Ординатура / Офтальмология / Английские материалы / Hyperopia and Presbyopia_Tsubota, Boxer Wachler, Azar_2003

from the anterior stroma, with subsequent weakness of the cornea. Denervation of the flap or subclinical epithelial ingrowth may exacerbate this mechanical uncoupling. Other factors that may predispose to corneal ectasia include excessive ablation with less than 250m of residual stromal bed, a thicker than normal flap with consequent ablation at a deeper than planned level, and irregular corneal thickness (46). One can attempt to prevent corneal ectasia with preoperative pachymetry maps to detect borderline cases. One must also identify patients with keratoconus and prevent them from undergoing H-LASIK because they, of course, would be at great risk for postoperative corneal ectasia.

g. Loss of Best Corrected Visual Acuity

Loss of BCVA is more likely to occur after H-LASIK performed on high hyperopes. Choi notes that 50% of eyes with attempted corrections greater than 5 D lost two lines of BCVA. These high rates of loss of BCVA in eyes with high hyperopia may be due to induced irregular astigmatism (27–28,30–31,35). Irregular astigmatism can result from poor centration of the ablation. Even small levels of decentration can cause irregular astigmatism, leading to degraded vision quality or monocular diplopia.

Knorz performed a pilot study on eyes with hyperopia and hyperopic astigmatism. In eyes with 5.1 D to 10 D of hyperopia (15 eyes), 53% had lost one line at 1 month, and 20% had lost two or more lines of BCVA at 1 month. For 12-month follow-up, 6 eyes were available, and 50% of these had lost one line while none had lost two or more lines of BCVA. No significant intraoperative or postoperative complications were noted. However, it was felt that the loss of acuity was due to image degradation by significant optical aberrations caused by the new corneal surface. Knorz concluded his study by suggesting that LASIK should not be used for hyperopia 5 D.(28) Studies of myopic LASIK procedures have identified other causes of loss of BCVA to include flap folds, epithelial defects, lamellar keratitis, and epithelial ingrowth (30).

3. Conclusion

As we gather more experience with hyperopic PRK and LASIK, we can achieve higher rates of predictability and accuracy by creating nomograms adjusted for preoperative refraction, keratometry, and age. Also, more sophisticated equipment can decrease complication rates for both PRK and LASIK: more sophisticated ablation profiles and better eyetracking systems can reduce decentrations. For LASIK, newer, larger microkeratomes that produce flap diameters of at least 9.0 mm should be used.

C.COMPLICATIONS OF NONCONTACT LASER THERMAL KERATOPLASTY

1. Background

Thermal keratoplasty (TK) was first performed in 1898 by the Dutch ophthalmologist Lendert Jan Lans in an attempt to treat astigmatism (47). Lans demonstrated that thermal energy, applied with a cautery, altered the structure of the corneal stromal collagen and changed the anterior corneal curvature. Unfortunately, using simple cauteries and probes, it was difficult to control the amount of energy applied, and TK resulted in unpredictable results and regression (48,49). Interest in TK was rekindled with the development of lasers that could heat the cornea in a more controlled manner.

Figure 5 Slit-lamp photograph of a cornea immediately after treatment with noncontact holmium: YAG laser thermal keratoplasty. (From Ref. 55.)

In 1990, Seiler first described laser thermal keratoplasty (LTK), which utilizes the holmium:yttrium aluminum garnet (Ho:YAG) laser to correct hyperopia (50). Ho:YAG LTK avoids damage to the corneal epithelium by delivering infrared radiation to the midstroma. LTK changes the anterior corneal curvature because corneal collagen shrinks by 30 to 45% of its original length at temperatures of 55 to 60 C (51). Local, peripheral flattening causes central steepening, which corrects for hyperopia. Initially, both contact and noncontact LTK were performed. However, contact LTK, performed by directly applanating the cornea with a probe, tended to cause irregular astigmatism, regression and undercorrection; this form of LTK was withdrawn from U.S. Food and Drug Administration (FDA) trials (52–54).

Noncontact LTK, on the other hand, has been approved by the FDA. It is traditionally performed by projecting one to three concentric rings of eight laser spots each onto the cornea through a slit lamp–mounted, fiberoptic delivery system (Fig. 5). FDA phase IIA clinical trials with 2 years of follow-up showed the uncorrected visual acuity (UCVA) was improved by one or more lines in 19 (73%) of 26 treated eyes (55).

2. Complications

While a variety of complications may occur following LTK, the most common is regression of effect (Table 3). Short-term complications include discomfort immediately after LTK treatment or for 1 to 3 days post-LTK; some patients complain of mild pain (18–20%), tearing (41–43%), mild photophobia (33–41%), mild foreign-body sensation (41–54%), and other mild discomfort (29%). These side effects of laser-induced epithelial injury typically resolve within 3 days of treatment (56,58). Corneal opacities and epithelial haze and staining are common in the first week post-LTK treatment. However, by 2 years after treatment, corneal opacities at the treated sites and golden-brown intraepithelial deposits (presumably iron deposits) in or adjacent to inferior treatment spots are typically the only evidence of change to the cornea (56). Long-term damage to the central cornea has not been reported as a complication.

Clearly, the principal limitation of noncontact LTK is regression. Reported rates of regression vary from 27 to 45% (55–58). In one study, 70.1% had an UCVA of 20/20 at

302 Glazer and Azar

Table 3 Complications of Noncontact LTK for Correction of Spherical, Primary Hyperopia

3 months, but only 50.8% maintained this level at 15 months. In fact, by 15 months, only 57.8% were within 1.00 D of the intended refraction (57). In addition to regression of effect, astigmatism may occur as a result of noncontact LTK.

3. Etiology of Regression

Some researchers feel that regression is inherent to the current technique for LTK. The Ho:YAG LTK technique delivers pulses of energy to the cornea. The pulses themselves may trigger a mixed shrinkage/relaxation pattern. For example, if the energy pulses are too low, an insufficient amount of collagen shrinkage is achieved, and the initial refractive change may gradually be lost. On the other hand, if the laser heats the collagen fibrils to 65 to 70 C, collagen relaxation occurs.

Regression after noncontact LTK is more common in younger patients and patients with thicker central corneas (57). Regression may be due to the elasticity of Bowman’s membrane and stromal collagen in younger patients, which causes the cornea to return to its previous shape. Similarly, thicker corneas may be more likely to resume their previous configuration. At least in rabbit models, noncontact LTK provokes procollagen synthesis by fibroblastic keratocytes, causing stromal remodeling which can produce irregularities in the anterior corneal surface leading to epithelial hyperplasia. This in turn, results in an

altered corneal curvature (60). While the precise wound healing response to noncontact LTK in humans is not known, it is possible that both regression and astigmatism may result from a similar response.

4. Prevention

Investigators are speaking optimistically about a new continuous-wave diode laser that can change the shape of the cornea without the peaks and troughs of the pulsed Ho:YAG laser (61,62). The continuous-wave diode laser is expected to avoid tissue overheating, thereby improving long-term refractive stability. In addition, FDA trials are under way on a device that uses radiofrequency energy to the peripheral cornea; this may produce more controlled shrinkage of collagen lamellae (63).

5. Conclusion

One point to remember is that while regression and, less frequently, astigmatism may result from noncontact LTK, it is rare for patients to lose even one line of BCVA. No eyes have been reported to have lost two or more lines of BCVA from noncontact LTK (55–58). For risk-averse low hyperopes ( 0.75 to 2.50 D), noncontact LTK is a procedure to consider because it causes very few BCVA-threatening complications.

D.COMPLICATIONS OF PHAKIC INTRAOCULAR LENSES AND CLEAR LENS EXTRACTIONS WITH INTRAOCULAR LENS IMPLANTS

1. Background

While most types of refractive surgeries alter the cornea, the refractive power of the eye can also be changed by implanting an intraocular lens (IOL) with or without extraction of the crystalline lens. Barraquer implanted the first phakic intraocular lens in the 1950s (64). Unfortunately, many of these anterior chamber lenses were poorly finished and had sharp edges. After Barraquer had implanted almost 500 lenses, significant complications such as corneal edema occurred, and over 300 of the lenses had to be removed (65). After this experience, interest in phakic IOLs waned until labs were better able to guarantee the quality of IOLs.

Intraocular lenses being made today are of much better quality than those used in the 1950s. A recent study used a scanning electron microscope to analyze the surface quality of new-generation phakic IOLs; the study showed that these lenses did not have any defects that would contraindicate their use as phakic IOLs (66). This study examined the three major types of lenses currently used as phakic IOLs: anterior chamber lenses (currently used only in myopic eyes), iris-fixated anterior chamber lenses, and posterior chamber lenses.

2. Complications

Even when perfectly constructed IOLs with smooth surfaces are placed, there is still a risk of progressive corneal endothelial cell loss secondary to phakic IOLs (67–71). Other

304 Glazer and Azar

Table 4 Complications of Phakic Intraocular Lens Implantation for Correction of Hyperopia

potential complications of IOL implantation include cataract formation, pupillary-block glaucoma, endophthalmitis, and retinal detachments (Table 4) (72–75).

Currently the most popular phakic IOL for the treatment of hyperopia is the Collamer Staar Posterior Chamber IOL, also called the implantable contact lens (ICL) (Fig. 6). A recent phase I trial of silicone plate posterior chamber lenses, implanted in hyperopes, reported that 100% of patients had 20/40 or better UCVA, and 70% had 20/20 or better UCVA (80).

In one study of hyperopes with phakic IOLs 1 year after implantation, opacities in the area of lens contact with the capsule developed in two eyes (6%). Pigment dispersion occurred in three eyes (9%), but without intraocular pressure elevation. One eye (3%) required a lens replacement because of a calculation error (81). Another study reported

Figure 6 The STAAR Collamer posterior chamber phakic intraocular lens implant. (From Ref. 79.)

an anterior subcapsular cataract developing immediately after surgery in one eye (6.7%), causing a loss of two lines of BCVA (79).

Because hyperopic eyes tend to be shorter, they are more prone to pupillary block after implantation of posterior chamber lenses. One study using the Staar Collamer Implantable Contact Lens (ICL) reported 2 of 15 eyes (13%) developing a severe pupillary block despite two iridotomies that had been performed 2 weeks prior to surgery. The increased intraocular pressures due to the pupillary block necessitated removal of the implants (79). Another study of the Staar ICL reported a 12.5% incidence of postoperative pupillary block. In addition, IOL decentration of more than 1 mm occurred in 2 of the 24 eyes (76).

Sight-threatening complications such as endophthalmitis have been reported to occur in phakic IOL procedures for myopia and could theoretically occur for hyperopic phakic IOL implantation procedures as well (75). Occasionally, silicone plate phakic intraocular lenses need to be removed due to incorrect sizing of the lens and poor fixation within the sulcus (82). Retinal detachments after phakic IOL implantation have been reported in 4.8% of myopic eyes (74). This complication has not yet been reported in hyperopic eyes.

Iris-fixated phakic IOLs for the correction of high hyperopia can be associated with serious complications such as corneal decompensation and glaucoma (Fig. 7) (78). Other risks include cataract formation and glaucoma (pupillary block glaucoma, pigmentary glaucoma, narrow-angle glaucoma, and malignant glaucoma) (76). Peripheral iridotomies can treat or prevent pupillary-block glaucoma. Shallow anterior chambers should be a contraindication to performing an ICL because of the risk of narrow-angle glaucoma. Lens decentration may also occur.

3. Clear Lens Extraction with IOL Implantation

Clear lens extraction (CLE) with IOL placement has been studied as a surgical correction of hyperopia. Some of the disadvantages associated with this procedure as a treatment for myopia are not as a relevant when it is considered as a hyperopic treatment. For example,

Figure 7 The Fechner iris-claw intraocular lens implant. (From Ref. 78.)

myopes are more prone to retinal detachments (RDs). But the increased risk of an RD after clear lens extraction surgery is less relevant in hyperopes. In addition, the loss of accomodation that accompanies removal of the crystalline lens is a moot point in the high hyperope, who can see neither at distance nor at near without correction. One problem of CLE with IOL placement encountered with hyperopes, which is not relevant in myopes, is the potential need to implant more than one IOL (piggyback IOLs) to correct for hyperopia.

Several recent studies on clear lens extraction for hyperopia demonstrate that this is a safe and effective procedure. Kolahdouz-Isfahani performed clear lens extraction on 18 eyes. Two eyes lost two lines of BCVA, but no reason for the loss of BCVA was found after a complete ocular examination was performed. Complications included one case of postcapsular opacification requiring one YAG capsulotomy, one case of a lens dislocation requiring an IOL exchange, and one case of malignant glaucoma (83). Another study of 35 eyes reported that no eyes lost BCVA postoperatively. Additional procedures consisted of one IOL exchange and one PRK for overcorrection, both due to IOL miscalculations. Posterior capsular opacification developed in 19 eyes (54.2%), requiring 19 YAG capsulotomies (84). One study of 20 eyes that underwent clear lens extraction and IOL implantation reported no complications; there was no loss of BCVA and no need for further procedures. The authors did find, however, that the procedure was less accurate and less predictable for less than 3.00 D of hyperopia (85).

Pop et al. performed CLE with IOLs followed by PRK or LASIK. The only postCLE complication in this study was interlenticular opacification (ILO), which occurred in 14 eyes that had piggyback polyacrylic lenses. Of the initial 65 eyes in the study, 40 eyes received two IOLs (piggyback IOLs) because the lens power needed was higher than 30 D. Thus, 35% of all the piggybacks developed interlenticular opacification. There were no other reported complications from the CLE surgery (86).

In April 1999, the FDA approved Intacs for myopic correction of 1.00 to 3.00 D with 1.00 D or less of astigmatism. Intacs can also be used to create central corneal steepening to correct for hyperopia. Studies are currently investigating the use of small linear segments placed in the peripheral cornea to create shortening of the peripheral length of the corneal arc, with subsequent central corneal steepening. By altering the thickness of the insert, one can titrate the refractive effect.

Although there have been no published studies on Intacs for hyperopia, clinical trials are currently under way in Germany and Spain. These trials have produced promising preliminary results: study 1 enrolled 19 patients, and at 1 year 95% (18 of 19) achieved an UCVA of 20/40 or better. Of note, an induced astigmatism of 1.00 D or greater was seen in 32% (6 of 19) of the cases. Eleven patients were enrolled in study 2, with 6 months of follow-up. Ten of the 11 eyes (91%) were 20/40 or better, and 4 eyes (36%) experienced an induced astigmatism of 1.00 D or more. Finally, study 3 enrolled 9 patients with 6 months follow up. All patients had an UCVA of 20/40 or better; only 1 patient had an induced astigmatism equal to or greater than 1.00 D (87).

2. Complications

Published studies of Intacs today are for the correction of myopia. However, the complications of Intacs would be similar whether the segments were placed for the correction of myopia or for hyperopia. In the FDA phase II and III studies, the incidence of adverse events was 2% of the 452 eyes enrolled. Complications of the ICRS procedure include accidental perforation into the anterior chamber (2 eyes), surface perforation of the epithelium anteriorly (3 eyes), significant decentration of the rings requiring removal or repositioning (5 eyes), and infectious keratitis (1 eye). All eyes in the group of patients with complications returned to preoperative BCVA by their 6-month follow-up appointment (88).

Schanzlin reported no serious complications in the 125 eyes that received ICRS in his study. Minor postoperative problems included one case of transient conjunctivitis, three cases of filamentary keratitis, and one case of transient iritis. One patient, whose incision had gone into a region of superior pannus, developed deep stromal blood vessels. At 12 months follow-up, four patients had a two-line loss of BCVA, from 20/12.5 to 20/ 20. All four of these patients had a substantial improvement in their UCVA (89).

Postoperative astigmatism is clearly a significant potential problem, with 20 of 102 patients in one group experiencing post-ICR astigmatism of 1.0 D or more at 3 months follow-up. Various theories exist as to the cause of the astigmatism; it may be related to suture tightness (90). Induced astigmatism may also result from postoperative movement of the intracorneal ring segments. Finally, Intacs-induced astigmatism can result from irregular stromal and epithelial thickening between the Intacs rings (91).

Reports describe one patient with persistent focal edema due to a small Descemet’s tear from a lamellar dissection that was too deep. Although the edema necessitated ICR removal, the patient’s BCVA was 20/20 at exit from the study. One of 102 patients incurred an intraoperative perforation of Descemet’s membrane, requiring an ICRS explantation (90). Channel deposits associated with Intacs are occasionally seen but are not associated with impaired visual acuity (88,89).

3. Prevention

One can attempt to prevent postimplant complications through meticulous attention to positioning, proper incision depth and pocketing, and sterile technique. In addition, proper

attention to wound architecture along with adequate closure and tissue approximation with suturing can minimize the frequency of wound-related complications such as wound gape and epithelial cysts. One can prevent corneal neovascularization status post-ICRS by avoiding incisions that make contact with pannus or a limbal blood vessel and by warning against eye rubbing so as to prevent wound dehiscence.

4. Conclusion

Intacs may prove to be a valuable tool for the correction of hyperopia. Advantages over procedures such as LASIK and PRK include the fact that the Intacs insert is placed in the peripheral cornea and the central cornea is never violated during the surgical procedure. In addition, the Intacs devices can easily be removed if necessary. Finally, the refractive effect can be adjusted by replacement of any of the implanted radial segments. The complication of induced astigmatism may become less of an issue as more Intacs devices are implanted: the cause of induced astigmatism may become better understood and thus better prevented. In addition, surgical technique will be improved as more of these surgeries are performed.

F. CONCLUSION

Clearly, since hyperopic refractive surgery is entirely elective, the surgeon must have a thorough understanding of any potential complications of each type of procedure. The risk/benefit balance is tipping in favor of H-PRK, H-LASIK, or noncontact LTK for low to moderate hyperopes and toward intraocular lens implantation with or without clear lens extraction for moderate to high hyperopes. The use of ICRS for hyperopia may be useful for low to moderate hyperopes; however, long-term results of current studies have yet to be reported.

Thorough preoperative evaluations and preventive techniques such as those described above can help to avoid complications. However, even with the most prepared surgeon and in the best of hands, complications may occur. Thus, it is essential to provide patients with a clear understanding of the potential risks of a procedure before proceeding.

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