Keywords

When performing cataract surgery in children, implanting an IOL is the silent wish of all pediatric cataract surgeons. A child provided with an IOL will be easier to manage during the postoperative visual rehabilitation than a child who is left aphakic. The reduced need for contact lens correction, improved comfort and image quality (even when some additional supplementary spectacle correction is required) will help to the goals of visual rehabilitation, provided the child does not develop posterior capsule opacification (PCO) during the follow-up. The largest group of children presenting with congenital cataract are newborns, requiring cataract surgery as soon as 2 to 3 months of age. Studies such as IATS and IOLunder2 have recently advised against IOL implantation in these very young children because of a higher risk of secondary surgery in pseudophakic children of a young age using a classic lens-in-the bag (LIB) IOL technique [1, 2].

When using classical LIB implants, these recommendations should be strictly followed because a classical LIBhas a diameter of 12 to 13 mm, which is extremely large for those small eyes. The authors have worked extensively on a novel technique since the late 1999s/early 2000s. This technique, called the Bag-In-the-Lens (BIL) technique (Fig. 1) uses an IOL of 7.5 mm in total width [3]. The IOL is not implanted in the capsular bag but rather is supported two matching diameter anterior and posterior capsulorhexes in the capsular bag. Recently, an entire book dedicated to this implantation technique has been published [4]. Motivated by the the very encouraging results in eyes of adults, the BIL implantation technique was then applied to younger patients with cataract; first in teenagers and toddlers and later in babies [5, 6].

Fig. 1
figure 1

Bag-in-the-lens implant 8 years postoperative, surgery at 6 months of age

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The results of the clinical outcomes using this lens [7], as well as those published by Swedish [8, 9], German [10, 11], French [12] and Norwegian [13] pediatric cataract surgeons have demonstrated that, if the BIL is correctly implanted,, there is a very low rate of complications and need for secondary interventions. Pediatric cataract is a rare disease with a high potential for visual disability if not corrected on time. It is therefore particularly important to know and use the most suitable technique that allows immediate visual rehabilitation and reduces the need for repeated examinations under anaesthesia. Being the inventors of this technique, our surgical experience in pediatric cataract is primarily based on the use of the BIL. This chapter will therefore focus on our strategy in calculating the IOL power when using the BIL IOL device in childhood cataract.

In addition to the more complex the technical aspects of implanting an IOL in children, particular care should be taken to define the biometrical parameters of the child eye on which the calculation of the power of the IOL will be based. Firstly, it is not easy to obtain reliable measurements of the biometrical parameters of the eyes in young patients. The younger the child, the lesser the reliability. In contrast with adults or older children, in whom optical biometers can be used, small babies or children with an intellectual disability do not have the capacity to fixate long enough to allow capture of measurements. Biometry in these patients must therefore be performed under anaesthesia, using an A-scan for axial measurement and a hand-held keratometry device. For the A-scan, it is not always possible to measure along the exact optical axis of the eye and, when using the contact method (if an immersion-based device is not available) care should be taken not to push on the cornea. Contact measurements depend on the expertise of the surgeon/ nurse/ technician performing them. When using an immersion probe, contact with the eye is avoided but orientation of the measurement can be even more difficult to define. In the authors opinion, the axis of measurement is extremely important, we prefer to use the contact A-scan. In small eyes, the A-scan is frequently unable to automatically recognize the anatomical landmarks. The manual mode, therefore, should be used in these eyes, which might result in a lower degree of measurement accuracy.

When performing keratometry, care should be taken to perform the measurement as well centered as possible and to avoid parallax measurements. In addition, care should be taken that the eye speculum is not exerting any pressure on the eye while measuring. It is also important not to forget about the potential impact of the surgery on these biometrical parameters. A good example is the centripetal forces exerted by retrolental fibrovascular membranes in primary fetal vitreous accounting for the very curved corneas measured preoperatively, which after dissection and cutting will gradually flatten. The degree of flattening can only be generally estimated preoperatively and included in the calculation strategy when choosing the IOL power. Secondly, a child’s eye will continue to grow postoperatively, so the target of the IOL-calculation will have to take this expected growth into account. Some data on eye growth progression are available in the literature though they are not conclusive.

The SRK/T formula, which was considered in the IATS to be one of the more accurate formulas in children [14], is also our preferred formula to calculate the BIL power. We will discuss the different factors that are considered in this formula:

The effective lens position, reflected by the a-constant of the IOL is, in our experience, not extremely different from that of adult eyes, provided there is a normal anatomical development of the eye but as mentioned previously, abnormal anatomical variations are very prevalent in children presenting with cataract. For example, in anterior vitreolenticular interface dysgenesis (AVLID) [15] there might be a clearly defined and intact posterior capsule but a dysgenic anterior hyaloid (or vice versa). Both membranes may also show significant dysgenesis. When these structures that are so crucial in determining the lens position in the eye are abnormal, it is evident that the estimation of the final lens position will be extremely unpredictable and adds to the unpredictability of the balance of the relationship between corneal curvature and axial length growth.

The corneal curvature is steeper in babies. It will flatten in harmony with the elongation of the eye to obtain emmetropia by ageing. If both parameters evolve differently however, it will result in ametropia. As previously mentioned, normal eye growth will cause an increase of the axial length. However, postoperative intraocular hypertension adds to the natural and relatively rapid growth of the eye during the first year of age, which might result in quite important long term postoperative ametropia. It is thus important to differentiate the cause for the eye growth, especially in children younger than 1 year. The growth of the eye also slows down somewhat in the second year of life but will continue to progressively increase until the age of 7 to 10 years, at which normally emmetropia is reached.

It is not yet known whether the growth of pseudophakic eyes follows the same curves as normal eyes. To answer this question and evaluate the degree of evolution in eye growth in the phakic and pseudophakic eyes of children, a registry has been developed under the auspices of the ESCRS-EUREQUO platform. This registry, called the European Registry of Childhood Cataract (EuReCCa) is ready to be launched shortly and the authors of this chapter are actively involved in the development of this EuReCCa registry as such data will be essential in improving outcomes for children after cataract surgery. It is therefore evident that the selection of the IOL power is clearly much more complicated in children than in adults. The choice of the IOL power must be guided by the specific eye characteristics considering the underlying type of cataract as well as the specific age of the child at the time of surgery. Furthermore, in unilateral cases, care should be taken to avoid the induction of large anisometropia to help mitigate the inherent high risk for amblyopia.

The BIL has the enormous advantage over standard lenses in that while it not only prevents PCO (or visual axis re-opacification, VAR, in cases where a posterior capsulorhexis has been performed) it can also be easily explanted and exchanged, even may years after explantation [16]. This major advantage has allowed us to challenge the current paradigm on IOL calculation. However, in our series, BIL exchange has only rarely been required in children so far. When the biometric calculation results in an IOL power of over 30 diopters, we opt to implant a 30D IOL and to correct the remaining refraction error with a contact lens, or spectacles if the refraction error is reasonable, and the child’s nose is sufficiently developed. We have adopted this rule for three reasons: firstly, the thickness of the IOL makes it more difficult to implant in these small eyes, secondly, to avoid myopic overcorrection with time, and thirdly to avoid spherical aberrations that are extremely prominent in high power IOLs and in addition may turn to coma aberrations in case of IOL decentration or tilt. For unilateral cataract in children under 6 years of age, we aim for a light hyperopia, depending on the refraction of the opposite eye. We strive to stay within a 2D difference between both eyes to minimize the amblyogenic anisometropia. For bilateral cataract in children under 6 years of age, we aim for hyperopia, decreasing with age. Over 6 years of age we aim for emmetropia, unless in cases of unilateral cataract with high ametropia where we aim for an aniseikonia of less than 5% between both eyes.

Case illustration

This case is one of a 10-month-old girl who underwent surgery for a unilateral cataract due to posterior lenticonus on the left eye. The axial length was 20.36 mm on the right eye and 19.57 mm on the left eye. The values for keratometry were as follows: right eye K1: 43.00 @ 13° - K2: 45.75 @ 103°, left eye K1: 41.12 @ 168° - K2: 43.12 @ 78°. Refraction of the right eye was + 2.25 (-1.25 @ 16°).

Because of the difference is axial length, a slightly more hyperopic target refraction was chosen for the left eye: + 4.5. Using the SRK/T formula, this resulted in the implantation of a Morcher 89A IOL of + 29.0 Diopters. The remaining refractive error (+4.5D) for the left eye was then corrected with a contact lens and occlusion therapy was started. Three months after the surgery, a switch was made to spectacle correction because the contact lens had to be replaced frequently due to spontaneous loss of the contact lens. Refraction of the left eye at that time was + 4.5 (-2.25 @ 175°). Over time, the hyperopia gradually decreased. At the most recent follow-up, the child was 5 years old and the refraction was + 0.75 (-0.50 @ 179°) on the right eye and + 0.25 (-2.75 @ 171°) on the operated left eye. Corrected distance acuity was 0.63 in both eyes using the Tumbling E-chart. The axial length was 22.25 mm on the right eye and 21.81 mm on the left eye. The values for keratometry were as follows: right eye K1: 42.27 @ 176° - K2: 43.22 @ 186°, left eye K1: 40.92 @ 174° - K2: 43.52 @ 84°.

FormalPara Author’s recommendation

For a detailed overview of the bag-in-the-lens IOL we recommend the book: “Innovative Implantation Technique: Bag-in-the-lens Cataract Surgery” (Springer).