

REVIEW ARTICLE 

Year : 2018  Volume
: 56
 Issue : 4  Page : 232236 

Pediatric intraocular lens power calculation
Sandra Chandramouli Ganesh, Shilpa G Rao, Farhadul Alam
Department of Pediatric Ophthalmology and Strabismus, Aravind Eye Hospital, Coimbatore, Tamil Nadu, India
Date of Web Publication  19Feb2019 
Correspondence Address: Sandra Chandramouli Ganesh Department of Pediatric Ophthalmology and Strabismus, Aravind Eye Hospital, Coimbatore, Tamil Nadu India
Source of Support: None, Conflict of Interest: None  1 
DOI: 10.4103/tjosr.tjosr_105_18
Pediatric cataracts pose multiple challenges in terms of management and postoperative rehabilitation. Difficulties in obtaining accurate measurements for axial length and keratometry are encountered due to poor cooperation in children and instrumentation errors. There exist multiple formulae for intraocular lens (IOL) power calculation, which are based on various factors and have varying degrees of accuracy. Children exhibit a tendency for myopic shift due to the anatomical differences from adult eyes and due to the growth of eyeball, as a result of which they require undercorrection, when IOL implantation is planned.
Keywords: Intraocular lens, intraocular lens power calculation formulae, myopic shift, pediatric cataract, undercorrection
How to cite this article: Ganesh SC, Rao SG, Alam F. Pediatric intraocular lens power calculation. TNOA J Ophthalmic Sci Res 2018;56:2326 
Introduction   
Pediatric cataract affects around 200,000 children worldwide, with an estimated prevalence ranging from three to six per 10,000 live births,^{[1]} accounting for 12% (range: 7%–20%) of preventable blindness^{[2]} in children; their treatment, by surgical removal of the lenticular opacity, is of paramount importance, as failure to do so results in irreversible visual handicap. Visual impairment produces an adverse impact on the scholastic performance of the child and his or her professional abilities and quality of life, which translates into further economic loss and social burden.
Multiple challenges are encountered by the surgeon in the management of pediatric cataract. In addition to being technically exacting, there exists a fair possibility that functional or visual rehabilitation maybe suboptimal following a meticulously performed surgery producing a good anatomical outcome.
Several perioperative factors may be responsible for the above phenomena of early and delayed refractive surprises. Difficulty in obtaining precise measurements in children with respect to corneal curvature, anterior chamber depth (ACD), and axial length (AL) both in terms of cooperation and accuracy of instruments used can cause errors in intraocular lens (IOL) power calculation. This is further confounded by the fact that IOL power prediction formulae in common usage today are based on the theoretical models or regression from normative data from adult eyes. The application of these formulae to pediatric eyes may not hold true for all biometric aspects in children.^{[3]} Pediatric eyes are expected to behave differently compared to the adult eyes owing to postoperative growth in the size of the pediatric eyeball, and with an IOL of constant power implanted in the eyes, there exists a higher possibility of myopic shift in pediatric pseudophakes and chance of delayed refractive surprises.^{[4]}
Instrumentation   
Measurement acquisition in pediatric biometrics is challenging because young children are not cooperative, and a few of the measurements may need to be taken under general anesthesia, immediately after induction and before insertion of the eye speculum. Among the twin parameters of AL and keratometry, it has been noted that errors in AL estimation have a greater impact on power calculation and the final refractive outcome as compared to erroneous keratometric readings. Inaccurate keratometric values cause errors of 0.8–1.3 diopters in both adults and children. Inaccurate AL measurement can account for 3–4 diopters of error for each millimeter difference in IOL power in adults and 4–14 diopters or higher in pediatric eyes.^{[5]}
Instruments using partial coherence interferometry such as IOLMaster (Carl Zeiss Meditec, Jena, Germany) and the Lenstar (HaagStreit AG, Koeniz, Switzerland) are used in older children and younger adults. Measurement of AL under anesthesia is performed with Ascan ultrasound biometry, using either applanation or immersion techniques. Applanation involves holding the probe in contact with the cornea which may induce measurement error in the form of shorter AL and ACD measurements due to corneal compression, leading to incorrect IOL power calculations. Immersion Ascan uses a coupling fluid between the probe and the cornea to reduce indentation and has been shown to be more accurate than the applanation method^{[6]} and in fact is considered the gold standard.
Keratometry and errors in its measurement produce a comparably less unfavorable outcome on the final IOL power. Handheld keratometry can be used under anesthesia in young children, but results may be flawed owing to the lack of fixation and centration. Obtaining multiple readings of the same and recording the average reading are considered helpful in overcoming the error.
Formulae for Intraocular Lens Power Calculation   
Since their origin in the 1950s, formulae for IOL power calculation have been subject to constant evolution. There exist two basic kinds of formulae: theoretical, determined by application of geometrical optics to the schematic and reduced eyes using various constants, and regression, using the actual postoperative results of implant power as a function of the variables of corneal power and AL or formulae which include a combination of both of the above. Various parameters, such as net corneal power, AL, effective lens position, and vertex distance, are involved in the determination of implant power and expected postoperative refraction.
Sanders, Retzlaff, and Kraff developed SRK formula, which was the most widely used formula for a long duration. Following this, various changes were suggested and practiced taking into account the effective lens position. Holladay, Holladay 2, Hoffer Q, SRK/T, or Haigis were derived as a result of the newer modifications that were made to the existing formulae.
Existing formulae are mainly derived from studies on adult eyes and are known to be accurate over a range of ALs between 22 and 26 mm. Data regarding accuracy based on AL are shown in [Table 1].  Table 1: Recommended intraocular lens power calculation formulae as per axial length
Click here to view 
Application of the above data to pediatric eyes is seen to produce inconsistent outcomes and conflicting results exist between studies conducted by various authors. Apart from inaccurate measurements, other possible sources of error in short eyes are related to a steep cornea, shallow ACD, short ALs,^{[7]} dense cataracts which may influence the final measurement to a greater extent in shorter eyes,^{[8]} and denser vitreous which may reduce ultrasound transmission and hence affect the results.^{[9]} Pediatric IOL calculator^{[10]} is a computer program using the Holladay 1 algorithm and pediatric normative data for AL and keratometry readings as established by Gordon and Donzis.^{[11]} It aims to calculate the postoperative pseudophakic refraction of a child during the immediate postoperative period and later to predict the refractive change as the child grows.
Most of the literature available for pediatric eyes consists of retrospective studies and not many measure refractive changes occurring overtime. Mezer et al.^{[12]} evaluated the refractive outcome in the postoperative period in 49 patients using two regression formulas (SRK and SRK II) and three theoretical formulas (Holladay 1, Hoffer Q, and SRK/T). Children of 6–7 years' age group at the time of surgery were included and mean difference between the predicted and actual postoperative refractions with all formulas ranged from 1.06 to 1.2 diopters. They concluded that all of the five IOL power calculation formulas were unsatisfactory in achieving target refraction.
In a retrospective case series conducted by Nihalani and VanderVeen^{[13]} in 2011, 135 eyes that underwent uncomplicated pediatric cataract surgery with IOL implantation using formulae SRK II, SRK/T, Holladay 1, and Hoffer Q, prediction error (PE) (PE = predicted refraction – actual refraction) was calculated and compared among the above formulae. It was seen that though in cases where PE was insignificant (<0.5), it was similar for all the formulae; among those cases (PE >0.5), Hoffer Q was most predictable of the formulae, while the others tended to produce an undercorrection. They also reported that there was a trend toward greater PE in the eyes of younger children (<2 years), shorter AL (AL <22 mm), and steeper corneas (mean K >43.5 diopters [D]). As pediatric eyes are shorter, it is expected that the Hoffer Q formula would have better accuracy in these eyes as it was formulated for shorter ALs. Hoffer had reported a greater accuracy of Hoffer Q formula as compared to SRK/T^{[14]} in adults with ALs >22 mm and Holladay 1 and Holladay 2,^{[15]} but these results were based on studies done in small populations and their reliability was called into question. Subsequent studies by Gavin and Hammond^{[16]} found Hoffer Q to be of greater accuracy than SRK/T in smaller eyes, whereas MacLaren et al.^{[17]} suggested that both Hoffer Q and Haigis performed equally well for these eyes. Neely et al.,^{[18]} though, reported that among the youngest group of children with ALs <19 mm, SRK II regression formula gave the least amount of variability, whereas the Hoffer Q gave the greatest.
Holladay 2 formula uses additional factors such as whitetowhite corneal diameter, ACD, age, and lens thickness. Holladay 2 formula was compared with that of the Holladay 1, Hoffer Q, and SRK/T formulas by Trivedi et al.^{[19]} and found to have the least PE specifically for the subgroup of the eyes <22 mm in length, following which the authors concluded that Holladay 2 formula can be reliably used despite the lack of preoperative refraction.
Further studies evaluating the PE with different IOL power calculation formulae produced inconsistent and mixed results.
Vasavada et al.^{[20]} conducted that an observational case study on 117 eyes of patients, of an average of 2 years of age, compared the PE for refractive outcome for each formula and they were evaluated based on initial AL. They proved with statistically significant data that SRK/T and Holladay 2 had the least PE in pediatric eyes. Personalizing the lens formula constant reduced the PE significantly for all formulae except Hoffer Q. In eyes AL <20 mm, SRK/T and Holladay 2 gave the best PE. These conclusions were also supported in a comparative case series by Vanderveen et al.,^{[21]} which showed that Holladay 1 and SRK/T gave equally good results and had best predictive value for infant eyes.
O'Gallagher et al.^{[22]} evaluated children under the age of 8 years undergoing cataract surgery with IOL implantation and compared Hoffer Q, Holladay 1, SRKII, and SRK/T in a small sample of patients and noted that mean absolute error was lesser in SRK/T, which is in agreement with results of Vasavada et al. They differ from findings of Vasavada et al. in that they found values obtained using Hoffer Q to have improved accuracy.
IOL power calculation in children of age lesser than 2 years is especially challenging. Retrospective case series done by Kekunnaya et al.^{[23]} compared 128 eyes of 84 children for SRK II, SRK/T, Holladay, and Hoffer Q. They found that the absolute PE tended to remain high with all the formulae, but it was significantly lesser with SRK II than with other formulae. PE with SRK II formula was not affected by any factor such as age, keratometry, or AL. AL influenced the absolute PE with Holladay and Hoffer Q formulae. Mean keratometry influenced PE with SRK/T formula. Children <2 years of age comprise only a subgroup, and most of the studies are underpowered to detect statistically significant differences. This age shows a rapid elongation of the eyeball and flattening of the cornea, thereby causing a significant myopic shift.
As prevalent practices and observations stand now, and in face of rapidly changing techniques and lack of consensus regarding preferred IOL formulae in children, there exists the need for improved IOL power calculations in pediatric cataract surgery, especially in children of younger age and smaller eyes.
Need for Undercorrection   
The prediction of longterm refractive outcomes among pediatric pseudophakes remains one of the biggest challenges in the management of pediatric cataracts. Growth of the eyeball and changing curvature of the eyeball produce a tendency for myopic shift.^{[24],[25]} Hence, an undercorrection is usually planned at the time of surgery, and the residual refractive correction is provided by means of contact lenses or glasses. Another school of thought advocates planning an initial small undercorrection or emmetropia to allay the possibility of an initial hypermetropia which might in itself be amblyogenic. They propose to correct the phenomenon of an expected myopic shift by a possible IOL exchange or refractive surgery later in life.^{[26]}
In spite of the lack of comparative studies between the above two approaches, undercorrection at the time of IOL implantation remains the widely accepted and practiced approach. Among the proponents of undercorrection, two of the most popular (guidelines were those proposed by Enyedi et al.^{[25]} and Dahan and Drusedau.^{[27]} Dahan proposed implantation of IOL which is 20% less than the emmetropic IOL power for children <2 years of age and 10% less for children >2 years of age, to allow for myopic shift occurring during the emmetropizing process. Enyedi proposed what is popularly known as “the rule of 7,” where the sum of postoperative refractive goal and age of the child is 7, and target refraction is decided accordingly: +6 for a 1yearold, +5 for a 2yearold, +4 for a 3yearold, +3 for a 4yearold, +2 for a 5yearold, +1 for a 6yearold, plano for a 7yearold, and −1–−2 for patients >8 years of age [Table 2] and [Table 3].  Table 2: Desired postoperative target refraction for different age groups according to Enyadi et al.
Click here to view 
 Table 3: Desired postoperative target refraction for different age groups according to Trivedi and Wilson
Click here to view 
Chen^{[26]} and companions recommended matching of the IOL power based on the spherical equivalent of the other eye (children 2–4 years, 1.25 diopters less power than SE of the fellow eye) and, in children >4 years of age, match the spherical equivalent refraction of the fellow eye.
Multiple recommendation tables have been published by experts and pediatric ophthalmologists to guide surgeons in selecting IOL power in children, including those by Plager et al.^{[28]} and Trivedi and Wilson.^{[29]}
In our practice, we follow guidelines set out by Trivedi and Wilson.
A retrospective observational study conducted by Sachdeva et al.^{[30]} on 84 eyes of 56 children who had undergone undercorrection according to Enyedi et al. and were followed up in the long term showed that most children achieved an acceptable final refractive error. Myopic shift was seen highest in the youngest age group (0–2 years of age) and least in 4–7 years of age group. Multivariate analysis suggests that the most important factor which might influence the results is the age at surgery, with reduction in error by 0.31 with every passing year.
Apart from a fixed undercorrection, some authors like McClatchey^{[10]} aimed to develop a computerbased software to predict the refractive error of the children undergoing cataract surgery in both aphakia and pseudophakia, called the pediatric IOL calculator. He also proposed a table outlining expected postoperative target refraction based on the age.^{[31]} Pediatric IOL calculator^{[11]} was openaccess computer program which was written for Windows and was based on average eyes and refractive changes, and this did not entertain the “outliers.” It calculates the initial refraction planned based on the keratometry, AL and IOL, and the Holladay formula and was found to give good predictions in initial trials in pseudophakic children and older children.
Jasman et al.^{[32]} conducted a comparative study of 31 eyes (24 patients) among children under 12 years of age that underwent cataract surgery and IOL implantations. Patients were randomized into two groups: SRK II group and pediatric IOL calculator group. At the end of 3month followup, no statistically significant differences were found in PE and accuracy of predictability of postoperative refraction between the two groups, hence proving the IOL calculator as a new tool in predicting refractive outcomes. Nevertheless, caution has to be exercised while using the same as very few studies regarding the pediatric IOL calculator are available and the above study is based on a very small sample size.
Myopic Shift   
The phenomenon of myopic shift is often discussed in the context of age at surgery, initial AL, and laterality of the cataract.
A retrospective cohort study by Valera Cornejo and Flores Boza,^{[33]} over a period of 3 years following cataract surgery and IOL implantation, demonstrated no statistically significant difference between initial AL and myopic shift. The same study showed a greater tendency toward myopic shift in the eyes with bilateral cataract and shorter initial AL; these findings were in support to a study by Trivedi and Wilson.^{[34]} It also demonstrated a significant relation between laterality and the shift, with statistically significant occurrence in unilateral eyes as compared to bilateral ones. Kora et al.,^{[35]} Vasavada et al.,^{[36]} and Hoevenaars et al.^{[37]} concurred with the above findings in independent studies. Many authors have reported that the greatest myopic shift occurs in the early years of life, at younger than 2 years of age.
Children enrolled in the Infant Aphakia Treatment Study who underwent IOL implantation were evaluated for the refractive changes at 5 years of age. It showed that the rate of myopic shift occurs most rapidly during the first 1.5 years of life. It was suggested that for a goal of emmetropia by 5 years, then the immediate postoperative hypermetropic targets should be +10.5 D at 4–6 weeks and +8.50 D from 7 weeks to 6 months.^{[38]}
Another surgical innovation proposed by Wilson et al.^{[39]} is the concept of pediatric polypseudophakia or the pediatric piggyback IOL, which involves an in the bag placement of IOL along with a second “piggyback” lens in the sulcus, both of which may be calculated using a piggyback pediatric IOL calculator.^{[40]} As the child attains emmetropia or myopic shift, the piggyback IOL, accounting for 20% of the total power, can be removed,^{[41]} to provide a more emmetropic refraction. Other technological breakthroughs such as mechanically adjustable or light adjustable IOLs which are being tested among adults have not yet come into pediatric practice.
Rehabilitation   
Socioeconomic situation, educational status, and parental concerns are factors to be taken into consideration while planning rehabilitation in pediatric pseudophakes and attempts must be made to involve parents and provide firsthand knowledge of various aspects pertaining to the child's wellbeing such as use of glasses, amblyopia therapy if needed, and need for regular visits to the doctor. The choice of spectacles or contact lenses may be given to the parents, but spectacles are advocated in most cases in view of safety.
In the age of newer and evolving instrumentation and scientific techniques and IOL often times being implantated in younger children; studies on the ideal postoperative refractive state and novel approaches to a dynamic refractive solution appear to be need of the hour.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]
