Visual axial opacity clearing
VAO clearing was reported in 5 studies11,22,34,37, and the I2 was 23%. As shown in Figure 9, the primary IOL had a significantly higher incidence of VAO clearing compared with primary aphakia, [OR=9.33, 95%CI: (5.21, 16.73), p=0.27].
Discussion
The present meta-analysis demonstrated that the postoperative VA in primary IOL group was better than that in primary aphakia group. These results were consistent with previous studies. Birch et al showed that IOL may support better VA development when a cataract was extracted after age 1, while the difference would taper off with the age reaching 4 years39,40. Vasavada et al22. reported that more infants in primary IOL group had documentable VA than aphakia group during early postoperative follow-up, and this trend continued until age 5. This result indicated that the visual rehabilitation was faster in pseudophakia group, especially in early postoperative follow-up. However, in the IATS study, the VA was similar in IOL group and contact lens group both in the first (1 year old)6 and last follow-up (4.5 years old)11. This could be explained by better compliance of contact lenses or spectacles in the aphakia group. The IATS study offered free contact lenses and spectacles for the enrolled subjects. Moreover, trained personnel assessed the vision and monitored the compliance through regular home visits. 41 This may not be carried out in the real world, particularly in developing countries. Actually, constant optical correction during this crucial period is very important for children. A faster visual rehabilitation will impact the activity and overall functional development of the children. Therefore, primary IOL implantation may provide better VA for pediatric patients, especially those under age 5.
Our meta-analysis did not see any difference in glaucoma incidence. Many hypotheses proposed the mechanisms of glaucoma after infantile cataract surgery. However, there is no unanimous conclusion. The mechanisms of secondary glaucoma of pseudophakia eyes and aphakic eyes might be different. The filtration angle of infants is susceptible to postoperative inflammation. Thus, IOL implantation in infants will cause chronic inflammations that induce changes of iris root or trabecular meshwork (TM)42-44. This might explain the glaucoma in pseudophakia eyes. As for aphakic glaucoma, since the mechanical absence of lens in the eye, TM cells would be exposed to the lens epithelial cells (LEC). Michael et al. specified that LEC induced changes in TM cells that resembled changes in primary open-angle glaucoma45,46. It is worth noting that the mechanism of glaucoma is still unclear. But we could not make any attempt to investigate the mechanisms of secondary glaucoma after infantile cataract surgery in this meta-analysis.
Analysis of unilateral subgroup indicated there were much fewer infants with strabismus in primary IOL group compared with primary aphakia group. This was consistent with the studies by Autrata et al31. and Lambert et al. Previous studies also suggested that the incidence of strabismus in IOL group was relatively low47-49. The absence of natural lens in unilateral eye will lead to anisometropia and aniseikonia, which are related to secondary strabismus following congenital cataract surgery. Primary IOL implantation offers a stable retinal image with minimal aniseikonia as well as full-time optical correction for surgery eye. Moreover, children have to wear contact lenses or spectacles to get optical correction in case of absent lenses. The compliance of wearing contact lenses will be affected by the complications associated with contact lenses like corneal infection. Hence, this might explain the higher incidence of strabismus in primary aphakia group.
Postoperative VAO is very common in infants after congenital cataract surgery. According to the previously published literatures, IOL implantation is associated with higher VAO incidence in congenital cataract children, especially those younger than 6 months50-52. The average incidence of VAO after IOL implantation is 44.0%, while the incidence is up to 80% when the patient is younger than 6 mouths53. The IOL in capsular bag acts as an obstacle and prevents the anterior and posterior capsule leaflets from fusing, while the capsule edges will seal more effectively in aphakic eye. In our meta-analysis, primary IOL group had a noticeably higher incidence of VAO clearing than primary aphakic group, which was consistent with the RCT study conducted by IATS and other previous studies.11,34,54,55.
Our meta-analysis has some limitations. First, even though this meta-analysis is the first systematic review on this topic, it included retrospective studies rather than just RCTs. Thus, the level of evidence will be weakened by the inclusion of retrospective studies. However, there is no denying that it is very difficult to conduct RCT among paediatric cataract children. Second, the pooled effect of complication, especially the glaucoma incidence, might be affected by the inconsistent diagnostic criteria adopted in these studies. Third, the control group of these included studies was either primary aphakia followed by contact lens or primary aphakia followed secondary IOL implantation. Third, given the number of patients in each study is relatively small, we could not draw explicit conclusion on the difference in postoperative visual performance.