October 15, 2006 Refractive Errors When we view an image, light rays coming into the eye are bent by the cornea (the transparent outer covering of the eye) and focused by the lens onto the retina. The retina is a light-sensitive membrane at the back of the eye. Specialized cells convert the information into electrical signals. The information passes to the optic nerve and is sent to the brain for processing. Refractive errors occur when light coming into the eye isnt bent properly. The image fails to focus onto the retina, causing blurred vision. There are several common types of refractive errors. Myopia (nearsightedness) occurs when the eye is elongated or the cornea is too steep. Images are focused at a point in front of the retina. People with myopia see near objects more clearly than distant ones. Hyperopia (farsightedness) occurs when the eye is shorter or the cornea is flatter, causing images to fall on a spot behind the retina. People who are farsighted see distant objects more clearly than near ones. In a condition called astigmatism, the shape of the cornea is irregular - more like a football than a baseball. The abnormal curvature causes light to come into the eye at different focal points, blurring vision. Astigmatism can occur in combination with myopia or hyperopia. Presbyopia is a condition in which the lens of the eye gradually loses its ability to quickly change focus between near and distant images. It is a natural condition that begins to occur in most people around 40. Reading fine print becomes more difficult and the eyes may become fatigued during close up work or reading. Eyewear for Refractive Errors The traditional method of treatment for refractive errors is corrective lenses (eyeglasses or contacts). According to the Vision Council of America, about 155.7 million Americans wear corrective eyewear; 48 percent of them are men and 52 percent are women. The majority (143.2 million) wear prescription eyeglasses. The most common reason for eyewear is myopia (61.3 percent of those needing vision correction). About 18.3 percent of Americans wear corrective eyewear for farsightedness. The remaining wearers (31.3 percent) use corrective lenses for presbyopia and other age-related eye conditions. iZon™ Lenses Myopia, hyperopia and other common refractive errors can usually be compensated with corrective lenses. However, even with correction, sometimes images are still not sharp. Experts say many people have tiny imperfections, or irregularities, that can affect how light enters the eye. These microscopic irregularities are called higher order aberrations. No two people have the same pattern of aberrations. The pattern is even different between the left and right eyes. Up until the last several years, there has been no way to measure these fine irregularities. But new, wave-front technology, recently used to plan LASIK eye surgery, is now being used to "map" the eye and measure these higher order aberrations. A company called Ophthonix, Inc. has a device, called the Z-View™ Aberrometer, which emits a low-power laser beam onto the retina. Images from the reflected beam are recorded and analyzed. The data provides a detailed map of all the irregularities in the visual system. The result - a unique kind of fingerprint (called an eyeprint) for each eye. The information gained in the images is used to produce custom corrective lenses, called iZon™ Wavefront-Guided Lenses. Optometrist, Lilien Vogl, O.D., F.A.A.O, says by accounting for and correcting the higher order aberrations, clients are able to see clear, crisp images. These images are described as "High-Definition Vision." Vogl recommends the iZon lenses for patients who, despite use of corrective lenses or refractive surgery, still dont see clear, crisp images. The lenses are also good for patients who have difficulty seeing at night or experience a lot of glare. The imaging process with the Z-View™ Aberrometer takes about one minute. The eye information is sent to a lab, where the customized lenses are produced. The manufacturer estimates about 50 percent of patients who need corrective eyewear could benefit from the iZon lenses. The custom eyeglasses cost more than traditional eyewear - about $100 more for traditional lenses and $200 for progressive lenses. Since they are a specialty lens, they may not be fully covered by insurance. The company is currently working on custom contact lenses and hopes to have them available sometime this year. AUDIENCE INQUIRY For information on the iZon technology and lenses, or referral to the nearest practitioner: http://www.izonlens.com For general information on vision problems: American Academy of Ophthalmology, http://www.medem.com Vision Council of America, http://visionsite.org BIBLIOGRAPHY Alio, J., and M. Shabayek, "Corneal Higher Order Aberrations," Journal of Refractive Surgery, June 2006, Vol. 22, No. 6, pp. 539-545. Artal, Pablo, et al., "The Human Eye is an Example of Robust Optical Design," Journal of Vision, January 10, 2006, Vol. 6, pp. 1-7. Atchison, D., et al., "Refraction and Aberration Across the Horizontal Central 10 Degrees of the Visual Field," Optometry and Vision Science, April 2006, Vol. 83, No. 4, pp. 213-221. Buhren, J., and T. Kohmen, "Factors Affecting the Change in Lower-Order and Higher-Order Aberrations After Wavefront-Guided Laser in situ Keratomileusis for Myopia with the Zyoptix 3.1 System," Journal of Cataract and Refractive Surgery, July 2006, Vol. 32, No. 7, pp. 1166-1174. Callina, T. and T. Reynolds, "Traditional Methods for the Treatment of Presbyopia," Ophthalmology Clinics of North America, March 2006, Vol. 19, No. 1, pp. 25-33. Carvalho, L., and J. Castro, "The Placido Wavefront Sensor and Preliminary Measurements on a Mechanical Eye," Optometry and Vision Science, February 2006, Vol. 83, No. 2, pp. 108-118. Dayan, Yosefa, et al., "The Changing Prevalence of Myopia in Young Adults," Investigative Ophthalmology and Visual Science, August 2005, Vol. 46, No. 8, pp. 2760-2765. Frederick, Douglas, "Myopia," British Medical Journal, May 18, 2002, Vol. 324, pp. 1195-1199. Lai, S., et al., "Method of Determining a Patients Subjective Refraction Based on Objective Measurement," Journal of Refractive Surgery, September-October 2004, Vol. 20, No. 5, pp. S528-S532. Navarro, R., et al., "On the Prediction of Optical Aberrations by Personalized Eye Models," Optometry and Vision Science, June 2006, Vol. 83, No. 6, pp. 371-381. Plainis, Sotiris, et al., "The Effect of Ocular Aberrations on Steady-State Errors of Accommodative Response," Journal of Vision, May 23, 2005, Vol. 5, No. 5, pp. 466-477. "Refractive Errors and Refractive Surgery," San Francisco: American Academy of Ophthalmology, downloaded from public website (http://www.medem.com), September 25, 2006. Saw, S., et al., "Myopia," British Journal of Ophthalmology, 2002, Vol. 86, pp. 1306-1311. "Vision Facts and Statistics," Alexandria: Vision Council of America, downloaded from website (http://visionsite.org), September 22, 2006. "The Wave of the Future is Here," San Francisco: American Academy of Ophthalmology, downloaded from public website (http://www.medem.com), September 25, 2006. Yoon, G., et al., "Large-dynamic-range Shack-Hartman Wavefront Sensor for Highly Aberrated Eyes," Journal of Biomedical Optics, May-June 2006, Vol. 11, No. 3, p. 30502.