A Very Different Kind of Monovision
By Robert A. Koetting, O.D., F.A.A.O
Join this O.D.'s search to discover whether contact lenses and refractive surgery for monovision have as much in common as you think.
It has been 35 years since Dr. Robert Lester first described the controversial technique of monovision. Fortune magazine recently quoted a disgruntled LASIK patient who says that his post-surgical monovision is inadequate for midrange seeing. The Federal Aviation Administration (FAA) charged that Delta Airlines' flight #554 crashed at LaGuardia Airport because the pilot was wearing monovision contact lenses.
Except for facing the same sort of scrutiny and criticism, are there any other similarities between contact lenses and refractive surgery when it comes to correcting presbyopia? My personal discussions with fellow contact lens practitioners who have managed thousands of presbyopia cases indicate that the two options don't have as much in common as you might think.
Surgery is Different
As an example of how monovision contact lenses and refractive surgery differ, consider the mechanics of each option. Although compensation for spectacle refractive vertex correction is programmed into the laser, there are other optical considerations that are rarely mentioned.
For instance, when a patient goes from a spectacle correction to contact lenses, there is a change in the amount of accommodation required in order for them to focus on a near object. A myope doesn't have to accommodate as much with spectacles, while a hyperope will find reading to be more efficient when he wears contact lenses. The effect can amount to a diopter or more of difference when patients have a high refractive error, but this rule apparently doesn't hold true for refractive surgery patients.
The aspheric optics of a post-surgical cornea seem to correct for extra increased optical or age-related accommodative effort.
Fortunately, even if the math is correct, most patients who are accepted as candidates for surgery are presently in a range which limits the correction to about �0.50D. The asphericity most likely accounts for acceptance by younger presbyopic patients.
Although surgical correction of hyperopia has only been approved for up to +4.00D at this time, it seems unlikely that there would ever be real success with patients who have greater amounts of hyperopia. Also, monovision surgery is discouraged in hyperopes because amblyopia is common.
Trial and Error
Surgeons usually only recommend surgical correction to patients who have successfully experienced monovision with contact lenses. Such caution assures a favorable outcome, but many feel that this prerequisite eliminates a large segment of the market. Following candid counseling, most patients are willing to sweat out the adaptation period with refractive surgery.
Furthermore, post-surgical problems can usually be corrected by enhancement LASIK or by replacing the Intacs (KeraVision) ring. Even though one-fourth of patients seem to require such enhancement procedures within a month after surgery, only six percent need it after that time. Changes or enhancements are best performed within the three-month healing period when the flap can still be lifted. After this time, the epithelium must be surgically removed.
The aspheric distortion accompanying present- day corneal surgery could actually be a
major benefit. In most practices, about 99 percent of patients over the age of 40 are
given an arbitrary mild
(-0.50D) near correction in one eye. This method conflicts with the philosophy of forcing adaptation by making it difficult to use the same eye for distance and near vision.
Some surgeons are reluctant to leave patients with seriously compromised vision, even for a short period of time. Of course, these surgeons have happily learned that surgery actually seems to reduce the need for a near point add.
An implantable contact lens, now under investigation, rests in front of the crystalline lens but is not an anterior chamber device. The lens introduces new optical considerations because it behaves like a traditional contact lens, which suggests the need for further studies. The effect of increasing plus or decreasing minus power in one eye to provide more comfortable vision for esophoric patients is one topic that requires further investigation.
Are surgical monovision patients likely to be more successful than those who wear contact lenses for monovision? No one knows for sure at this point, but the following current observations will certainly aid in the future selection of presbyopic candidates for refractive surgery or co-management.
- A decreasing number of surgeons feel that bilateral operations shouldn't be recommended for patients over 50 years old. In fact, about 90 percent of the surgeries are now bilateral.
- Patients who have opted for LASIK, and who have spent several thousands of dollars on it, are less likely to publicly express dissatisfaction with the outcome. Nevertheless, most myopic borderline presbyopes report their post-surgical near vision to be much improved, even when monovision was not intentionally employed.
- With effective adds of less than +1.00D, moderate to high myopes, over 45 years old, are reported to adapt quite readily. The same can be said of low hyperopes who are younger.
- High hyperopes and those with adds of +2.00D or greater generally have unsuccessful refractive surgery procedures. For that reason, only about one percent are given a full near-point correction during surgery. As many as nine out of 10 have spectacles prescribed for part-time wear for reading, and about eight percent wear spectacles to drive.
- Select the right surgeon. Experience counts. Many say 300 procedures may be required of a surgeon before his results are truly dependable. A success rate of 99 percent has been reported for surgeons who have performed 3,000 or so procedures. It should be noted that these figures are skewed because advanced instrumentation now assures greater success than expected from less experienced surgeons.
As it was in the early days of contact lens wear, monovision has become acceptable because there is no other choice. But, the comparison of monovision approaches may stop there.
Dr. Koetting is a contributing editor of Contact Lens Spectrum and has written extensively on subjects involving monovision procedures.