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H. Park, D. Robins, D. L. Guyton; A New Way to Measure Potential Visual Acuity in Nystagmus Patients: Full-Field E Test. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1996.
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Conventional visual acuity measurement is inadequate in measuring the potential visual acuity in patients with nystagmus. A "Full-Field E" test is compared to the conventional methods of vision assessment in patients with congenital idiopathic nystagmus (CIN) and oculocutaneous albinism (OCA).
A Full Field card measures 24 cm square and is covered with an array of multiple, identical, illiterate E’s. Each E’s on the card have the standard Snellen format of five equal segments with a letter height chosen such that the "20/20" E subtends a visual angle of 5 minutes of arc in height and width at a distance of 35 cm. The staggered pattern of E’s are separated by 1 ½ letter widths horizontally and vertically. This arrangement and angular subtense ensures the simultaneous presentation of letters of the foveal and parafoveal areas when the eye attempts to fixate near the center of the card. The cards have E’s that range from 20/20 to 20/200 equivalent acuity. The cards are turned randomly to switch the orientation of the E’s. Same size cards with equivalent E’s were made for linear illiterate E’s and Single E’s. A headband with an attached rod was worn to fixatethe cards at 35 cm from the patient. Refractive correction and presbyopic correction were given when necessary. A retrospective review of charts was performed in diagnosed with CIN and OCA and included patients that were tested with the Illiterate Linear, Single and Full Field E cards at the same distance.
21 patients (42 eyes) with CIN and 11 (22 eyes) patients with OCA were identified. The mean vision improvement was 2.4 (SD ±2.3) lines when using the Full Field E test compared to the Linear E test in CIN, and 0.9 line (SD ± 1.3) in OCA. The difference in vision observed between these groups were statistically significant (p=<0.001) in CIN and in OCA (p = 0.01). Vision measured using Full Field E test was better compared to Single E test by average of 1 line in CIN and by 0.1 line in OCA. The difference in vision measured between the Single E and Full Field E is statistically significant in CIN (p=< 0.001), but not in OCA (p= 0.9).
Conventional visual testing using Snellen lines underestimates the visual acuity measured in nystagmus patients. Full Field E test is an easy tool to use in office to test potential visual acuity in patients with CIN. Small difference observed in patients with OCA may be related to an underlying foveal hypoplasia.
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