Uncorrected Visual Acuity and Manifest and Cycloplegic Refraction
The refractive elements of the preoperative examination are extremely critical because they directly determine the amount of surgery to be performed. Visual acuity at distance and near should be measured. The current glasses prescription and visual acuity with those glasses should also be determined, and a manifest refraction should be performed. The sharpest visual acuity with the least amount of minus (“pushing plus”) should be the final endpoint (see BCSC Section 3, Clinical Optics). The duochrome test should not be used as the final endpoint because it tends to overminus patients. Document the best visual acuity obtainable, even if it is better than 20/20. An automated refraction with an autorefractor or wavefront aberrometer may be helpful in providing a starting point for the manifest refraction.
A cycloplegic refraction is also necessary. Sufficient waiting time must be allowed between the time the patient’s eyes are dilated with cycloplegic eye drops and measurement of the refraction. Tropicamide, 1%, or cyclopentolate, 1%, are the most commonly used cycloplegic drops. For full cycloplegia, waiting at least 30 minutes (with tropicamide, 1%) or 60 minutes (with cyclopentolate, 1%) is recommended. The cycloplegic refraction should refine the sphere and not the cylinder from the manifest refraction, as it is done to neutralize accommodation. For eyes with greater than 5.00 D of refractive error, a vertex distance measurement should be performed to obtain the most accurate refraction.
When the difference between the manifest and cycloplegic refractions is large (eg, >0.50 D), a postcycloplegic manifest refraction may be helpful to recheck the original. In patients with myopia, such a large difference is often caused by an overminused manifest refraction. In patients with hyperopia, substantial latent hyperopia may be present, in which case the surgeon and patient need to decide exactly how much hyperopia to treat. If there is significant latent hyperopia, a pushed-plus spectacle or contact lens correction can be worn for several weeks or months preoperatively to reduce the postoperative adjustment that may result from treating the true refraction.
Refractive surgeons have their own preferences for whether to program the laser using the manifest or cycloplegic refraction, based on their individual nomogram and technique and on the patient’s age. Many surgeons plan their laser input according to the manifest refraction, especially for younger patients, if that refraction has been performed with a careful pushed-plus technique.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.