Refraction problems after refractive surgery

SURVEY OF OPHTHALMOLOGY

VOLUME 32 * NUMBER 6. MAY-JUNE 1988

PERSPECTIVES

IN REFRACTION

MELVIN L. RUBIN, EDITOR

Refraction J. JAMES

Problems

ROWSEY,

M.D.,’

AND

After Refractive MELVIN

L. RUBIN,

Surgery

M.D.2

‘Dean McGee E_ve Institute, L’niversi
Abstract.

While refractive surgery such as radial keratotomy, epikeratophakia, and cornea1 relaxing incisions offer many potential benefits to patients, they can also generate optical problems such as overcorrection or undercorrection of the prior refractive error and variable vision. The authors offer suggestions for avoiding problems through proper patient selection; candid preoperative communication with the patient about possible difficulties and limitations; understanding of the physiological changes that may occur; and optical techniques for alleviating postoperative vision problems. (Surv Ophthalmol 32:414-420, 1988)

Key words. refractive

cornea1 surgery

relaxing

incisions

l

epikeratophakia

Remember,

the average

recall that the normal

cornea, at its airltear of optical power

corneal surface

keratotomy

l

Most of the optical problems resulting from refractive surgery are related to simple overcorrection or undercorrection of the prior refractive error. For example, after a radial keratotomy, which purposely flattens the central cornea more than the periphery, it is not unusual to be left with a residual error of 0.5 D-5 D of either myopia or hyperopia. Thus, after bilateral RKs, a patient could conceivably need a correction of, say, - 2 D in the right eye and + 3 D in the left. Such an antimetropia is likely to be particularly disturbing, and the problem is hardly mitigated by the fact that the patient meticulously read your informed consent form twice before sur-

interface,

to the eye. Also,

is not spherical;

radial

Overcorrection and Undercorrection

Refractive surgery is designed primarily to eliminate, or at least to reduce the need for corrective spectacles or contact lenses. Surgery of this type is directed to the cornea, whose surface is so highpowered even minor shape changes significantly affect the eye’s refractive state. adds about 45 diopters (0)

l

its

central Zone is about 4 D steeper than its peripher_y. (One inherent advantage of this corneal aspheric+ is that it helps compensate for spherical aberration of the eye.)

Several different surgical procedures have recently been developed to alter the cornea1 power by modifying the central and/or the peripheral cornea1 topography. These include radial keratotomy (RK), epikeratophakia, and cornea1 relaxing incisions. While each procedure offers potential optical benefits, each can also induce unique refractive (and other) problems. Some of these will be considered here.

gerY . When a residual refractive error is present after clinicians prescribe pharmacologic some RK, agents to constrict the pupil. By doing so they hope to reduce the myopic shift that results from pupillary dilation and to increase the eye’s depth offocus. This solution, however, is useful only in theory. Not 414

REFRACTION

PROBLEMS

AFTER

REFRACTIVE

SURGERY

only does the miotic pupil reduce the amount of light reaching the retina and thereby cause the patient to complain about visual “dimness,” but it is also associated with an induced ciliary spasm that in itself can create even more myopia. We therefore suggest avoiding miotics for this purpose.

Variable

Cornea1 Steepening (Fluctuation)

POST-OP

Another post-RK problem, common in the early postoperative period but sometimes lasting for years. is a duil_y regression ofsurgical effect - a cycle of cornea1 steepening (progressive myopic shift) occurring between morning and evening, followed overnight by cornea1 flattening (progressive hyperopic shift). These cornea1 shifts may be markedly diff‘ercnt, or occur on different “schedules” in the two eyes, and this obviously makes possible a multiplicity of refractive problems and symptoms. You can determine whether or not a variant of this post-operative condition affects your RK patient simply by asking, “Is your vision better in the morning or in the e\rening?” The myope whose RK ouercorrected him (making him hyperopic) will tell you his vision gets much better in the evening. (It is then that the diurnal cornea1 steepening auto-corrects his new hyperopia). The undcrcorrected myopr, on the other hand. has much better vision on awakening in the morning. but as his cornea steepens toward evening, his acuity will gradually worsen. As you might anticipate, the patient with such cornea1 fluctuations who also has even mild antimetropia after bilateral RKs will really have a problem! Pity the poor woman who postoperatively has 1 D of hyperopia OD and 1 D of residual myopia OS. She is likely to find the vision in her right eye adequate in the morning (if she is not presbyopic) and improving as the day progresses toward evening; hut the vision in the myopic left eye, which may be only marginally adequate in the morning, will gradually diminish throughout the day, perhaps to a myopic error of 2 or 3 1). This situation is understandably difficult to tolerate. In our practice, we have encountered daily refractive shifts of as much as 3 D ofincreasing myopia, a cycle that has persisted for three years following an 8-incision RK with a 3 mm optical zone. One such case was especially disconcerting,

since

it happened

to a gracious

nurse

in our own hospital. J.V’., a 32-year-old female nurse, presented to us in March, 1977. with marked contact lens intolerance. G:vcloplegic Refraction 01); OS:

-6.00 + - 5.75

1.25 x

142 20/20 20.20

R.K. I/

formed by peripheral cornea position best composite image (dilated pupil)

--‘-of

position of image formed by _--l)central cornea

(cor&s$cted Fig. 1. After RK, as the pupil dilates, the peripheral

cor-

nea, now steeper than the newly flattened central zone, adds more convergent light to the image and results in shifting the composite focus into the vitreous; hence, mow myopia.

She was highly motivated for surgery, even after our long discussions with her about the potential problems the procedure could generate. She underwent an 8-incision 3.0 mm optical zone RK in the right eye in March 1981 and a similar procedure in the left eye six months later. Even two years after the surgery, though her right eye had an escellent result with a stable correction of - 0.50 + 0.50 x 170, the refractive error in her left eye continued to fluctuate. Cvcloplegic R@action OS (morning) OS (afternoon)

- 0.25 -2.75

+ 11.75 x 165 20/20 + 0.75 X 175 2(1/20

Needless to say, J.V. was not at all happy with her surgical result. She is not one of the staunchest advocates for RK.

Effect of Pupil Size After RK, even with a stable, non-fluctuant cornea, a myope who was mildly undercorrected by the surgery may find his blurred vision becoming even worse in the evening, when his pupil dilates. With pupil enlargement, the planned optical effect of the zone of central cornea1 flattening may he compromised by the added optics induced by the peripheral “knee” of relative cornea1 steepening (Fig. 1). (The “knee” adds more plus vergence power to the in-

416

Surv Ophthalmol

32(6) May-June

ROWSEY,

1988

RUBIN

c fGg. 2. Retinoscopy

before RK (prior linear reflex streak. The cornea1 profile is normal. to neutralization)

shows a regular

Fig. 3. After satisfactory with motion

RK, retinoscopy streak narrows peripherally, where the cornea is now steeper than centrally.

with mot ion

ret i noscopy

FL coming light, thereby increasing the overall myopia.) This phenomenon was called to our attention by a surgeon who himself had an RK. His refractive error fluctuated between 1 D and 4 D of myopia as the light energy in the operating room changed. The amount of his error was directly related to the size of his pupil. In evaluating a patient prior to RK surgery, you should compare the size of the pupil relative to the size of that cornea’s optical zone. This will help you anticipate the potential optical effect of changes in pupil size. Just as with the surgeon above, after RK a patient may achieve an optimal degree of central cornea1 flattening, (with even perfect emmetropia postoperatively), yet still have a significant (apparently undercorrected) myopia when the pupil dilates. To prescribe an appropriate post-operative lens correction for the residual error, you need to be able to identify the zone of exaggerated cornea1 asphericity produced by the surgery (as shown in Fig. 1) and note its position relative to the pupil.

Retinoscopy Preoperative retinoscopy of a typical eye routinely shows a regular linear pupillary light streak (Fig. 2). Post RK, however, retinoscopy will show something new - the knee of steepening in the cornea1 midperiphery that we’ve already mentioned, This knee will create peripheral narrowing of the retinoscopic reflex (Fig. 3). (By rotating the sleeve of the retinoscope, you can easily recognize the unusual reflex, which can indicate unequal cornea1 flattening in various meridians Fig. 4.). In such instances, with plus cylinder retinoscopy, the steepest cornea1 quadrant will be the last area neutralized. After full neutralization of the cornea1 periphery, you will likely find an “against motion” remaining

in the cornea1 center, sors-like” pattern.

resulting

in an overall

“scis-

Refracting and Prescribing To provide the early post-RK patient with a reasonable estimate of the potential visual acuity, Dr. recJack Holladay (U niversity of Texas, Houston) ommends that, as part of the cycloplegic refraction after surgery, the clinician put up a large “pinhole” (3 mm) set in a trial frame. This pinhole approximates the resting pupil size, eliminates the aberrant peripheral cornea1 topography, and usually yields a good approximation of the eventual acuity. We prefer to approximate the potential acuity by performing the post-RK manifest refraction in a dimly lit room that allows the pupil to dilate normally. If the vision is stable, we “crowd” as much plus power as the patient will accept without blurring his distance vision. Then we purposely overminus that refraction by 0.50 D to partially compensate for the induced myopia that accompanies pupillary dilation, and if the vision is still fluctuating (as determined by history), we will sometimes overminus by as much as 1 D. When there is wide visual fluctuation during the day, we do a cycloplegic refraction (along with corneoscopy and keratometry) in both the morning and late afternoon, not only to determine if it is the cornea1 fluctuation that is contributing to the symptoms, but to document the extent. If there is a marked difference, two sets of glasses may need to be prescribed, although that is rare.

Presbyopes and Refractive Surgery Ifyour preoperative-RK patient happens to be an early presbyope, you would be wise to anticipate post-operative trouble with a capital T! Sure, the

REFRACTION

PROBLEMS

AFTER

REFRACTIVE

SURGERY

417

full spectacle correction of an_)’ undercorrected myope) and that reading glasses will very likely he necessary. Despite the careful explanations, demonstrations, and admonitions. we still find that patients rarely understand presbyopia. After surgery. they typically blame the operation itself’ for an)’ presbyopic symptoms. (To help reduce the impact in these patients, we try to achieve a refractive correction of’ about - 1 D; however. in all honest)-. we’ve been so thoroughly burned by this problem. we are compelled to make the following cate,qorical statement: Prepresbvopic low m)‘opes are poor candidates for RK.

POST- OP R.K i%EEL

Cornea1 Curvature

VL

rettnoscopy

----+

near

neutralization

\ Kg. 4. Al‘ter unsatisfactory RK. retinoscopy streak narrows asymmetrically. ;4s)immetry becomes even more apparent as neutralization is approached. (If this pattern occurs aftcar a cornea1 transplant, the narrow reflex edge points directly to the tightest suture.) The cornea1 profile shows thr asymmetric steep zone superiorly.

Measurements

The change induced in the central cornea by refractive surgery is ohviously important because of that area’s key role in forming the retinal image. Thus. it is important to be ahle to monitor the curvature, especially those changes stemming from surgery. Unf‘ortunately, the optics of‘the central cornea are not yet easily measurable h!. small-mire krratometry or corneoscopy. Howe\.er. we anticipate that new techniques ti)r better determining cornea1 curvature should soon become available.

Contact Lens Correction

surgery is likely to make her presbyopia more obvious, but that is only part of the problem; the major difficulty is getting that patient to really understand beforehand the nature of problem she can anticipate. The average myope. especially an early presbyope, habitually removes his glasses to read or to inspect small objects. After surgery, even if the RK perfectly corrects the myopia, but especially if it overcorrects him (making him hyperopic), his remaining accommodation may not be sufficient to afford comfortable focusing for near. In other words. hr was made frankly presbyopic and unable to read without having some type of optical help. To avoid this surprise, do everything possible before surgery to make any near-presbyopic patient aware of this e\,entuality. One way is to show him what to expect by placing a - 1 D lens over his full spectacle correction in a trial frame and asking him to read. At least this will let him appreciate presbyopic symptoms. Such firsthand pre-operative experience for a preprrshyope is particularly valuable and enlightening. In addition, we make a major pre-operative effort to explain carefully that surgical correction of myopia will alrvlqys stress accommodation Cjust as does

Contact lenses might seem reasonahlc ti)r correcting residual refractive error after RK surgery, hut they are not ideal. In fact. contact lens fitting after refractive surgery is far more problematic than you might helievc. The new cornea1 shape, with a flatter center and a peripheral knee. doe5 not allow for adequate tear exchange beneath the edge ofthe lens. The prripheral cornea1 touch o\‘er the knee makes it diflicult to prevent a “sucked-on 1~s“ >yndrome: moreover, the knee of cornea1 topog;raph) may he associated with subsequent incisional infections and a risk 01‘ abrasions or anosia at the contact lens pressure points. Further, pressure beneath the peripheral lens bevel may induce reflex coniunctival injection. tearing, and cornea1 astigmatism or other topographic shape changes. Despite these problems. you c-an sometimes he successful. Use a lens of lar,qer diameter than normal to help with centering. hut with a smaller optical zone to avoid the knee of cornea1 topography. Begin with a trial lens having the same hasc curve as the patient’s prc-operative K-reading. (RK incisions typically do not change the curvature of the peripheral cornea.) The RK-flattened central cornea, when coupled with the overlying contact lens, often generates a plus-powered tear meniscus that needs compensation, so VW n/ill likely need to add some extra minus power to the contact leans corrtwion. In any case. try fitting with a diagnostic contact lens of

418

Surv Ophthalmol

32(6) May-June

ROWSEY, RUBIN

1988

known power that is close to the patient’s tion. That lens can then be over-refracted mine the final contact lens power.

prescripto deter-

Postoperative Symptoms After RK, even if no daily refractive shifts occur, the new refractive state may not be stable over the long term. Over months to years, the refractive error can gradually shift back toward myopia; sometimes, the shift even moves in the opposite direction, toward hyperopia. Between six months and one year after surgery, about ten percent of patients experience between 1 D and 3 D of late shift in one direction or the other, so do not rejoice too soon. After a “perfect” optical result three months postoperatively, a patient may need no spectacle correction; yet in six months, one year, or even three years after surgery, he may need to wear glasses again. Considering that the primary goal for RK surgery is to eliminate the need for glasses, an outcome that requires that they be used again cannot be considered a romping success. Right at the outset, all pre-RK patients must be helped to understand the possible adverse refractive results as well as the optimal, desirable one. Patients who have had oblique RK incisions may complain of persistent glare and haloes around lights whenever the pupil dilates. These problems become ours as well as our patients; because we can’t seem to solve them! While some surgeons prescribe an iris-colored contact lens for complaints of glare, we do not, as we feel this solution is worse than the problem. After RK, epithelial debris or cysts sometimes form within the cornea1 incisions and can produce glare, haloes around lights, or secondary images. If such defects occur near the incision surface, you can remove them with a 23-gauge needle; however, removal is not usually necessary since, eventually, the cysts extrude from the incisions and spontaneously reduce some of the glare problem. Irregular astigmatism that follows any type of refractive surgery has many causes: inadvertent corneal perforation, unequal depth of cuts, incisions traversing the visual axis, peripheral cornea1 vascularization (ifincisions have crossed the limbus), corneal erosions, infections along the incisions, a contact lens that presses on or reshapes the cornea postoperatively, or a misplaced epikeratophakia lenticule. A careful exam of the retinoscopy pattern, or of Placid0 disc photokeratoscopy, is likely to reveal the cause of this astigmatic problem.

Problems with Epikeratophakia In epikeratophakia, designed lathe-ground

a 8.5 mm diameter customcornea1 lenticule is sutured

APEX

GRAFT DEHISCENCE

‘-

Fig. 5. Cornea1 graft after suture removal.

The graft edge has slipped forward. Diagram of the corneascope rings shows them centered on the high point (apex) of the anteriorly displaced edge. The ring pattern shows the marked astigmatism that has been generated.

onto the cornea1 surface. This optical modification of the cornea is designed to correct refractive errors of high degree. During surgery, the lenticule must be well-centered over the visual axis, lest a “kneeeffect” of the peripheral cornea (with myopic lenticules) induce asymmetric cornea1 astigmatism. If the lenticule-created knee is too close to the visual axis, glare and visual distortion will be evident every time the pupil dilates, similar to that seen following radial keratotomy (Figs. 1 and 4). Visual fluctuation is not as common after epikeratophakia as after RK, but over months, progressive scarring may resteepen the lenticule and produce even more myopia than the patient had before the surgery. We have followed a patient whose refractive correction improved after surgery from - 8.00 D to -2.00 D seemingly, a nice result. After wound healing, however, the myopia drifted back to - 12.00 D - not so nice. Following epikeratophakia, patients may also experience monocular diplopia or ghost images. These symptoms are rare after RK, unless the incisions pass close to the visual axis.

Problems with Relaxing Incisions A retinoscopy reflex similar to that seen in Fig. 4 may be found either before or after making purposeful cornea1 relaxing incisions to reduce cornea1 astigmatism. Whenever the cornea has been steepened by trauma, a cornea1 transplant, or tight sutures, retinoscopy can help identify the steepest corneal meridian and guide your surgical intervention

REFRACTION

APEX

PROBLEMS

AFTER

REFRACTIVE

SURGERY

419

:

Tangential

Keratotomy

Fig. 7. A radial cornea1 incision produces llattening both parallel to the meridian of the relaxing incision and 90 degrees to it. .L\ tangential (circumferential) cornea1 inc-i-

GRAFT

REPOSITIONED

sion flattens the cornea parallel to the meridian incision, but steepens the cornea in thr maridian grees awa!

01‘thr 90 dr-

Fig. 6. Clorneal graft has been resutured. The cornea1 apex has been shifted centrally, moving the corneascope rings correspondingly and showing a reduction in astigmatism.

(or suture removal) to help flatten the cornea appropriately. Note that the narrow side of the retinoscopy reflex points in the direction of the cornea1 pathology that requires treatment (Fig. 4). Clinical hint: To enable you to recognize the dim, astigmatic retinoscopic reflex more easily, try moving closer to the patient - to about twelve inches away, or even as close as two inches. “Mr. Retinoscopy,” the late Jack Copeland, called this, radical wtinoscojy, a technique that often permits easier visualization and neutralization of astigmatism rcflexes when retinoscopy at the more typical armslength distance of 66 cm (20 inches) might not. (Because of the uneven cornea1 power, when you add enough plus to neutralize the reflex in the area of cornea1 steepness. you will reverse the reflex centrally. ) Relaxing incisions that correct astigmatism created hy RK or hy suture removal can result in too much movement of‘the cornea1 surface and an overcorrection of the original astigmatic problem. Figs. 5 and 6 diagram Placid0 disc and corneascope imagery in a case where sutures were removed from a cornea transplant and a wound dehiscence occurred. Fig. 5 shows one edge of the graft shifted foward, increasing the astigmatism, and markedly decentering the corneascope rings toward the high graft edge. (The badly centered cornea1 apex forces the patient to view the environment through the “side of the cornea1 mountain.“) Such a wound must he resutured, since an elevated wound edge is precarious for a cornea1 transplant. (It produces

epithelial healing problems.) Resuturing the wound will shift the cornea1 apex (peak) fi-om the marginal edge hack to the center of the cornea where it helongs (Fig. 6). Proper wound repair, then, provides the patient with better vision and a hetter contact lens fit and the surgeon with much hetter sleep. Cornea1 incisions can be designed to correct astigmatic errors and may indeed produce marked astigmatic shifts. Unfortunately, the results arr not reliably reproducible, and the incisions may cause continuous cornea1 shape fluctuation. Still. radial and tangential incisions can he usef-ul surgical tools. Both ty-pes of cuts produce similar (complementary) cornea1 flattening in the meridian parallel to the incisions, but opposite efrects in thca cornea1 meridian 90 degrees away. Fig. 7 (left) shows the radial cornea1 incision. Basically, this type of incision flattens the cornea in the meridian parallel to the incision. Thus it tends to reduce the astigmatism (fiom 1 to 3 D) without rotating the original axis. Radial incisions are considered somewhat fhrgivirl,q of surgeon variability. Also shown in Fig. 7 (right) is a tangential (circumferential) cornea1 incision, which similarly flattens the cornea parallel to the meridian of the incision, but in contrast to the radial cut, it steepens the cornea 90 degrees away and is unforgi\.ing of a surgeon’s miscalculation, since it is more likely to rotate the c.orrective axis hy 90 degrees. (-4 good discussion of the mechanics of astigmatic ketatotom! presented hy Lindquist, et al.‘) Any surgery for astigmatism can radically alter the preexisting refraction. Some ofour most unhappy patients arc those who have had a 90” shift in axis. It astigmatism with a vertical axis in an unoprratcd t-ye turns out to be paricd with astimatism having a hori-

420

Surv Ophthalmol

zontal

32(6) May-June

axis in the operated

well tolerated. in most clearly

eye, the result

In surgery

other

types

preferable

for astigmatism

of surgery,

ROWSEY,

1988 will not be then, just

as

undercorrection

is

to overcorrection.

Summary The omy,

optical

problems

epikeratophakia,

overcorrection rection Though

these

fraction,

prescription and contact

ticularly

difficult

pil can

markedly

patient’s states

alter

refraction of pupil

lenses,

dilation

the

should -

undercor-

during

be managed

to control

be

more

problems

(e.g., glare). refractive

the day.

by careful

(sometimes some

include

error,

vision

of glasses

keratot-

incisions

refractive

and variable

can usually

by radial

relaxing

of the prior

of the error,

one pair)

generated and

and

appropriate of refractive

prescriptions surgery,

investment

you make in discussing

lems ahead

of time will help lead to realistic

consent

and

venting

his ire by making

automobile

will help

avoid

the potential

the disgruntled radical

given.

the preoperative

incisions

prob-

informed patient’s on your

tires.

Reference 1. Lindquist TD, Rubenstein JB, Rice SW, et al: Trapezodial matic keratotomy, Arch Ophthnlmol 104: 1534-39, 1986

astig-

re-

are par-

results,

checked

-

For all types

than

A dilated

in the morning

evening

RUBIN

pu-

so the

in different and

in the

This paper was supported in part by an unrestricted departmental grant to the University ofFlorida by Research to Prevent Blindness, Inc., New York. Reprint requests should be addressed to Dean .4. McGee Eye Institute. 608 Stanton L. Young Drive, Oklahoma City, OK 73104.