The accuracy of a measuring instrument is an essential factor when selecting a device for clinical purposes. None of the three devices studied require contact with the eye and all can be operated by a trained technician. We found significant differences in the anterior chamber depth measurements between IOLMaster, SPAC, and AS‐OCT. In general, AS‐OCT measured the highest values and the IOLMaster the lowest. Our results are consistent with a report by Baikoff et al
who found that AS‐OCT measurements of anterior chamber depth were generally deeper by 110 microns than those made by IOLMaster. The 95% limits of agreement were narrowest for AS‐OCT and IOLMaster, suggesting that these two instruments have good agreement. There was also a constant bias of deeper anterior chamber depth with the AS‐OCT compared with the IOLMaster (fig 1).
There are several possible explanations for these differences. First, cycloplegic agents were not used in this study. For AS‐OCT imaging, accommodation was minimised by adjusting the fixation target using the subject's refractive correction, whereas IOLMaster and SPAC do not have a non‐accommodative fixation target. Thus patients undergoing evaluation by the SPAC and IOLMaster may have had different states of accommodation. Accommodation would be expected to lead to reduction in anterior chamber depth, and indeed the IOLMaster and SPAC tended to give shallower anterior chamber depth measurements than the AS‐OCT. A second possible explanation is pupil size. Vogel et al13
reported that the IOLMaster can give inaccurate measurements in subjects with small pupils. We did not measure pupil size in this study, but it is conceivable that differences in the isolation of the central axis may have resulted in variations in anterior chamber depth measurements. The use of an infrared light source in the AS‐OCT may keep the pupil size unaltered, thereby presumably giving a more accurate anterior chamber depth value. A third possible explanation for the detected differences is the selection of the axis of measurement. The IOLMaster and SPAC measure anterior chamber depth along the visual axis, whereas AS‐OCT measures it along the geometric axis, by adjusting the fixation angle setting in the device. A recent study reported that a centring error of 0.5 mm away from the eye's geometric centre gave a 20 μ undervaluation for anterior chamber depth by AS‐OCT.22
Being in the geometric axis, the AS‐OCT measurements probably reflects a more accurate estimation of anterior chamber depth, and this explains to some degree why the AS‐OCT values tended to be the highest. Finally, IOLMaster and SPAC both measure anterior chamber depth by optical methods whereas AS‐OCT uses infrared energy. Relying on different physical principles should not alter the findings, however, and we believe this is an unlikely explanation for the differences.
It was also interesting to note that the differences in anterior chamber depth measurement between SPAC and the other two devices could be positive or negative depending on the mean anterior chamber depth (figs 2 and 3). For example, when comparing SPAC with the IOLMaster, SPAC appeared to give larger values when anterior chamber depth was deeper (fig 2). A possible explanation is that SPAC measures anterior chamber depth at various points from the visual axis along the anterior iris (reference plane). The central anterior chamber depth is measured automatically by the instrument by drawing a vertical line connecting the corneal endothelium at the centre of cornea and this reference plane. This method may have led to variations in the measurements at different anterior chamber depths, possibly due to different iris profiles in subjects with shallow and deep anterior chambers.
Our study had some limitations. The reproducibility of anterior chamber depth measurements was not evaluated. The examiner dependent variability in the results was not examined, and studies have shown that optical measuring devices like IOLMaster and SPAC are, to a large extent, operator independent.13,17
Anterior chamber depth measurements by AS‐OCT are also not automatic and hence examiner subjectivity cannot be ruled out. However, a recent study has reported that AS‐OCT gave consistent results with better reproducibility in measuring anterior chamber depth than IOLMaster.22
Our study showed that there was good agreement between the three non‐contact devices for measuring anterior chamber depth, although there were small but significant differences in the measurements. AS‐OCT gave systematically deeper anterior chamber measurements than SPAC and IOL‐Master. There appears to be a systematic bias towards higher AS‐OCT values with deeper anterior chambers. However, as the differences found were small they are unlikely to be clinically important.