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Surgeon considers meaning of optimum optical performance
There is tremendous variation in the human ocular system, and surgeons are only approximating the visual potential of most eyes.
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The eye is far more complex and variable than we previously thought. As with all biological systems, there are inconspicuous, obscure measurable differences that are not always evident. Many ophthalmologists and the public at large think of the eyeball as being shaped like a basketball with set, regular curved surfaces that should perfectly refract and conduct light rays. In astigmatism, the eye may be described as resembling an American football. Methods of measurement and evaluation have grown more sophisticated, revealing the eye’s nuances in structural variation and processing that make our visual world function in the face of such tremendous complexity.
I often refer to portobello mushrooms to illustrate ocular variance. We know shiitake mushrooms are not baby bellos. However, if you carefully observe a box of baby bellos, they are all different, as in every biological system. Some are flatter in the center, some are peaked, and some have steep edges, not to mention all the variation in size. Why do we think eyes and the visual system are any different? Mandelbrot’s book on fractals explains that the formulas we use to systemize and smooth results are actually fictitious. Things get measured at a coarser level to incorporate the small variances that always exist.
In eyes, as in other biologically linked systems, each component is “matched” as best as possible to correct and improve the end result. Just like tall people have big feet and little people have little feet, our eye parts are linked together. Teeth, for example, normally occlude, but embryonic programming of maxillary facial anatomy may cause malocclusion. With vision, our cornea, anterior and posterior surfaces, angle of incidence, angle kappa, visual and optical axes, pupil position, lens position, tilt and variance in optical qualities all conspire to work together to either perfect our sight or, for the majority of the population, result in some form of visual disability. This has become more apparent to me since I began correcting presbyopia using corneal inlays along with advanced topography- and wavefront-guided refractive surgery. Moreover, with the Scheimpflug photography in my Lensar femtosecond laser, the positional variance, tilt and lens density variations are even perceptible in clear lens extractions.
In analyzing the mathematical models that best approach the actual optical qualities of the human eye, Noel Alpins’ vector analysis best captures this variance (ocular residual astigmatism) in the endpoint. Reaching the exact optical match of our visual system will improve surgical results. It is work that has yet to be done in science.
I have often seen that removing a cataract and inserting a monofocal implant results in a sudden and opposite shift in astigmatism. This occurs because lenticular-correcting astigmatism is replaced with a “perfect” but unmatched lens. It is even truer when a CustomVue LASIK patient ages out into cataract surgery. After the cornea has reached the best approximation of visual performance with the distorted human lens, it then becomes totally “unlinked” when replaced with a near perfect aspheric implant. Another CustomVue wavefront-guided LASIK touch-up will be needed to bring a patient’s vision back to the level he enjoyed after having LASIK.
Behind the cornea is a macular area that may or may not be perfectly aligned with the visual axis. It may be slightly sloped or skewed as a result of the sclera’s posterior surface, possible variance in ganglion cell and/or photoreceptor density. Those with denser optical sensors see finer, less pixilated images.
Besides the biological functioning processes, science has developed a new way of thinking about vision and the brain. Fifty years ago, we were taught that the visual cortex interpreted incoming cues, allowing us to see. Modern neuroscience has shown us that that could not be further from the truth. Vision is a cortical function, the result of filtering, comparing and generating perceptions to function effectively. A good analogy is that the optical system delivers an AM radio “image” with static and noise. The brain takes in the information and uses its various components to make sure we “see” an FM radio image free of the distractions (static) delivered by our optical system.
I routinely demonstrate this to patients by asking them to look in the distance as they hold up one finger. They then observe the appearance of a second finger. The demonstration is reversed by asking patients to fixate on their finger and note that the distant chart now becomes double. My explanation is that they are seeing things this way because of the instructions given how to look. If we saw these things all the time, the world would be difficult to navigate. Our brain suppresses images we do not need or want, doing amazing things that calibrate our perception. Neuroplasticity only occurs if patients allow it to happen. Cortical function integrates incoming cues, improving visual perceptions.
The Snellen chart is an archaic measure of vision that now needs to be replaced so that our patients can achieve their maximum visual performance, which is different for everyone. This goes hand in hand with finally developing models and simulations that better demonstrate visual performance for patients, both as it exists and in improvements or changes that come with advanced refractive surgery.
On top of all this are patients’ subjective concerns. One way to help them understand neuroadaptation is to point out the phenomenon of hypervigilance. For example, after buying a new car, one begins to notice a multitude of the same car suddenly on the road. Many visual observations were present before the “purchase” of new vision, but patients are convinced the sights are novel, a result of the procedure. It is due to their state of hypervigilance. Pointing it out gets most patients through this period and on to appreciating their new vision.
There is tremendous variation in the human ocular system, and we are only approximating the visual potential of most eyes. As measurement systems are refined, analyses and treatment parameters will progress to achieve visual results that approach the ideal for any individual eye.
- For more information:
- Steven B. Siepser, MD, FACS, can be reached at Siepser Laser Eyecare, 860 E. Swedesford Road, Wayne, PA 19087; email: ssiepser@siepservision.com.
Disclosure: Siepser reports no relevant financial disclosures.