This study demonstrated the feasibility of using the Visante OCT to image the crystalline lens and to measure lens thickness as a function of age and accommodation. Despite the limited number of subjects and measurements, the calculated increase in lens thickness with age and accommodation compares well to previous studies using Scheimpflug photography, ultrasound, PCI and MRI (). Our cross-sectional findings of an increase of 21 μm in lens thickness with age are within the range of values reported using other techniques (Dubbelman et al, 2003
; Bullimore et al., 2007
; Garner and Yap, 1997
; Jones et al., 2007
). To our knowledge, there is no reported measurement of a change in lens thickness with age using PCI. Our calculated increase in lens thickness of 51 μm with each diopter of accommodative stimulus agrees well with 1.5T MRI results (Jones et al., 2007
), but are slightly higher than those found by Scheimpflug photography (Dubbelman et al., 2003
) and ultrasound (Garner, 1997
). Recent studies using PCI to measure changes with accommodation report disparate values, likely due to one group referencing to the accommodative stimulus and another measuring the accommodative response. Tsorbatzoglou et al (2007)
, using the ACMaster, reported an increase of 36 μm per diopter of accommodative stimulus. In one of the few studies to measure accommodative response simultaneously, Bolz et al (2007)
used the ACMaster and a photorefractor and reported an increase in lens thickness of 63 μm in emmetropic subjects and 72 μm in myopic subjects for each diopter of accommodative response.
There are many reasons for the reported differences in lens thickness between techniques. First, many of the studies, including this one, did not record accommodative response, but only assumed that the subject was accommodating accurately to the target. Each study may have used different accommodative stimuli. Depending on the amount of accommodative lag for each subject and with each target, the calculation for change in lens thickness per diopter of accommodative stimulus would be artificially reduced. The internal fixation target in the Visante OCT system (a pinwheel pattern), may not be an adequate accommodative stimulus, and is different than the letter target we used to measure accommodative response with the autorefractor. Current studies using the Visante OCT and photorefraction are being conducted to improve the precision of the measurement and confirm accurate accommodation with the internal target.
The resolution of the techniques available to measure lens thickness also vary widely with PCI and OCT being the best at approximately 10 to 20 μm and ultrasound and 1.5T MRI at over 100 μm (Drexler et al., 1997b
; Jones et al., 2007
; Guell et al., 2007
). The addition of repeated measurements could improve the precision of any of the techniques. In this pilot study, only one measurement at each accommodative stimulus was taken, and a statistically significant change was found which agreed well with other reports. Although repeatability was not assessed as part of this study, our colleagues (Lehman et al. 2007
) have found 95% limits of agreement of −0.030 to +0.030 mm for images that contain the light reflex (manuscript in preparation).
OCT, like other optical imaging techniques, assumes a single refractive index. A refractive index of 1.42 was chosen for these calculations based on previous studies, but this value may underestimate the true lens thickness. Changing the refractive index from 1.42 to 1.40 increases the calculated lens thickness, but does not alter change in thickness with age or accommodation (). Dunne et al (2007)
measured lens thickness in a known model eye and found that the Visante overestimates lens thickness, and presented formulas that could be used to improve accuracy. Future studies could combine phakometry with OCT to calculate the individual subject's equivalent refractive index and apply a unique correction factor. Still, this method could not account for the gradient refractive index of the lens. Likewise, ultrasound assumes a single speed of sound. Numerous studies have demonstrated that the speed of sound and refractive index of the lens vary throughout the population (Dubbelman and van der Heijde, 2001
; Glasser and Campbell, 1999
; Beers and van der Heijde, 1994a
). When comparing findings between different studies, differences in assumed refractive index and speed of sound should be considered.
The Visante OCT provides another useful tool to examine the crystalline lens in vivo. Although there are limitations to this technique, comparisons of findings to other measurement techniques allow for a better understanding of the changes in the lens with age and accommodation. Future studies combining OCT with phakometry and photorefraction are needed to improve the accuracy of the measurements.