We need to think in detail about corneal optics after refractive surgery because the normal cornea is relatively trouble-free. The cornea after refractive surgery is not trouble-free. It frequenlty has a more aberrated optical performance than its preoperative counterpart. It is often unstable during diurnal cycles and in time periods measured in months and years. Its optical performance can even change instantaneously with changes in pupil size. It can impair peripheral vision more than central vision. Most seriously, it can impair night vision more than day vision by a pernicious combination of factors, and it is irreversible. Once done, there is no return to the preoperative state. Contact lens and spetacles do not permanently alter the physiologic optics of the eye. Refractive sugery does.

When one alters irreversibly the most trouble-free component of the human visual system, one runs the risk of compounding the visual aberration caused by components of the visual system that characteristically show dysfunction with age (the lens and macula.) The young to middle-aged population now seeking refractive surgery will in 30 to 40 years be part of the swelling population over age 55. This population is predicted to increase by 82% between 1980 and 2030. How many keratorefractive patients who can compensate for their aberrated cornea will be able to do so when the lens and macula develop age related changes? How much sooner will they require cataract surgery or visual aids for macular degeneration? These questions have important public health implications.

Consumer groups are concerned about these issues. They note that most patients who undergo refractive procedures are responding to advertising. In 1988 Consumer Reports magazine reviewed the data on radial keratomy and advised its readers against the procedure. They reconfirmed that judgment in their 1992 health letter.

Many reasonable refractive surgeons will listen politely to the various types of arguments, but they will say the arguments are moot now that refractive practice is so successful. Emmetropia is achieved in ever increasing numbers of patients. Patients are happy. The number of refractive procedures increases as even the staunchest critics of refractive surgery are forced to acknowledge its success or find their practice at a competitive disadvantage. These statements are made almost weekly in the trade journals. In short, why worry about refractive surgery as a potential public health hazard when everyone is happy and basking in success? To answer that question, one needs to understand the following ideas.

Optical aberration occurs commonly after refractive surgery - Refractive surgery commonly aberrates human vision because it is difficult for refractive procedures to generate a regular optical surface over a large enough corneal surface to avoid aberrations in the cental field, and even more difficult to prevent aberration in the peripheral visual field. To avoid aberration in the center of the visual field, the cornea must be regular over the entrance pupil . For example, a patient with a 2-mm pupil will have nonaberrated central vision if the cornea overlying that pupil is regular. A patient with a 5-mm pupil needs a regular corneal surface over that larger area.

When the cornea is irregular over the entrance pupil, the image generated by the cornea loses contrast and edge definition. Glare and ghost images can occur that are particulaly disturbing under conditions of low illumination. The degree and quality of the aberration will depend on the degree and quality of the corneal irregularity. Topographic studies show corneas develop a heterogenous group of topographic patterns even when they undergo uniform surgical procedures. The final result is that corneal irregularity from refractive surgery can cause optical degradation; and performance in the central field can change with pupil size.

To avoid aberration in more peripheral portions of the visual field, the cornea must be regular over the cornea adjacent to the entrance pupil as well as over the entrance pupil itself. Let us use it to calculate the diameter of corneal regularity needed over the pupillary area to avoid aberration over a given portion of the visual field. If we want our patient with a 2mm pupil to retain glare free vision to 30 degrees from fixation, we require a regular cornea 5.04 mm in diameter and it must be centered on the pupil. Our patient with a 5mm pupil needs a regular area 7.43mm in diameter. The surface area requirements are slightly less stringent for refractive procedures, but not appreciably so. Applegates's work document that these distortions in peripheral fields do occur.

Can keratorefractive procedures produce regular surfaces of sufficient size and place them properly relative to the entrance pupil? They occur inconsistently for central vision requirements; and less consistently for peripheral vision requirements. One finds in corneal topograrphy studies of keratorefractive procedures smaller than predicted optical zones, procedures centered away from the pupil center, and irregularity within the bounds of the pupil, or in the areas just peripheral to the pupil margin. These effects occur in disturbingly high numbers of cases. For example, a recent topgraphic study found that only 42% of 97 consecutive cases of excimer laser photorefrative keratectomy for myopia showed a uniform pattern of refractive power within the abalation zone; 10% had distortion severe enough to cause spontaneous complaints of aberration in central vision. The center of the pupil can also change in location during mydriasis further aggravating problems with central aberration and glare in the peripheral field. Fortunately topographic analysis provides this type of feedback and allows for much tighter quality control than occurs when best-corrected-visual-acuity and residual refrative error are the major indicators of success. Unfortunately many authorities continue to assume that these procedures are an unqualified success as long as uncorrected visual acuity is 20/40 or better.

Diameter of Clear Optical Zone for Glare-Free Distance Vision
[Top Row is the Entrance Pupil Diameter/Size in miilimeters. Radius (column numbers) is the angle of additional light rays to consider. For example, an entrance pupil of 5mm using a radius of 15 degrees requires an ablation optical zone of 6.28mm to minimize optical aberrations.]

Across - Pupil Entrance Size ==> Down - Visual Field Radius In Degrees	2.00 mm	3.00 mm	4.00 mm	5.00 mm	6.00 mm	7.00 mm
0 (fixation point only)	2.00	3.00	4.00	5.00	6.00	7.00
5 degrees	2.52	3.50	4.48	5.45	6.41	7.37
15 degrees	3.53	4.46	5.38	6.28	7.17	8.04
30 degrees	5.04	5.86	6.66	7.43	8.18	8.91
45 degrees	6.62	7.29	7.93	8.55	9.14	9.71
60 degrees	8.36	8.83	9.27	9.70	10.12	10.51

Compensatory mechanisms mask surgically induced optical aberration - The human visual system has sophisticated backup systems that can dampen the effect of corneal aberration especially under photopic conditions. The Stiles-Crawford effect is a mojor component of that defense. It acknowledges that the human visual system uses with more efficiency light rays that pass through the center of the pupil than rays passing through the peripheral pupil. As a result of this phenomenon, refractive patients may perceive an object as well resolved despite conrneal irregularity within the pupil area if most of the irregularity overlies the peripheral portion of the pupil. In other words, their optical degradation, because the visual system can compensate for at least a portion of it. They may notice mild glare subjectively and have minor loss of contrast and simultaneously have mooderate or moderately severe corneal aberration. They may be happy with the results despite the quality of the optics rather than because of it.

Compensatory mechanisms fail at night - Humans are not nocturnal beings. As a result, human vision is not as well adapted for night vision as it is for day. We are required to distinguish between much subtler degrees of contrast. We are presented with sudden and unexpected sources of intense glare especially when operating motor vehicles. Our peripheral vision becomes more important. It is during night conditions that one would expect optical aberration from refractive surgery to occur most commonly, and indeed such patients are more likely to complain about night aberration than day aberration.

What factors raise concern about visual performance at night? First, the pupil enlarges. As it does, aberration of central vision increases as more distorted paracentral cornea falls within the pupillary space. The peripheral vision suffers to a greater extent for the same reason.

Refractive error changes as well in many patients as a result of the increase in spherical aberration. Holladay and associates' study found pupil-induced changes in refractive error in 36% of the Prospective Evaluation of Radial Keratotomy subgroup he studied compared with 9% of controls. Degraded image quality can cause changes in the resting tone of the lens which can further aggravate night myopia.

The problems with pupil-related aberration are further magnified by the reality that the Stiles-Crawford effect is negated in night vision. As a consequence, any corneal irregularity overlying the pupil will affect central vision more at night than during the day.

I hope the reader will now understand how a patient may have clinically acceptable 20/20 visual acuity in the daytime and still suffer from clinically dangerous visual aberration at night if that patients's visual system must cope with an altered refractive error, increased glare, poorer contast discrimination, and preferentially degraded peripheral vision. People die at night in motor vehical accidents four times as frequently as they do during the day and these figures are adjusted for miles driven. Night driving presents a hazardous visual experience to adults without aberration. When we discuss aberration at night we are considering a possible morbid effect of refractive surgery.

A keratorefractive patient may simultaneously be happy wwith the result of surgey and have degraded vision - How can refractive surgey be a potential public health problem if patients are happy with the results? Inherent in this quetion is the assumption that a patient without complaint is a patient without optical degradation. That argument does not hold up to closer scrutiny. The keratoreractive literature contains disturbing examples of patients who have visual handicaps that place themselves and others at significant risk for nighttime driving accidetns and yet that are happy with the results.

A recent study of patient satisfaction after radial keratotmy consisted of 100 former contact lens patients who underwent radial keratotomy. Comlaints after radial keratomy were rarely mentioned spontaneously, but when elicited, the study found a high complaint rate. Of the patients 10% complained of glare, 7% of "suboptimal visual acuity," 5% of astigmatic blur and headache, 3% near-vision problems and 2% photophobia. Fifty percent complained of "impaired night vision." Half of that group needed spectaces for night driving. Six percent decided against operation in the fellow eye. Despite these findings, 93% of the patients perferred radial keratotomy to contact lenses.

In a study of excimer laser photorefractive keratectomy for myopia, 6% of patients complained of "severe trouble" with night driving, but all said they would have the procedure again. In a rcent study of multizone excimer laser ablation for severe myopia, 14 of 17 patients with six-month follow-up complained of halo and sunburst effects and degraded night vision. The authors are disturbed enough about the findings to suggest that multizone treatment should be discontinued. Despite these findings, five of the patients elected to have the fellow eye treated, and six more are planning such treatment. These findings document that patients who do develop disabling night aberrations may nonetheless be happy with their results.

We are led to the conclusion that patients will tolerate degraded optical quality (including functionally significant aberrations in night vision) for improved uncorrected vision. They are happy now but how safe are they on our highways at night? How safe will they be when cataract-induced aberration compounds the glare and aberration effects of the cornea; and when early macular disease begins to reduce compensatory mechanisms in the retina?

We need answers to these questions because in 30 years one in four drivers on the road will be older than 65 years.

In summary, keratorefractive surgery changes refractive error by permanently altering the optical performance of the cornea. These new surfaces differ optically from their preoperative counterparts. Topographic and psychophysic studies show clinically significant differences in structural stability, quality of resolution (central and peripheral vision), depth of field, reproducibility of clincal refraction, pupil effects on refractive error, and pupil effects on glare and blur in both the central and peripheral visual field. Any surgeon or design error will exacerbate the optical problems inherent in the surgery. Visual acuity and refractive information is of limited value in assessing surgical success because such measurements cannot assess central and peripheral glare, ghost images, or the dynamic shifts in optical performance caused by changes in pupil size and ambient illumination.

Keratorefractive surgeons can be proud of the strides that have been made in improving shortterm refractive accuracy. Unfortunately, they and their patients must understand that the optical changes induced by the surgery allow the conditions for visual compromise to exist even if 100% of their patients have at least 20/40 uncorrected visual acuity and everyone is happy.

In the late 1990s the innovators in the keratorefractive field will be those who understand the optical shortcomings of the present state of the art. They will measure results with indicators more sensitive than visual acuity and residual refractive error. In the process they will identify the extent and the nature of the problems that remain and will work to remedy them. As a results of those efforts, the patients we serve will feel increasingly secure that surgeons are consistently preserving optical quality while correcting refractive error. The work has already begun. We must ensure it continues.

End Text
Excerpted from American Journal of Ophthalmology journal,"Keratorefractive Surgery, Success and the Public Health", March 1994, P. 394 by Dr. Leo Maguire, Mayo Clinic.

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