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January 2006

 
Ophthalmic Pearls: Pediatrics
Selecting Intraocular Lens Power in Children
By Rupal H. Trivedi, MD, MSCR, and M. Edward Wilson Jr., MD
Edited By Ingrid U. Scott, MD, MPH, and Sharon Fekrat, MD
 
 

The accurate determination of IOL power is one of the major challenges for the long-term care of children undergoing cataract surgery. Selecting a fixed-power lens for implantation into an eye that is still growing requires a complicated decision-making process. Recent advances in instrument technology have helped us obtain reliable axial length (AL) and keratometry (K) measurements, from which emmetropic implant power calculations using formulas for the adult eye can be derived. In children, however, implantation of an IOL at the emmetropic power risks a significant myopia at ocular maturity. Yet if we aim for early hyperopia with the expectation that the eye will come to be emmetropic during adulthood, then this early hyperopia may create hurdles in the management battle against amblyopia. Both approaches have pros and cons; the best solution probably lies in finding a compromise. What is the ideal compromise? Unfortunately, there is no one answer. We present four key steps to guide you in selecting an IOL for a child.

Take Measurements
Axial length. We measure AL with an immersion technique, often under sedation. Readers should refer to the specific technical instructions for the A-scan unit they are using. Errors in AL measurement affect the IOL power calculation the most, with an average error of 2.5 D per millimeter of AL in adults. However, this error jumps to 3.75 D per mm in short eyes (i.e., AL 20 mm or less).

Keratometry. For the K measurement, we use a handheld keratometer. Although the reliability of the handheld keratometer may be suboptimal for axis measurements, we feel that averaging several readings is a practical way to obtain an acceptable K value in sedated children.

Calculate the IOL Power
It is preferable to use theoretical formulas because they are generally more accurate for small eyes, and in pediatric studies they appear to be slightly more accurate than regression formulas. While errors in AL measurement, K measurement and IOL power calculation show up in the immediate postoperative refraction, these errors are not the main source of unexpected refractive error. Growth of the eye, which is generally not very predictable, is responsible for these “refractive surprises.”

For long-term care of children, an additional focus should be on how much undercorrection should be the goal (based on the likely growth of the eye in a specific child).

Determine the Target Postoperative Undercorrection
This is a straightforward decision in adult eyes, as visual needs assist in the determination of those calculations. For children, another factor to consider is the projected growth of the eye. The ideal IOL power should give the best help for fighting amblyopia in childhood while inducing the least refractive error in adulthood. This power can be calculated by anticipating the expected myopic shift and undercorrecting the eye that needs IOL implantation. This initial hyperopia should be low enough to be corrected by either a contact lens or glasses to facilitate compliance with amblyopia therapy. We prefer a customized step-by-step approach that starts by considering age.

Age at cataract surgery. When an IOL is implanted in a child, a marked myopic shift must be expected. Therefore, IOLs implanted in children are usually selected to produce undercorrection. The closer to birth the implantation is performed, the more marked the undercorrection will need to be. How much should one undercorrect and at what age? Two approaches can be used for reduction from the calculated emmetropic power: percentage (e.g., 20 percent undercorrection) or absolute value (e.g., 10-D undercorrection). The former approach is more accurate; the latter approach is more practical. Our current recommendations, based on the latter approach, are shown in the table “Age at Cataract Surgery and Residual Refraction.” These recommendations must be balanced with the following additional factors:

Status of the fellow eye. In a child with a monocular cataract or with a pseudophakic fellow eye, it is important to determine the refractive status of the fellow eye to minimize the aniseikonia. When surgery will be done in both eyes, a larger amount of hyperopia may be acceptable, since aniseikonia can be avoided by targeting a nearly equivalent refraction in both eyes.

Visual acuity. Current advances suggest that it will probably be easier in the future to manage myopia than amblyopia. Dense amblyopia may prompt a decision to leave less hyperopia (or even to aim for emmetropia) to facilitate compliance with amblyopia therapy. In such a case, late myopia is acceptable if it helps the child recover vision during the amblyopia treatment years.

Expected compliance. It is better to leave less refractive error if the child and/or family is expected to comply poorly with glasses, contact lenses or occlusion therapy.

Parents’ refractive error. It is also important to ask about high refractive error (especially myopia) in the parents. Because children with myopic parents can be expected to undergo more eye growth, these eyes may be left with more hyperopia than stated in the table.

IOL power. In general, the higher the IOL power, the more undercorrection is needed. For example, if at 1 month of age, one child has an emmetropic power need of 50 D and another child needs 40 D, the first child will require a higher early residual refraction. We may use an approximate expected postoperative refraction in the first child of +12 D, while in the second child the expected refraction would be +10 D.

Microphthalmia. In microphthalmic eyes, the target refraction may not be reached even with the highest available IOL power. In such eyes, the highest available IOL power should be chosen.

How to Adjust the Correction During Surgery
If the site of the IOL implantation needs to be changed after the start of surgery, an appropriate power adjustment should be made. The IOL intended for capsular bag placement should be decreased by 1 D to 2 D (depending on the IOL power; the higher the power, the greater the reduction), when placed in the ciliary sulcus.

Conclusion
Our approach attempts to take into account all known factors affecting axial growth after cataract surgery. Several additional factors, such as gender and race, have been reported to affect growth of the normal eye and may influence eye growth after cataract surgery as well. A multicenter, prospective trial might clarify the effect of these factors on postop eye growth.

Despite our best efforts, it may be that some children may eventually need an IOL exchange or refractive surgery. Surgeons who implant IOLs in young children must be prepared for a wide variance in the long-term myopic shift. Both the magnitude and the variability in this shift are likely to be greatest in children having surgery in the first few years of life. Nevertheless, we must assume that an IOL implanted in a child’s eye will remain there for many years. The long-term refractive error outcome of the operated eyes of today will help us develop formulas suited for IOL power calculation for the pediatric patient of tomorrow.

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Rupal H. Trivedi, MD, MSCR, is a research assistant professor and M. Edward Wilson, Jr., MD, is professor and chairman of ophthalmology at the Medical University of South Carolina.

Age at Cataract Surgery and Residual Refraction
Our current recommendations if late myopia is to be minimized.*
Age
< 1.9 months
2.0–3.9 months
4.0–5.9 months
6.0–11.9 months
1.0–1.9 years
2.0–3.9 years
4.0–4.9 years
5.0–5.9 years
6.0–6.9 years
7.0–7.9 years
8.0–9.9 years
10.0–13.9 years
> 14 years
Residual Refraction
 +10 D
 +9 D
 +8 D
 +7 D
 +6 D
 +5 D
 +4 D
 +3 D
 +2 D
 +1.5 D
 +1 D
 +0.5 D
 Plano
*Other factors must also be taken into consideration before selection of the IOL power: fellow eye status, degree of amblyopia, assumed compliance and parental refractive error.