Cornea

Refractometry in the cornea clinic

Part three of a seven-part corneal/anterior segment survival guide.

BY STEPHANIE D. MCMILLAN, MHA, COT

Imagine being assigned to work in a busy cornea clinic. Now imagine that you have to refract every patient with diagnoses as complicated as keratoconus, corneal grafts, and Fuch’s dystrophy. What if the physician relies on you to obtain precise astigmatism correction in anticipation of a toric intraocular lens? How do you handle these difficult cases, and what tools and techniques can you use to provide reliable results?

In general, refracting doesn’t have to be complicated; think of it as a puzzle that you need to assemble. You follow the same steps, looking at the overall picture rather than just a small piece. As with refractometry, we observe the same steps in sequence every time while also picking up clues, which you find by using tools in the office.

The importance of refractive history

First, you need to establish a refractive history, which is important in the cornea clinic. The patient’s prior correction, whether glasses, contact lenses, or refractive surgery, is essential to obtaining an accurate final result. Comparing to a previous history helps to document or verify progression or stability of disease.

Also, we compare previous refractions to see if the patient experienced “minus shift” as we sometime see with progression of cataracts. After a corneal transplant, we will expect the astigmatic power and axis to change with every suture that is removed. A latent hyperope will need careful cycloplegic refraction while pushing plus power.

We receive these pieces of information from the patient that may appear to be small, but each has a purpose in the bigger picture.

Stephanie D. McMillan, MHA, COT, educates technicians on refraction skills.COURTESY MARTHA TELLO, COMT

Where do we start?

This age of technology has provided us with many tools at to assist us in beginning our refractions. It is inefficient and ineffective to start a refraction from “zero.” Here are some of the methods most commonly used in the cornea practice.

• Retinoscopy: My favorite refractometry tool is the retinoscope. Unfortunately, this skill is the least used by many technicians. You can only gain proper technique and results with constant use and practice, but many techs do not have the time. Start by attending a retinoscopy workshop where you can get hands-on practice observing the various light reflexes in the eye. Another alternative is to watch a video. Once you get the hang of it, you can obtain an objective refraction in as little as 20 seconds.

• Auto-refractor: The auto-refractor acts as a guide for subjective refractometry. If time permits, take the patient to the auto-refractor; it can help with difficult and irregular corneas. Keep in mind that the auto-refractor gives an approximation — you still need careful refinement to obtain the final result. Auto-refractor measurements can be influenced by the patient’s accommodation and can be skewed by any corneal irregularities.

• Corneal topography: This is useful in determining the astigmatic correction of the cornea. Use the topography image as a guide to dial in axis and cylinder power correction into the phoropter for those with corneal grafts, high astigmatism correction, or keratoconus.

Tips for subjective refractometry

Here are some tips for refracting the cornea patient.

• Always adjust the sphere in the plus direction first. This avoids inadvertently over-minusing the patient. Offer choices until the patient states that they are about the same. Then, move on to the next step and remember to push plus because myopes “eat minus.” Keep in mind that for every 0.25D of spherical power, the patient should be able to read one additional line. If he or she is unable, do not give the extra power.

• Use the Jackson cross-cylinder (JCC). This is a great tool for finding the axis of astigmatism. Also, using the JCC keeps the images focused on the patient’s retina during the refraction. An astigmatic cornea projects multiple images on the retina, resulting in a blurry image until these images are fused. At this step, we “follow the white dots” by refracting in plus cylinder and adjusting by smaller and smaller degrees until the patient cannot differentiate between the choices. When refracting in minus cylinder, we “chase the red dots.”

• Use the JCC to find the cylinder power. Align the JCC so that the axis aligns with the white dots. When adjusting the cylinder power, use the mnemonics “WAP” (white add plus) and “RAM” (red add minus). In other words, if the patient prefers the choice in which the white dots line up to the cylinder axis, add +0.50D to the cylinder power. If the patient prefers the choice in which the red dots line up to the cylinder axis, add -0.50D to the cylinder power. Perform this step until the patient cannot differentiate between the choices. The same mnemonic is true for minus cylinder refractions, but consider that we take minus away when the white dot is on the “P.”

• Maintain the spherical equivalent. This is important to keep the images focused on the retina. To do this, if you add +0.50D to the cylinder power, simply subtract -0.25D of spherical power. Conversely, if you remove -0.50D of cylinder power, then you should add +0.25D to the sphere. When refracting in minus cylinder, if you add -0.50D of cylinder, then add +0.25 sphere.

• Use the trial frame. This helps to avoid costly and unnecessary refraction rechecks and to prevent spectacle remakes. Trial frames are particularly useful when you notice large changes in refraction from previous refractions and anisometropia. It may be helpful to use the trial frame for patients to visualize a perceived change.

• Use the duochrome test to prevent over-minusing. Remember: “RAMGAP: Red add minus power. Green add plus power.” To prevent over-minusing, ask which lens makes the letters “sharper” rather than “smaller and darker.” Another method is to add at least one diopter of plus sphere to the best correction so the chart is blurred. Ask the patient to indicate when he or she can read that same line.

For patients with keratoprosthesis, the retinoscope may give you a retinal reflex to guide the refraction.COURTESY ELIZAMA LOPEZ, COA

Sequence of subjective refractometry

Yes, the sequence is important. We follow the same steps every time for reliability and results. The sequence in refractometry can be remembered with another simple mnemonic, SACS: initial Sphere correction, adjust the Axis, find the Cylinder power, and the final Sphere adjustment.

Conclusion

Refracting the cornea patient is a challenging yet rewarding experience for the technician. By remembering to use the tools available to us in the clinic in conjunction with proper technique, we can help the technician and physician arrange the puzzle pieces of refraction for difficult cornea cases. OP

	Kendall E. Donaldson, MD, MS, is associate professor of ophthalmology and medical director of Bascom Palmer Eye Institute in Plantation, FL.
	Stephanie D. McMillan, MHA, COT, is the lead ophthalmic technician and a clinical and informatics trainer at Bascom Palmer Eye Institute in Plantation, FL.