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Wavefront may replace traditional methods of refraction
With more precise measurement capabilities, wavefront may one day supersede subjective refraction and autorefraction.
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The applications of wavefront technology in eye care extend beyond custom ablation. When used purely for diagnostics, wavefront can surpass traditional methods of refraction, measuring higher-order aberrations of the eye and, in combination with corneal topography, the internal optics of the eye, researchers say.
“Wavefront gives a more precise refraction than any other technology,” Raymond A. Applegate, OD, PhD, professor and director of the Visual Optics Institute at the University of Houston’s College of Optometry, told Ocular Surgery News. “The precision with which the low-order refraction (sphere, cylinder and axis) can be determined from knowledge of the total wavefront error of the eye is higher than with a subjective refraction and better than current autorefractors.”
Francesco Carones, MD, of the Carones Ophthalmology Center in Milan, Italy, said that wavefront sensors should be considered “advanced autorefractometers.”
“Wavefront sensors … provide a more precise measurement of not only aberrations that can be measured with traditional methods (sphere, cylinder) but also higher-order aberrations that could otherwise not be measured,” he said in a paper in September’s Ophthalmology Clinics of North America.
If more precise wavefront measurements prove to be more beneficial to clinicians than subjective refraction, Dr. Applegate said wavefront methods will likely become a new standard for specifying conventional spherocylindrical corrections.
Advanced autorefractometry
“I suspect that in the future neither ophthalmologists nor optometrists will be doing routine traditional subjective refractions,” Dr. Applegate said.
Dr. Carones agreed. “In the same way that corneal topography has replaced keratometry and keratoscopy in the diagnosis of corneal surface, wavefront analysis will most likely be substituted for other diagnostic technologies and examinations, including autorefractometry and skiascopy,” Dr. Carones said in his journal article.
In addition to recognizing higher-order aberrations that cause poor nighttime vision and glare, Dr. Applegate said, wavefront may supersede traditional refraction because the measurements are highly reproducible.
“Variation in refraction is higher with a traditional subjective refraction than with a wavefront refraction. If multiple examiners perform the refraction, the variation in the subjective refraction is higher still,” he said.
Detailed findings in this regard will be reported at the American Academy of Optometry meeting this month by Katrina Parker of the College of Optometry at the University of Houston, Dr. Applegate said.
In a study in the April Journal of Vision, Dr. Applegate and colleagues compared the accuracy and precision of 33 objective metrics of optical quality based on whole eye wavefront sensing as a criterion for the endpoint of an objective wavefront-guided spherocylindrical refraction. The study was an effort to determine which optical quality metric based on wavefront sensing best predicted the subjective refraction. Five of the metrics tested provided refractions that were within 0.125 D of the subjective refraction, Dr. Applegate said.
Dr. Applegate said these two studies led him and others to believe that wavefront-guided refraction will one day emerge as the objective gold standard for quantifying refractive errors of the eye.
“As this [wavefront] technology becomes fine-tuned to diagnostic applications, more diffused, less expensive … it will probably become a routine ophthalmologic examination performed on all patients,” Dr. Carones said in his paper.
Corneal, lenticular problems
Wavefront will be used not only to assess the refractive errors of the eye but also to determine what visual disorders are present, Dr. Applegate said.
“It will be used in all forms of diagnostics, from assessing diseases to optic research in retinal imaging and accommodation,” he said. According to Dr. Applegate, experts in accommodation such as Adrian Glasser, PhD, of the University of Houston are currently using wavefront to examine the change in the aberration structure of the eye over time in order to learn about the function of accommodation.
Dr. Applegate said that wavefront maps are helping surgeons and clinicians to determine whether a patient has a corneal disease such as keratoconus, marginal degeneration or corneal dystrophy, or differing degrees of cataract.
A wavefront diagnostic study led by Robert Montes-Mico, OD, MPhil, appearing in the May/June issue of Journal of Refractive Surgery [See related article.], outlined differences between the corneal profiles of patients with dry eye syndrome and patients with normal eyes.
Eyes of dry eye patients showed greater optical aberrations compared with normal eyes, the study authors said. The increase in higher-order aberrations in patients with dry eye resulted from increased tear film irregularity, the researchers said.
Another study, appearing in August in the Journal of Cataract and Refractive Surgery, led by Nisha Sachdev, MB, ChB, found that wavefront technology could determine different types of lenticular opacities based on differences in aberration profiles.
“Cortical opacification produced an increase in coma, and nuclear opacification induced an increase in spherical aberration compared to eyes without opacities. Both types of lenticular opacities also induced a higher amount of tetrafoil,” the researches said.
“This could explain the significant visual symptoms in patients with early cataract and relatively good high-contrast Snellen acuity,” the authors said.
Visual symptoms and aberration
Dr. Carones said in his article that wavefront analysis may help surgeons to better understand the reason for patients’ visual complaints.
Examining the wavefront profiles of enhancement patients, Dr. Carones was able to correlate specific visual disturbances to corresponding higher-order aberrations. He found, for instance, that spherical aberration was related to peripheral halo and that coma was related to double vision.
By determining which aberration causes which visual disturbance, surgeons can choose the most appropriate refractive procedure to target and correct the aberration, he said.
For Your Information:References:
- Raymond A. Applegate, OD, PhD, can be reached at the College of Optometry, University of Houston, 4901 Calhoun, 505 J. Armistead Building, Houston, TX 77204-2020; 713-743-1957; fax: 713-743-2053.
- Francesco Carones, MD, can be reached at Carones Ophthalmology Center, via Pietro Mascagni 20, 20122 Milan, Italy; 39-02-7631-8174; fax: 39-02-7631-8506.
- Carones F. Diagnostic use of ocular wavefront sensing. Ophthalmol Clin North Am. 2004;17(2):129-133.
- Thibos LN, Hong X, et al. Accuracy and precision of objective refraction from wavefront aberrations. J Vis. 2004;4(4):329-351.
- Montes-Mico R, Caliz A, et al. Wavefront analysis of higher order aberrations in dry eye patients. J Refract Surg. 2004;20(3):243-247.
- Sachdev N, Ormonde SE, et al. Higher-order aberrations of lenticular opacities. J Cataract Refract Surg. 2004;30:1642-1648.
- Nicole Nader is an OSN Staff Writer who covers all aspects of ophthalmology, specializing in pediatrics/strabismus and neuro-ophthalmology.