We thank Nihalani and associates for their interest and their comments on our paper. They have raised concerns regarding the large prediction errors and the SRK II formula being the most accurate and the Hoffer Q formula the least accurate in our series. However, the available literature is suggestive of large prediction errors in small children, especially those aged <2 years and those with an axial length <20 mm. Tromans and associates reported a prediction error of 2.56 ± 2.2 diopters (D) (range 0.24–9.19) in children less than 36 months, which was similar to observations in our study of 2.27 ± 1.69 D.
We would like to highlight observations from the Infant Aphakia Treatment Study (IATS) that, although the mean prediction error was about 1.8 ± 1.3 D using Holladay formulae (which is significantly less than in our study), this was not as high as if compared with the prediction error using the SRK II formula (2.27 ± 1.69 D, P = .47, CI −0.99 to 0.05 D). In addition, the distribution of the prediction error in these patients was similar to our series within an absolute prediction error of 0.5 D (12% vs 9.4% for Holladay and 27.1% for the SRK II formulae) and within an absolute prediction error of less than 2.0 D (59% vs 57% using the SRK II formulae). This suggests that the accuracy of the intraocular lens (IOL) power calculation could be nearly the same in both the series using the different formulae.
Nihalani and VanderVeen previously reported that the Hoffer Q formula gave the least prediction error, but in another series Neely and associates showed that the Hoffer Q formula had the maximum predication error even in the older children. We believe that, as suggested earlier, observation of high prediction error with the Hoffer Q formula in our study could be attributed to the use of adult anterior chamber depth values and there is a need for use of customized anterior chamber depth values in patients with shorter axial length.
As reported by us and by Nihalani and VanderVeen, our series had some limitations, but data were carefully reviewed and standard precautions maintained while doing retinoscopy and biometry. Although applanation biometry was used instead of the immersion biometry, there are conflicting data (including from IATS) regarding the supremacy of immersion biometry.
We have stated in our paper that the refraction performed at 4 weeks postoperatively, after removal of sutures, was considered for the analysis. The IOL powers implanted ranged from 11 to 30 D (24.5 ± 4.0 D); however, for IOL power undercorrection was done using Enyedi’s rule of 7.
In conclusion, we would like to state that at this time there is a similar degree of uncertainty with all the IOL power calculation formulae and a need to modify the IOL power selection using results of the refractive outcomes. This will continue until appropriate modifications can be made to the current IOL power formulae to decrease the prediction error to the minimum.