtained from the patient’s current spectacle correction. In addition, the autorefractor reading can be used as the starting point. Enter the autorefraction or lensometry measurement into the phoroptor.
CHECKING THE VISION
The next step is to check the patient’s vision with the correction placed into the phorotor at the starting point. Occlude the left eye and make sure the right eye ocular is set to “O.” Check the patient’s vision.
REFINING THE SPHERE
Present the patient with a choice of two lens powers either 0.25D or 0.50D apart, asking “which is clearer, one or two?” Remember to offer plus power before offering minus power. You want to give the patient the most plus or the least minus power for the best vision. If the patient prefers the more minus power lens, ask the patient to read the chart. The patient will need to read the chart better in order to “earn” the more minus power. It is important to be stingy with giving minus power so as to not over-minus the patient. Once you have given the most plus or least minus lens to bring the patient to 20/20 (or close to it), you have reached the end point of finding the best sphere lens.
USING JACKSON CROSS CYLINDER
Refine Cylinder Axis. Once you have refined the sphere, the astigmatism correction should be tested next. Adjust the Snellen chart to two lines above the last read line and instruct the patient to let you know which lens flip is clearer. Be sure to inform the patient that even though the lenses shown may be both blurry, you would like the patient to indicate which of the two lenses are clearer or if they are of equal clarity or blurriness. Rotate the Jackson Cross Cylinder (JCC) lens into place, orienting the dial so the red and white dots straddle the axis of the cylinder. The JCC is a single lens that is a combination of plus and minus lens.
Flip the JCC and ask the patient which lens flip is clearer or do the lenses seem to have the same clarity. Note the lens choice the patient prefers and adjust the cylinder axis 5° or 10° in the direction of the red dot (if working in minus cylinder), or 5° or 10° in the direction of the white dot (if working in plus cylinder).
Repeat showing the patient the lens choices and follow or “chase” the red dot (minus cylinder) or white dot (plus cylinder) until the patient reverses and prefers the red dot in the reverse position or states that both lenses are equally blurry. This is the stopping point.
Refine Cylinder Power. Now that the cylinder axis has been found, the amount of astigmatic power required by the patient needs to be established. Orient the JCC so the red or white dots are aligned with the axis. Again, you will ask the patient to identify which lens flip is clearer, “one or two?” If you are working in minus cylinder, “follow the red dot.”
If the patient prefers the red dot, then add -0.25D to the cylinder power. And if the patient prefers the white dot, then add +0.25D to the cylindrical power. Remember, in order to maintain spherical equivalency for every -0.50D added to the cylinder, you must also add +0.25D to the spherical power.
Repeat asking the patient their preference until the patient reverses and prefers the white dot, or states that both lenses appear to be the same. This is the end point.
|The Jackson Cross Cylinder is a single lens that is a combination of plus and minus lens. – PHOTO COURTESY OF GULDEN OPHTHALMICS.
RECHECKING THE SPHERE
By rechecking the sphere power, you ensure you haven’t over-minused the patient. Start by “pushing plus” or “fogging” the patient. Add a small amount of plus power and have the patient read the smallest line she is able to discern. Then slowly remove the “fog” lenses while asking the patient to identify which of the two lenses is clearer.
CHECKING DUOCHROME BALANCE
Before testing the other eye, the duochrome balance test can also be performed to ensure that you haven’t over-minused the patient. The test is based on chromatic aberration, with red light having a longer wavelength than green light.
First isolate the 20/40 line on the Snellen chart and then dial in the red/green filter. Ask the patient which side are the letters clearer and darker—the red side or the green side, or are they the same clarity?
If the patient states both sides of the chart are equally clear, then the red and green focal points are equal on either side of the retina.
If the patient chooses the red side, you have made the patient hyperopic. You now need to add minus sphere lenses until the patient states both sides of the chart are clear or until the patient prefers the green side.
If the patient chooses the green side, you have then made the patient myopic, and minus sphere lenses must be added until the patient notes that both sides appear equal or reverses.
READING ADD DETERMINATION
Finally, the appropriate reading add is given based on age, hobbies, occupation, and preferred reading distance.
FINAL REFRACTION THOUGHTS
The ultimate goal in performing the refraction is to bring the focal point of parallel light rays onto the retina by placing different lenses in front of the patient’s eye.
Keep in mind, it is extremely easy to over-minus patients, so “push plus” and when in doubt, give the more plus lens correction.
Janet Hunter is president of Eye Source LLC and specialized in ophthalmic technician training.