Accuracy of axial length measurements from immersion B-scan ultrasonography in highly myopic eyes

Abstract

Aim: To evaluate the accuracy of axial length (AL) measurements obtained from immersion B-scan ultrasonography (immersion B-scan) for intraocular lens (IOL) power calculation in patients with high myopia and cataracts.

Methods: Immersion B-scan, contact A-scan ultrasonography (contact A-scan), and the IOLMaster were used to preoperatively measure the AL in 102 eyes from 102 patients who underwent phacoemulsification and IOL implantation. Patients were divided into two groups according to the AL: one containing patients with 22 mm≤AL<26 mm(group A) and the other containing patients with AL≥26 mm (group B). The mean error (ME) was calculated from the difference between the AL measurement methods predicted refractive error and the actual postoperative refractive error.

Results: In group A, ALs measured by immersion B-scan (23.48±1.15) didn't differ significantly from those measured by the IOLMaster (23.52±1.17) or from those by contact A-scan (23.38±1.20). In the same group, the standard deviation (SD) of the mean error (ME) of immersion B-scan (-0.090±0.397 D) didn't differ significantly from those of IOLMaster (-0.095±0.411 D) and contact A-scan (-0.099±0.425 D). In group B, ALs measured by immersion B-scan (27.97±2.21 mm) didn't differ significantly from those of the IOLMaster (27.86±2.18 mm), but longer than those measured by Contact A-scan (27.75±2.23 mm, P=0.009). In the same group, the standard deviation (SD) of the mean error (ME) of immersion B-scan (-0.635±0.157 D) didn't differ significantly from those of the IOLMaster (-0.679±0.359 D), but differed significantly from those of contact A-scan (-0.953±1.713 D, P=0.028).

Conclusion: Immersion B-scan exhibits measurement accuracy comparable to that of the IOLMaster, and is thus a good alternative in measuring AL in eyes with high myopia when the IOLMaster can't be used, and it is more accurate than the contact A-scan.

Keywords: axial length; high myopia; immersion; measurement; ultrasonography.

Figure 1. First quality control measure for immersion B-scan ultrasound biometry

The vector an ultrasound sampling line was determined to be exactly perpendicular to the double bands of the cornea and the center of the lens capsule, and directed at the center of the macula.

Figure 2. Second quality control measure for immersion B-scan ultrasound biometry

The intersection of the vector an ultrasound sampling line and the retina was determined to be 4.5 mm from the optic disc or 3 mm from the optic disc edge.

Figure 3. Axial length measurement by immersion B-scan ultrasound

AL was defined as the distance from the corneal vertex to the macula or from the crest of the anterior corneal surface to the crest of the macular retinal surface with an electronic measuring scale in B-mode ultrasound.