Handheld device reveals macular acuity, predicts postop vision when macula cannot be seen
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Ophthalmologists can usually examine the cornea, lens and vitreous with the slit lamp to learn if there is sufficient clarity to allow good vision.
However, learning the potential visual acuity of the macula when the cornea, lens or vitreous is cloudy is presently impossible. This is because the ophthalmologist cannot see the macula adequately, and the cloudy cornea, lens or vitreous blocks the macula from viewing the visual testing chart on the wall. To remedy this impossibility, we propose using the blue field entoptic test.
Source: John Karickhoff, MD
The blue field entoptic phenomenon was first described in 1924 by Richard Scheerer. It consisted of using an electric light in a box as a projector to shine its light through a pure blue light pass filter into the eye of the patient. There, the blue light was absorbed by the red blood cells in the microvessels of the perifoveal area of the macula, but the light was not absorbed by the white blood cells. This created what appeared to the patient as about 50 tiny white lights, each moving for about 1 second in its curved pattern and all the lights pulsating when the patient’s heart beat.
We have studied this blue field entoptic phenomenon for 30 years and have learned or done six things with the test.
1. We learned clinically that if the patient sees their retinal white blood cells in their perifoveal area, their macular acuity is 20/40 or better, which, by happy coincidence, is the level of vision commonly used as the standard of surgical success.
2. We learned that this test is independent of the presence of a cataract, vitreous haze or corneal haze when a filter with the proper optical frequency bandpass, bandwidth, diameter and optical density is used and some of the pure blue light reaches the retina.
3. We learned that the test worked when the cornea, lens or vitreous opacities were dense enough to prevent reading any conventional wall chart. This is possible because both the visual receptors and the visual target (the white blood cells) are in the retina.
4. We adapted the blue field entoptic test from using the light in the box as the projector to using the slit lamp as the light projector. This was accomplished by simply placing the filter on a handle. This adaptation made the test less expensive, no table space was used, the patient did not have to be moved from the slit lamp to the box, and the test could be performed in 10 seconds.
5. We performed a 10-month clinical study to learn the accuracy of the test in predicting postoperative vision.
6. We worked with a manufacturer and made this test commercially available.
The diagram (Figure 1) shows how the test works. The bright white light of the slit lamp shines through the filter so as to get adequate blue light to the retina but not to dazzle the patient. The blue light displays the patient’s perifoveal white blood cells as a dramatic, unique experience.
In our clinical study of the device, 70 consecutive cataract patients were examined preoperatively with the Macular Acuity Tester (Good-Lite). The test accurately predicted in 69 of the 70 patients which ones would have better or worse than 20/40 vision after removal of the cataract. Thirty-eight of the 70 patients had preoperative vision of 20/200 or worse. Sixteen patients had preoperative vision of count fingers at 5 feet or worse. Six patients had hand motion or light perception vision preoperatively. Therefore, the test maintained its high accuracy in the densest of cataracts when no fundus details could be seen and the usual office tests were useless.
From our 30 years of clinical experience and testing, we have concluded that in patients whose maculas cannot be seen, by using this 10-second slit lamp examination, the ophthalmologist can learn before surgery the macular visual potential and give an accurate prediction of the patient’s postoperative vision.
- For more information:
- John Karickhoff, MD, volunteer director of research at the Corrales Ocular Innovation Institute, can be reached at jrk538@gmail.com.
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