REFRACTIVE SURGERY UPDATE

New Surgical Options in Presbyopia

by Jeffrey M. Augsutine, O.D.

More than 100 million Americans have presbyopia. They've had at their disposal many treatments for managing this visual problem, but surgery hasn't been one of them.

The surgical correction of presbyopia is fundamentally difficult. The corneal structure and ocular architecture make it much more difficult to project surgical outcomes in presbyopia than for hyperopia or myopia. Research has focused on two theoretical areas of presbyopic surgical correction: creating a multifocal cornea with excimer laser technology or corneal inlays, and special PMMA scleral expander segments.

Hermann von Helmholtz's hypothesis of accommodation has long been used to explain presbyopia. It states that in distant vision, the ciliary muscle is relaxed and the zonules of the lens are under tension. When the eye accommodates, it relaxes the tension on the zonules. This reduced tension allows the elastic capsule of the lens to contract, causing a decrease in equatorial diameter, a decrease in radii of curvatures of the anterior and the posterior surfaces of the lens, and an increase in central corneal thickness. According to Helmholtz, presbyopia results from the loss of elasticity of the lens capsule and sclerosis of the lens with age. So, when the zonules relax, the lens does not change its shape.

Scleral Expansion Theory
A new theory for presbyopia has been put forth by Ronald Schachar, M.D., Ph.D., of Dallas. This states that zonular tension increases during accommodation, and changes in the lens curvature can be attributed to zonular forces. Presbyopia occurs as a result of normal lens growth. Being of ectodermal origin like skin, hair and nails, the lens does not shed but continues to grow throughout life, at approximately 0.02mm/year. This growth causes the distance between the ciliary muscle and the lens equator to decrease over time. As a result of this growth, the muscle does not work as well and cannot exert enough force to accommodate properly. To confirm this, Dr. Schachar used an ultrasound probe to measure the movement of the lens equator in accommodated and unaccommodated states.

A new investigational surgical procedure based on Dr. Schachar's theory involves increasing the distance between the ciliary muscle and the lens equator. This is accomplished by stretching the sclera in the region of the ciliary muscle 1.5mm posterior to the limbus. The procedure uses a specially designed diamond blade with which the surgeon makes a tiny belt loop incision in the sclera. He does this in all four quadrants and inserts PMMA scleral expansion segments. These segments elevate or stretch the sclera and move the ciliary body away from the lens. This gives the ciliary body enough room to function, thus restoring accommodation. The surgery takes about 40 minutes. Most surgeons operate on the dominant eye first; most patients don't need the procedure on the second eye.

The procedure is now being investigated at five sites in the United States, with five patients at each site. As with any surgery, there are risks of complications. Some of the potentially serious ones involve conjunctival erosion, scleral erosion, anterior segment ischemia and malignant glaucoma. Side effects may include subconjunctival hemorrhage, photophobia, foreign body sensation, excessive tearing and irritation. Scleral expansion has been shown to be reversible; the surgeon simply removes the PMMA expander segments. The eye returns to its original pre-surgical state after such removal. Postoperatively, the patient will use a topical antibiotic-steroid for one week along with artificial tears and bland ointment for three weeks.

Multifocal Corneal Theory
Clinical observation after radial keratotomy has revealed that some presbyopic patients have both excellent uncorrected distance and near acuity when variations in corneal power produce a multifocal effect. However, patients who undergo laser vision correction and its large, flat, homogenized treatment zone do not exhibit a multifocal cornea. Is there is a simple laser sculpting technique to produce the multifocal corneal?

There are many questions regarding the visual system and the effects of multifocal imagery. The brain only recognizes one image—the object of regard—and parallel perception is not possible. In Germany, Till Anschutz, M.D., is investigating various techniques for laser ablation to correct for presbyopia. Some of these include the creation of central islands and/or sectorial zone treatment. The technique has resulted in ghosting and diplopia, the same side effects that we try to avoid during standard laser vision correction. Further investigation of how the visual system interacts with multifocal imagery is necessary before researchers develop presbyopic laser correction.

Luis A. Ruiz, M.D., of Bogota, Colombia, is treating presbyopia by multifocal LASIK, a technique that offers patients full near vision correction, regardless of age. Dr. Ruiz' technique incorporates various masking techniques to produce an aspheric cornea, creating a steep zone in the central cornea for near vision, with intermediate and distance vision in the peripheral zone. He concludes that further work is needed in algorithms and the accuracy and predictability of the multifocal LASIK cornea.

These two surgical techniques for the correction of presbyopia are currently under investigation. The safety and efficacy of these techniques have yet to be established. As the U.S. population ages, the demand for these procedures will increase in the coming decades. Expect technology to meet and exceed these demands.

Dr. Augustine is president of the Optometric Refractive Surgery Society and director of refractive consultative services at the Cleveland Eye Clinic and Toledo LASIK Center.