Myopic Shift in Pseudophakic and Aphakic Eyes of Children
Rupal H. Trivedi
M. Edward Wilson
Pediatric eyes have a rapidly developing visual system.1,2,3,4 The growth of different components of the eye is regulated by a process known as emmetropization.5 Alterations of the process of emmetropization after cataract surgery in children are complex and poorly understood. Both active and passive components of the emmetropization process may become altered in these eyes compared with phakic eyes. Increasing axial length (AL) can lead to a myopic shift of refraction in both phakic eyes and eyes operated for cataract surgery. However, in phakic eyes, compensatory changes occur in the optical system making large refractive errors uncommon. For example, to compensate power changes due to increasing AL, lens power declines from 34.4 to 18.8 diopters (D).2 After cataract surgery, in the absence of the crystalline lens, these eyes are not able to compensate through change in lens power, and thus, a large myopic shift of refraction is observed.6,7,8,9 Note that this is a myopic shift of refraction, not necessarily myopia. Postoperative refraction is attributed to the refraction immediately after cataract surgery and the amount of myopic shift since surgery (postoperative refraction = initial refraction + shift of refraction, which is myopic). If the initial postoperative refraction is “0” and the shift of refraction is −5 D, the postoperative refraction would be −5 D. However, if the initial refraction is “+10” and the shift of refraction is −5, the postoperative refraction would be +5. In this case, although shift of refraction is in the myopic direction, the actual postoperative refraction is hyperopic.
A tendency toward axial elongation and a myopic change of refraction after pediatric cataract surgery has been reported.6,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34 Studies on animals and humans have focused on various factors influencing axial growth and refractive change of the eye after cataract surgery. A cautionary note is needed at this point. Studies describing the growth of an eye can be problematic to interpret. Results from different studies are difficult to compare because of inconsistencies in inclusion and exclusion criteria, surgical technique, and length of follow-up. The impact of length of follow-up can be slightly minimized if refractive shift per year is reported as compared with total refractive shift.35 Some studies report growth as axial growth24 and others, as refractive change.15,24,25,26,27,28 Some studies have reported growth as an absolute value (AL in millimeters or refraction in diopters), while others have reported rate of growth (depending on refractive or axial rate of growth—[final refraction or AL minus initial refraction or AL] divided by log of ratio of age at which initial and final refraction or AL are observed).
A word of caution is also needed here, as many of the factors described below may not occur individually. For example, when aphakic and pseudophakic eyes are compared, several factors may confound the interpretation of results. Aphakic eyes may have been operated at a younger age, may have more eyes with bilateral etiology, may have more eyes with poor visual outcome, etc. Some of these factors may have been responsible for the appearance of retardation of growth in pseudophakic eyes. When comparing unilateral and bilateral cataract, again, unilateral cataractous eyes may have been operated at an earlier age (more elongation) and may be more likely to be amblyopic (more elongation), to receive intraocular lens (IOL) implantation, and to have more initial interocular axial length difference (IALD)36 between eyes, etc. These confounders may alter the results when true differences in growth between unilateral and bilateral cataractous eyes are being sought in a retrospective analysis.
It is important to differentiate “refractive change” and “axial growth.” Refraction determines the final optical correction the child needs to use. However, as it reflects not only axial changes but also changes produced by corneal curvature and IOL-related factors (more refractive change per millimeter of growth when a higher power of IOL is implanted), it is not the best way to document and report the growth of the eye. Thus, to understand the growth pattern of an eye, the reporting of axial growth is recommended.
Table 52.1 FACTORS THAT MAY AFFECT GROWTH OF APHAKIC AND PSEUDOPHAKIC EYES | |||||
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Table 52.1 lists factors that may affect the growth of the eye. Some of them are identified to affect growth after cataract surgery in children, while others have been reported in the literature to affect the growth of phakic eyes, which might affect aphakic/pseudophakic eyes. In this chapter, we discuss the available literature on myopic shift in aphakic and pseudophakic eyes.
GENERAL FACTORS
Age of the Child at the Time of Cataract Diagnosis and Cataract Surgery
It is well documented in the literature that the normal phakic human eye undergoes extensive growth in the postnatal period.1,2,3 Larsen1 reported a rapid postnatal growth phase, with an increase in AL of 3.7 to 3.8 mm in the 1st year and a half, followed by a slower infantile growth phase from the 2nd to the 5th year of life, with an increase in AL of 1.1 to 1.2 mm, and, finally, by a slow juvenile growth phase lasting until the age of 13 years, with an increase in AL of 1.3 to 1.4 mm. Longitudinal growth is reported to be minimal after this age. Gordon and Donzis2 noted that the AL increases from an average of 16.8 mm at birth to 23.6 mm in adult life. Although most studies have observed rapid axial growth in infantile eyes, there is no sharp cutoff point when axial growth stabilizes.
It is reasonable to believe that eyes with cataracts follow a similar triphasic curve—before surgery as well as after surgery. However, we have noted that the mean AL of our patients’ cataractous eyes is different (20.52 ± 2.87 mm) from that of the noncataractous eyes in the Gordon and Donzis2 data (21.9 ± 1.6 mm).37 Not only did the mean values differ, but more importantly, the standard deviation was double that of the normal population. Also, the younger the age at the time of measurement, the more the variability of the AL.37
The age at onset and the duration of cataract-related visual deprivation also influence axial growth. Lambert17 reported that age at the time of lensectomy appears to be a critical factor in determining subsequent axial growth in monkeys. When surgery was done at a very early age, retardation of axial growth occurred compared to the normal eye. However, this retardation was not seen with lensectomy in slightly older monkeys. Below we review the available literature on the influence of age on the axial growth of aphakic/pseudophakic eyes.
Axial Length Studies
Flitcroft et al.14 reported a mean increase in AL of 3.41 mm in congenital cataracts (<1 year) versus 0.36 mm in developmental cataracts (>1 year) at mean follow-up of 2.7 and 2.86 years, respectively.
Vasavada et al.38 noted that the rate of axial growth (RAG) in children when operated at ≤1 year of age (23.5%) was significantly higher than in those operated between 1 and 3 years (4.8%; P = 0.0001) and 3 to 10 years old (4.3%; P = 0.0001). In children operated at ≤1 year of age, the temporal profile of RAG was higher in the first 2 years after surgery. Temporal profile of RAG was the difference between two consecutive mean ALs with respect to the previous reading.
Fan et al.39 reported that although eyes operated before 6 months of age have shorter preoperative AL compared with eyes operated between 7 and 12 months of age (18.9 mm versus 20.3 mm), 3-year follow-up AL was longer in infants operated before 6 months of age (22.7 mm versus 21.2 mm).
We40 reported AL elongation even during the second decade of life. Globe AL was 23.4 mm (mean age 11.5 years) at the initial measurement and 23.9 mm at the last measurement (mean age 15.2 years).
Refractive Error Studies
Moore15 noted that the refractive error of the aphakic eye of patients treated for a unilateral congenital cataract decreases most rapidly during infancy and less rapidly during the next few years of childhood.
McClatchey and Parks6 reported that the average refraction tended to follow a logarithmic decline with age. The average rate of myopic shift was −5.5 D. A stepwise regression analysis showed that age at surgery had a small but significant effect on rate of the growth. Much of the observed myopic shift in aphakic eyes is due to normal growth of the eye.
McClatchey and Parks7 used aphakic refraction at last follow-up to calculate the final pseudophakic refraction, and these values were compared with the prediction of a logarithmic model of myopic shift. They reported a median calculated pseudophakic refraction at last follow-up of −6.6 D, with a range of −36.3 to +2.9 D. Children who underwent surgery in the first 2 years of life had a substantially greater myopic shift (11.9 D) than did older children (4.7 D) and a larger variance in this myopic shift. The logarithmic model accurately predicted the final refraction within 3 D in 24% of eyes undergoing surgery before 2 years of age and in 77% of eyes undergoing surgery after this age.
Dahan and Drusedau13 reported an average elongation of 19% for age <18 months and 3.4% for age >18 months.
Enyedi et al.29 reported that children operated on at ages 0 to 2, 2 to 6, 6 to 8, and >8 years had refractive shifts of −3.0, −1.5, −1.8, and −0.38 D, respectively (2.5, 2.5, 3.0, and 1.8 years postoperatively). The authors noted a statistically significant difference in the average total change in refraction between the youngest age group (0-2 years) and the oldest age group (>8 years).
Plager et al.26 reported that children operated on at ages 2 to 3, 6 to 7, 8 to 9, and 10 to 15 years had mean myopic shifts of −4.60, −2.68, −1.25, and −0.61 D, respectively (5.8, 5.3, 6.8, and 5.7 years postoperatively).
Crouch et al.10 reported that children operated on at ages 1 to 3, 3 to 4, 5 to 6, 7 to 8, 9 to 10, 11 to 14, and 15 to 18 years had mean myopic shifts of −5.96, −3.66, −3.40, −2.03, −1.88, −0.97, and −0.38 D, respectively, with an average follow-up of 5.45 years.
