Think you don’t over-minus you patients? It’s hard to tell unless you check. Here are a few quick tests to check your monocular subjective refraction for accuracy.
I hear you saying to yourself. These tests take too long and my patient has trouble understanding what I’m telling them. Try this first technique, its fast, easily understood, and can be preformed with the lights on. If you like the other techniques better, I’ve included a quick review.
Plus to Blur:
Once you have completed your monocular subjective refraction, the monocular subjective lenses are left in the phropter. Occlude one eye. Plus lens power is added monocularly until your chosen acuity line is blurred. Start by separating an acuity line of letters (either 20/20 or 20/25). Ask your patient to first tell you when “the letters become blurry and difficult to read”. With the 20/20 line in place it should take 2 clicks of plus for the patient to notice the letters are “difficult to read”. With the 20/25 line in place it should take 3 clicks of plus for the patient to notice the letters are “difficult to read”. If you find more plus than suggested you have likely given your patient too much minus.
The Bichrome test – Monocular End Point:
The bichrome test should be done in a completely darkened room. Starting with the results of your monocular subjective, +0.75 of spherical power is placed in front of each eye. Use the red-green filter along with the side-by-side letter chart.
Light rays from a green source are refracted to a greater extent than those from a red source. Therefore, if the patient is adequately fogged, the focus of the red rays will be closer to the retina than that of the green rays. We use this to our advantage.
Ask the patient “which letters are sharper, blacker, and more distinct”. The patient is expected to report that the letters on the red background are more distinct than those on the green background. Plus lens power is reduced 0.25 at a time. If the original monocular subjective end point was correct, the patient should report that the backgrounds are equally distinct. Adjust your refraction according to the endpoint and recheck visual acuities if changes are necessary.
Prism Dissociation Test:
Once you have completed your monocular subjective refraction, the monocular subjective lenses are left in the phropter. Plus lens power is added OU until the 20/20 letters are blurred but the 20/25 letters are easily resolved. This normally requires an increase in plus of 0.25 or 0.50. The patient is then asked to compare the clarity of the 20/25 letters for the two eyes.
Place three prism diopters of base-down prism in front of the right eye and three prism diopters of base-up prism in front of the left eye. Your patient will see two charts, separated vertically, the upper chart visible to the right eye. Call the patient’s attention to the 20/25 line of letters and ask “are the letters more distinct and easier to read in the upper chart or the lower chart”. If the two 20/25 lines are equally distinct for the two eyes, the refractive state of the two eyes is considered balanced, and the test is over.
If your patient reports a difference in clarity of the letters for the two eyes, +0.25 is added in front of the eye with the better vision and the test is repeated. Once the patient’s acuity is balanced at 20/25, the patient is defogged binocularly to the criterion “maximum plus for best acuity”.
-Some information obtained from Theodore Grosenor’s “Primary Care Optometry” fifth edition.
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