Optical Rehabilitation of Aphakia with Epikeratophakia
KEITH S. MORGAN
Table Of Contents
COMPLICATIONS AND THEIR MANAGEMENT
|Epikeratophakia is a form of refractive surgery wherein the anterior curvature
of the cornea is altered by the addition of a tissue lens created
from donor cornea. The tissue, procured through eye banks, is frozen
and shaped on a cryolathe in a modification of one of the keratorefractive
surgical procedures originated by Barraquer.1 In Barraquer's keratomileusis procedure, a disc of anterior cornea
is shaved off with the microkeratome, frozen on a cryolathe, carved
to the required specifications intraoperatively, based on theoretic and
empiric formulas, and replaced in the stromal bed. Epikeratophakia differs
in that donor tissue rather than the patient's own tissue
is used, the tissue is precarved, and the resulting tissue lens is placed
on the anterior surface of the recipient cornea (Figs. 1 AND 2). No microkeratome cuts are made in the patient's cornea; the only
modifications are the removal of the epithelium and the trephination
of a partial-depth, circular incision into the stroma in the peripheral
cornea. Therefore, Bowman's layer remains intact across the central
cornea, which makes the procedure extraocular, potentially reversible, and
more easily adaptable to corneas of different sizes and shapes.|
Epikeratophakia tissue lenses are generally supplied from a commercial source, which processes the tissue to order and lyophilizes it for shipping and storage until surgery. During surgery the lyophilized tissue lens is hydrated and then sutured to the patient's cornea. Alternative tissue preparation techniques include carving unfrozen tissue with the use of microkeratome instrumentation (BKS 1000, Allergan Medical Optics, Irvine, CA), as described by Krumeich and Swinger,2,3 or possibly by means of the excimer laser.4
|Epikeratophakia has been used to correct aphakia in adults5 and children,6,7 as well as myopia,8 and to treat keratoconus.9 However, only epikeratophakia for pediatric aphakia is discussed here.|
The first pediatric patients to undergo epikeratophakia were children with unilateral congenital or traumatic cataracts who were either intolerant to contact lens wear or who were considered to be poor candidates for successful contact lens wear.10–14 Although the procedure was originally envisioned as a permanent optical correction for young children with unilateral congenital cataracts, it soon became apparent that epikeratophakia was not well suited as a first or sole solution to this problem because it could not achieve the very high power correction required for neonates and infants.15 Also, the rapid changes in refraction in the first year of life required a more flexible and easily changed optical prosthesis. For these reasons, epikeratophakia has been recommended for contact lens-intolerant, aphakic children who are 1 year of age or older. In these cases, the changes in dioptric power requirements that occur with the passage of time can be corrected with changes in spectacle overcorrection.15
Children who have traumatic cataracts with corneal lacerations are well served by this procedure because the injury often destroys the capsular support that can be used to stabilize an intraocular lens.16–18 The epikeratophakia graft provides structural support for the weakened cornea and minimizes the induced irregular and regular astigmatism, in addition to correcting the optical error (Figs. 3 AND 4).
Although epikeratophakia is primarily a secondary procedure for children who cannot tolerate a contact lens, it may be combined with cataract extraction in children with acquired cataracts in whom contact lens compliance is unlikely. The results in these cases have been mixed.19,20 The accuracy of the achieved correction may be decreased with the combined procedure, compared with epikeratophakia alone, possibly because of the variation in intraocular pressure that occurs with a combined procedure, but there is no increase in nonoptical complications. The best example of a candidate for this usage would be a 2-year-old child with a subluxed lens who will not tolerate a trial of a contact lens prior to surgery.
Epikeratophakia is also useful in older children who are contact lens-noncompliant.7 The results actually seem to be somewhat better in these older children, perhaps because they are better able to cooperate with not rubbing their eyes but are still able to benefit from the more rapid healing common to young patients. Also, amblyopia is generally not a factor, so the problems of patching do not arise. In uniocular aphakia in older children, many would argue the advantage of an intraocular lens. However, epikeratophakia is likely safer, with less potential for severe complications, is reversible, and has good but not excellent power predictability. Optical rehabilitation is slightly slower than with intraocular lens implantation. The advantages that may be expected with an intraocular lens (faster rate of recovery and better power predictability) must be balanced against the possibility of long-term risks in a young person, and the difficulties associated with the removal or exchange of an intraocular lens.
Epikeratophakia has been used in selected cases of bilateral aphakia in children.20 Such use must be approached with caution, however, because amblyopia could be induced if one eye develops complications that reduce vision. This is a special concern in that most bilaterally aphakic children do tolerate spectacle correction, so the parents must be fully informed and the risk-benefit ratio of surgical correction must be weighed carefully.
In a contact lens-intolerant child who has an intraocular lens in one eye, an epikeratophakia graft may be an appropriate option for the other eye. In these cases, epikeratophakia may avoid the long-term problems that could be associated with bilateral intraocular lens implantation, while leaving the options open for future surgery in the adult years, if necessary or desired.
|As a procedure that involves the surface modification of the cornea, epikeratophakia
would be contraindicated in patients with severe ocular
surface problems, including uncontrolled blepharitis and keratitis sicca. Also
any child who exhibits poor wound healing or has a compromised
metabolic status would not be a good candidate for this procedure. The
most crucial part of healing is the reestablishment of the epithelial
cover. If the child has extremely poor vision and is prone to ocular
digital stimulation, this habit may prevent proper healing.|
Children who have had radiation treatment for ocular or orbital disease or who have radiation-induced cataracts will often develop surface ocular problems from radiation keratitis.21 These children seem to do poorly with contact lenses and epikeratophakia over the long haul and they should be considered for intraocular lens implantation.
One is often confronted with a child who has had a severe corneal laceration and requires penetrating keratoplasty. Correcting these children's eyes with an epikeratophakia tissue lens seems to be almost impossible. A variety of techniques such as placing the epikeratophakia graft inside the corneal transplant or combining it with the corneal transplant have been tried but the results have been poor. Although the tissue lens can be coaxed into healing, it often does so at the cost of the viability of the corneal transplant. For these reasons I am conservative in recommending corneal transplantation for young children who are in the age group at risk for amblyopia because of the difficulties in correcting aphakia in conjunction with penetrating keratoplasty. In such patients, an epikeratophakia lens over the corneal scar may provide adequate visual rehabilitation, or if a corneal transplantation is unavoidable, an intraocular lens implant may be the best option for optical correction of aphakia.
Extremely small eyes with corneal diameters less than 9 mm present a special problem for epikeratophakia.20 In these cases, there appears to be a higher incidence of complications because the keratectomy into which the edge of the graft is sutured extends out to the limbus, leading to problems with vascularization.
After a complete ophthalmologic examination, the child may be sedated with chloral hydrate, if necessary, and keratometry and retinoscopy are performed. A-scan axial lengths must be obtained when retinoscopy is not possible. The measurements provide the parameters for ordering the tissue lens, which is prepared at a commercial facility and shipped to the hospital.
There have been technical difficulties in calculating powers for young children. The most commonly utilized formulas were designed for adult eyes and when they were used for short eyes with steep corneas, as seen in infants, the results were erroneous.22 Newer formulas seem to alleviate this problem.23,24 However, no formula is ever as accurate as retinoscopy in children's eyes. Thus, retinoscopy should be used whenever possible-either with trial lenses alone or with trial lenses over an aphakic trial lens.
The power obtained with retinoscopy is corrected to the corneal plane and, in an older child (perhaps after the second birthday), one may wish to add 2D to achieve a mild overcorrection. This addition is not necessary in a child under the age of 2, who will continue to become more myopic over the ensuing years (Fig. 5). The maximum power achievable with an epikeratophakia tissue lens appears to be in the vicinity of +25D.
Tissue preparation has evolved considerably since the days of the early Barraquer procedures. For epikeratophakia, the commercial provider obtains fresh donor tissue, dehydrates it by means of a cornea press, shapes it to the correct optical power, and lyophilizes it for transport and storage.
Unfrozen tissue that is shaped and stored in a moist chamber for shipping is also available commercially. Some individuals who have access to a cryolathe still prefer to use either Barraquer's original formulas or some modification thereof for preparing their own tissue.
At the time of surgery the child is taken to the operating room. No specific premedication or antibiotic regimen is required. Since children undergo this procedure under general anesthesia, there is no reason to mark the visual axis with the child awake.
If a lyophilized lenticule is to be used, it is placed in a balanced salt solution to be rehydrated. Care should be taken to keep the tissue lens away from contact with any surface that could be contaminated with soap or other chemical residues that could be absorbed by the lenticule and hinder epithelialization. A low concentration of gentamicin (0.3 ml of 40 mg of injectable gentamicin per milliliter in 20 ml of balanced salt solution) is sometimes added to the hydration solution, although the value of this in preventing infection has not been proved. The lenticule generally is hydrated for 20 minutes prior to suturing.
The first step in the surgical procedure is to move the epithelium from the patient's cornea with gentle mechanical abrasion. A 4% solution of cocaine that has been millipore-filtered or absolute alcohol may be used to facilitate deepithelialization. Drying the epithelium has been advocated to ease removal, but this approach results in changes in the texture of the corneal surface which make it difficult to attach the trephine in the next step.
A Hessburg-Barron vacuum trephine (Fig. 6) measuring 7 mm in diameter is used to make a partial depth (0.2 to 0.3 mm) incision into the corneal stroma centered on the optical axis (Fig. 7). In adults, the optical axis is marked prior to surgery while the patient is still awake. In children, this procedure may be difficult because their amblyopia may prevent central fixation or the fixation may be unsteady. One trick that is often employed is for the surgeon to note the position of the visual axis in the fellow eye and recall that as a landmark in the eye to be operated on. For example, if the fellow eye shows a large positive or negative angle kappa, the surgeon may take that into account and shift the centration of the epikeratophakia lens on the surgical eye appropriately. It has empirically evolved that a 1.5-mm difference in size between the trephine and the lens works well to achieve the correct power; thus, with a 7-mm trephination, the tissue lens is generally 8.5 mm in diameter. In the past, trephination was followed by the creation of an annular keratectomy, in which a wedge of tissue was removed from the inner aspect of the trephine cut. The use of this keratectomy has been abandoned, however, because of the technical difficulties involved in making the thin incision and the scarring that sometimes occurred as a result of this procedure.
At this point, if intraocular surgery, that is, cataract extraction, pupilloplasty, or membranectomy, is to be done, it is performed through a limbal incision with a vitreous cutting instrument such as the Microvit or Ocutome (Fig. 8). The limbal incision may be closed with 8-0 polyglactin 910 (Vicryl) sutures. After the intraocular procedure is completed and before the epikeratophakia procedure is resumed, it is important to return the eye to normal pressure by injecting balanced salt solution. I prefer to accomplish this with a 30-gauge sharp paracentesis injection through clear cornea.
Next, an angled lamellar dissector such as one would use to dissect a trabeculectomy flap is used to create a 1-mm lamellar pocket in the peripheral direction for 360° around the cornea at the base of the trephined incision. In the past a small strip of cornea was excised centrally from the inner aspect of the trephine incision. This keratectomy has been found unnecessary and the procedure may be more easily reversed without the keratectomy. Then the tissue lens is draped over the cornea stroma-side-down and centered on the annular incision (Fig. 9).
Sixteen to 22 interrupted 10-0 nylon sutures are placed to secure the edge of the lens in the peripheral corneal stroma trough. The first eight sutures are placed with the edge of the lens lying over the circular trephine mark (Fig. 10). As with any corneal transplant surgery, the second suture is the most important in centering the lens. All knots must be tied in such a way that the knots may be buried easily. At the same time, the sutures must not be tightened so tightly that compression occurs with the lens. My personal preference is for a 2-1-1 surgeon's knot. Care should be taken to prevent the lens from being sewn eccentrically into the bed, to prevent astigmatism. After the first eight sutures are placed, the edge of the lens is tucked into the stromal bed all around (Fig. 11) and the remaining 8 to 14 sutures are placed to fasten the lens securely in place (Fig. 12). The sutures are rotated so that the knots are buried in the recipient cornea. This allows the surgeon to remove the sutures at a subsequent examination under anesthesia (EUA) by means of traction directed peripherally without dehiscence of the wound.
An antibiotic, for example, gentamicin (0.5 ml, injectable, 40 mg per milliliter), is injected into the inferior fornix. Generally neither topical nor injectable steroids are required. The eye is pressure-patched and a plastic shield is placed over the bandage. Thereafter, the eye is not disturbed for 24 hours, after which the pressure patch is removed, the eye is examined, and the protective shield is replaced (Fig. 13).
Children are usually discharged from the hospital the afternoon of surgery. They are given oral antibiotics (amoxicillin 20 mg/kg/day in three divided doses) and topical atropine and gentamicin ointment are squeezed onto the eye through the holes in the plastic shield. Parents are instructed not to allow the child to rub the eye or to remove the shield. Parents may remove the shield for cleaning and the physician removes the shield in the office for examination during postoperative visits.
The eye is usually examined on the first and second day after surgery and then daily until the epithelial cover is complete. Fluorescein may be instilled in the eye to delineate the extent of the epithelial cover. If the epithelial defect is closing quickly, a pressure patch may be left in place for up to 48 hours. Two weeks after surgery the child is again put under general anesthesia, the eye is stained with fluorescein to determine if the epithelial cover is complete, and the sutures are removed. At the end of the procedure the eye is pressure-patched for 24 to 48 hours. The application of topical atropine and gentamicin ointment and the use of the plastic shield are continued, but systemic antibiotics are generally stopped at this time.
Amblyopia therapy is instituted within 1 to 2 weeks of suture removal. The epithelium is generally somewhat hazy; however, once a retinoscopic reflex can be achieved it is then appropriate to institute amblyopia therapy. The parents are instructed to begin patching the sound eye 3 hours the first day (“warm-up” patching), 4 hours the second day, and so on, increasing the patched time progressively to 8 to 10 hours a day. This level of patching is generally maintained until a plateau in visual acuity is reached-usually 4 to 6 months after surgery. If a significant spectacle overcorrection is needed, the over-refraction is generally given in spectacles with a bifocal 4 to 6 months after surgery when visual acuity has stabilized.
|Continued follow-up of the patient population in our series has demonstrated
a lower success rate (graft retention and accuracy of dioptric correction) for
children younger than 1 year of age. With the original
formula of Retzlaff and colleagues (SRK),24 there were also inaccuracies in calculating required tissue lens power
from axial length and keratometry readings in short eyes. It also has
become apparent that epikeratophakia tissue lenses do not produce the
desired change in corneal refractive power in very young patients.15 This may be because there is an absolute limit to the curvature that can
be achieved with this surgery. In neonates and young children, there
is a shift in the direction of myopia which is caused by ocular growth
and which results in rapidly changing dioptric requirements. This rapid
growth period seems to stabilize after the first year of life. Specifically, it
was found that patients younger than I year developed an
average of 4.75D of myopia over a 20-month postoperative period, whereas
those older than 1 year developed only 2.25D of myopia over the same
follow-up period.25 From the keratometry and axial length measurements we have concluded that
the myopia is solely a result of changing axial length and not secondary
changes in the corneal curvature from the epikeratophakia procedure.|
The changes in corneal curvature over time were not significantly different from the normal changes in corneal curvature that occurred with the passage of time. In other words, if one achieved a corneal curvature of 65D in a 2-year-old child, the corneal curvature remained essentially the same 10 years later. There has been no evidence of any deterioration or loss of power with time in patients followed to date.26 (The longest follow-up in my experience has been 13 years.) In children younger than 2 years of age, slight changes in corneal curvature parallel the normal changes seen in the growing eye.
Analysis of the accuracy of the procedure showed the achieved correction to be approximately 85% of the intended correction in children older than 1 year of age.25 In children younger than I year of age the achieved correction was only 50% of that intended. It appears that the younger corneas behaved differently perhaps because of their already steep curvature. This is further evidence that epikeratophakia is probably not the procedure of choice for neonates and young infants.
For all of these reasons, it seems prudent to remove a cataract in a neonate or infant as soon as possible and to fit these very young patients with a contact lens. Contact lens compliance is usually quite good during the first year of life. Thereafter, when and if contact lens compliance becomes more difficult, epikeratophakia may be performed. The probability of achieving an accurate optical correction is much greater in older children and the correction, once obtained, is more likely to remain appropriate as the child grows.
In the nationwide study of epikeratophakia in younger children,6 335 procedures were performed by a pool of nearly 100 surgeons. There was an 89% graft success rate in this series. Most of the problems were related to failure of the graft to epithelialize and these were treated by removal of the graft. Postoperatively, 73% of the patients were within 3D of emmetropia. There appears to be a learning curve in which the complications appear to decrease with increasing surgeon's expertise.20
Among the difficulties of dealing with younger children are the problems associated with obtaining accurate verbal responses for visual acuities both preoperatively and postoperatively and the complexities of assessing the impact that amblyopia had on visual acuity. For these reasons, older children in the nationwide study were analyzed separately.7 All 65 children in this group were between 8 and 18 years of age and were capable of verbal responses in the measurement of visual acuity.
The graft success rate in the older children was 100%. None of these grafts was removed. The achieved accuracy in correction of dioptric powers was again 73% within 3D of emmetropia. Analysis of preoperative and postoperative visual acuity results showed that there was an average loss of one half of a Snellen line in the subgroups with traumatic cataracts and congenital cataracts. The rate of return of visual function in these children was plotted and compared with the rate in aphakic adults (Fig. 14).
|COMPLICATIONS AND THEIR MANAGEMENT|
The most difficult part of healing is the growth of epithelial cover over the surface of the tissue lens. This aspect of healing is usually complete within 3 to 7 days. If epithelial cover is delayed much past this time, changes occur in the lens which usually prevent it from becoming a good optical surface.
Since the tissue is acellular because of freezing and lyophilization, host keratocytes migrate in from the periphery. Histologically this migration seems to be complete by 6 months. At this time the epithelium and the stroma are indistinguishable histologically from normal cornea.
Problems with epithelialization constitute most of the reasons for graft removal (Table 1). One possible cause of slow epithelial healing is a child who is rubbing the eye. in such a case, a pressure patch with adequate tape and an eye shield is the first line of defense. If there is significant ocular inflammation, systemic steroids may be given. When there is a persistent epithelial defect, however, topical corticosteroids should not be given because of the increased risk of bacterial infection.
* Age at surgery.
(Morgan KS, McDonald MB, Hiles DA et al: The nationwide study of epikeratophakia for aphakia in children. Am J Ophthalmol 103:366, 1987. Published with permission from The American Journal of Ophthalmology. Copyright by The Ophthalmic Publishing Co.)
The new epithelium appears slightly hazy. Epithelium must go through a period of maturation that leads to clarity. If epithelial cover is complete and clarity is delayed, topical corticosteroids can be used and often have the effect of increasing graft clarity.
Other devices such as collagen eye shields or soft contact lenses have been employed to assist with epithelial cover, with limited success. Tarsorrhaphies have been used in adults26 but are rarely if ever used in children.
Bowman's layer in the tissue lens seems to undergo some deterioration if the surface is not covered by epithelium within 7 to 10 days.14 Therefore, if an epithelial defect persists beyond this period (Fig. 15), it is worth considering removing the lenticule and replacing it with a different lenticule when the eye is no longer irritated.
Rests of epithelium have been noted in approximately 20% of patients undergoing epikeratophakia. Occasionally they enlarge and significantly impede vision. These cysts have been successfully curetted27 but occasionally result in the need to replace the lens.28
Historically, there had been a strong tendency toward undercorrection in children under 1 year of age. In children over 1 year of age, the change in refractive error was more predictable with the nationwide study demonstrating 73% within 3D of emmetropia. Low amounts of residual refractive error are often successfully corrected with spectacles.
When the achieved power is inaccurate by an amount too large for spectacle correction, a lens exchange can be performed. In the case of lens exchange, the parameters for ordering the new lens should include the same diameter and the same original corneal curvature. The edge of the healed lens can be teased with a combination of traction and blunt dissection to loosen the peripheral attachment, and the new lens is sewed into place. Since the patient's Bowman's layer is intact, there should be no scarring across the central cornea. However, if the cornea had undergone trauma or had a corneal laceration, there may be scarring to the lens and a clean removal may be impossible.
Bacterial infections of the lenticule have constituted the most feared complication in this form of refractive surgery. Since devitalized acellular tissue is placed on a deepithelialized cornea, an infection is potentially quite damaging. In the past, bacterial infections have been found to consist of organisms that are normal nasopharyngeal flora in children, that is, Hemophilus and pneumococcus. Parents should be alerted to respond immediately to complaints of pain or signs of discharge with a visit to the physician so that an early diagnosis can be made. Infections are treated by removal of the lens and the sutures and the application of topical antibiotics. If a lens has been in place for several months, the tissue can be treated much like normal cornea; an infected peripheral corneal ulcer caused by trauma, for example, may be treated with antibiotics without removal of the lens.
Because of the potential for bacterial infections, I do not routinely use topical corticosteroids in children with epithelial defects. Instead, the children are given oral antibiotics such as amoxicillin or an amoxicillin/clavulanate potassium combination (Augmentin) and the parents are instructed to instill ointments onto the lashes through the holes in a plastic eye shield (see Fig. 13). This prevents the parents from manipulating the eye and lessens the chance of the child's inoculating an organism.
A portion of the wing of the lens may become elevated out of the stromal bed before the sutures are removed. If this occurs, and the lens edge is not repositioned, the result may be permanent astigmatism and degradation of visual acuity. The lens edge may be repositioned at the slit lamp or with the patient under anesthesia; an additional suture should be added in the area to prevent recurrence. Occasionally, removing a suture at examination under anesthesia may cause the edge of the graft to dehisce, allowing an air bubble to form between the cornea and the lens. Small sections of dehiscence may not require treatment, but several clock hours of separation probably should be dealt with by replacing sutures and removing them later at a separate sitting.
|The correction of pediatric aphakia is a dynamic field which is changing constantly. Twenty years ago, it was advocated that children with uniocular congenital cataracts receive no rehabilitation since the visual results were so disappointing at that time. Today, for the contact lens-noncompliant child, epikeratophakia offers an alternative to uniocular aphakic spectacle correction or an intraocular lens. There are special circumstances in which one of these options may be the most desirable choice. The relative indications will certainly change as greater experience with intraocular lens implantation becomes available. Epikeratophakia will continue to be an attractive option for a young child with corneal trauma or a child in whom the posterior capsule is no longer available for lens support. There also will likely continue to be an age limit below which an intraocular lens will not be appropriate. At this time, each child must be considered individually and all of the factors weighed carefully before the optimal choice of rehabilitation is decided upon.|