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Influence of the signal-to-noise ratio on the accuracy of IOLMaster measurements

Suto, Chikako MD; Sato, Chiaki COT; Shimamura, Emiko COT; Toshida, Hiroshi MD; Ichikawa, Kazuo MD; Hori, Sadao MD

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Journal of Cataract & Refractive Surgery: December 2007 - Volume 33 - Issue 12 - p 2062-2066
doi: 10.1016/j.jcrs.2007.07.031
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Abstract

Preoperative measurement of the axial length (AL) is important in the calculation of intraocular lens (IOL) power and selection.1–3 However, inaccurate AL measurements are the greatest cause of errors in the prediction of postoperative refraction.4 Thus, a more accurate measurement method is required. Although A-mode ultrasound (US) has generally been used for measuring AL, a method that uses laser interferometry was recently developed.5–7 The laser interferometry is done using a noncontact device, the IOLMaster (Carl Zeiss Meditec AG), that enables calculation of the required IOL power and optimization of the A-constant by measuring the AL, radius of corneal curvature, anterior chamber depth, and transverse corneal diameter. The IOLMaster is simple to operate and gives highly accurate measurements irrespective of the user.8–15

The accuracy of AL measurement by the IOLMaster can be easily assessed using the waveform and signal-to-noise ratio (SNR). A higher SNR value indicates greater accuracy16; thus, the SNR can help inexperienced operators confirm the accuracy of their measurements. According to the manufacturer, an SNR ≥2 indicates the measured values are reliable, although the results obtained sometimes show variation in this SNR range. Conversely, an SNR<2 indicates that measurements are not reliable, although the results can sometimes be used to calculate IOL power if repeatability is obtained.

The IOLMaster, however, has a significant drawback. It cannot be used in some patients because laser interferometry is impossible to perform in eyes with corneal opacity, vitreous hemorrhage, macular disease, severe posterior subcapsular cataract (PSC), hypermature cataract, and poor fixation. In such cases, the AL must be measured by A-mode US. Furthermore, AL measurements with the IOLMaster are influenced by the severity of the cataract.17,18

To our knowledge, there is no published study of whether the accuracy of the IOLMaster is related to the SNR value. The goal of the present study was to evaluate the relationship between the SNR and the reliability of values obtained with the IOLMaster.

PATIENTS AND METHODS

This clinical prospective study comprised 216 eyes of 144 consecutive patients scheduled for routine cataract surgery. Preoperatively, all patients had a complete evaluation including measurement with the IOLMaster performed by 2 certified ophthalmic technicians (COTs) using the regular settings for phakic eyes. The IOLMaster was connected through the network port to download all individual measurements, including the corresponding SNR, to a custom-built electronic case-record system. The COTs were instructed to obtain at least 5 measurements and a maximum 20 measurements, if possible. The best measurement for each patient, defined as the measurement with the highest SNR, was selected for further analysis. No attempt was made to manually select the right peak of a low SNR measurement. The AL and corneal radius curvature were measured in undilated eyes. The IOL power was calculated with the SRK/T formula. An A-constant of 119.8 was used, as published by the User Group for Laser Interference Biometry (available at http://www.augenklinik.uni-wuerzburg.de/eulib/const.htm. Accessed September 19, 2007); 119.8 was also the optimal A-constant value at Saiseikai Kurihashi Hospital.

All surgery was performed by the same experienced surgeon (C.S.) from August 2003 to July 2004. A continuous curvilinear capsulorhexis was created and small-incision phacoemulsification performed. An AcrySof MA60BM acrylic IOL (Alcon Surgical Inc.) was then implanted.

The AL was measured with the IOLMaster preoperatively and 3 months postoperatively. The pseudophakic AL was calculated with the correction factors of the IOLMaster based on the material of the implanted IOL.7

The preoperative SNR was defined as the highest SNR obtained during measurement. Eyes were divided into 5 groups based on the SNR values as follows: SNR<2, 2≤SNR<5, 5≤SNR<8, 8≤SNR<11, and SNR≥11.

The following parameters were compared between groups: preoperative logMAR best spectacle-corrected visual acuity (BSCVA), difference in AL from preoperatively to postoperatively (preoperative AL − postoperative AL), error in postoperative predicted refraction (postoperative spherical equivalent [SE] − predicted target refraction], and severity of cataract assessed with the Lens Opacities Classification System (LOCS III),19 a slitlamp-based grading method.

The postoperative SE values were obtained by subjective refraction 3 months after surgery. Based on the LOCS III system, the lenses were graded according to 3 major types of cataract: nuclear, cortical, and posterior subcapsular. The same examiner (C.S.) graded the cataracts at the slitlamp with the pupil dilated the day before surgery. The grade of nuclear color (NC) was used for nuclear cataract evaluation, and the grade of PSC (P) was assessed.

Statistical analysis was performed with SPSS for Windows (SPSS Japan, Inc.). The results are presented as means ± SD. The Kruskal-Wallis U test was used for preoperative logMAR BSCVA, difference in AL between preoperatively and postoperatively, and postoperative error. The Spearman rank correlation was used to evaluate the severity of cataract. Differences with a P value less than 0.05 were considered statistically significant.

RESULTS

The mean age of the 52 men and 92 women in the study was 72.2 ± 6.4 years. There were no significant differences in sex or mean age between the 5 SNR groups.

Twenty-eight eyes were excluded from analysis for preoperative astigmatism greater than 2.0 diopters (D), preexisting macular disease (eg, epiretinal membrane, macular edema, macular hole, age-related macular degeneration [ARMD]), or postoperative BSCVA worse than 20/25. Measurement of the AL with the IOLMaster was possible in 87.8% (244 of 278 eyes) of the total cataract population entering the hospital during the study period.

Preoperative Signal-to-Noise Ratio

The mean preoperative SNR in all patients was 8.35 (range 1.3 to 20.7). The mean was 1.69 ± 0.17 in the SNR<2 group (n = 18 eyes), 3.24 ± 0.87 in the 2≤SNR<5 group (n = 51 eyes), 6.50 ± 0.79 in the 5≤SNR<8 group (n = 56 eyes), 9.52 ± 0.93 in the 8≤SNR<11 group (n = 43 eyes), and 13.90 ± 2.49 in the SNR≥11 group (n = 48 eyes).

Preoperative Best Spectacle-Corrected Visual Acuity

The SNR correlated significantly with the preoperative logMAR BSCVA. It was significantly worse when the SNR was low. It was worst (mean 0.62 ± 0.53) in the SNR<2 group. It improved in the 2≤SNR<5 (mean 0.47 ± 0.47), 5≤SNR<8 (0.25 ± 0.29), 8≤SNR<11 (0.15 ± 0.22), and SNR≥11 (0.11 ± 0.18) groups. There were significant differences between the SNR<2 group and the 5≤SNR<8, 8≤SNR<11, and SNR≥11 groups (P<.05, Kruskal-Wallis u test).

Axial Length

Although there were no differences in the mean AL between the 5 SNR groups before surgery, the AL was shorter after surgery in all groups. The decrease in AL was largest in the SNR<2 group (mean 0.10 ± 0.09 mm). There were significant differences in the change in AL between the SNR<2 and 2≤SNR<5 (mean 0.06 ± 0.04 mm) groups and the 5≤SNR<8, 8≤SNR<11, and SNR≥11 groups (0.04 ± 0.06 mm, 0.04 ± 0.03 mm, 0.04 ± 0.03 mm, respectively) (P<.05, Kruskal-Wallis test). No significant differences were observed between the 3 groups with an SNR<5.

Postoperative Error in Predicted Refraction

Patients in the SNR<2 group became a mean of 0.31 ± 0.66 D hyperopic, and those in the other 4 groups became slightly myopic. There were no significant differences between the groups (P = .09) (Figure 1).

Figure 1
Figure 1:
Postoperative refractive prediction error in relation to SNR value. The SNR<2 group became slightly hyperopic, and those in the other 4 groups became slightly myopic (SNR = signal-to-noise ratio).

Type and Severity of Cataract

The percentage of patients with P4 or worse increased significantly with a decrease in SNR (P<.01, Spearman rank correlation) (Table 1). However, there was no correlation between the percentage of patients with an NC4 or worse and the SNR (P = .88, Spearman rank correlation) (Table 2).

Table 1
Table 1:
Type and severity of PSC in relation to the SNR value.
Table 2
Table 2:
Type and severity of nuclear sclerosis in relation to SNR value.

DISCUSSION

Our study found that the quality and reliability of AL measurements with the IOLMaster were influenced by the SNR value. The SNR correlated significantly with preoperative logMAR BSCVA and the frequency of P4 or worse PSC. With a low SNR, the IOLMaster exaggerated the AL, leading to a risk for postoperative hyperopia. Therefore, concomitant use of A-mode US is essential in eyes with an SNR<2. However, in eyes with a high SNR (≥5), the IOLMaster gave more accurate and reliable AL measurements.

Our results are supported by those of Olsen and Thorwest,16 who report that the SNR correlates significantly with visual acuity with considerable scatter and that the quality of IOLMaster AL readings is influenced by the SNR. They confirm that the error between US and IOLMaster measurements decreases significantly with increasing SNR, showing a minimum error in eyes with an SNR>2.1. All our patients had a complete preoperative evaluation including conventional contact biometry of the AL with US and measurement with the IOLMaster. If sufficiently reliable values can be obtained with the IOLMaster, US measurement is unnecessary except in patients in whom the IOLMaster cannot be used. Therefore, we also investigated the clinical indications for concomitant use of A-mode US.

As previously reported,11–18 the preoperative AL measurements were longer than the postoperative measurements in all groups of patients. The SNR<2 group had the largest difference in AL, and the preoperative measured value was longer than the actual length. Therefore, errors in predicted postoperative refraction tended to be larger, leading to a risk for postoperative hyperopia. The probable reasons for this outcome include that the percentage of patients with P4 or worse PSC was highest in the SNR<2 group because PSC causes the scattering of light reflected from the ocular fundus and decreases detectability. Accurate measurement of the AL cannot be expected at SNR<2; therefore, such results should be used as reference values, as suggested by the manufacturer, and concomitant US measurement should be performed. However, the error in postoperative predicted refraction was only 0.31 D, while IOL power is selected at 0.50 D intervals. Although the clinical impact appeared to be minimal, a tendency toward hyperopia was observed only in the SNR<2 group, which suggests it is better to increase the IOL power by 0.50 D when in doubt about the measured values. The error in postoperative predicted refraction in the 2≤SNR<5 group was comparable to that in the groups with higher SNR values. However, there were significant differences in the change in AL after surgery between the 2≤SNR<5 group and the groups with an SNR≥5. Based on these findings, we recommend the concomitant use of US measurement in eyes with an SNR<2 or 2≤SNR<5 to obtain better postoperative results. Also, in this study, similar patients were observed in the SNR<2 group (18 eyes, 8.3%) and the 2≤SNR<5 group (51 eyes, 23.6%). Therefore, it can be predicted US measurement will be necessary in 20% to 30% of patients scheduled for cataract surgery at Japanese general hospitals. There were no significant changes in AL after surgery in the 3 SNR≥5 groups, with the postoperative refractive prediction errors being −0.10 D or less. These findings indicate that accurate measurements can be obtained by using the IOLMaster without US measurement if the SNR is ≥5.

However, there are cases with an SNR≥5 in which exceptional conditions are present. These include cases of epiretinal membrane, macular edema, macular hole, ARMD, and poor fixation. We excluded preexisting macular disease in this study; however, when the data in such cases are used, the measurement with good waveform must be confirmed. Our results suggest it is possible to extend the use of preoperative optical biometry with higher accuracy and greater efficiency.

The IOLMaster has a significant disadvantage in that it will not work in every patient. In our study, the AL could not be measured by the IOLMaster in 12.2% of patients (34/278 eyes). This is similar to previously reported percentages (approximately 8%16 to 20%11,15) and suggests that further studies of US measurement are necessary.

Tehrani et al.12 report that uncorrected visual acuity and lens opacity are predictors of successful measurements. Similarly, we found that patients with severe PSC (P4 and worse) and poor preoperative vision (logMAR BSCVA 0.62) accounted for the majority of those with an SNR<2. We found no correlations between the SNR and severity of NS, as reported by Freeman and Pesudovs17 and Prinz et al.18 These findings indicate that IOLMaster measurements are affected by the severity of PSC.

In conclusion, we found the SNR value to be useful in confirming the good quality of the AL readings with the IOLMaster, even though the SNR value correlated significantly with the preoperative logMAR BSCVA and severity of PSC. Accurate measurement of AL can be achieved using the IOLMaster alone in eyes with an SNR≥5; however, the concomitant use of US measurement is preferable when in eyes with an SNR<5 to confirm IOLMaster readings.

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