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Elizabeth Richter, Jan Kylstra; Preliminary assessment of a new device to test potential visual acuity when compared to gold standard PAM. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5017.
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Assess a newly-developed potential visual acuity device compared with the gold standard Guyton-Minkowski Potential Acuity Meter (PAM)
The rationale for potential acuity assessment is to determine if vision loss is due solely to cataract or due to other ocular pathology. The current gold standard is the Guyton-Minkowski PAM. This method has several problems, including high cost and difficulty in use. The physician cannot see the projected chart on the retina and has to rely on patient cooperation. When this method is unsuccessful, the cause of failure may result from true retinal pathology or difficulties with equipment use. We have developed a new device (the SCALE) which combines current indirect ophthalmoscopy with a 20 diopter lens and transparent ETDRS-style eye chart. The virtual image of the retina is formed 5cm in front of the lens, and in this same plane the eye chart is held. This allows the physician to directly project the eye chart onto the patient's retina while the practitioner also views the patient's retina to make sure the image is directed on the macula. This is the first comparison study to determine the ability of visual acuity measurement when compared to the current gold standard in 23 patients with preoperative cataracts. After conversion to logMAR acuity, paired t-test was used to compare differences amongst the data. The goal of this study was to determine ease of use and correlation with other accepted methods.
Of 23 eyes, only one patient was unable to see letters on either device. Another subject was unable to see anything on the PAM device (>20/800), but could see one line (20/252) with the SCALE. Analysis of the other patients yielded a significant difference between the visual acuities measured by these two potential acuity methods (t(20)=3.4916, p=0.00230). The mean PAM logMAR acuity was 0.551 (SD=0.384), while the SCALE showed a 0.335 acuity (SD=0.246); this is approximately 2-line increase on the ETDRS chart.
The SCALE is another method to assess potential acuity. From preliminary study, it appears to overestimate visual acuity when compared to the gold standard PAM. However, this new device may be an excellent and inexpensive addition to the pre-cataract toolkit to assess retinal function. Further studies are needed to assess post-surgical acuity to determine if this is a good estimator of prospective surgical outcome.
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