Abstract
Purpose:
To evaluate the impact of pharmacologic pupil dilation on axial length (AL) measurement in patients with dense cataracts and previous failed AL measurements carried out without pupil dilation.
Design:
Retrospective case series.
Methods:
All participants underwent swept-source optical coherence tomography (SS-OCT) biometry. Patients with unsuccessful AL measurements due to dense cataract underwent an additional SS-OCT biometric evaluation after pupil dilation, and the SS-OCT AL measurement was compared to the immersion ultrasound.
Results:
The study included 3668 eyes of 3668 patients who underwent SS-OCT between October 2021 and March 2023. Of them, 102 eyes (2.8%) had failed AL measurements because of dense cataract. Eighty-seven of those 102 eyes underwent a repeat SS-OCT biometric exam following pharmacologic pupil dilation, after which AL measurements were successfully achieved in 27 (31.0%) of the 87 eyes. These measurements were found to be consistent with immersion ultrasound, supporting the validity of SS-OCT biometer measurements post dilation.
Conclusions:
Pharmacologic pupil dilation improved the rate of successful SS-OCT biometrically measured AL in patients with failed AL measurement due to dense cataract.
Keywords: Dense cataract, failed AL measurement, pupil dilation, SS-OCT biometry
The progress made in cataract surgery has led to high expectations from patients for clear postoperative vision without eyeglasses. Highly precise ocular biometric measurements, especially of the axial length (AL), are crucial in order to attain optimal refractive outcomes.[1] Even a slight variation of 0.1 mm in AL measurement can result in an error of 0.28 diopters (D) in intraocular lens (IOL) power calculation.[2] Various noninvasive optical biometers that use different technologies have been developed to improve precision.[3] However, patients with severe media opacity, particularly dense and advanced cataracts, pose a special challenge in ensuring accuracy of the preoperative evaluation due to poor quality of the findings or failure altogether of conventional ocular biometers in measuring AL.[4]
Ultrasound (US) can be used to assess AL, but it is less accurate, more time-consuming, less comfortable, and more costly than optical biometry.[5,6,7] The IOLMaster 700, a swept-source optical coherence tomography (SS-OCT)-based biometer, has been shown to enhance the success rate of AL measurements in eyes with dense cataracts.[3,8,9] However, AL measurements with an SS-OCT biometer may still fail in some patients, resulting in unsuccessful acquisition of AL.[8] An earlier study from our group showed that pharmacologic pupil dilation improved the quality of SS-OCT biometrically measured AL in patients with low-quality AL measurement due to dense cataract.[10] The current study aimed to evaluate the effect of pharmacologic pupil dilation on AL measurements by means of SS-OCT biometry in patients who had failed AL acquisition due to dense cataract.
Methods
This retrospective observational clinical investigation was carried out at our Department of Ophthalmology. Approval for the study was granted by the institutional review board (IRB) of the center, and the research adhered to the principles outlined in the Declaration of Helsinki. Informed consent was not required for this anonymized study as per the IRB's waiver.
Medical records of all consecutive patients referred for preoperative assessment for cataract surgery at our ophthalmology department from October 2021 to March 2023 were reviewed. Patients with dense cataract and failed AL measurements on their SS-OCT biometric assessment who underwent a repeat measurement after pupil dilation were enrolled in the study. Only the eye scheduled for cataract surgery of each patient was included. Patients under 18 years of age, those unable to fixate on the target light, and those with any media opacity other than cataract were excluded.
Ocular biometry was performed without pupil dilation at the preoperative visit under the same light conditions and by means of the same SS-OCT ocular biometer (IOLMaster 700, Carl Zeiss Meditec AG, software v. 1.80.6.60340) as those tests conducted with pupil dilation. The IOLMaster 700 provides 18 OCT scans (3 times 6 meridians) to obtain AL measurements.[8,11] The device is equipped with integrated indicators and alerts designed for the operator, such as a green checkmark indicating good signal quality, a red “X” indicating a failed measurement, and a yellow exclamation point indicating impaired signal quality. A measurement that falls below the internal quality criteria of the SS-OCT biometer should be reviewed or repeated, as per the manufacturer's recommendation.[12]
As part of our preoperative clinic's protocol, patients with failed or low-quality AL measurements taken using the SS-OCT biometer underwent repeated biometry measurements typically between 30 minutes and 50 minutes after pharmacologic pupil dilation (unless contraindicated). Pupil dilation was performed by the instillation of topical tropicamide 0.5% and phenylephrine 10%, 1 drop each. Following biometric evaluation, the patients underwent a comprehensive ophthalmologic assessment, including applanation intraocular pressure measurement (Goldmann tonometer), slit-lamp biomicroscopic examination, cataract classification and grading, and dilated fundus biomicroscopy. Cataract was classified as nuclear, cortical, and subcapsular. Additionally, these patients routinely underwent AL measurement by immersion US (iUS) with the ABSolu US biometric device (Quantel Medical, v1.04) in our preoperation clinic. The device utilizes a Hansen immersion shell that houses the US probe tip. This shell is positioned over the operative eye and filled with a balanced salt solution to obtain an accurate AL measurement. Three default US velocities are used: 1532 m/s for the anterior chamber (including corneal thickness), 1641 m/s for lens thickness (LT), and 1532 m/s for vitreous depth.[13] The US device is operated in the automatic image-freezing mode during the acquisition, and the gain adjustment is dynamic, with a range of 20–110 dB. The AL measurement process is conducted several times, and the reading with the most precise alignment pattern and gate placement is selected. The difference between AL measurements obtained by optical biometry and iUS has been evaluated in a previous study by Cooke et al.,[14] and a regression formula has been developed to account for these differences.
Statistical analysis
Normality was assessed using the Shapiro–Wilk test. Paired t-tests were used to compare post-pupil dilation ALs measured using the SS-OCT biometer with standard iUS and the calibrated iUS AL values.[14] Data were recorded in Microsoft Excel and analyzed using XLSTAT (version 2023.2.1414).
Results
In total, 3668 eyes of 3668 consecutive patients underwent preoperative ocular biometry and clinical evaluations during the study period [Fig. 1]. One-hundred and twenty-four eyes (3.4%) had failed AL measurements, of which 102 (82%) were due to dense cataract (the others were due to noncataract ocular media opacity and other factors). Eighty-seven of these 102 (86%) patients had undergone repeated ocular biometry after pharmacologic pupil dilation, and they constituted the current study group. The decision was made not to perform a second biometric assessment with pupil dilation in 11 patients due to a diagnosis of narrow angle, and the post-dilation measurement of four patients was missing. The baseline clinical and demographic characteristics of the study population and the rate of successful AL measurement post dilation according to cataract type are summarized in Table 1. An example of the biometric printout and the changes in AL measurement parameters before and after pharmacologic mydriasis as well as postoperatively is illustrated in Fig. 2. After pharmacologic pupil dilation, successful measurements were obtained with the SS-OCT device in 27 out of the 87 eyes (31%).
Figure 1.
Flowchart of the study results. AL = axial length
Table 1.
Clinical and demographic characteristics of the study population and post-dilation AL success rate
| Characteristic | Value | Post-dilation AL measurement |
|
|---|---|---|---|
| Successful | Unsuccessful | ||
| Number of patients (eyes) | 87 (87) | 27 (27) | 60 (60) |
| Age (years, mean±SD [median]) | 70.2±11 [69] | 68.8±12 [68] | 70.8±11 [70] |
| Female, n (%) | 51 (58) | 13 (48.1) | 38 (63.3) |
| Type of main cataract, n | |||
| Nuclear sclerosis | 36 | 21 | 15 |
| Posterior subcapsular cataract | 7 | 6 | 1 |
| White | 44 | 0 | 44 |
SD=standard deviation; AL=Axial length
Figure 2.
Printout of the IOLM700 SS-OCT biometer before (a) and after (b) pupil dilation. (a): Failed AL measurement (arrow) due a dense posterior subcapsular cataract (arrowhead). (b): Successful AL measurement post dilation (arrow) with 3 successful OCT scans per measurement (star). ACD = anterior chamber depth; AL = axial length; CCT = central corneal thickness; LT = lens thickness; LVC = laser vision correction
The values obtained by SS-OCT biometry following dilation and those obtained by iUS were compared [Table 2]. The analysis revealed a significant difference in AL values between these two measurement techniques, with a mean difference of 0.1 ± 0.25 mm (24.16 ± 1.16 and 24.06 ± 1.14 mm for the SS-OCT biometry and for the iUS, respectively, P < .05). After applying the calibration formula to our results, we found no significant difference between the measurements obtained using the SS-OCT biometer and those obtained by the recalibrated iUS AL (24.16 ± 1.16 and 24.15 ± 1.15 mm, respectively, P = .87), as shown in Table 2.
Table 2.
Comparison between IOLMaster 700 and iUS AL measurements (n=27)
| Post-dilation IOLMaster 700 (mm) | iUS (mm) | Difference | P |
|---|---|---|---|
| 24.16±1.16 | 24.06±1.14 | 0.1±0.25 | <0.05 |
| Recalibrated iUS AL=1.0228 x iUS AL - 0.4556 | |||
| 24.16±1.16 | 24.15±1.15 | 0.008±1.15 | =0.8 |
AL=axial length; iUS=immersion ultrasound; SD=standard deviation. Values are given as mean±SD
Discussion
Achieving optimal refractive outcomes after cataract surgery relies upon a precise preoperative evaluation of ocular biometrics, especially AL measurements.[2] While SS-OCT biometry generally has a high success rate for measuring AL in patients with dense cataracts, there are also instances of failure of these measurements.[3] In the present study, 2.8% of cataract surgery candidates had preoperative failed AL SS-OCT biometric measurements due to dense cataract. Following pharmacologic mydriasis, repeated AL measurements resulted in successful acquisition in 31% of those cases. Our group had previously demonstrated the beneficial impact of pupil dilation on improving low-quality AL measurements. To the best of our knowledge, this is the first study to demonstrate the advantageous effect of mydriasis on AL measurement in eyes with totally unsuccessful AL measurements due to dense cataract.
Several reports on eyes with clear media showed no significant changes in AL measurements by means of an SS-OCT biometer after pupil dilation.[15,16] To assess the accuracy of AL values obtained following mydriasis in eyes with dense cataracts, we compared these measurements and those obtained by means of iUS. Our results showed a significant difference of 0.1 mm between these two techniques; however, there were no significant differences between the measurements obtained using the SS-OCT biometer and those obtained using the recalibrated iUS AL.[14] These findings support the validity of SS-OCT biometer measurements acquired after pupil dilation in these eyes.
The precise mechanism by which pupil dilation facilitates the achievement of successful AL scans remains somewhat unclear. A plausible explanation for the observed improvement in acquisition rate is that pharmacological mydriasis allows an increased amount of light to pass through, thereby enabling a higher number of scanning SS-OCT beams, particularly in areas where the cataract is relatively less dense. Notably, similar to our own findings, Cheng et al. documented that pharmacologic mydriasis resulted in improved signal strength in retinal nerve fiber layer OCT scans using time-domain and spectral-domain techniques.[17] However, reports on investigations of the influence of light conditions on SS-OCT signal quality are currently lacking. Finally, several reports on eyes with clear media showed no significant changes in AL measurements by means of an SS-OCT biometer after pupil dilation.[15,16]
Noncontact optical biometry has emerged as the preferred approach to AL measurement due to its accessibility, ease of use, accuracy, and reproducibility, thus establishing it as the gold standard.[18] This method may nevertheless encounter challenges to obtain successful AL measurements in patients with opaque ocular media.[14] While alternative techniques, such as applanation or immersion-based ultrasonographic AL measurements or intraoperative aberrometry, can be utilized in such cases, they are associated with drawbacks that include reduced accuracy, increased time requirements, the need for additional staff and training, and close contact with the eye.[3,7,19,20] By utilizing simple pharmacologic pupil dilation, it is possible to potentially achieve accurate AL measurements by SS-OCT biometry in about 30% of these eyes, thereby reducing the reliance upon alternative techniques. It should be noted that successful acquisitions of AL measurements were limited to eyes with dense nuclear sclerosis and posterior subcapsular cataracts. None of the eyes with white cataracts yielded measurable readings with optical biometry, indicating that for these eyes, iUS biometry remains the best option.
Our study bears some limitations other than its retrospective nature. First, it was specifically conducted on eyes with advanced cataracts and free of other forms of media opacity. As a result, caution must be exercised when extrapolating our findings to different patient populations. Second, pupil dilation was avoided in patients at high risk for acute angle-closure attack and their post-dilation AL measurement was missing.
In conclusion, this study demonstrated successful AL measurement after pupil dilation in 31% of patients with previously failed AL acquisition using an SS-OCT biometer due to dense cataract. The provision of pupil dilation can potentially provide more accurate AL measurements and avoid the need for iUS measurements in this subset of patients.
What is already known on this topic
SS-OCT-based biometry improves the rate of successful axial length (AL) measurements.
AL measurements may still fail in the presence of dense cataract.
Biometry after pharmacologic pupil dilation may improve the quality of SS-OCT-measured AL in patients with dense cataract.
What this study adds
Pupil dilation achieved successful AL measurements in 31% of patients with dense cataracts and previous failed AL measurements by SS-OCT.
Post-dilation AL measurements were determined to be precise and reliable.
How this study might affect research, practice or policy
SS-OCT biometric AL acquisition after pharmacologic pupil dilation can potentially provide successful and accurate AL measurement in eyes with dense cataract and earlier failed measurement and avoid the need for alternative techniques like ultrasound.
The other authors have no financial or proprietary interest in any material or method mentioned.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Acknowledgments
The authors thank Mrs. Yehoudit Berdugo and her technical staff for their help with data collection.
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