Radial Keratotomy — side effects

ll surgical procedures, regardless how benign or “safe”, carry with them certain side-effects and/or complications. RK is no exception. Side effects, for the purposes of this discussion, are defined as the usual or expected sequelæ of a surgical procedure that disappear with time and require minimal or no treatment. These sequelæ are usually self-limiting but for the time they are present can create severe problems for the patient and may, subsequently, have to be dealt with by the surgeon. The side effects of RK are:

Glare and/or photophobia

It is a matter of some amusement to me to find that patients are often puzzled by the fact that they are experiencing some degree of sensitivity to light after eye surgery. I have to admit to being somewhat bemused that such an individual can't figure that out when they find it perfectly natural to experience the same phenomenon after getting something in their eye. Go figure. Nonetheless, it is a fact that practically all RK patients experience some degree of photophobia (light sensitivity) following surgery as well as some degree of glare at night. The photophobia lasts just as long as it takes the corneal epithelium to heal over - which is about 6 to 12 hours. However, the sensation can last longer especially if the patient has “leaky epithelial syndrome”. The corneal epithelium usually heals tightly together, producing a relatively water-tight surface. This is not always the case. Sometimes it can weeks for a tight bond to form. During this time the patient's cornea is typically thicker due to excess fluid and also the corneal nerves are exposed producing considerable discomfort. In these cases Muro-128TM 5% drops can help by drying the cornea out; though in severe cases prolonged patching of the eye may be necessary. Glare is not the same thing as photophobia and is experienced at night manifested by oncoming headlights or other point sources of light showing a starburst pattern or a flare or both. This situation is aggravated by the fact that there is normally a certain degree of loss of effect of the surgery (especially in the first few months) in the evening because of decreased intraocular pressure. Hence any point source is out-of-focus which increases the sensation of a smeared or flaring image. In severe cases, the fitting of temporary spectacles to be worn while driving at night solves the problem.

The treatment of glare can take many forms with the main object that of diminishing its effect upon the patient. It is not possible to totally eliminate glare and patients vary in their response both to glare and its treatment. Some patients never complain at all. Pre-operative testing with a device such as the Miller-Nadler glare tester, while alerting the surgeon to a possible problem, has not proven useful in screening out patients with increased glare sensitivity. It is not unheard of for patients to complain of less glare post-operatively and show this in their test results. Some glare is associated with residual myopia or “against-the-rule” astigmatism and can be treated by simply fitting temporary spectacles.

Commercially available absorptive lenses will reduce the amount of visible light reaching the retina and may reduce some of these symptoms. The following may be of value:

1. Anti-reflective coatings typically consisting of an ultra-thin layer of magnesium fluoride create light wave interference, thus reducing reflected light. This will improve the optical performance of any spectacle lens and may reduce the multiple image reflections caused by oncoming headlights and other point sources of light.

2. Tinted spectacles are popular with the public for reduction in glare although their effectiveness in the treatment of glare may be minimal. In the truly light sensitive (or photophobic) individual, a dense pink tint allowing approximately 52% light transmission (cruxite C) may be of value. Neutral gray tints are available in densities as dark as 12% transmission. These absorb as much as 98% of ultraviolet along with infrared, and do not distort colors. If a significant reduction of light intensity during the day is required, these are a satisfactory lens. Green tints may be similarly used, but do not reproduce colors as well. Brown tints may also be of value, have some cosmetic appeal, and do slightly enhance contrast. They, however, cause color distortion. Yellow tinted spectacles absorb 100% of ultraviolet, transmit infrared, and about 83% of the visible spectrum. They tend to absorb heavily in the blue region and, therefore, reduce haze and enhance contrast slightly.

3. Polaroid sunglasses, alone or in combination with dense neutral gray coating, are of value in reducing total incident light as well as eliminating annoying surface reflections such as occurs on water or snow. Such lens are particularly effective in treating both glare and photophobia.

4. Photochromic lenses darken when exposed directly to ultraviolet light and lighten when ultraviolet light is withdrawn. In those patients exposed to particularly bright sunlight on snow or water, a combination of photochromic glass and a Polaroid(tm) filter as found in the photopolar glasses may be helpful.

Topical pilocarpine in dilute strength (0.25%-0.50%) may also be used to reduce glare-dazzle symptoms through pupillary miosis. However, such use is of limited value and some patients will experience brow ache or difficulty in reading.

Oftentimes the patient will complain of “glare” at night when what they are really experiencing is an enlarged blur circle resulting from the natural shift toward myopia experienced late in the day. In these cases, the problem might be eliminated by fitting the patient with spectacles to be worn only when driving at night. Sometimes repeat surgery may be necessary.

Corneal edema

Corneal edema is seen in 93% of patients in the immediate post-operative period. It usually extends from the bottom of the incisions anteriorly, spares the area within the optic zone, and is usually of mild degree. Greater edema is most often seen in cases which have had 12 or more incisions and in those cases in which the incisions have been stepped or otherwise re-deepened or manipulated. It will also occur in those cases in which the cornea has been kept wet, or which have received excess irrigation of the incisions following the surgery. The use of hypertonic saline solutions have been suggested (and tried by some investigators) but have not proven to be of much help. The use of such treatment is quite uncomfortable for the patient and of questionable value. Generally the edema subsides rapidly without treatment and is completely gone within 2 weeks.

Ghost images

In some patients there is a complaint of ghost images or rarely, discrete monocular diplopia. Usually the patient describes images as having an associated secondary image best described as similar to an out-of-convergence television picture. Occasionally these secondary images will be sufficiently separate to produce frank diplopia. In all occurrences of this phenomenon, vertical folds in Descemet's membrane can be faintly seen within the optic zone. These are more prominent in cases of diplopia and are closer to the optic center. Circumferential folds can also be found between the incisions outside the primary optic zone. If these are prominent they may add to the complaint of glare. The circular folds spread centrifugally and usually disappear by four months. Upon their disappearance, the vertical folds also leave taking the diplopia with them. Contrary to the case with glare, correcting any residual myopia may make the symptoms worse. The problem is compounded by the presence of “against-the-rule” astigmatism either pre-existing or iatrogenic. In the latter case, the symptoms may diminish as the astigmatism “perambulates” or disappears.

Fluctuation of vision

More than 90% of all RK patients will experience some degree of fluctuating vision throughout the day for the first 4 to 6 months. This phenomenon is infrequently seen in 8 incision cases and is undoubtedly related to the diurnal variation in intraocular pressure seen in the human eye. [8] Other factors such as splinting of the malleable cornea by the lids during the night may be a factor as well. Corneal edema occurring during the night has been suggested as an underlying cause. [9] This has not been borne out by repeated ultrasonic thickness measurements carried out on randomly selected patients at different hours of the day — although such changes in thickness have been seen in patients with severe epithelial disease. While the cornea is increased in thickness post-operatively to some degree, no significant changes in this thickness have been found to occur diurnally (see Under-correction, below).

Some studies have suggested that corneal shape changes due to diurnal pressure fluctuation are not a factor, by demonstrating no significant corneal curvature changes through the day. [10-12] In a series of patients examined by the author over a period of three months — applanation pressures were measured on both the operated and the unoperated eye at varying times of the day including late evening and early morning. In addition, ophthalmometry was performed on each eye. It was found that it was possible to detect small changes in the central corneal curvature by this method. While not conclusive because of the inherent inaccuracies within the method of measurement and the smallness of the changes, these changes were, nevertheless, significant in their own right. In every case in which the visual acuity was affected, the K-readings were measurably different in the operated eye as opposed to those in the non-operated eye for a given time period. In those patients, for example, who stated that their vision was better in the AM as opposed to the PM, the central corneas were uniformly flatter centrally in the morning, than in the control eye. This observation was well demonstrated by Deitz [13] in a series of patients in whom the IOP was increased by use of topical steroids and has been corroborated by Fyodorov and co-workers[14] as well as Feldman and co-workers. [8]

The “Bates effect” [15] or phenomenon has not been suggested as a possible causative agent but may well play a part because the corneas of these patients are extremely malleable. It has been observed that many patients demonstrate good unaided post-operative visual acuity despite their having a residual myopia which ordinarily would drop the vision to 20/100 or less. If the lids are held away from the eye in these same patients their vision falls to the expected levels. It is possible that varying degrees of orbicularis spasm or lid pressure exerted during the day (with more in the morning when fresh) produce varying amounts of flattening and hence varying acuity.

This side effect, like glare, is self-limiting but may not go away completely for a number of years. [16] Large fluctuations — those inclined to induce patient complaints — last for a much shorter time and usually subside by 4 to 6 months. The fitting of a hard contact lens to reduce the fluctuation has been tried with limited success. These patients are often lens intolerant in the first place. Further, an exact corneal curvature measurement, required in hard lens fitting, is not often possible during this period. The fitting of a soft contact lens for the same purpose is not recommended before 8 weeks because of the danger of neovascularization of the incisions. Since the lens is extremely flexible, its effect upon the variation in vision is small but some patients notice a diminution of visual variation.

Treatment is “tincture of time” and re-assurance. The fitting of any visual appliance for this purpose before 12 weeks is an exercise in futility and in any case unnecessary unless there is significant residual myopia and/or astigmatism. In those cases the patient should be told that he/she may require additional changes somewhere down the line.

Diplopia (double vision)

This symptom will usually occur between surgeries as a result of anisometropia. Treatment is surgery of the fellow eye. Transitory binocular diplopia may occur shortly after surgery in young individuals. This may be as a result of over-convergence associated with over-accommodation. No cases of persistent binocular diplopia following this surgery have been reported to date.

Asthenopia (weak eyes)

Certain patients, particularly those in their early 30's, will complain of some difficulty with “tired” eyes or intermittent blurring while reading. These symptoms are generally seen in cases of pre- operative myopia greater than 4 D. These patients may have normal near points of accommodation upon initial testing both pre-operatively and post-operatively but tire upon repeated testing. Consequently, these patients will be unable to perform close work or to read comfortably for a time. The treatment is re-assurance — reading glasses are not recommended — unless in the presbyopic age range. The problem can be likened to that of a short distance runner being suddenly expected to run 10K or more. These patients have not been called upon to fully utilize their accommodation until now — some “getting into shape” is what's required. Accommodative effort is increased in contact lens wearing myopes. [3]

Premature presbyopia (loss of reading ability)

This is actually a misnomer. A better term perhaps would be “unmasked presbyopiaIt should also be borne in mind that many myopes experience some degree of asthenopia for near when wearing contact lenses. Such patients should be warned that after a successful RK procedure (or for that matter any procedure for myopia) some difficulty with near vision — particularly reading — may be experienced.

Interestingly, some patients who were previously unable to read comfortably while wearing a distant correction are able to read after having their myopia fully corrected by the surgery. This could be as a result of a multi-focal condition of the cornea occurring after surgery. The author reported a series of cases in which the cause for the seeming “loss” of presbyopia appeared to be pseudo- accommodation wherein the pliable post-RK cornea was flexed by the ciliary muscles. Helmholtz demonstrated that the corneal curvature does change slightly in the normal during accommodation — the incised cornea could be more susceptible to this effect. [7]

Upper lid edema (swelling)

Approximately 16% of patients who have corneal abrasions demonstrate edema of the upper lid within 24 hours. The incidence of this side effect after RK is approximately the same, and no special significance is placed on its occurrence. It lasts — for the most part — only for a few days and rarely becomes severe enough to produce complaints. On those occasions, cool compresses and aspirin analgesia suffice. Tylenol (acetaminophen) does not seem to be as effective.

Over-correction (see also: Complications — Over-correction)

All RK patients are usually over-corrected to some degree for the first few months. This occurs chiefly because of corneal edema but also because a certain amount of over-correction is planned on to offset a certain loss of effect that can be expected within the first few months. Generally this over- corrected effect is no more than 17% of the preoperative myopia ( 0.75 D in a 5 diopter myope). This amount should be less in an patient over 40 after 30 days but may be more in the first few weeks.

Under-correction

A patient cannot be said to be undercorrected after RK surgery for the first week or so. Typically, the opposite is the case. Generally speaking a patient who complains of good vision first thing in the morning is one who is likely to end up undercorrected after 6 months. To paraphrase an old saying:

“Good vision in morn — doctor forlorn.”

However, if the vision is satisfactory at about 11 to 12 am, the likelihood is that after 6 months it will still be satisfactory. It should be noted that it is not unusual for the vision to take a nose-dive at around 5 months only to recover at the 6th month. This is one of the reasons that the author cautions to wait for 6 months before having repeat surgery. It is also not unusual for the vision to get blurry after a bout of the flu or severe allergies.

Increased color saturation (Wizard of Oz phenomenon)

Along with the reduction of the myopia and/or astigmatism, this is one of the pleasant side effects of this surgery and was first described by the author in 1979. It is experienced as an increase in the brightness and density of color in objects. Vision is described as “less pastel” in nature. The patients rarely complain of this and all exclaim about it — except that artists may find it a problem initially. One such patient described it like Dorothy walking out into the land of OZ, hence its more popular name: "the Wizard of Oz phenomenon”.

Spatial displacement (out-of-body sensation)

This side effect is a psychological reaction which occurs most often in patients who have had successful surgery in one — usually the non-dominant — eye. They describe scenes as if they were viewing them from one side (usually the operated side) and slightly above their actual position. Usually very transitory, disappearing when the second eye has been operated, it occasionally occurs even then. Sometimes the displaced feeling is sufficiently strong as to produce vertigo. Vertiginous patients may need medication. Fortunately, the phenomenon occurs very rarely and produces no lasting effects.

© Leo D. Bores, MD - 2002