Evaluating The Accuracy Of Predicted Near Magnification Compared To Patient Selected Devices In Vision Rehabilitation Patients With A Primary Diagnosis Of Age Related Macular Degeneration.

Background: In low vision practice the majority of patients have a primary diagnoses of
age related macular degeneration. Device selection can be overwhelming to patients
due to the abundance of options available and is often conducted over several visits.
Predicted magnification formulas are used by practitioners to determine the amount of
magnification needed for the patient to achieve near acuity goals. By determining the
accuracy of these formulas practitioners will employ the most effective and efficient
device and power selection strategies to meet the demands of the patient’s near
activities of daily living.
Methods: This study is a retrospective chart analysis of 68 patients that presented for examination at the University Eye Center, Vision Rehabilitation Service between the years of 2000 to 2013. Patients charts were selected with a primary diagnoses of age related macular degeneration and had a uncorrected near visual acuity recorded in metric notation (M). Recording of final near device selection was an additional prerequisite for inclusion in the study. The formula V = M x D was utilized where needed mag (D) to read 1M, M is the smallest letter size read in M notation and D is the dioptric working distance. The result of this formula was then compared to the final near device selected by the patient. Accuracy of the predicted magnification was characterized by the following criteria. An accurate prediction demonstrated a difference of +/- 4D. A non-accurate prediction was categorized into> + 4D (greater than 4D higher than predicted) or >- 4D (greater than 4D lower than predicted).
Results: The results were analyzed using a distribution plot that represents the spread of subjectively chosen magnification as compared to predicted magnification. An accurate prediction was made for 31 of 97, or 32% of the time. A nonaccurate prediction was made for 66 of 97, or 68% of the time. Of the non-accurate predictions 3 of 97, or 3% were greater than 4D below the predicted value, and 63 of 97, or 65% were greater than 4D above the predicted value. The mean difference is +5.525 D from exactly predicting the value of selected magnification. This places the mean only +1.525 D outside of the criteria set for an accurate prediction.
Conclusions: According to our criteria, the formula does not accurately predict near magnification, patients trend toward powers above what the formula predicts. However most results were within 6D of predicted magnification, and the Pearson Correlation Coefficient shows a strong correlation between the predicted magnification and the prescribed magnification.