Ctrl + Shift + IOL
Controlling for myopic shift following cataract surgery in infants: too many variables?
Roisin McGuigan |
Pediatric cataract surgery poses its own special challenges – the eye is still growing, with the cornea and crystalline lens flattening at the same time as axial elongation occurs. So when you operate to remove a cataract and implant an IOL, the long term results can be difficult to predict. It can be especially difficult in younger children – the eye undergoes 3–4 mm of axial elongation before a child is one year old, meaning young infants require a very different approach to older children. Calculating IOL power is tricky, as the amount of myopic shift can vary significantly as the eye grows, and there is currently no agreement on just how much doctors should undercorrect vision by when implanting IOLs in infants.
The Infant Aphakia Treatment Study Group sought to better understand the best approach by studying myopic shift in a group of infants with unilateral congenital cataract, who were treated with primary IOL implantation or contact lenses. The team studied 43 eyes of infants who underwent unilateral IOL implantation at one to six months, and followed them from the time of cataract surgery to the age of five. They found that myopic shift followed a piecewise, linear relationship; the most rapid shift occurred in the first year and a half of life (mean of 0.35 D/month) before slowing after this age (mean of 0.08 D/month) (1).
None of the characteristics the group measured – including age at cataract surgery, IOL power, and axial length – affected the rate of the shift. However, only a small percentage of eyes showed the myopic shift the researchers predicted, and only ~25 percent were within a diopter of the expected change, demonstrating that accurate prediction is extremely difficult.
So what can be done to offset this effect? For their study, the authors had a goal of emmetropia at five years – so postoperatively, they used hypermetropic targets of +8 D in children aged four to six weeks, and +6 D for children aged seven weeks to six months. When the children studied reached five years old, the mean refractive error was calculated at -2.5 D – suggesting that to have a better chance of achieving emmetropia, an additional 2.5 D of postoperative hypermetropia may help to more accurately compensate for myopic shift. But they also offer a word of caution: there are many factors to be considered, such as refractive error in the fellow eye, and other conditions such as glaucoma. They conclude that although targeting an extra 2.5 D might be a beneficial approach, “the variability in myopic shift among patients will continue to result in unanticipated anisometropia at later ages.”
- DR Weakley et al., “Myopic shift 5 years after intraocular lens implantation in the infant aphakia treatment study”, Ophthalmology, [Epub ahead of print] (2017). PMID: 28215452.