Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology Current Thought and A Practical Guide_Wilson, Saunders, Trivedi_2008

dicated for the better eye. A frame approved by the ANSI (American National Standards Institute Standard No. Z87.1) with polycarbonate lenses should be worn for daily wear and low-eye-risk sports. Polycarbonate sports goggles complying with American Society for Testing and Materials (ASTM) should be worn for most ball and contact sports, and head and face protection should be added for higher-risk activities, as described in a Joint Policy Statement by the American Academy of Pediatrics and the American Academy of Ophthalmology (2003).

Monocular patients are at special risk for injury, and ophthalmologists should strongly recommend that athletes who are functionally one-eyed wear appropriate eye protection during all sports activities, including physical education during school. Monocular patients should not participate in full-contact martial arts or boxing, as adequate eye protection is not available. Wrestling was previously contraindicated for monocular patients, but custom eye protection can be fabricated to reduce the risk of eye injury for wrestlers.

Parents of athletes, and older student athletes themselves, should be carefully apprised of the risks associated with athletic participation and the availability of a variety of appropriate and/or certified sports eye protection. Even though eye protection cannot completely eliminate the risk of ocular injury, appropriate eye protection has been demonstrated to dramatically reduce the risk of significant eye injury. Somesports(forexample,icehockey)havemandated eye protection as a condition for sports participation, and there has been a dramatic reduction of significant eye and related injuries after institution of such policies. Sadly, not all sports have taken such a proactive stance.

Students and athletes with a history of intraocular surgery or significant eye trauma may be more susceptible to injury with even minor trauma; these children may require additional eye or face protection for athletic activity, or may need to be restricted from

certain sports. It is important to inform the child’s pediatrician or family medicine physician of necessary eye protection or limits on participation so that sports participation forms can be filled out properly.

2.6Contact Lenses for Children

Contact lenses are important in the visual rehabilitation of unilaterally aphakic infants and children, and should be fit as soon after surgery as reasonably possible so that amblyopia treatment can begin. A contact lens reduces the image magnification that an aphakic spectacle lens produces, though the more important barrier to binocular vision is usually dense amblyopia due to the unilateral cataract and aphakia. Many bilateral aphakes use contact lenses with additional plus power to correct for near in infancy, graduating to bifocals as toddlers. Extended-wear soft or rigid lenses are generally well tolerated, though the frequent power changes and lost lenses are a significant financial barrier for many families. Decreased acuity secondary to corneal opacities and scarring after corneal laceration can usually be improved with a rigid contact lens if a good fit can be achieved.

Unilateral high myopia is another situation when contact lenses are sometimes recommended for young children, citing that the anisiekonia caused by the spectacle correction would prevent sensory fusion. In clinical practice, however, these children usually have dense anisometropic amblyopia that, even when successfully treated, prevents development of sensory fusion. Contact lens correction of unilateral high myopia does prevent anisophoria caused by anisometropic correction, but children are rarely bothered by this problem. Finally, children with unilateral high myopia typically have amblyopia and poor vision in the weaker eye, and so should be wearing spectacles for protection even if contacts can be successfully worn.

Take Home Pearls

•Spectacles alone improve best-corrected amblyopic eye visual acuity by about three lines, so many patients do not need additional treatment with patching or penalization.

•Penalization is a viable option to occlusive patching in most patients with amblyopia.

Penalization methods include atropine eye drops, optical penalization using a plano (blank) lens or excessive plus power lens, atropine with a plano lens, and Bangerter foils.

•Atropine may successfully treat amblyopia even when there is no apparent fixation switch to the amblyopic eye and when the atropinized sound eye near visual acuity remains better than amblyopic eye acuity.

•After stopping patching or atropine, about one in four patients will lose two or more lines of amblyopic eye visual acuity over the next 1 year.

•Amblyopia treatment may have its greatest benefit in later life, when sound eyes

can sustain injuries or be afflicted by diseases of the macula or optic nerve.

•The rapidity of the response from the child is often a good indicator of her confidence in the answer: a rapid response in the anticipated direction is usually a good indication of a reliable response.

•Refraction over the current spectacle correction is the most accurate way to control for errors that result from vertex distance.

•A quick way to locate the optical centers of a lens while in the exam lane is to hold the lens below a ceiling spotlight, and align the reflections of the light from the front and rear surfaces of the lens (see Fig. 2.3). If a problem with the optical center of the lens is suspected based on the rapid chair-side

assessment, the precise location of the optical center can be confirmed with a lensmeter.

•In children with difficult media and a poor retinoscopic reflex, it is often necessary to move closer to the eye to “enhance” the reflex.

An assistant can measure the working distance while the doctor performs the refraction,

and the working distance (in diopters) is subtracted from the retinoscopic findings.

•When parents seem to resist spectacle correction of refractive errors, it can be useful to “demonstrate” the child’s refractive state by placing error lenses in front of the parent. Minus lenses are used to simulate hyperopia, and parental attention is directed first to a distance, and then near, target to stress accommodation. Anisometropia can be demonstrated in a similar fashion, simulating the refractive errors with disparate lenses.

References

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Group. Factors associated with high myopia after 7 years of follow-up in the Correction of Myopia Evaluation

Trial (COMET) Cohort. Ophthalmic Epidemiol 2007; 14:230–7

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Mitchell P. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology 2008, 20 February.

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12-year old Australian school children. Invest Ophthalmol Vis Sci 2008, 9 May. [Epub ahead of print]

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J, Lindstrom JM, Quinn GE. Associations between childhood refraction and parental smoking. Invest Ophthalmol Vis Sci 2006; 47:4277–87

7.Woodhouse JM, Cregg M, Gunter HL, Sanders DP, Saunders KJ, Pakeman VH, Parker M, Fraser WI, Sastry P. The effect of age, size of target, and cognitive factors on accommodative responses of children with Down syndrome. Invest Ophthalmol Vis Sci 2000;41:2479–85

8.McClelland JF, Parkes J, Hill N, Jackson AJ, Saunders KJ. Accommodative dysfunction in children with cerebral palsy: a population-based study. Invest Ophthalmol Vis Sci 2006; 47:1824–30

9.Guyton DL, O’Connor GM. Dynamic retinoscopy. Curr Opin Ophthalmol 1991; 2:78–20

10.Hunter D. Dynamic retinoscopy: the missing data. Surv Ophthalmol 2001; 46:269–74

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12.ANSI Z80.1-2005 Ophthalmics – Prescription Ophthalmic Lenses – Recommendations. The Accredited Committee Z80 for Ophthalmic Standards, Optical Laboratories Assocaition, Fairfax, Virginia, 2006

13.Wallace DK, Carlin DS, Wright JD. Evaluation of the accuracy of estimation retinoscopy. J AAPOS 2006; 10:232–6

14.Chung K, Mohidin N, O’Leary DJ. Undercorrection of myopia enhances rather than inhibits myopia progression. Vision Res 2002; 42:2555–9

15.Adler D, Millodot M. The possible effect of undercorrection on myopic progression in children. Clin Exp Optom

2006; 89:315–21

16.Gwiazda J, Hyman L, Hussein M, Everett D, Norton TT, Kurtz D, Leske MC, Manny R, Marsh-Tootle W, Scheiman M. A randomized clinical trial of progressive addition lenses versus single vision lenses on the progression of myopia in children. Invest Ophthalmol Vis Sci 2003; 44:1492–1500

17.Gwiazda JE, Hyman L, Norton TT, Hussein ME, Marsh-

Tootle W, Manny R, Wang Y, Everett D; COMET Group. Accommodation and related risk factors associated with myopia progression and their interaction with treatment in COMET children. Invest Ophthalmol Vis Sci 2004; 5:2143–51

18.Walline JJ, Jones LA, Mutti DO, Zanik K. A randomized trial of the effect of rigid contact lenses on myopia progression. Arch Ophthalmol 2004; 122:1760–6

19.Miller JM, Harvey EM. Spectacle prescribing recommendations of AAPOS members. J Pediatr Ophthalmol Strabismus 1998; 35:51–2

20.Harvey EM, Miller JM, Dobson V, Clifford CE. Prescribing eyeglass correction for astigmatism in infancy and early childhood: a survey of AAPOS members. J AAPOS 2005; 9:189–91

21.American Academy of Ophthalmology. Pediatric eye evaluations, preferred practice pattern. American Academy of Ophthalmology, San Francisco, 2002

22.Donahue SP. Prescribing spectacles in children: a pediatric ophthalmologist’s approach. Optom Vis Sci 2007; 84:110–4

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Contents

3.2The Excimer Laser  . . . . . . . . . .   22

3.3Excimer Surgical Procedures  . . . . . .   24

3.4Setting up a Pediatric Excimer Refractive

Surgery Program  . . . . . . . . . . . . . . . . . . . . . .   26

3.4.1Preoperative Guidelines  . . . . . . . .   26

3.5Refractive Lens Exchange

Core Messages

•Anisometropic amblyopia is an important cause of visual impairment worldwide. Bilateral ametropic amblyopia can be severe enough to cause legal blindness in both eyes (20/200 or worse in the better seeing eye). Traditional treatment measures are ineffective in a significant number of affected children.

•Excimer refractive surgery is effective for correcting amblyopiogenic levels of severe anisometropia and bilateral

ametropia when traditional treatment fails.

•Excimer refractive surgery in children usually requires general anesthesia.

Specific modifications to the typical anesthesia protocol are needed to avoid the escape of inhalational anesthetic agents that will affect the excimer laser/tissue interaction.

•Refractive lens exchange and phakic intraocular lenses are just starting to be investigated in children with extremely high refractive errors that are outside the treatment range for excimer laser.

3.1Introduction

Children are born with immature visual systems, and for normal visual development to occur, they need a clear, focused image to be projected onto the retina where it is converted to neuronal signal and then transmitted to the developing occipital cortex via the optic nerve. Uncorrected refractive errors cause image blur, impeding this process, and may result in failure of normal visual maturation (amblyopia).

Amblyopia is commonly caused by anisometropia, the condition in which unequal refractive error between fellow eyes results in image blur in one eye (form vision deprivation) and/or abnormal binocular interaction via projection of dissimilar images onto the fovea of each eye [16]. In general, anisomyopia of more than 2 diopters, anisohyperopia of more than 1 diopter, and anisoastigmatism of more than 1.5 diopters can result in amblyopia [54, 55]. Studies of anisometropic amblyopia indicate a 100% prevalence of amblyopia in patients with 4 diopters or more of uncorrected hyperopia or 6 diopters or more of uncorrected myopia [26, 50]. Anisometropic amblyopia associated with anisometropia of more than 4 diopters is also less successfully treated with traditional amblyopia therapy [20]. The severity of amblyopia is directly related to the degree of anisometropia [13, 20, 26]. Successful treatment of anisometropic amblyopia with traditional therapy varies widely among practitioners and has been reported to be between 48 and 82% of children [16, 20, 23−26, 28, 56].

Bilateral uncorrected high refractive error can also cause amblyopia. This condition, called bilateral ametropic (or isoametropic) amblyopia, though less common than anisometropic amblyopia, is even more of a disability as it affects both eyes. Unsuccessfully treated bilateral ametropic amblyopia is typically a disorder that affects children with neurobehavioral disorders and high refractive error who are tactilely averse and refuse to wear their prescribed spectacles. It is becoming a more common problem today as more extremely premature infants are surviving with the sequelae of severe ROP and subsequent high myopia. Visual impairment in these children with multiple special needs further isolates them. Tychsen has coined the term “visual autism” to refer to the resultant severe visual isolation in these children [51].

Traditional therapy for anisometropic amblyopia includes refractive correction with spectacles or con-

tact lenses, minimization of aniseikonia with contact lenses, and amblyopia management with occlusion therapy and/or pharmacologic and optical penalization of the sound eye [21, 24−26, 43]. Though these treatment strategies appear simple, they are frequently problematic and unsuccessful due to induced aniseikonia or diplopia with spectacles, psychosocial stress, unacceptable cosmesis with spectacles in which one lens is much thicker than the other, impracticality of contact management, and poor compliance with occlusion therapy [9]. Management of large magnitude bilateral ametropia is similar to anisometropia with regard to spectacles or contact lenses, though occlusion and penalization are not needed as both eyes have equal image blur.

Severe amblyopia causes a lifetime of visual handicap with its associated economic and social costs. Refractive surgery is now being used with good results for severe anisometropia and ametropia in children when traditional therapy fails.

3.2The Excimer Laser

The excimer laser was invented in 1970 by Basov, Danilychev, and Popov in Moscow [31]. Excimer is short for “excited dimer”, as the laser uses a combination of electrically stimulated inert gas (argon, krypton, or xenon) and reactive gas (fluorine or chlorine) to create energized molecules which generate ultraviolet (UV) laser light. The UV light emitted by excimer laser is absorbed by organic material and disrupts molecular bonds, resulting in ablation, rather than burning, of surface tissues; thus, excimer lasers can remove exceptionally fine layers of surface tissue with almost no heating of or damage to the underlying tissue, which is left intact. These properties make the excimer laser well suited to precision corneal refractive surgeries.

For almost two decades, excimer refractive surgery, performed as an outpatient procedure under local anesthesia, has been an accepted method of treating myopia, hyperopia, and astigmatism in adults.

Prudent selective use of excimer laser refractive surgery is now being investigated in carefully designed study protocols for children with severe anisometropia or bilateral ametropia in which traditional therapy failed and the visual results without any other treat-

ment would be effective blindness. This conservative approach is being used because of concerns regarding both the long-term refractive stability in the growing eye and the long-term corneal health.

With the advent of the excimer laser, corneal refractive surgery has evolved from incisionally based techniques to ablative procedures. Rather than altering the corneal curvature at discrete locations, photoablative procedures provide smoother and more uniform reshaping of the corneal surface. In myopia, excimer treatment flattens the central cornea to decrease its refractive power. In hyperopia, treatment indirectly steepens the central cornea by removing tissue from the periphery and flattening the peripheral cornea, which increases the cornea’s focusing power. Astigmatism is corrected by differentially steepening the flattest meridian or flattening the steepest meridian.

Modern refractive surgery treatments are performed either as conventional or custom procedures. Conventional laser ablation patterns are calculated based on a patient’s manifest or cycloplegic refractions, while customized (or “custom”) laser ablation patterns incorporate computer-aided wavefront technology to deliver a more precise treatment. Wavefront science is a newer technology that has been incorporated into laser refractive surgery since the early twenty-first century. Using wavefront-guided techniques, a patient’s subtle refractive errors, known as

aberrations, which may not be detected by traditional refraction can be measured. In custom laser refractive surgery, the laser ablation pattern can be programmed to treat a patient’s individual aberrations, as well as those induced by the refractive procedure itself. Unfortunately, aberration measurement requires a very cooperative patient and, for this reason, is often not possible in children. Fortunately, conventional ablation treatments still provide excellent results and are the most commonly used form of refractive surgery in the pediatric population today. Both conventional and custom treatments are available for a wide range of refractive errors (Table 3.1).

To date, approximately 270 amblyopic children (Table 3.2) who have undergone excimer refractive surgery for severe anisometropia and 15 who have undergone these procedures for bilateral severe ametropia have been reported (Table 3.3). Much more is now known about how children respond to treatment, making photorefractive keratectomy (PRK), laser insitu keratomileusis (LASIK), and laser-assisted subepithelial keratectomy (LASEK) increasingly viable alternatives to conventional management in the subset of children with severe anisometropia or bilateral ametropia. In current practice, refractive surgery is generally considered in the severe bilateral ametropic and anisometropic pediatric population only when (1) traditional treatment measures have been exhausted

or fail, or (2) chronic noncompliance with, or intolerance of, traditional treatment endangers normal visual development.

Visual and refractive results of excimer laser procedures in children have been moderate to excellent depending on the study evaluated [3, 5−10, 32, 33, 35, 36, 42, 44, 51, 52]. By and large, the younger children tended to have better visual results while the refractive results were equivalent at any age. In some studies, social interaction and behavior were reported to have improved.

3.3Excimer Surgical Procedures

PRK and LASIK are the most common ablative procedures performed in adults and children. For PRK, a topical anesthetic is first applied and the corneal epithelium in the ablation zone is removed mechanically with a spatula (which may be combined with alco- hol-enhanced epithelial loosening), brush, microkeratome or, less commonly, via laser scrape. The laser is then applied to the corneal stroma to resurface it (Fig. 3.1). After the procedure, a topical antibiotic and

Fig. 3.1  In PRK ablation is performed directly on the corneal surface. (Courtesy of J. Van Luu)

Fig. 3.2  In LASEK, the epithelium is moved over to the side after it is loosened with alcohol (shown here). Then a surface ablation like in PRK is performed and the epithelium is repositioned. (Courtesy of J. Van Luu)

Fig. 3.3  In LASIK, the laser is applied to the stroma after a flap is cut and lifted. The flap is then replaced. (Courtesy of

J. Van Luu)

steroid are applied and a disposable bandage contact lens is applied. Until the corneal epithelium recovers the patient may experience some discomfort and photophobia (3−5 days). A topical antibiotic is used for a week and topical steroid is used for 6 months in children. Visual acuity gradually improves as the epithelial defect and stroma heal and typically stabilizes by 3−6 months. Reliable refractive results are limited to 10−12 diopters of myopia, 5 diopters of hyperopia, or 4 diopters of astigmatism.

PRK carries the risk of several complications. Glare, halos, and dryness are common initially but generally diminish or resolve by the sixth postoperative month. Corneal haze is a potentially serious complication of PRK that tends to occur more often in patients with high myopia who require a larger laser ablation. The risk for developing this complication appears to be related to higher expression of collagen type-IV α3. Children may be at risk for this complication for a longer period of time postoperatively than adults. For this reason, they may require topical steroids for a longer period of time (4−6 months).

LASEK is a procedure similar to PRK. In this procedure, the corneal epithelium is first preserved with an alcohol solution and then peeled back as a single sheet to expose the corneal stroma (Fig. 3.2). The corneal stroma is then reshaped by excimer laser. After photoablation is complete, the preserved epithelium is replaced so that no corneal epithelial defect occurs. Postoperatively, patients are managed similarly to those who receive PRK. LASEK is supposed to be less painful than PRK postoperatively, though this has not been extensively studied. Visual and re-

fractive outcomes and potential risks of LASEK are equivalent to those of PRK.

LASIK is a procedure in which a microtome or femtolaser is used to create a corneal flap typically 100−180 µm thick and 8−9 mm in diameter. This flap is reflected to reveal the underlying stroma. The excimer laser is then used to photoablate the exposed stroma to reshape the cornea (Fig. 3.3). Afterward, the flap is returned to its original position. The flap is stabilized by the natural tissue dehydration that results from the action of the corneal endothelial pump rather than by sutures. Because no corneal epithelial defect is created, patients report less postoperative pain and faster recovery. LASIK, however, carries the risk of more serious complications such as flap dislocation, corneal striae, and keratectasia [5, 14, 15, 18, 22, 27, 40, 42, 46, 48, 49]. Because children tend to rub their eyes frequently, which increases the risk of lamellar flap dislocation following LASIK, PRK and LASEK are currently preferred in the pediatric population.

The newer excimer laser models may include infrared cameras that image the eye intraoperatively.

Image processing software identifies the pupil, and the excimer laser tracks the pupil margin during treatment to accurately align the ablation pattern with the cornea. The laser machine also has a fixation target for the awake patient to fixate on during the procedure. The risk of treatment decentration increases with loss of fixation or the inability to fixate.

This risk is naturally higher in children who require general anesthesia and must have the eye manually centered (see below).

3.4Setting up a Pediatric Excimer Refractive Surgery Program

3.4.1Preoperative Guidelines

The first step in establishing a program to perform

PRK, LASIK, and/or LASEK on pediatric patients mustbetoclearlydefinethecriteriausedtodetermine candidacy for such procedures. In general, refractive surgery in children should be considered for patients with anisometropia greater than 3−4 diopters when chronic noncompliance with and/or intolerance of traditional treatment endangers normal visual development or when standard therapy has failed to improve vision. The refractive treatment dose needed should ideally be within the limits of the specific excimer laser being used, typically 12 diopters of myopia, 5 diopters of hyperopia, and 4 diopters of astigmatism. Until recently, because there was no other treatment to offer, children with even higher levels of refractive error would still undergo these procedures up to the maximum dose, even though it would leave some residual refractive error because the lower residual refractive error would create less image blur which in turn would translate into less severe amblyopia. There are some new refractive surgical procedures, such as refractive lensectomy with or without lens exchange and phakic intraocular lenses, that are just beginning to be investigated in children with severe refractive error falling outside the dose range for the excimer laser. These procedure had great promise.

Absolute and relative contraindications to excimer refractive surgery include glaucoma, uveitis, recurrent conjunctivitis, tear film insufficiency, endothelial dysfunction, corneal scarring, keratitis, significant macular or optic nerve anomalies, or systemic inflammatory disease.

During the preoperative evaluation, care must be taken to ensure that patients and their families have reasonable expectations of what excimer laser surgery can achieve. During the preoperative consultation, the physician should discuss the range of visual acuity that the patient can expect to achieve but also explain that refractive surgery is not riskor compli- cation-free and that some patients may have some residual postoperative refractive error that would benefit from some refractive correction, especially if the amblyopia is severe and safety is an issue.

A comprehensive ophthalmologic examination should be performed on all pediatric patients being considered for refractive surgery. Special attention should be paid to the following:

1.External examination. This is necessary to rule out anatomic abnormalities and inflammatory conditions such as acne rosacea, though this is rare in young children.

2.Visual acuity. This should be measured at distance and near with and without correction, if possible.

3.Slit lamp examination. Corneal scarring within the treatment zone is a contraindication to the surgery.

Herpetic viral infections can be reactivated by the ultraviolet radiation of the excimer laser.

4.Corneal pachymetry. This should be measured after the refraction has taken place to prevent surface disruption. Excimer refractive surgery may be contraindicated if the cornea is too thin to allow the needed treatment dose which varies depending on the refractive error and procedure type. Controversy exists regarding the minimal required residual thickness of the postoperative stromal bed necessary to minimize the risk of keratectasia in LASIK and haze in PRK. Currently, for LASIK, a residual stromal bed thickness of 200−300 µm with an absolute corneal thickness of 500 µm is considered reasonable. For PRK, a residual stromal bed thickness of 360 − 400 µm with an absolute residual corneal thickness of 475 − 500 µm is considered reasonable.

5.Pupils. Patients should be checked for a relative afferent papillary defect and pupil diameter should be measured in the dark, preferably using a pupillometer. Some surgeons feel that the planned treatment zone should extend beyond the edge of the dark-adapted pupil to minimize night vision symptoms such as halos and glare postoperatively.

6.Keratometry. These values are used to calculate the postsurgical corneal contour to avoid overflattening the cornea to less than 35 diopters, which will cause aberrations [19].

7.Corneal topography. This test is used to screen for an abnormal corneal contour, asymmetry in astigmatism, inferior corneal steepening, or other evidence of forme fruste keratoconus, which can be difficult to diagnose, especially in children. Patients with keratoconus should not have excimer refractive surgery as it increases the risk of postoperative keratectasia.

their vision loss should not undergo the procedure as the likelihood of visual improvement is low. Mild abnormalities however, do not preclude proceeding with refractive surgery.

An explanation of the refractive procedures and possible complications should follow the physical examination on an initial visit, and patients and their families should have the opportunity to ask questions and consider the risks and benefits of all traditional and surgical options. Physicians should take care to emphasize that while refractive surgery is expected to improve vision and decrease dependence on refractive correction, it may not eliminate a patient’s need for spectacles entirely. Furthermore, it should be emphasized to parents that long-term results of pediatric excimer laser procedures beyond 5 − 6 years are not known, and there is a possibility of unknown complications occurring later.

3.4.2Operative Guidelines

In our practice, pediatric PRK is performed as follows: General anesthesia is usually required for children under 11 − 12 years of age.Anesthesia is first induced in an induction room using sevoflurane and nitrous oxide and oxygen by mask inhalation.An intravenous line is placed after the child is asleep, and a laryngeal mask airway is inserted into the posterior pharynx; sevoflurane is discontinued, and a propofol infusion is started. Intravenous ketorolac tromethamine and/ or rectal acetaminophen are administered for analgesia. An adhesive, nonporous drape is placed over the laryngeal mask airway to minimize escape of the inhalational anesthetic agents which may interfere with laser-tissue interaction. The child is then transported to an adjacent operating room fully monitored and breathing oxygen through a Jackson-Rees circuit.

Before entering the operating room, the inhalational anesthetic is discontinued.

In the operating room, the child’s head is positioned in the supine position with the plane of the iris perpendicular to the laser beam. For the children requiring general anesthesia, the surgeon fixates the eye manually with forceps, taking care to avoid

lium. For hyperopic PRK, the entire epithelium is removed manually. PRK is then performed. The laser beam is centered on the entrance pupil using the laser machine’s own tracking mechanism. Centration is assured by the laser tracking mechanism during the procedure. To further ensure that the iris plane remains perpendicular to the laser beam during the procedure under general anesthesia, an observer positioned on one side of the patient continually monitors the eye position.

After the procedure is completed, topical ketorolac 0.5%, fluorometholone 0.25%, and a fourth-gen- eration fluoroquinolone are placed in the treated eye and a disposable contact lens is placed on the cornea.

A fox shield is then placed over the eye.

3.4.3Postoperative Guidelines

Postoperative medications for PRK include the fourthgeneration fluoroquinolone and fluorometholone

0.25%, four times a day in the treated eye until the corneal epithelium heals (3 − 5 days). Topical ketorolac and tetracaine can be used up to four times a day as needed for discomfort for the first two postoperative days only. Hydrocodone oral elixir is also prescribed as needed for severe discomfort for the first few days. The fluoroquinolone is discontinued after 1 week, and fluorometholone 0.25% is continued four times a day for 2 months, followed by a slow taper over the next 4 months. Oral vitamin C (age-depen- dent dosing) is also prescribed to decrease the risk of corneal haze.

The children are examined postoperatively 4 − 7 days after the procedure, then at 1 month and 2 months postoperatively and then every 2 − 3 months for 12 months and again at 24 months following the surgery. Cycloplegic refractive correction is prescribed as needed at the 1-month postoperative examination and updated as needed thereafter. Occlusion therapy of the sound eye is recommended if needed up to full time except 2 h a day based on the child’s age and visual deficit. Postoperative corneal topography is performed as indicated as patient cooperation allows to assess for centration and healing changes.