Purpose
To evaluate the effect of pupil dilation on biometric measurements and intraocular lens power calculation using the IOLMaster (Carl Zeiss Meditec).
Design
Prospective, observational case series.
Methods
In this prospective study, 2 consecutive optical biometry measurements, before and after pupil dilation, were obtained using the IOLMaster on 318 eyes of 214 patients at the cataract presurgery clinic. The parameters compared were axial length, corneal power, cylinder, and the corresponding intraocular lens power, which was calculated using the Sanders-Retzlaff-Kraff/Theoretical formula.
Results
This study found no statistically significant difference before and after dilation in axial length (0.005 mm; P = .476), corneal power (0.001 diopters [D]; P = .933), or calculated intraocular lens power (0.011 D; P = .609). A statistically significant difference was shown in cylinder measurements before and after dilation (0.102 D; P < .001).
Conclusions
This study demonstrated there is no clinically significant effect of pupil dilation on the IOLMaster measurements of axial length, corneal power, and corresponding theoretical intraocular lens power calculated using the Sanders-Retzlaff-Kraff/Theoretical formula.
Cataract extraction and intraocular lens (IOL) implantation is one of the most frequently practiced and successful ophthalmic surgical procedures carried out today. Among the few remaining challenges associated with cataract surgery is the achievement of optimal postoperative refraction. To calculate the appropriate IOL power, several parameters, such as axial length (AL) and corneal power (K), must be measured accurately.
The gold standard device for ocular biometry is the IOLMaster (Carl Zeiss Meditec, Jena, Germany), and new devices are being tested against it. It essentially has replaced ultrasound biometry because of its high intrasession, intersession, and interobserver precision; better refractive outcomes; and reduced risk for infection by being a noncontact method. The IOLMaster measures AL by partial optical coherence interferometry. The measurement is based on a refracted laser beam used to estimate the distance to the posterior pole while the patient fixates. The AL for IOL power calculation is best measured on the visual axis because the focus is on the macula. Accommodation changes and fixation may be more difficult in the presence of mydriasis, which may lead to errors in the measurement of AL.
The IOLMaster uses automated keratometry to measure the anterior corneal curvature. Six spots of light are projected onto the cornea in a hexagonal pattern within a 2.5-mm diameter. The position of each pair of reflection spots is detected and measured, and the relative positions are compared to determine corneal curvature and astigmatism as radial measurements. It then converts the radial measurements into diopters (D) using a keratometry index to determine the total corneal power. Change in corneal shape and thus corneal power may occur because of mydriasis.
A number of preliminary studies examined the effect of pupil dilation on IOLMaster measurements. In 2002, Heatley and associates showed that mydriasis does not affect the accuracy of the IOLMaster measurements, but their study had some limitations: one was that the size of the study group was small, and another is that no inclusion or exclusion criteria were given. The latter comprises a crucial factor because cataract patients often have comorbidities that involve the macula and that may influence the ability of the patient to fixate even with myosis. An additional limitation is that the control group was not matched to the study group, a feature that may have introduced confounders. Two other small studies examined the effect of cycloplegia on AL measurements with the IOLMaster: one was by Sheng and associates in 2004 and the other by Bansal and associates in 2008. Both showed that there was no difference in AL measurements after pupil dilation. A more recent study by Huang and associates examined the effect of cycloplegia on the Lenstar LS900 (Haag-Streit AG, Koeniz, Switzerland) and the IOLMaster biometry. Their study was better defined in terms of both exclusion criteria and the control group, although the study group also was small.
The primary aim of the current study was to evaluate further the effect of pupil dilation on the accuracy of IOLMaster measurements in a large, well-defined study group with a carefully powered sample size. The secondary aim was to examine the effect of pupil dilation on the corresponding IOL power prediction.
Methods
This prospective study comprised patients attending the cataract presurgery clinic at the Tel Aviv Medical Center, Tel Aviv, Israel. The study protocol was reviewed and approved by the Institutional Review Board of Tel Aviv Medical Center, and written informed consent was obtained from all participants. A total of 214 patients met the inclusion criteria and were enrolled. Two consecutive optical biometry measurements were performed before and after pupil dilation on the 318 studied eyes. The predilation measurements formed the control group and the postdilation measurements formed the study group. In this way, each eye was its own control, and all measurements were paired.
All patients underwent a visual acuity test as well as a predilation biometric measurement with the IOLMaster according to the manufacturer’s instructions. Pupil dilation was achieved with tropicamide 0.5% and phenylephrine 10%. After mydriasis was attained, the patients underwent a complete ophthalmologic examination, including slit-lamp biomicroscopy and intraocular pressure measurement, which was immediately followed by a second biometric measurement with the IOLMaster. Both biometric measurements were performed for each participant by the same experienced examiner.
The recorded data included demographics, AL, mean K, and cylinder. The IOL power required for emmetropia, with a target refraction of −0.5 D, was calculated using the Sanders-Retzlaff-Kraff/Theoretical (SRK/T) formula with an A constant of 119.
Exclusion criteria included uncorrected visual acuity of worse than 6/30, mydriasis of less than 5.5 mm, previous ocular surgery, or penetrating trauma. Also excluded were patients who were physically unable to be positioned at the IOLMaster, those who could not open the eyelid widely enough so that all measurements could be performed, and those who could not fixate because of ocular disease. Patients who had corneal or media opacities other than cataract that cause acquisition failure and those with active ocular infection, an inflammatory process of the eye, or any macular disease were excluded as well.
Sample size was determined based on the maximum difference between predilation and postdilation AL that had no clinical significance, and thus would not change the IOL calculation using the Sanders-Retzlaff-Kraff formula. Assuming that the average AL is 23.4 mm with a standard deviation of 1.36 mm in both measurements, the correlation between both measurements would be 0.8, and a difference of less than 0.2 mm between both measurements would be clinically insignificant, with a statistical significance of 5% and a statistical power of 80%. A total of 318 eyes were needed to prove the lack of any difference between both measurements.
The paired t test was applied to test changes between predilation and postdilation measurements. The difference between the changes in 2 independent subgroups was determined by using the independent samples t test. All tests were 2-tailed, and a P value of .05 or less was considered statistically significant. Data were analyzed using SPSS for Windows software version 20.0 (SPSS, Inc, Chicago, Illinois, USA).
Results
A total of 318 eyes (214 subjects) met the inclusion criteria. The subjects’ ages ranged from 20 to 95 years (average, 71.93 years). There were 153 (48.1%) right eyes and 165 (51.9%) left eyes. For all 318 eyes, all the selected parameters were measured. The results of the IOLMaster measurements before and after dilation and the differences between them are shown in Tables 1 and 2 , respectively.
| Before Dilation | After Dilation | |
|---|---|---|
| AL (mm) | 23.3573 ± 1.014 | 23.3620 ± 1.014 |
| Mean K (D) | 44.3705 ± 1.479 | 44.3690 ± 1.509 |
| Cyl (D) | 0.8987 ± 0.67 | 1.0004 ± 0.721 |
| IOL (D) | 21.6494 ± 2.749 | 21.6384 ± 2.752 |
| Measurement Before vs After Dilation (Mean ± SD) | P Value | |
|---|---|---|
| AL (mm) | 0.00478 ± 0.119 | .476 |
| Mean K (D) a | 0.00145 ± 0.305 | .933 |
| Cyl (D) | 0.10176 ± 0.431 | <.001 |
| IOL (D) | 0.01101 ± 0.383 | .609 |
Neither the difference in AL before and after dilation for each eye (mean of differences, 0.005 mm; P = .476) nor the difference in mean K before and after dilation for each eye (mean of differences, 0.001 D; P = .933) reached a level of statistical significance. In contrast, the difference in cylinder before and after dilation for each eye (mean of differences, 0.102 D) was statistically significant ( P < .001).
The difference in theoretical IOL power calculated before and after dilation within each eye (mean of differences, 0.011 D) was not statistically significant ( P = .609). The results were slightly different when examining the IOL power calculated as a discrete variable with changes in intervals of 0.5 D. Of the 318 studied eyes, 211 (66.4%) had no change in the calculated IOL before and after dilation. In 107 cases (33.6%), the IOL that was calculated with the SRK/T formula was different for the predilation and postdilation measurements. The IOL changed by 0.5 D in 97 (30.5%) of these 107 cases, by 1 D in 6 cases (1.9%), by up to 2 D in 3 cases (0.9%), and by more than 2 D in only 1 case (0.3%).
Results
A total of 318 eyes (214 subjects) met the inclusion criteria. The subjects’ ages ranged from 20 to 95 years (average, 71.93 years). There were 153 (48.1%) right eyes and 165 (51.9%) left eyes. For all 318 eyes, all the selected parameters were measured. The results of the IOLMaster measurements before and after dilation and the differences between them are shown in Tables 1 and 2 , respectively.
| Before Dilation | After Dilation | |
|---|---|---|
| AL (mm) | 23.3573 ± 1.014 | 23.3620 ± 1.014 |
| Mean K (D) | 44.3705 ± 1.479 | 44.3690 ± 1.509 |
| Cyl (D) | 0.8987 ± 0.67 | 1.0004 ± 0.721 |
| IOL (D) | 21.6494 ± 2.749 | 21.6384 ± 2.752 |
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