Hyperopia is much more amblyogenic than myopia.
18 Hyperopic anisometropia greater than 1.00 to 1.50 D is considered to produce anisometric amblyopia, and hyperopia greater than 4.00 to 5.00 D is generally thought to induce refractive amblyopia
14,15; this study used these cutoffs to calculate the period of amblyogenic hyperopia. We anticipated that temporary piggyback IOL implantation for congenital cataract surgery would enable the patient to avoid a hyperopic period, minimize amblyopia, and ultimately improve visual acuity. However, this study found that temporary piggyback implantation yields worse visual outcomes than single IOL implantation in congenital cataracts and is associated with higher frequencies of other ocular comorbidities and additional surgery.
In our study, the polypseudophakia group had inferior visual acuity in bilateral congenital cataract cases, while there was no difference in unilateral cataracts. Because unilaterality is a powerful prognostic factor for worse visual outcome,
19,20 it might have affected visual acuity to be poorer and less variable, thus making it less likely that statistical significance could be attained. In addition, the sample size for unilateral congenital cataract was small, so it was difficult to reveal differences; larger samples might have been able to present differences in unilateral congenital cataract.
The additional surgery frequency was significantly higher in the polypseudophakia group compared with the monopseudophakia group. The main causes of additional surgery in the polypseudophakia group were posterior synechia, ILO, and glaucoma. None of the cases in the monopseudophakia group had these complications. Wilson et al.
10 reported that piggyback patients and single IOL patients had comparable rates of anterior segment complications that required additional surgery (26% and 22%, respectively). Complications in piggyback patients consisted of IOL tilting, IOL capture in the pupil, and posterior cortex reproliferation.
10 Differences of anterior segment complication rate were exacerbated in our study compared with the previous study, possibly due to our longer follow-up period.
ILO has been reported frequently after implantation of piggyback IOL in adults
21,22 and is thought to be associated with two IOLs in the bag. We expected that ILO could be avoided as long as the piggyback IOL was located in the ciliary sulcus as hypothesized by Wilson et al.,
10 and therefore performed cataract surgery according to the method described. However, despite meticulous capsular polishing, pearl-type ILO developed in 4 of 32 (12.5%) of the polypseudophakic cases. This result is inconsistent with the result of Wilson et al.,
10 in which none of the cases had ILO postoperatively.
Because glaucoma is a well-documented and serious complication after congenital cataract removal, all patients in the study were monitored closely for development of glaucoma during the follow-up period. According to a previous study, eyes with primary IOL implantation have a lower risk of developing glaucoma compared with eyes left aphakic.
23 However, retrospective reports might be biased and thus inaccurately conclude that IOL has a protective effect.
24 In a recent prospective randomized clinical trial (IATS 5-year outcome study), the incidence of glaucoma was similar in groups with primary IOL implantation and in those left aphakic.
9,12,13 The current study revealed that the polypseudophakia group had a higher rate of glaucoma-related adverse events (40.6%) compared with the monopseudophakia group (14.3%), and this is as expected, because postoperative elevation of IOP, pigment dispersion syndrome, and secondary glaucoma after piggyback IOL implantation have been reported in adults.
25,26 However, the difference of glaucoma-related adverse event was not significantly different in the two groups (
P = 0.086, effect size = 0.550). With the sample size of our study, large effects of at least 0.89 can produce a statistical power greater than 80%; we believe that more samples would have proven the significance. Due to the retrospective nature of our study and the lack of gonioscopic and sonographic angle evaluation, it is not possible to specify the exact mechanism of glaucoma development. Further studies are warranted to examine this point. To this end, we are planning further patient follow-up that includes angle evaluation.
The rates of reoperation and adverse events were lower in this study than in the IATS. Overall development rates of glaucoma and glaucoma-related adverse events in our study were 5.0% and 28.3%, respectively. In the IATS 5-year outcome study, the IOL implantation group had a 19% glaucoma rate and a 28% glaucoma-related adverse event rate. The rate of glaucoma-related adverse events in our study is comparable to that of the IATS, but the glaucoma rate is lower. Regarding additional surgery, in addition, the IOL implantation group in the IATS had a 72% reoperation rate, whereas we demonstrated a reoperation rate of 30.0%. We speculate that these lower glaucoma and reoperation rates are due to age differences of participants and the retrospective nature of our study. The IATS only enrolled infants younger than 7 months of age, whereas the mean age of the children in our study was 11 months at the time of surgery. Because the risk of complications is known to be higher in young infants, difference of baseline age may have produced lower rates of complications and associated additional surgeries in the current study. In addition, regarding glaucoma, our study was a retrospective chart review, and axial length and corneal diameter were not routinely examined, so possible glaucoma cases might have been categorized as glaucoma suspect rather than glaucoma.
The amblyopic hyperopic period was significantly longer in the monopseudophakia group than in the polypseudophakia group during the critical period of visual development. However, somewhat paradoxically, the final vision of the polypseudophakia group was worse. This might have been produced by several factors. First, the higher rates of anterior segment complications and visual axis opacification such as synechia or ILO could have interfered with image focusing and aggravated the development of amblyopia in the polypseudophakia group. Second, although this hypothesis has not been proven, polypseudophakia might have produced severe higher order aberration that affected the quality of vision and eventually deteriorated into amblyopia. Because it is almost impossible to exactly align the anterior temporary IOL and the posterior permanent IOL at the central axis point, temporary polypseudophakia is more likely to induce higher order aberration compared with monopseudophakia. In addition, the monopseudophakia group was aggressively treated with glasses for amblyogenic refractive error, so they might have had less amblyopia with better visual outcomes than expected.
To the best of our knowledge, this is the first study to report long-term outcomes of temporary piggyback IOL in congenital cataract. The strengths of this study are the long follow-up period, the similar baseline characteristics of the eyes in each group, and the uniformity in surgical procedures, as evidenced by the uneventful surgeries of all cases. Because the study shows long-term clinical outcomes and considering that ages at the last visit in both groups were above 10 years, the visual acuity and refraction of our study are accurate and reliable. A previously reported comparative study of polypseudophakia and monopseudophakia for congenital cataract followed patients for only 2 years and was unable to accurately analyze visual acuity due to the young age of the study participants; this merits deeper study.
The current study had certain limitations. First, the study was designed to be retrospective. Hence, our study is prone to compounding factors, such as indication, time, and surgeon experience; this is an inherent limitation of this type of study. However, the baseline characteristics of age at diagnosis, age at operation, preoperative axial length, and keratometric value were not significantly different in the two study groups; this at least suggests that the indication of surgery did not greatly differ between those two groups. Additionally, the two groups did not seem to be different regarding time and surgical expertise; the median date of cataract surgery was November 5, 2004 in the monopseudophakia group and September 8, 2005 in the polypseudophakia group. Hence, we believe that surgeon experience for these procedures would not have significantly influenced the differences of surgical outcomes between the two groups. Another limitation is the small sample sizes of the study groups. Even though we included the largest number of piggyback IOL implantations for congenital cataract reported in the literature, the sample size was still relatively small. This, in addition to the retrospective nature of the work, small sample size is a limitation to interpretation, and future prospective studies with larger sample sizes will be required to confirm the results.
In conclusion, our study suggests that temporary piggyback IOL implantation in patients with congenital cataract yields worse final BCVA, more additional surgeries, and a higher rate of glaucoma development compared with single IOL implantation. Temporary polypseudophakia does not have benefit in congenital cataract and single IOL implantation.