Calculating IOL power for children

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Determining the optimal IOL power for children is as much an art as a science.

Dr. Trevidi

By Fred Gebhart; Reviewed by Rupal Trivedi, MD MSCR

Charleston, SC-Implanting an intraocular lens has become a more common practice during pediatric cataract surgery, but while the surgical management of pediatric cataract has improved, the accuracy of IOL power calculations has not shown similar improvement.

“Selection of an IOL power is one of the major challenges for the long-term care of children undergoing cataract surgery,” said Rupal Trivedi, MD MSCR, research associate professor, Storm Eye Institute, Medical University of South Carolina, Charleston. “Many of the late refractive surprises are attributed to a myopic shift in refraction from axial eye growth, however, early refractive surprises can be attributed to inaccuracy in IOL power calculations.”

The ultimate goal of pediatric IOL power selection is to provide a manageable course of refraction between IOL implantation and adulthood with the best possible adult visual acuity.

The axial length measurement is the single most important factor in making an accurate IOL power calculation regardless of patient age, Dr. Trivedi said, but accurate axial measurement is far more important in children than in adults.

In adults, every millimeter of error in axial length measurement translates into approximately 2.5 D error. In children-who typically have short eyes of 20 mm or less-every millimeter of error in axial length translates into approximately 3.75 D error. That difference in effect has a direct impact on how axial length is measured.

Immersion ultrasound measurements are standard-of-care in adults, Dr. Trevidi said. Ophthalmic surgeons recognize that physical contact between the ultrasound probe and the cornea can result in corneal depression and an artificially shortened axial length measurement.

Diving deeper

Multiple studies have shown that immersion A-scan measurement eliminates corneal compression and produces more accurate axial length measurements compared to contact biometry.

An informal survey of pediatric ophthalmologists found that 82% use the contact ultrasound method to measure pediatric eyes.

Dr. Trivedi reported data from a prospective study that compared contact and immersion ultrasound and the resulting IOL errors. Axial length measurements using contact ultrasound were 0.27 mm shorter compared to the same eyes measured with immersion ultrasound (p < 0.001).

The resulting IOL power needed for emmetropia was significantly different, with 28.62 D for contact and 27.63 for immersion (p < 0.001).

“If axial length measure by the contact technique is used, it would have resulted in the use of an average 1 D stronger IOL power than is actually required,” Dr. Trivedi said. “This can lead to induced myopia in the postoperative refraction. If possible, use immersion A-scan . . . (and) use the measurements from the scan with the best waveform and the highest peaks with a perpendicular retinal spike.

“If you must use applanation biometry, rely on the measurement with the greatest anterior chamber depth,” she said.

Guidelines and recommendations

Like adults, manual keratometry can be used in children to obtain K-value. However, for most children, K-value is obtained under anesthesia using handheld keratometer.

One of Dr. Trivedi’s recommendations is to take K-value as soon as possible after the induction of anesthesia and immediately following IOP measurement to avoid corneal dryness.

A handheld keratometer is not the preferred tool for axis measurements, she said, but the literature suggests that it is a practical tool to obtain an acceptable K measurement. The measurement should be taken without the use of an eyelid speculum.

Dr. Trivedi suggested instilling a balanced salt solution to maintain a smooth corneal surface during measurement. Each eye should be measured twice as a way to improve accuracy, and the two readings averaged to obtain the final K-number.

The two K readings should be within 1 D. If the two K readings are more than 1 D different, take a third reading and average the closest two readings.

Like keratometry, A-scan biometry should be performed for both eyes.

IOL power calculations are more variable in children, as adults’ visual needs help determine the calculation. But in children, the surgeon must also take into account the expected growth of the eye.

The ideal IOL power would provide the best correction for amblyopia in childhood while inducing the least refractive error in adulthood.

Aiming for emmetropia in childhood is likely to result in a large refractive error in the adult eye, which is likely to require a second surgery to exchange IOLs. At the same time, a near-zero refractive error in childhood simplifies treatment for amblyopia and results in better visual outcomes, Dr. Trivedi said.

Aiming for high hyperopia reverses those advantages and disadvantages. Better adult refraction avoids the need for repeat surgery but complicates childhood amblyopia and produces worse visual outcomes.

Targeting moderate hyperopia immediately after implantation is more likely to yield a more acceptable lifelong balance, she said.

Rupal Trivedi, MD MSCR

E: trivedi@musc.edu.

Dr. Trivedi had no relevant financial disclosures.

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