Biometric calculation of intraocular lens power for cataract surgery following pupil dilatation

The ability to perform biometry accurately on a dilated pupil can greatly facilitate the efficiency of a cataract service as it can be done on the day of surgery. The purpose of this study was to assess the repeatability of axial length (AL) calculations in undilated pupils and measure the difference in predicted and actual refractive outcomes in dilated pupils compared with undilated pupils.

First, intraobserver repeatability was assessed by taking two consecutive recordings of AL using applanation A-scan ultrasonography in undilated pupils in 21 eyes. The mean AL for each eye was compared with a measurement made following pupil dilatation. Second, we audited the mean spherical equivalent refractive errors following routine cataract surgery in 38 patients with undilated pupils and 36 patients with dilated pupils.

The mean difference in intraobserver measurements was -0.05 mm (standard deviation [SD] 0.15) with pupils undilated. Following pupil dilatation, the mean dilated AL differed from the mean undilated AL reading by only 0.03 mm (P > 0.05). The mean differences between planned and actual refractive error were 0.71D (SD 0.54) and 0.55D (SD 0.41) in dilated and undilated patients, respectively. This was not statistically significant (P > 0.05). The range of differences between target and actual refraction was -1.45D to 2.70D for undilated patients and -1.88D to 1.18D in dilated patients.

Although there was a greater spread of postoperative refractive errors in the dilated group, there were no statistically or clinically significant differences in postoperative refractive errors between the two categories of patients. Our study shows that applanation biometry may be safely performed for the purpose of cataract surgery after pupil dilatation.