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D.C. Fletcher, R.A. Schuchard; Visual Acuity Letter Errors in Low Vision Patients With Macular Scotomas . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2780.
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Purpose: To determine if errors demonstrated in letter-by-letter visual acuity (VA) scoring were related to Scanning Laser Ophthalmoscope (SLO) determined macular dense scotomas (DS) characteristics in low vision patients. Methods: 48 patients referred for low vision rehab evaluation had VA with letter-by-letter scoring and SLO macular perimetry performed. Light box illuminated EDTRS acuity charts with a 0.5 to 2.0 meter test distance were used with no direction to correct for misidentifications or omissions. Total letter errors and letter errors in each of the 5 vertical columns were noted. Macular scotomas were determined with a hybrid perimetry protocol and a 50,000 troland stimulus on the SLO. DS were grouped by location in the field relative to fixation as None (N), Right only (R), Left only (L), Vertical only (V), and All directions (A). Results: Patients were from 9 to 92 years old (median 78); had visual acuity (VA) with range 20/20 - 20/1837 (median 20/200); and had macular scotomas with a diameter of 0 to 25 degrees (median 3). Macular scotoma location relative to fixation incidence was N= 21, R=20, L=17, V=14, A=20. The N and V groups had the lowest letter error rates for all columns. Compared to the N and V groups (Student’s T-test): the A group had significantly higher letter errors for all columns, the L group had significantly higher letter errors in the first two left columns, and the R group had significantly higher letter errors in the last or right column. 35% of variance in VA was explained by location of scotoma and there was a wide spread of VA in all scotoma groups. Considering only errors in lines above the last line attempted, the N group had 0.2 mean letter errors, while the R, L, V, and A groups had significantly higher mean letter errors (2.3, 4.2, 1.2, 6.9, respectively). The letters "C", "D", and "O" account for 29% of the letters tested but make up 41% of the errors. 23% of variance in C/D/O errors is explained by scotoma categorization. Conclusions: Macular scotomas play a significant role in VA measurement. In low vision patients with no scotomas, letters were read accurately up to the limits of resolution while any scotoma decreased large letter accuracy significantly. The letter errors noted in the letter-by-letter scoring system (especially confusion and omission) may give clinicians a clue as to the location of macular scotomas relative to fixation.
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