The PRESBYLENS™ Corneal Inlay: An Adjustable Device for Presbyopia

Presbyopia is a complex phenomenon. There are still debates about the mechanism of action, and a host of "treatments" have been tried. Right now there are many companies working on many therapies, and hundreds of millions of dollars have been raised and spent. This article, while interesting from a financial point of view, doesn't make sense without an understanding of the history of presbyopic treatment. The method being proposed is by no means novel, the first corneal inlays were tried in the 1980s, and the first multifocal lenses were tested in the 1970s.

Treatment for presbyopia ranges from conservative to very speculative.

Most conservative are bifocal or reading spectacles and bifocal contact lenses.

Monovision, using a near focus lens for one eye and distance for the other, can be tried with contact lenses or any of the optical surgical interventions listed below. Some people adapt to monovision, some don't, so patient adaption should be tested first with contact lenses before progressing to invasive methods.

Then come multifocal contact lenses, which have two or more diopters so that part of the optic improves near vision and part of the optic improves distance vision. All multifocal approaches exploit the ability of the visual system (not just the eye) to seperate out the "well focused" image from the "poorly focused" blur of the other focal zones of the added lens. In all such treatments, there is a trade-off that becomes most obvious under low light (mesopic) conditions: only part of the incoming light is "well focused". These methods are problematic.

The classic test for these methods is the so called "night driving simulation" which is still required in some cases by the FDA. A pubmed search today turns up 542 papers on "night driving". Many of these discuss the limitations of multifocality.

Multifocal lenses are currently being placed on the cornea (contact lens), under the epithelium or in a stromal flap (as in this article), in the anterior chamber of the eye (phakic IOL), and in the posterior chamber of the eye (phakic IOL). In addition, there are treatment algorithms for most lasers used in LVC (laser vision correction) to create multifocality in PRK, epi-LASIK, and classic LASIK. All attempt to trade off the optical limitations of these approaches: pupil size (from 2 mm photopic to 7 mm mesopic) limits placement of near and distance focal zones - is it more important to have distance vision under low light or to be able to read a menu under low light?

Other treatments being investigated include the use of lasers to create incisions in the sclera outside the limbus, in a sort of analogy to the early RK relaxing incisions used to reshape the cornea. In some cases these incisions are filled with silicone or other inserts which are sutured into place. Implants are also being inserted into tunnels in the sclera, using various incision methods, ranging from simple "trench" incisions to manual tunneling through automated systems somewhat analagous to the microkeratomes used for LASIK. These are conceptually related, in that they treat presbyopia not with a multifocal optic but by changing the physical ability of the eye to accomodate. In some cases, these have shown excellent results, none are approved by the FDA though some are in late clinical trials.

The last family of treatments are the "accomodative IOLs". These replace the natural crystalline lens with an IOL which is designed to simulate the optical behavior of the young natural lens. The single lens systems have received a lot of press in the last few years, but some simple math shows that they cannot achieve significant improvement - the available millimeter of motion is only good for a couple of diopters of accomodation, where young eyes have 8 to 12 diopters of accomodation.

Some newer designs of accomodative IOL use multiple lens elements, and are at least theoretically able to achieve 4 to 6 diopters of accomodation. It will be interesting to see how these perform in large populations over extended periods of time.

For over 25 years there have been attempts to create "gel" type implants that have the same mechanical and optical properties as the natural crystalline lens. This is the most promising approach, but is beset by complications. Current work at many centers is targeting some sort of polymerize-in-place so that a liquid can be injected, formed, and then polymerized in-situ - preferably with closed loop control with a wavefront system to achieve optimum correction. This requires further development of the materials, of course, and significant changes to surgical methods of phacoemulsification, lens injection, lens forming, and so on.

And still, there is incomplete understanding of presbyopia. Too many of the interventions either neglect or dismiss the many factors involved. Crystalline lens growth continues through life, which is the usual explanation of presbyopia. But what must also be considered is that pupil size changes with age, retinal function changes with age, various low-level neural control mechanisms change subtly with age, many medical conditions and medications effect muscle performance in the eye, scleral properies can change with age or disease, and neural plasticity varies between patients for reasons not yet understood.

As an ophthalmic engineer I've worked on many of these interventions over the last 20 years. While I congratulate ReVision for their successful fundraising, I think the jury is still out on whether or not ReShaping and ReContouring the cornea is as good for patients as it is for marketing. The conservative treatment, which is 100% reversible with no side effects, is still bifocal spectacles or reading spectacles. I'll keep mine for a while yet, thank you.