Laser Vision Correction
Part 2: Pretreatment, Treatment & Posttreatment
BY RONALD SNYDER, O.D., & PAUL KLEIN, O.D.
FEB. 1996
Since Part 1 of this article appeared in the November 1995 issue of Contact Lens Spectrum, the FDA has approved the Summit Technology SVC Apex Excimer Laser for photorefractive keratectomy. The approval specifications include:
- minimum patient age of 21;
- refractive stability of plus or minus one diopter difference over the previous year's findings;
- spherical myopia correction from -1.50 to -7.00 diopters;
- a minimum waiting period of three months before a patient's second eye can have PRK;
- an ablation zone of 6mm.
As significant members of a comanagement team, optometrists will usually participate in the pre- and posttreatment phases.
PRETREATMENT EVALUATION AND CONSULTATION
Pretreatment evaluation includes a comprehensive eye examination with attention to factors that relate to PRK (Table 1).
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Motives and Expectations -- What is a patient's principal motivation for and expectation of PRK? Patients who decide to have PRK feel they need it because they find their dependence on eyeglasses or contact lenses psychologically, physically or vocationally unacceptable. A disproportionate number of people who have specific vocational needs want PRK because spectacles and contact lenses are not suited to their occupation. Others have recreational needs, such as waterskiing, where wearing soft contact lenses may pose a risk. Some patients in their late 40s and early 50s want PRK because they have developed age-related dry eyes and can no longer wear contact lenses but don't want to wear eyeglasses. Practitioners must understand what motivates patients and then set realistic expectations for what PRK can achieve.
A summary of U.S. clinical results using the Summit SVC Apex Excimer Laser with a 6mm ablation zone indicates what patients can expect on average. The average attempted correction was about 4.00D with a range from 1.50D to 7.80D. Although the original Phase III protocol had a cut-off at 6.00D, the FDA approved up to 7.00D. About three-quarters of the patients were in the 4.00D to 6.00D range, and about half were presbyopic.
At six months posttreatment, about two-thirds of the patients were 20/20 or better uncorrected, with the lower range of myopia somewhat better. About 95 percent of the patients were 20/40 or better and able to pass a driver's license test without correction. At 12 months posttreatment about 80 percent were 20/20 or better uncorrected, and 99 percent were 20/40 or better uncorrected.
Patients must understand that there's no guarantee they'll completely eliminate their need for eyeglasses for distance vision. Explain that they will still need reading glasses, perhaps sooner than they would have if they are left hyperopic. One reason larger optical zones are better is that there isn't as much initial hyperopia and subsequent instability.
In this study, most patients at six months and essentially all patients at 12 months were within plus or minus one diopter of the attempted correction, although this was slightly greater with higher myopia. A conventional standard of plus or minus one line is used for other refractive procedures and epidemiologic studies. With PRK, at six months it was about 87 percent and at 12 months nearly 99 percent. About 6 percent to 7 percent of the patients improve two to three lines, and an equal number have a similar loss. The majority of people who "lost" best corrected vision were those who were better than 20/20 and became 20/20 or 20/25 best corrected.
Medical History -- Every patient should have a preoperative topography to screen out keratoconus and establish a baseline. Even slight, incipient keratoconus is a contraindication for PRK, because the thin cornea and abnormal stroma create problems (Figs. 1, 2 & 3).
FIG. 1: NORMAL CORNEAL MAP OF STEEP CORNEA. |
FIG. 2: CORNEAL MAP OF INCIPIENT KERATOCONUS. |
FIG. 3: POST PRK CORNEAL MAP. |
History of collagen-vascular disease, e.g., rheumatoid arthritis or Lupus, is important because the corneal stroma and the immune system are abnormal and predispose patients to corneal melts. If such patients insist on refractive surgery, consider other types of corneal refractive surgery because exposure of the cornea to UV radiation increases risk of corneal melt. Note too that exposure to UV light can also reactivate herpes. Diabetics, who have poor epithelial attachment and wound healing, should approach this procedure with caution because in PRK the corneal epithelium is removed and then re-epithelializes.
Be sure the patient has no history of adverse reactions to the medications that are typically used after PRK, such as oral pain medications, non-steroidal anti-inflammatories and topical steroids used to control the healing process.
Patients should be relatively stable in refraction before PRK. Those who still have significant progression should not be treated until there is greater refractive stabilization. The FDA recommends that patients should be within plus or minus one diopter of their last refractive status from a year ago. If refractive progression is uncertain, evaluate the patient again in about six months.
Contact Lens Wear History -- Contact lens failures comprise a significant percent of the people interested in PRK, as do patients with contact lens compliance or hygiene problems and those who are just psychologically unsuited for inserting something into their eyes. Patients with a persistent history of GPC or who have had small, sterile, peripheral ulcers from extended wear may benefit from PRK. The best candidate is one who needs convenience, uses daily disposable lenses, and would prefer PRK.
When patients discontinue contact lens wear, especially rigid lens wear, corneal topography and refraction may not be stable. Before they are evaluated for PRK, they should discontinue contact lens wear until there is relative stability of refraction and topography. For refraction, cycloplegia is done routinely, although the PRK attempted correction is predicated upon the manifest subjective refraction. Cycloplegia will ensure there are no unusual refractive factors.
Current Ocular Health -- Make sure there is no active inflammation, although this might not be a contraindication if it's curable and not recurrent. Evaluate epithelial and basement membrane integrity or other corneal disease. Evaluate the anterior chamber and make certain the angle is open and that there is no excessive pigmentation nor a predisposition to pigmentary glaucoma. Patients may be treated with steroids after PRK, so there may be moderate intraocular pressure elevations. Determine that IOP is normal and that the patient neither has glaucoma nor is a glaucoma suspect. A baseline IOP reading is essential to compare with postoperative IOP related to subsequent steroid therapy. Examine the crystalline lens. Patients with incipient or frank cataracts are often not refractively stable and will likely need cataract surgery and intraocular lens implantation.
Set Realistic Vision Goals -- Set realistic expectations with the best spherical equivalent refraction and a demonstration of 20/40 acuity. This is important because in the United States only spherical ablations are being done. Use either trial ophthalmic lenses or disposable contact lenses to demonstrate what a patient's uncorrected vision might be with a spherical, rather than a sphero-cylindrical, endpoint.
The current FDA-approved range for PRK candidates is from -1.50 to -7.00 in sphere and up to -1.50 in cylinder. However, it's important that patients have a realistic representation of what their vision might be without a cylindrical component in their correction. Many patients who wear spherical soft lenses have about three-quarters of a diopter of uncorrected astigmatism and function satisfactorily. Be aware that cylinder axis orientation as well as the magnitude of the cylinder can degrade the vision.
In the patient consultation (Table 2), review your clinical findings and inform the patient if his degree of myopia and astigmatism are in the right clinical range for PRK. Explain that we treat only one eye at a time, as the FDA approval dictates, and there is a three-month waiting period before the other eye can be treated. Therefore, the patient may experience some unusual binocular stresses during the first several months. Also, patients undergo a transient initial hyperopic shift in the treated eye during the first three months or so. Explain that this is a normal part of the healing process, as are subjective binocular symptoms from anisometropia, aniseikonia and prismatic imbalances. Such explanations and managing patients optically are extremely important. Depending on the magnitude of the refractive error disparity, patients may need temporary spectacles or a disposable contact lens on the contralateral eye until it also has PRK.
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Explain the treatment procedure thoroughly, including the differences between radial keratotomy and PRK. Explain that the surgeon does not actually touch the eye, but uses a beam of light to reshape the cornea and restore the eye to proper focus without using knives or blades. Demonstrate their possible visual outcome. Relate this to their specific reasons for wanting PRK and explain how they will function. Be forthright and honest and tell them what will actually happen (e.g., pain they might experience, vision fluctuation), but reassure them that their practitioners will help them to manage the process.
Be sure patients understand that follow-up care involves medications and temporary changes of their eyeglasses or contact lenses. Explain that they may have some difficulty if their occupation requires a great deal of close work because near vision changes while far vision continues to improve. Discuss how the two eyes may be somewhat unbalanced, how that might affect them, and what you will do to help them with this. Describe how the eye will heal, how some patients experience a little bit of haze, and how it is of little visual impact. Inform them that there is minimal risk of infection and that very few patients experience untoward healing or lose their refractive effect and need to be re-treated. Describe how the refractive surgeon will take them through a very formal, informed consent process that thoroughly details the risks and benefits, and encourage them to ask questions.
Once you've completed your evaluation and consultation, help the patient through the decision process, but neither talk them out of or into PRK. Be open, honest, and thorough, and above all take the time to answer their questions.
TREATMENT
The surgical technique involves four key elements: treat the correct eye, program the computer accurately, remove the epithelium properly so it has a smooth surface without debris or nicks to Bowman's layer, and center the patient, which is extremely critical (Table 3).
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TABLE 3: TREATMENT PROCEDURE
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Treatment begins with a topical anesthetic to anesthetize the cornea. A pad is placed on the other eye to prevent the patient from cross-fixating during the procedure. The pad is tipped a little to prevent eyelash rubbing. Another pad is placed on the side of the face of the eye to be treated to prevent moisture from leaking into the patient's ear which may cause him to move (Fig. 4). A drop of tetracane is applied. The patient is positioned so that he can view the fixation light internally and so that the refractive surgeon can see the helium neon red aiming beam. The patient has a fixation light that is green in the center and surrounded by a red ring (Fig. 5). The patient looks at the green light and the surgeon sees the green light reflecting as a Purkinje image. The surgeon also sees intersecting external red aiming beams that are focused on the cornea. The corneal central red reflex is superimposed on the green reflex from the crystalline lens, which produces a three-dimensional orientation of the eye and guarantees good centration.
FIG. 4: PATCHES ON UNTREATED EYE AND OVER EAR ON SIDE OF TREATED EYE . |
FIG. 5: PATIENT'S VIEW OF FIXATION SYSTEM. |
Next, the surgeon demonstrates the sound and smell of the laser. The laser produces a distinct snapping sound when it ablates the cornea. The smell is similar to that of a toy cap-gun. A cobalt blue fluorescence occurs when the epithelium is removed, but patients don't notice it. The stroma does not fluoresce during treatment.
With the first few pulses on the epithelium, patients tend to move, so a lid speculum is inserted at the start. It takes 5 to 20 seconds for patients to accept the speculum without struggling, but the topical anesthetic helps them tolerate it.
A 6mm beam with 25 pulses is first applied to the epithelium. It would take about 150 pulses to remove the epithelium. The desired correction is input via the computer keypad, which calculates aperture movement and the number of pulses. After the computer verifies the program, the surgeon removes the epithelium in two steps. First, an optical zone marker is used to prevent removal of more epithelium than necessary. For a 6mm treatment zone, 7mm of epithelium are removed to provide a safety area of about half a millimeter on all sides to prevent epithelium removal in undesired areas (Fig. 6). The treatment removes stroma, and the epithelium regenerates in the healing phase. The surgeon uses a blunt spatula or the side of a blade to remove the epithelium taking care not to nick Bowman's layer and to leave a smooth surface (Fig. 7). Patients feel tugging, but no pain. The surface is cleared of debris with a cellulose sponge and any remaining small islands of epithelium are removed with a dry sponge. Next, most surgeons use 0.5% carboxymethyl cellulose to lightly moisten the end of the cellulose sponge and do a final polish, so that the surface is clean, with an exposed fresh, smooth Bowman's layer (Fig. 8). The treatment takes 15 to 30 seconds and patients can easily stay fixated.
FIG. 6: SPECULUM IN PLACE AND OPTIC ZONE BEING MARKED. |
FIG. 7: REMOVAL OF EPITHELIUM. |
FIG. 8: CLEANING AND POLISHING BOWMAN'S LAYER. |
Immediately after the treatment, the surgeon places three drops of the non-steroidal anti-inflammatory agent Voltaren in the eye. Some surgeons apply an antibiotic/steroid combination such as Tobradex ointment and a pressure patch. These patients are also given Voltaren p.r.n. and Tobradex ung q4h x 24h as well as oral pain medication, usually Mepergan Fortis. Mepergan Fortis is a combination of Demerol, to reduce pain, and Fenergan, to relieve the nausea from the Demerol. Anecdotally, the preferred method for pain management is to place a bandage lens (i.e. Bausch &Lomb SeeQuence II 9.0 B.C., -0.50 D) in the eye after treatment with Ocuflex and FML 0.1% immediately and then q.i.d.
Patients are advised to avoid vigorous activity and to stay in a darkened room due to photophobia. Discomfort may start about two hours after treatment and usually peaks the first night after treatment. With adequate sedation and pain medication, it is tolerable, and with the bandage lens regimen the discomfort goes virtually unnoticed.
At 24 hours, uncorrected vision and epithelial healing are evaluated. Within a day, about 90 percent of discomfort is gone. Re-epithelization takes a little longer. Data indicates that 91 percent of patients re-epithelialized in less than three days, 98 percent in less than four days, and 100% within one week. Vision is not at full recovery at the time of epithelial closure because of the irregular nature of the epithelium; vision is typically about 20/40 to 20/60 on day three.
Postsurgical Haze -- Consider the rates of corneal haze (pictured in Part 1, Nov. 1995). Trace haze would be missed on a casual exam, but with a broad oblique beam from about 30 degrees, the treatment area can be perceived. At one month, many patients have trace haze that consistently decreases. By the twelfth month, only corneal topography can detect that a patient had a treatment. About 50 percent are crystal clear, and almost all the others are trace. Nobody has severe haze at 12 months. The distinction between haze and scarring is that scarring does not go away, while haze is transient and in most cases it's not troublesome. The percentage of patients having the second eye done is a good measure of their satisfaction. This is difficult to quantify, however, because some patients choose not to have the second eye done for reasons other than dissatisfaction (e.g., monovision or low-level amblyopia). According to data submitted to the FDA for the first Phase III group, about three-quarters had the second eye done. Of the 700 patients in the first group with the smaller optic zone, 98.8 percent said they would do it again.
POSTTREATMENT MANAGEMENT
Posttreatment management considerations are summarized in Tables 4 and 5. There are three healing categories. About 85 percent of the patients are Type I healers who experience some overcorrection followed by regression to the desired refractive outcome. Type II healers are "non-healers, " i.e., they have little or no haze and tend to stay hyperopic. Type III healers are overly aggressive, regress the most, experience haze, and end up myopic with some haze. Both refractive changes and haze indicate the healing pattern.
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Patients are unaware of trace haze probably because there is a difference between forward scattering (light scattering that goes back to the retina) and back scattering which is what the practitioner sees when observing patients. There can be much back scattering without forward scattering. The normal cornea has back scatter or it would be invisible when we shine a slit beam through it.
Radial keratotomy has a lot of diurnal variation due to changes in stromal hydration during the day, but there is no diurnal variation in PRK. With refraction, plus acceptance is pushed to relax accommodation. Prior to surgery, you must educate patients about refractive changes and then plan for them. You may wish to prescribe a temporary soft lens for some patients. Even if they have a little astigmatism, vision may be good enough to get them through the three months between treating the second eye and the first eye. Some motivated patients tolerate the difference for three months without auxiliary optical aids.
Prescribing steroids postoperatively is somewhat controversial as the benefit is uncertain and there is potential for raising intraocular pressure. Generally speaking, the data indicate that for treatments under
-3.00D, steroids are not indicated unless there is an aggressive healing response. That decision is driven by the postoperative refraction and by the haze.
Nobody knows what causes haze, although it's probably the combination of new collagen deposits that are subsequently remodeled. We know from immunopathologic studies that Type III collagen is deposited as a wound repair mechanism and then it seems to disappear and presumably is converted to Type I collagen which belongs in the cornea. Another possibility is that the glycose-amino glycan moieties, especially keratinsulfate, are not normal. Haze goes away as it normalizes over the first six to 12 months. There is often some improvement by the third month, many eyes clear by six months and almost all are clear at 12 months. Vision is not usually affected in the long term.
Managing patients with haze depends upon healing patterns. The only way to push patients after treatment is to manipulate postoperative steroids. If patients are hyperopic and crystal clear at one month, quickly discontinue steroids. Later, if they remain clear and hyperopic, it may help to debride the epithelium to stimulate wound healing.
In normal wound healing, at one month there will be a little haze, which will dissipate. Patients should be just slightly hyperopic, within about a diopter, at one month at which time you should continue the normal steroid dosage.
By three to six months any residual haze will definitely decrease. There will be some regression and patients will no longer be slightly hyperopic, but close to emmetropic, with good uncorrected vision. At this time patients will either be off or almost off steroids. If there is still significant haze in three to six months, patients are undercorrected and may need re-treatment which consists of ablating through the epithelium and doing a further correction. Haze, accompanied by undercorrection, can be re-treated to remove the new tissue formation that has resulted in the undercorrection, thus both problems are handled simultaneously. These are aggressive healers, prototypical Type III, so treatment with steroids is aggressive and more prolonged than normal.
About 1 percent to 2 percent of PRK patients are Type III healers. Advise patients that there is a small chance they may need re-treatment, and that most patients do well with a second treatment. There is also a sub-group, probably 1 in 500 to 1 in 1,000 cases, who will have haze even after a second treatment accompanied by a very aggressive course of steroids. The best protocol is not to use steroids on patients who are three diopters or less after the epithelium has healed. On patients with higher myopia, use FML 0.1% four times a day for the first month, three times a day for the second month, twice a day for the third month, once a day for the fourth month, and then discontinue. For under-responders, Type II healers who are without haze but stay a little hyperopic, the taper is quicker or even "cold turkey." Keep Type III aggressive healers at four times a day for the second month. If they are still more aggressive healers, you might even switch them to a stronger steroid, such as prednisolone acetate 1% (Pred-Forte). For very aggressive healers, use Pred-Forte every two hours for a couple of weeks, and then taper to four times a day. CLS
Dr. Snyder and Dr. Klein are partners in a contact lens and refractive surgery practice in Ft. Lauderdale, Fla.
For references, write to:
Paul White, O.D.
The New England College of Optometry
424 Beacon St.Boston, MA 02115
QUESTIONS
1. Minimum patient age for PRK according to FDA approval is:
a. 17 years
b. 21 years
c. 25 years
d. 29 years
2. Minimum waiting time before the second eye can have PRK is:
a. one month
b. two months
c. three months
d. four months
3. Pretreatment should include slit lamp evaluation of:
a. corneal opacities
b. lenticular opacities
c. keratoconus
d. all of the above
4. Six months posttreatment, about what percentage of patients were 20/20 or better uncorrected?
a. 25
b. 40
c. 65
d. 80
5. Twelve months posttreatment, about what percentage of patients were 20/40 or better uncorrected?
a. 99
b. 89
c. 79
d. 69
6. Larger ablation optic zones (i.e., 6mm) are better because of:
a. less initial hyperopia
b. less subsequent instability
c. both "a" and "b"
d. neither "a" nor "b"
7. Because of UV radiation from PRK, patients with a history of which of the following should not have PRK?
a. rheumatoid arthritis
b. Lupus
c. herpes
d. all of the above
8. A history of adverse reactions to which of the following would be contraindications for PRK?
a. oral pain medications
b. non-steroidal anti-inflammatories
c. topical steroids
d. all of the above
9. The FDA guideline for refractive stability from the previous year should be about plus or minus how many diopters?
a. 1
b. 2
c. 3
d. 4
10. The FDA-approved refractive range for PRK candidates is:
a. up to -7.00 spherical
d. up to -1.50 cylindrical
c. both "a" and "b"
d. neither "a" nor "b"
11. As patients may be treated with steroids after PRK, which of the following must be considered?
a. IOP
b. angle openness
c. history of glaucoma
d. all of the above
12. Which of the following should be discussed in the pretreatment patient consultation?
a. temporary anisometropia
b. temporary haze
c. temporary hyperopic shift
d. all of the above
13. From the refractive surgeon's view, which of the following contributes to centration of PRK?
a. green light as a Purkinje image
b. intersecting red aiming beams
c. three dimensional orientation of the eye
d. all of the above
14. About how many pulses would be needed to remove the epithelium?
a. 75
b. 150
c. 225
d. 300
15. The PRK treatment itself occurs in about how many seconds?
a. 15 to 30
b. 35 to 50
c. 55 to 70
d. 75 to 80
16. Mepergan Fortis contains:
a. Demerol to reduce the pain
b. Fenergan to relieve nausea
c. both "a" and "b"
d. neither "a" nor "b"
17. The data for re-epithelization after PRK indicates that:
a. 91% re-epithelialized within 3 days
b. 98% re-epithelialized within 4 days
c. 100% re-epithelialized within 7 days
d. all of the above
18. Hyperopic shift will be noted about how long after PRK?
a. one week
b. one month
c. three months
d. six months
19. A moderate haze with pronounced iris detail visible would be graded:
a. 1
b. 2
c. 3
d. 4
20. About what percentage of PRK patients are Type III healers?
a. two
b. four
c. six
d. eight
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