Compensated versus uncompensated millimeters during myopia progression: Estimating H' in children

The second principal point (H') of the eye can be used to estimate the diopters of change in refractive error that result from each millimeter of axial elongation. Calculating the location of H' requires data from phakometry, a technique not included in most myopia control clinical trials. Previous estimates have ranged from 1.602 mm (Gullstrand) to 2.1 mm. The purpose of this analysis is to develop a method for estimating the location of H' in children using more readily available biometric data.

Children (n = 4913) from the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study provided the necessary biometric data to find the location of H'. Crystalline lens power was also estimated using Bennett's method. A generalized linear model was used to create an equation that could estimate the location of H' without phakometry data which was then validated against the CLEERE values.

The average (±SD) position of H' in CLEERE children was 1.90 ± 0.15 mm posterior to the cornea. Multivariate modeling resulted in the following equation: $H^{\text{'}}=0.439+\left(0.222\ast \text{LT}\right)+\left(0.293\ast \text{ACD}\right)-\left(0.030\ast \text{CP}\right)-\left(0.009\ast \text{AL}\right)+\left(0.049\ast \text{BLP}\right)$ (adjusted R² = 0.60). The position of H' using this model agreed well with the position calculated using the CLEERE complete set of biometric data, with an average difference (±SD) of 0.003 ± 0.094 mm.

Estimating the location of H' does not require phakometry. The position of H' was intermediate between the classic Gullstrand value and the value previously estimated for children. This method could be used with patients or in clinical trials to gauge the effects and effectiveness of myopia control by estimating the fraction of inhibition of eye elongation that comprises compensated elongation compared to uncompensated elongation, setting targets for slowing eye growth to the point of no net myopia progression.