The effect of off-the-visual-axis retinoscopy on objective refractive measurement.

Purpose: To determine the effect of off-axis retinoscopy on objective refractive measurement.
Design: Prospective experimental study.
Methods: Eight volunteers underwent cycloplegic retinoscopy of their right eye on-the-visual-axis, and 5, 10, 15, and 20 degrees off-the-visual-axis in adduction. A single masked examiner performed all retinoscopy with random order of the patient and axis refracted.
Results: The average spherical retinoscopic value at 0, 5,10,15, and 20 degrees of off-axis alignment was -0.40, -0.90, -1.00, -1.38, and -1.80 diopters, respectively. The average spherical equivalent retinoscopic value obtained for each of the above positions of eye alignment was -0.02, -0.59, -0.45, -0.64, and -0.98 diopters, respectively. The induced cylinder power increased by an average of 3% for each degree of off-axis retinoscopy, though the axis of the cylinder was not predictable.
Conclusion: Objective refractive measurement by retinoscopy is significantly altered by off-the visual-axis retinoscopy. The induced error may be clinically important even with small degrees of eccentricity.