Volume 1, Chapter 45. Optical Correction of Pediatric Aphakia

Chapter 45
Optical Correction of Pediatric Aphakia
SCOTT R. LAMBERT
Main Menu Table Of Contents

ANATOMY
AMBLYOPIA
INTRAOCULAR LENSES
POWER OF THE IOL
CONTRAINDICATIONS TO IOL IMPLANTATION
CONTACT LENSES
GLASSES
CONCLUSIONS
REFERENCES

The optical correction of childhood aphakia with an intraocular lens (IOL) has become more common in recent years. However, there are several ways in which the correction of aphakia differs between children and adults.¹ First, a child's eye is still growing during the first few years of life. As a result, these eyes usually experience a myopic shift, an important consideration when determining the power of the IOL to be implanted. Second, young children have immature visual systems and are at risk of developing amblyopia if visual input is defocused or unequal between the two eyes. Third, actuarial data would suggest that an IOL will remain in a child's eye for significantly longer than an IOL implanted in an adult. Given the cumulative incidence of many complications, certain risks that are acceptable in adults are unacceptable in children. Finally, a child's eye typically has a more exuberant postoperative inflammatory response, necessitating certain modifications in the surgical implantation technique.²

ANATOMY

During early childhood, the refractive elements of the eye undergo radical changes. At birth the cornea has on average a curvature of 52D. By the time a child is 18 months of age, the cornea has on average flattened to 43.5D.³^,⁴ From this age on the curvature of the cornea remains about the same. In addition, the crystalline lens undergoes a reduction in its power during early childhood. This process usually continues until a child is about 6 years of age.⁵ The most important change in a child's eye in terms of its aphakic optical correction is its axial elongation. At birth the mean axial length is 17.0 mm.⁶^,⁷ During childhood, a triphasic pattern of axial elongation has been noted. The first phase occurs during the first 2 years of life, when the eye grows on average 4.4 mm.⁴ The second phase occurs between the ages of 2 and 6 years, when the eye grows on average another 1.5 mm. The last phase extends through adulthood. During this phase, the eye grows on average another 1.0 mm, although this growth can be greater in eyes with a genetic predisposition to myopia.

AMBLYOPIA

Animal experiments have shown that visual deprivation during a “sensitive period” results in anatomic changes in the lateral geniculate bodies and visual cortex.^8–10 These changes result in a reduction in visual acuity referred to as amblyopia. The sensitive period in humans extends up to 7 years of age. Because of the visual inattention of infants during early infancy, visual deprivation during the first 6 weeks of life appears to have no lasting consequences.¹¹^,¹² However, after this 6-week “grace period,” even short intervals of visual deprivation can have profound effects on the development of the central visual pathways. In general, the earlier the visual deprivation occurs, the more severe the amblyopia induced. Unequal stimulation of the two eyes has been shown to be particularly damaging to the development of these central visual pathways.¹³ Thus, a unilateral cataract or unilateral uncorrected aphakia in a child has particularly profound effects on the visual development of the deprived eye.¹⁴ The need to provide equal visual stimulation to both eyes of a child to prevent amblyopia is one of the most important considerations when trying to decide how best to correct a child's eye optically.

INTRAOCULAR LENSES

Intraocular lenses have become the method of choice for optical correction of aphakia in children after the first year of life because of the superior visual outcomes associated with their use,¹⁵^,¹⁶ their convenience, and the constancy of the correction they provide.¹⁷ Numerous studies have shown improved visual outcomes in age-matched children corrected with IOLs versus contact lenses. Ben Ezra and coworkers¹⁸ reported that 65% of children with unilateral cataracts were able to see 20/40 or better after cataract surgery and IOL implantation, compared with only 35% of eyes corrected with contact lenses. Similarly, Greenwald and Glaser¹⁹ reported 20/40 or better visual acuity in 75% of children 2 years of age or older after cataract surgery and IOL implantation, compared with 48% of eyes corrected with contact lenses.

The improved visual outcome associated with IOLs is presumably related to the constancy of the optical correction. In addition, pseudophakic children have been reported to have superior binocu-larity and a lower incidence of subsequent strabismus than children corrected with contact lenses.¹⁸

Another important consideration is the convenience of IOLs. Whether worn on a daily-wear or an extended-wear basis, contact lenses require a considerable amount of care. Parents usually must care for the contact lenses worn by their child. The time spent caring for these contact lenses is time that could potentially be spent interacting in more positive ways with the child. IOLs also provide a constant correction, whereas even the most compliant child will have periods of time when the contact lens cannot be worn because of an ocular infection or a lost or damaged lens. In addition, an IOL provides an immediate optical correction after implantation, whereas a delay of several days or even weeks often occurs before contact lenses or glasses are dispensed.²⁰

Finally, IOLs, unlike contact lenses, are not associated with ongoing expenses and are therefore particularly well suited for families who have limited financial resources.²¹

Despite the widespread acceptance of IOLs for the optical correction of aphakia in children, there exists considerable controversy regarding the optimal technique for IOL implantation in a child's eye (Fig. 1). A child's eye differs in several significant ways from an adult eye. First, there is nearly universal opacification of the posterior lens capsule after cataract surgery.²¹ This opacification can occur a few weeks after cataract surgery or years later.²² In older children, the posterior capsule is frequently left intact at the time of cataract surgery and opened with a Nd:YAG laser when opacification occurs.²³ This approach requires that the patient return for regular follow-up visits and that the patient be sufficiently cooperative to allow the capsulotomy to be performed in the clinic. If treatment is delayed, excessive amounts of laser energy may be required to create an adequate capsulotomy.²⁴ In addition, opacification of the anterior hyaloid face frequently occurs after a YAG capsulotomy in young children, necessitating repeated YAG capsulotomies or a surgical membranectomy.²⁵ Alternatively, a primary posterior capsulotomy may be created either with or without an anterior vitrectomy.

Fig. 1. A. Intraocular lens (IOL) implanted in the capsular bag after a primary posterior capsulotomy and anterior vitrectomy through the limbal incision. B. Anterior vitrectomy and posterior capsulotomy being performed through a pars plana incision after implanting an IOL into the capsular bag through a limbal incision. C. IOL optic prolapsed through a primary posterior capsulotomy. The haptics remain in the capsular bag. The hyaloid face has been left intact. D. IOL implanted in the capsular bag after a primary posterior capsulorhexis. The hyaloid face has been left intact. E. Lensectomy and anterior vitrectomy being performed after implanting IOL in the ciliary sulcus. F. YAG laser posterior capsulotomy being performed as a secondary procedure after implanting an IOL in the capsular bag. (Lambert SR, Drack AV: Infantile cataracts. Surv Ophthalmol 40:427–458, 1996.)

Some authors have stressed the importance of combining an anterior vitrectomy with primary posterior capsulotomy,²⁶^,²⁷ However, Fenton and O'Keefe²⁸ reported a clear visual axis in 84% of children after a 19-month follow-up after a primary posterior capsulotomy alone. Gimbel²⁹ suggested that prolapsing the lens optic through the primary posterior capsulotomy reduced the incidence of secondary membranes forming across the pupillary space, but Koch and Kohnen³⁰ reported nearly universal opacification of the visual axis using this approach. The anterior vitrectomy may be performed through the limbal incision before IOL implantation or through the pars plana after IOL implantation.³¹ Although performing the posterior capsulotomy through the pars plana makes it technically easier to implant the IOL in the capsular bag, a randomized clinical trial failed to find any difference in the outcome between the two techniques.³²

The type of IOL implanted may also affect the incidence of posterior capsule opacification. IOLs that create a symmetric radial stretch of the posterior capsule may increase the contact between the convex IOL surface and the posterior capsule, thereby creating a barrier to the central migration of epithelial cells.³³ Acrylic lenses may be associated with both a lower incidence and a delayed onset of posterior capsular opacification. Possible reasons for the lower incidence of posterior capsular opacification with acrylic lenses include the square edge of the lens, greater biocompatibility, and increased capsular adherence. Finally, heparin-surfaced modified PMMA lenses have been reported to reduce the cellular deposits on IOLs in a child's eye.³⁴

Children also differ from adults in that they are more likely to traumatize their eye after cataract surgery. Therefore, a superior incision has the advantage over a temporal incision of allowing the brow to protect the incision site. A sutureless incision is inappropriate in children because of the greater likelihood that they will rub their eye and cause dehiscence of the incision site.⁵

POWER OF THE IOL

If an IOL is implanted in the eye of a child less than 6 years of age, the ongoing axial elongation in the presence of a relative stable corneal curvature means that a myopic shift will occur. Although some authors have predicted myopic shifts as great as 36D,³⁵ the actual myopic shifts observed in pseudophakic children have been quite small.³⁶^,³⁷ In 18 children with a mean age of 6.3 years (range 3 to 9 years), Hutchinson and colleagues³⁸ noted a mean myopic shift of only 1.0D after a 3-year follow-up. Only three of these eyes experienced a myopic shift of 2D or more. Similarly, Enyedi and coworkers³⁹ noted a myopic shift of only 1.5D after a 2-year follow-up in children 2 to 6 years of age. Although a larger myopic shift has been noted in infants after IOL implantation, Dahan and Drusedau⁴⁰ reported a mean myopic shift of only 6.4D after a 7-year follow-up in 68 eyes undergoing IOL implantation during infancy. The less-than-expected myopic shift that occurs in these eyes may be partially explained by the retardation of axial elongation that occurs in infant eyes after the removal of the crystalline lens.^41–43 The myopic shift also has been noted to be less in eyes with congenital and developmental cataracts than eyes with traumatic cataracts.⁴⁴

A variety of approaches have been used to compensate for the myopic shift that occurs in pediatric eyes after IOL implantation. In some cases, the IOL power is targeted for emmetropia with the expectation that the child will need to wear an additional optical correction when older. Proponents of this paradigm point out that these children are at greatest risk of amblyopia at the time of cataract surgery, making this the most important time for the aphakic eye to be clearly focused. More commonly, an IOL power is chosen that initially undercorrects a child's eye in hopes of making the eye emmetropic or only slightly myopic after the eye has elongated to its adult length.⁴⁵ An alternative approach is to place two lenses in a child's eye in a piggyback fashion.⁴⁶^,⁴⁷ One lens can then be removed when the eye has elongated sufficiently that the more posterior IOL can focus an image on the retina by itself. Ray tracing analyses have shown that there is no degradation of visual acuity with polypseudophakia, even in extremely short eyes.⁴⁸ Algorithms have been created that allow one to predict the final refractive error of an eye based on the axial length and keratometry readings at the time of cataract surgery.⁴⁹ Because genetic factors also influence axial elongation, they should also be factored into the decision as to what IOL power to implant.

CONTRAINDICATIONS TO IOL IMPLANTATION

Despite the popularity of IOL implantation after cataract surgery in childhood, in certain situations IOL implantation during childhood continues to be controversial. IOL implantation in a child's eye with active uveitis is an absolute contraindication. Even when an eye with uveitis is quiescent, most authors caution against IOL implantation.⁵ Microphthalmia is also generally considered a contraindication to IOL implantation because of the limited range of IOL sizes currently available. Although an anterior chamber or a suture-fixated posterior chamber IOL can be implanted in an eye without adequate cap-sular support,⁵⁰ both are associated with several po-tential problems. Anterior chamber IOLs may result in corneal endothelial decompensation or chronic inflammatory changes of the uveal tract over time.⁵¹ Although the lens may be fixated with sutures in the posterior chamber in the absence of adequate capsular support,⁵² long-term follow-ups are not available for these eyes. There are concerns that the sutures may in time erode either through the conjunctiva, resulting in endophthalmitis, or through the sclera, resulting in subluxation of the IOL.

IOL implantation during infancy is also controversial. It is associated with an increased incidence of postoperative complications compared with eyes treated with a lensectomy alone.^53–55 Moreover, it is uncertain whether this procedure is associated with sufficient visual benefits to compensate for the increased incidence of postoperative complications.⁵⁶ Although the risk/benefit ratio may warrant IOL implantation in children with unilateral cataracts, most authors do not believe the procedure is warranted in infants with bilateral cataracts.

CONTACT LENSES

Contact lenses have been used extensively in the past for the correction of aphakia in children. They are still the treatment of choice in infants with bilateral aphakia and in children of all ages with active uveitis or inadequate capsular support. They are also frequently used in conjunction with an IOL in eyes with corneal scars to neutralize irregular astigmatism.

Contact lenses have several distinct advantages over IOLs as a means of correcting aphakia in children. First, the power of the contact lens can easily be adjusted as a child's eye elongates. This is particularly useful in an infant's eye, which undergoes considerable growth during the first 2 years of life. Whereas infants require on average a 32D contact lens at the time of cataract surgery, by the time they are 1 year of age, only a 24D lens is needed.⁵⁷ One potential strategy for optical correction in an infant is to treat infants with a contact lens immediately after cataract surgery and then to implant an IOL once the eye has stopped elongating.⁵⁸^,⁵⁹

Second, cataract surgery is simpler if a lensectomy alone is performed. Smaller incisions can be made, reducing the risk of wound dehiscence. In the presence of a complicating factor such as ectopia lentis, the simplicity of the surgical procedure compared with that of suture fixation may be reason enough to warrant a lensectomy alone with contact lens correction.

In the United States, silicone contact lens have been the contact lens of choice for young children because of their ease of fitting and the option of wearing these lenses on an extended-wear basis. However, silicone lenses are very expensive. In addition, because of the hydrophobic nature of silicone, a hydrophilic surface must be applied to the lens, which requires that these lenses be replaced on a regular basis.⁵ In recent years, rigid gas-permeable contact lenses have become increasingly popular as a means of correcting aphakia in children because of their lower cost, their availability in a greater range of powers, and their ease of insertion and removal.⁶⁰ Keratometry readings can be taken during surgery to facilitate the fitting of these lenses.

When overseen by conscientious parents, contact lenses can be associated with a good visual outcome even in children with unilateral aphakia.¹³^,⁶¹^,⁶² Unfortunately, many children with unilateral aphakia have a poor visual outcome when corrected with contact lenses because of the time and constancy required for their use.²⁰^,⁶³^,⁶⁴

GLASSES

Glasses are rarely used to correct aphakia in children. The one exception is children with bilateral aphakia who are intolerant of contact lenses. Glasses have the disadvantage of producing several undesirable optical distortions, such as ring scotomas. However, in a child with microphthalmic eyes, they do have the advantage of magnifying the apparent size of the eyes. Although compliance is generally better with glasses than contact lenses, some children who are developmentally delayed may object to their use.

Glasses are still commonly used even in pseudophakic or contact lens-corrected children to provide a near point correction. In these situations, an IOL or contact lens is used to provide the distance correction and a bifocal segment the near correction. Bifocals are typically prescribed when children are 18 months to 3 years of age. Multifocal IOLs may reduce the future need for glasses for pseudophakic children.⁶⁵^,⁶⁶ However, the factor limiting their use in children is again the axial elongation that occurs, which would make it difficult to make these children free of spectacles.

CONCLUSIONS

During the past decade, the optical correction of aphakia in children has changed radically from contact lenses to IOLs. Superior visual outcomes have been achieved with IOLs so that many aphakic children can now live nearly normal lives. However, implanting an IOL in a child's eye differs from implanting an IOL in an adult's eye. Further studies will be necessary to determine whether multifocal IOLs should be used to correct aphakia in infants and children.

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