Determining the optimal IOL power for children is as much an art as a science.
The ultimate goal of paediatric IOL power selection is to provide a manageable course of refraction between IOL implantation and adulthood with the best possible adult visual acuity.
In adults, every millimetre of error in axial length measurement translates into about 2.5 D error. In children - who typically have short eyes of 20 mm or less - every millimetre of error in axial length translates into about 3.75 D error. That difference in effect has a direct impact on how axial length is measured.
Contact versus immersion
Immersion ultrasound measurements are the standard of care in adults, Dr Trivedi explained.
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.
Multiple studies have shown that immersion A-scan measurement eliminates corneal compression and produces more accurate axial length measurements compared with contact biometry.
An informal survey of paediatric ophthalmologists found that 82% use the contact ultrasound method to measure paediatric 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 with 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 measured 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. Use the measurements from the scan with the best waveform - the highest peaks with a perpendicular retinal spike," she said. "If you must use applanation biometry, rely on the measurement with the greatest anterior chamber depth."