Refraction is not an easy skill to learn. It takes patience and a lot of practice. As a beginner, stay away from uncooperative patients (babies and mentally handicapped) and those with significant corneal or lens pathology.
Figure 1. Components of a spectacle prescription.
Begin by understanding the components of a glasses prescription (Figure 1.) The first number indicates the spherical power in diopters, signifying the degree of myopia (minus power because a divergent lens is required to neutralize nearsightedness) or hyperopia (plus power because additional convergence is required to neutralize farsightedness). The second and third numbers refer to the astigmatism, if there is any. The second number indicates the power in diopters of the cylinder. Beware that the cylindrical power can be indicated in plus or minus cylinder form; lens transposition can convert one to the other. The third number indicates the axis at which the cylinder is neutralized. If the patient requires a bifocal, the additional spherical power is called the “ADD.”
First, ask the patient to fixate on a distant target or dilate the eyes with a cycloplegic agent in order to relax accommodation. Sit at arm’s length from the phoropter. You must take this into account when determining the patient’s final spherical error. To calculate how many diopters you need to offset, take the inverse of your working distance in meters. For example, if the working distance between your retinoscope and the phoropter is 50cm, then subtract 1/0.5m or 2.00 diopters from the final spherical correction to account for this.
Figure 2. Reflex motions in retinoscopy.
Starting with the right eye, shine the retinoscopy streak into the patient’s eye and move it from side to side. Determine if the light reflex in the patient’s pupil moves “with” or “against” motion (Figure 2). Rotate the axis of the streak and look at the reflex in different meridians. If the reflex is of a consistent width and brightness all around, then there is no astigmatism. However, if the reflex looks thicker/thinner or brighter/dimmer when varying the axis of your streak, then there is astigmatism to correct as well.
Figure 3. Anatomy of a phoropter. 1 Tilt adjustment, 2 Pupillary distance adjustment, 3 Strong sphere power, 4 Jackson cross, 5 Weak sphere power, 6 Viewing tube, 7 Cylinder axis, 8 Level, 9 Auxiliary lens dial, 10 Eye piece, 11 Risley prism, 12 Cylinder power
Assuming the refractive error is spherical only, determine if you need to add plus or minus power. Use the pneumonic “SPAM” to remember that Same (“with” motion) requires Plus power and “Against” motion requires Minus power. Using the spherical power dial on the phoropter (Figure 3), add the appropriate power until you see a bright red reflex without any motion that fills the pupil. You have achieved neutrality.
Figure 4. Orientation of streak in astigmatic eye.
What if there is astigmatism? As you move the streak from side to side, the reflex will appear to move obliquely (Figure 4). Rotate the beam until it is parallel with the reflex motion. Neutralize the “with” or “against movement” you see here. The secondary meridian is 90 degrees away and is at the axis of the patient’s astigmatism. Neutralize the “with” or “against movement” in this second meridian to find the cylindrical power. Do the exact same procedure with the left eye. Don’t forget to subtract your working distance before writing the prescription. Congratulations, you are finished with retinoscopy
* * *
About the author: Olivia L. Lee, MD, is a specialist in uveitis and cornea/external disease at the Doheny Eye Institute and Assistant Professor of Ophthalmology at UCLA. After completing medical school at Baylor, she completed her residency and uveitis fellowship at the New York Eye and Ear Infirmary as well as a cornea fellowship at Jules Stein Eye Institute. She joined the YO Info editorial board in 2015.