Multifocal and Accommodative Intraocular Lens Implantation in Children
M. Edward Wilson
Muralidhar Ramappa
Rupal H. Trivedi
One of the most intriguing challenges of modern cataract surgery is the restoration of the accommodative ability in a pseudophakic eye. Although monofocal intraocular lenses (IOLs) offer excellent visual function, they do not restore any of the remarkable accommodation that is lost when the crystalline lens is removed from a child. With that said, subjects with small pupil size, myopic astigmatism, corneal aberrations, and corneal multifo-cality and subjects with good visual perception can have an increased range of what has been called “pseudo-accommodation” with monofocal IOLs. Monovision techniques (one eye corrected for distance and the other for near) have been used as a partial substitute for the absence of accommodation, but it may be at the cost of binocularity. Typically, if a monofocal IOL power is selected for distance correction, the family is informed that glasses will be needed for near and intermediate vision. However, it is remarkable how many times parents report that their bilaterally pseudophakic child functions well at near even when looking over the bifocals or when they are not being worn at all. While this is not universal for all children, it is a common occurrence that cannot be explained simply by depth of focus with the pupillary miosis that occurs with attempted accommodation. The importance of this finding is that any study of pseudo-accommodation in children implanted with multifocal IOLs needs a comparison group of age-matched children with similar residual refractive errors who were implanted with monofocal IOLs.
Newer IOLs designs have enabled adult cataract surgeons to both neutralize corneal astigmatism and reestablish the range of near and far vision without spectacles. The success of multifocal IOLs in the adult population has lead to a growing interest in implanting them in children.1 Unlike most adults who have cataract surgery with IOL placement, children have excellent accommodation, so multifocal IOLs are often viewed as an attractive way to mediate the loss of the ability to zoom in and out at will. There are children who do not wear spectacles or contact lenses secondary to behavioral issues related to syndromes, neurologic issues, or other medical issues. Multifocal IOLs have been suggested for these children (or young adults) with special needs who develop cataracts requiring surgical intervention. There is a dearth of literature in children after multifocal IOLs with long-term outcomes.2,3 This chapter focuses on the multifocal and accommodative IOL designs available in the United States and Europe and those under investigation at this time.
MULTIFOCAL IOLS
Multifocal IOL designs are based on the concept of simultaneous vision. These IOLs focus light toward distant, near, and sometimes intermediate focal points to provide the patient near and distant targets.4 Visual disturbances can arise from light focused in multiple areas. The quality of the vision produced by a multifocal IOL is based on several factors, that is, pupil size, shape of the IOL, and the IOL’s refractive or diffractive characteristics. Haloes around lights, glare, and decreased contrast sensitivity are potential problems with multifocal IOLs.4 For example, when attention is to light rays focused for the distant focal point, the retina still receives light for the near focal point, and these out-of-focus light rays may be interpreted as glare and lower contrast sensitivity. Currently, many multifocal IOLs have adopted aspheric designs to improve contrast sensitivity.4 Advantages are that many designs are similar to current foldable monofocal IOLs implanted in routine cataract surgery; minimal modification of surgical technique is required. Multifocal IOLs
currently give more predictable results for near vision when compared with single optic accommodating IOLs.5
currently give more predictable results for near vision when compared with single optic accommodating IOLs.5
IOL DESIGN VARIABLES
Different areas of the IOL have different focal planes, usually for near and distant vision. At any given time, one image is in focus at the retina, and the second image is highly defocused with very little structure. Distant objects are focused by the distance power of the lens and defocused by the near power. For near objects, the opposite is true; near objects are focused by the near power of the lens and defocused by the distance power.
OPTICS
Multifocal IOLs can be broadly categorized as diffractive or refractive.
Refractive IOLs
Refractive multifocal IOLs direct the light at different focal points using concentric zones of varying dioptric power within the optic. The principle is similar to the bifocal spectacle. These are also referred to as multizonal refractive IOLs.6 As the pupil size changes, the number of zones in use varies, and eventually, the relative proportion of light directed to the near and distant focal points changes as well.6 Thus, image quality can vary depending on pupil size. The ReZoom lens (Abbott Medical Optics, Santa Ana, CA) is an example of a refractive multifocal IOL.
Diffractive IOLs
Diffractive IOLs have closely arranged concentric rings on one of the surfaces of the lens to divide incoming light into multiple beams; they add together in phase at a predetermined point on the optical axis for near focus, while the overall curvature of the lens provides the distance focus. The number of the rings, spacing, and step heights vary by IOL design and manufacturer. The apodized design consists of concentric rings showing a decrease in height from the taller central diffractive steps to the shorter outer steps.6 The perceived advantages of apodization include the fact that the gradual change in step height decreases sudden shifts in optical boundaries, reducing distracting out-of-focus light rays that produce glare and haloes when viewing distant objects through a large pupil. As the pupil size increases, more light is focused to the distant focal point. The rationale for this design is that in mesopic conditions with dim light when the pupils are large, such as driving at night, distance vision is the priority. In most situations, humans rely on near vision in well-lit environments, such as reading under artificial light or in daylight, when the pupil is constricted. In these situations, apodization directs equal light to both focal points.6 The ReSTOR IOL (Alcon Laboratories, Fort Worth, TX) is an example of an apodized diffractive multifocal IOL. Nonapodized diffractive multifocal IOLs have diffractive steps with fixed height from the center to the periphery. Thus, these lenses distribute light to the near and distant focal points in constant proportions, regardless of the pupil size.4 Current examples of nonapodized diffractive multifocal IOLs are the TECNIS Multifocal IOL (Abbott Medical Optics) and the AT LISA 809 IOL (Carl Zeiss Meditec Company, Hennigsdorf, Germany).
ACCOMMODATIVE IOLS
In accommodative IOLs, a shift in the focal length of the IOL-eye optical system is induced by a change in the ciliary muscle tension. In contrast to multifocal IOLs, accommodative IOLs are free from glare and haloes and result in improved contrast sensitivity since these lenses are designed to focus light at the desired focal point.7 Accommodating IOLs are the most diverse in terms of design.
SINGLE-OPTIC ACCOMMODATING IOLS
The presumed mechanism of accommodation is a forward axial shift in the optic of the IOL and changes in the lens architecture as a result of contraction of the ciliary muscle.7 Theoretically, the accommodation is achieved by combined mechanisms. It is believed that pseudoac-commodation might play a role in addition to changes in lens position and lens architecture.8 In the case of the Crystalens (Bausch & Lomb, Rochester, NY), the optic may curve anteriorly, which changes the radius of curvature of the anterior surface of the optic, resulting in more near vision.4,8 The Crystalens HD variant has a small refractive number add centered in the optic to improve depth of focus for intermediate and near distances.8,9 This essentially adds a little refractive multifocal feature to the accommodative lens design. The Crystalens AO is an aspheric version. One study comparing the Crystalens HD to a monofocal IOL found a mean accommodating range of 1.5 ± 0.0 D in the Crystalens HD group and 1.0 ± 0.0 D in the monofocal IOL group.9
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