Manifest refraction still crucial for LASIK surgery

Issue: October 2004

Manifest refraction

Louis J. Catania, OD, FAAO: When using an excimer laser platform, the manifest refraction is used to determine the accuracy of the wavefront aberrometry and can even be used to modify the treatment. This subjective measure facilitates a comparison between the patient’s cortical adaptation to their higher-order aberrations (HOAs) and the absolute quantitative levels of HOAs identified by the aberrometer. With this comparison, we have the ability to adjust the final treatment powers more accurately than with a wavefront aberrometry alone.

As the visual system develops, the brain begins dealing with some aberrations as adjunctive and complementary (“good” aberrations) and uses them to improve vision. For example, in one patient, spherical aberrations may diminish his or her functional vision, while spherical aberrations in another patient may improve the quality of vision.

The wavefront aberrometer provides very accurate information about the total aberrations of the human eye, but the instrument cannot discriminate which aberrations are beneficial and which are harmful (“bad” aberrations) to vision. Basically, good aberrations enhance a person’s vision, and the last thing we would want to do is remove them. Theoretically, wavefront-guided ablations could result in the correction of aberrations that actually enhance vision.

How do we identify which aberrations are good or bad? At the present time, we have no objective way to discriminate between them. But if we take a manifest refraction and find that there is a substantial difference between that subjective assessment and the wavefront refraction (e.g., more than 0.75 D), the difference is an indirect measure of the aberrations that are helping that individual’s vision.

Currently, the best, if not the only, way to measure these good aberrations is through the manifest refraction. Thus, the subjective examination procedure remains a critical ingredient in the preoperative analysis. It becomes the only source of clinical information that allows the surgeon to manipulate the difference between the patient’s subjective responses and his or her objective wavefront reading to potentially benefit the surgical outcome.

A cycloplegic refraction is valuable in that it is a measure of the accuracy of the manifest refraction. There should be no more than a 1-D difference between the manifest and the cycloplegic refractions. If there is a significant difference, it suggests that we are dealing with a latent element in the prescription, and it will be more difficult for the surgeon to establish a treatment plan, whether it is a custom or conventional treatment. By and large, it is best to go with the manifest refraction, which again speaks to the value of the subjective refraction in this whole custom ablation process.

Optometrists face the challenge of continuing to provide the safest, most effective and advanced vision correction solutions to their patients. The data from clinical studies and clinical experience seem to prove that custom ablations produce a better subjective outcome than conventional treatments, especially in mesopic and scotopic conditions. However, in custom ablations, it is critical to consider the manifest refraction and make adjustments when necessary in the final programming of the laser. This speaks to the importance of the preoperative manifest refraction by the optometrist.

Wavefront aberrometry is a very sophisticated and accurate tool, but it does not provide information about cortical adaptation. It is unable to distinguish which aberrations are helpful to the vision and which are not. Aberrometers are being developed that will identify the effects of specific aberrations on the visual system and eventually manipulate readings to correct only the bad aberrations. But until this discrimination between good and bad aberrations is possible, the manifest refraction will continue to remain a critical ingredient in preoperative patient care.

Louis J. Catania, OD, FAAO, is a Primary Care Optometry News Editorial Board member. He can be reached at Nicolitz Eye Consultants, 1235 San Marco Blvd., Suite 301, Jacksonville, FL 32207; (904) 398-2720; fax: (904) 398-6408; e-mail: lcatania@bellsouth.net.

Sometimes a compromise works well

Roy Rubinfeld, MD: Differences between the wavefront (aberrometer) refraction and the manifest refraction are not uncommon in clinical practice. The postop results of wavefront LASIK depend very much on how this situation is handled. Although there is no cookbook answer to managing this circumstance, the following tips may be helpful.

Don’t start with the manifest refraction. Start with a reliable, good-quality wavefront map, and be sure to suppress patient accommodation.
Then, do a WAMR (wavefront-adjusted manifest refraction) by putting up the wavefront refraction and seeing if the patient can tolerate this. Use the cross cylinder and encouragement to refine the refraction to achieve the optimum WAMR.
Try and get better than 20/20 corrected vision with the WAMR.
Compare results with the topography for cylinder magnitude and axis.
Consider what postop result is desired. For example, a 21-year-old myope might be best served with a +0.50 result because he or she might be expected to pick up a little myopia over the next few years, and the mild postop hyperopia may avoid the need for a future enhancement.

Although these guidelines may be helpful, there is no substitute for judgment and thought. Of course, if the magnitude of the cylinder is low, then axis differences between the manifest and the wavefront maps are not usually important. With more cylinder, the axis differences become more significant. Sometimes, a compromise between the wavefront refraction and the manifest refraction will work very well. In cases where the two are markedly different, then conventional ablation may be the patient’s best option if it is consistent with the old habitual refraction and not consistent with the aberrometer data.

As always, judgment remains invaluable. Contrary to what some of our patients may think, the computer doesn’t make the decisions or do the surgery.

Roy Rubinfeld, MD, is a clinical associate professor of ophthalmology at Georgetown University/Washington National Eye Center in Washington. He is in private practice with Washington Eye Physicians and Surgeons in Chevy Chase, Md., and is director of training and education for the Washington, D.C., region of TLC Laser Eye Centers. He can be reached at 5454 Wisconsin Ave., Suite 950, Chevy Chase, MD 20815; (301) 654-5114; fax: (301) 654-9132.

Establish patient preference

Scott MacRae, MD: When we evaluate the patient’s data, we want good “congruency” between the manifest refraction and the wavefront refraction in terms of sphere and cylinder. We make sure that the manifest refraction sphere and the wavefront are within 0.75 D of each other and that the manifest refraction cylinder and the wavefront are within 0.50 D of each other. We also want the manifest refraction cylinder axis and the wavefront to be within 15° of each other, especially if more than 0.50 D of astigmatism exists.

If the discrepancy between the manifest and wavefront sphere is more than 0.75 D, we treat with conventional LASIK based on the manifest refraction.

If a discrepancy exists between the manifest and the wavefront cylinder magnitude or axis, we bring the patient back for a post-wavefront manifest refraction. We then put the wavefront manifest cylinder up in the phoropter and let the patient compare that refraction to the previous manifest refraction. If the patient accepts the wavefront refraction, we recommend that the patient receive customized correction. If the patient has a strong preference for the manifest refraction compared to the wavefront refraction, we recommend conventional LASIK.

Scott MacRae, MD, is a professor of ophthalmology and visual science at the University of Rochester. He can be reached at Strong Vision Refractive Center, 100 Meridian Centre, Rochester, NY 14618; (585) 273-2020; fax: (585) 756-1975.

Manifest refraction

Jeff Genos, OD: The short answer is that we almost always use the manifest refraction (MR). However, before choosing a refraction, we first determine if there is a simple reason for the difference. For example, the most common reason for a difference (with the Visx platform anyway) is overminusing (aberrometer refraction more than the MR), so we will typically try to get the patient to relax or bring him or her back and try some more scans. This usually solves the problem or gets both refractions more in sync.

If the wavefront MR is less minus than the MR by more than 0.75 to 1.00 D, we simply bring the patient back and rescan. We usually like the scan to be equal to or slightly less than the MR with cylinder closely matching. If none of this works, we usually start with the closest aberrometer refraction to the MR, put that in the phoropter and refract the patient. In almost every case, the patient ends up back close to the MR. We almost always end up doing conventional LASIK in these cases as well.

Finally, don’t forget contact lens de-adaptation. In some of these cases, the patient wasn’t out long enough. Our protocol has been 5 to 7 days for daily-wear soft, 1 to 2 weeks for toric/extended wear and 4 weeks plus 1 week for every 10 years of gas-permeable wear.

Jeff Genos, OD, is clinical director at TLCVision Centers Cleveland. He can be reached at 6500 Rockside Rd., Suite 100, Independence, OH 44131; (216) 901-9393; fax: (216) 901-0953; e-mail: Jeff.Genos@tlcvision.com.