الفهرس 
Anatomy & optics of the corneaOnly 14 pages are availabe for public view
 |  | from | 172 |  |  |
| | | from | 172 | | |
Abstract
Presbyopia is the most common refractive error. It is an age-related visual impairment, resulting from the gradual decrease in accommodation. Without optical correction, presbyopia results in an inability to perform once were effortless near tasks at a customary working distance without experiencing visual symptoms.
A variety of options are available for correction of presbyopia. These options could be optical correction as; spectacle lenses, contact lenses. Or they could be surgical correction as; scleral expansion, conductive keratoplasty, small diameter corneal inlays, multifocal IOL implants and finally the use of excimer laser.
Although glasses and contact lenses represent the easiest and traditional method for correcting presbyopia, some patients are not willing to wear reading glasses and many patients can not tolerate contact lenses. Conductive keratoplasty is an effective and safe procedure for the treatment of presbyopia, but it is limited by its monocular application. Scleral expansion is another surgical approach to correct presbyopia but it is prone to regression. Corneal inlays are not popular and are not recommended in current practice due its dangerous surgical technique and potential complications. The performance of different types of multifocal IOLs is constantly being improved, but the IOLs cuase a decrease in near vision contrast sensitivity.
Researchers are increasingly optimistic about the potential of multifocal LASIK as a treatment for presbyopia. The concept first attracted attention when some presbyopic hyperopic patients reported both improved near and distance vision after undergoing LASIK for far vision correction. Careful corneal analysis of those cases led to the appreciation that it might be possible to use excimer laser to intentionally create a multifocal cornea. Studies show the multifocal ablation procedure is very safe and is able to provide reasonable reading vision without compromising distance outcomes.
Various presbyopic strategies using the excimer laser have also been brought forward. Beside monovision, the creation of multifocal cornea represents an attractive option. This procedure is also called “ Presby-LASIK”.
Presby-LASIK treatment uses the principles of LASIK surgery to create multifocal corneal surface aimed at reducing near vision spectacle dependence in presbyopic patients. There are two main different techniques for presby-LASIK treatment. In the first technique, known as central presby-lasik, a central area is created for near vision and a peripheral area is created for central vision. whereas in the second technique, known as peripheral presby-LASIK, the central area is for distance vision and the mid peripheral area is for near vision. Both techniques create a multifocal pseudoaccommodative corneal surface.
The ablation profiles of the first generation of presbyopic LASIK techniques were based purely on refractive data, with nomograms that required adjustment based on outcomes previously achieved. The more recent second-generation presbyopic LASIK algorithms are based on corneal geometry and wavefront data. The early results achieved with the second-generation of presbyopic techniques indicate that they may provide superior results. The new LASIK techniques use wavefront and corneal topography data to calculate an ablation profile that will change the peripheral cornea’s Q-value to provide an increased depth of field.
While the data are limited, the excimer laser offers some potential advantages over other methods to manage presbyopia. The procedure is less invasive than scleral expansion or a multifocal IOL, it can concomitantly correct the near and distance refractive errors. One of the advantages of this type of treatment is that it is centered on the visual axis of the eye rather than on the corneal apex like many multifocal contact lenses and in this way, decreases the induced loss of optical performance due to axis mismatching.
Despite encouraging initial clinical results, various questions are unsolved. There are no long-term results, and most trials include hyperopic patients. PresbyLASIK was often performed on relatively young patients, with no or little presbyopia (fewer than 2.00). It is unknown whether or not a retreatment is needed later. Safety issues including impairment of contrast sensitivity and unwanted optical aberrations with appearance of glare and halos should be addressed in future studies.