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Axial length measurement and detection rates using a swept-source optical coherence tomography–based biometer in the presence of a dense vitreous hemorrhage

Wang, Qinmei MD; Huang, Yingying MD; Gao, Rongrong MD; Chen, Hao MD; Song, Benhao MD; Tu, Ruixue MD; Zhao, Yune MD; McAlinden, Colm MD, PhD; Huang, Jinhai MD, MS, PhD

Author Information
Journal of Cataract and Refractive Surgery: March 2020 - Volume 46 - Issue 3 - p 360-364
doi: 10.1097/j.jcrs.0000000000000072
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Vitreous hemorrhage is the extravasation of blood into the vitreous body; it occurs when blood leaks from ruptured vessels into the vitreous cavity.1 Vitreous hemorrhage usually happens spontaneously and in cases of ocular trauma in adult patients. Spontaneous causes include proliferative diabetic retinopathy (DR), retinal break, retinal vein occlusion, and traumatic posterior vitreous detachment.2–4 The most common causes are DR and trauma. Vitreous hemorrhage in these cases is usually accompanied by cataract, or cataract is iatrogenically induced during vitrectomy in the management of vitreous hemorrhage. Typically, patients undergoing vitrectomy have biometry performed preoperatively either as part of the plan to remove cataract concurrently or in anticipation in cases of lens touch or poor intraoperative retinal view.5,6

Axial length (AL) is an important parameter of intraocular lens (IOL) power calculation before cataract surgery. Approximately 50% of refractive errors after IOL implantation can be accounted for by inaccuracies in AL measurements.7–9 A difference of 1.0 mm in the AL affects the final refraction by 2.50 diopters (D) in an average eye and up to 3.00 D in a short eye. Ultrasound (US) was the earliest technique to measure AL but has been superseded by optical instruments because of its deficiencies, such as poor repeatability, contact measuring, and lower degrees of resolution.10,11 The IOLMaster (Carl Zeiss Meditec AG), with biometry based on partial coherence interferometry (PCI), has been widely used in the clinic for AL measurements.12,13 The LENSTAR LS 900 (Haag-Streit AG), another biometer based on optical low-coherence reflectometry (OLCR), was introduced in 2009 and has proved to have high agreement with the IOLMaster.14 However, these 2 optical biometers have limitations when the media opacity is significant, such as dense vitreous hemorrhage and dense cataract, resulting in high AL measurement failure rates.15 In such cases of failure to acquire the AL, US is typically used, but it still encounters the above-mentioned shortcomings.

The OA-2000 (Tomey GmbH) is a swept-source optical coherence tomography (SS-OCT)-based biometer that has strong penetration with the 1060 nm wavelength.16 The SS-OCT is a Fourier-domain OCT technique in which the wavelength of the light source is tuned in rapid cycles to sequentially scan the eye.17,18 Compared with the broadband light source, SS-OCT has a higher signal-to-noise ratio because the reflections of the narrow-bandwidth wavelength light source are projected to the eye one at a time, which in turn improves image quality and tissue penetration.16,19 However, to our knowledge, there are scant data describing the validity of the SS-OCT biometer and other optical biometer measurements in patients with vitreous hemorrhage.

The aim of the present study was to evaluate AL measurement detection rates using the OA-2000 SS-OCT biometer. Furthermore, the study aimed to compare the agreement between the OA-2000, IOLMaster, LENSTAR, and US in eyes with vitreous hemorrhage.

METHODS

This prospective study comprised patients with vitreous hemorrhage at the Eye Hospital of Wenzhou Medical University. Inclusion criteria were adults with vitreous hemorrhage in one or both eyes. Exclusion criteria were severe corneal opacities, nystagmus, retinal detachment, or Lens Opacities Classification System III (LOCS III) score greater than 4.5.20,21 Informed consent was obtained from each patient after the study had been fully explained. This research conformed to the tenets of the Declaration of Helsinki. This study was approved by the Research Ethics Committee at the Eye Hospital of Wenzhou Medical University.

Each participant underwent a full ophthalmic examination including pupil dilation. Then the same experienced ophthalmologist (H.C.) used LOCS III to grade the type and severity of cataract and used the Forrester system to classify the grade of vitreous hemorrhage.20 The LOCS III decimal scale was used for each of the 4 possible components of cataract: nuclear opalescence, nuclear color, cortical, and posterior subcapsular cataract. The scale ranges from 0.1 (clear or colorless) to 5.9 (very opaque, in cases of cortical and posterior subcapsular) or 6.9 (very opaque or brunescent, in cases of nuclear opalescence and color).22 Patients who were pseudophakic were scored as zero. The 4 LOCS scores were added to represent the severity of cataract. The Forrester system divides the vitreous opaqueness into 5 grades, severity is gradually rising from grade V to grade I.23 Forrester grade V means the vitreous is clear, grade IV means the central vitreous is clear with a few small residual opacities, grade III means patches of the fundus are visible between discrete vitreous opacities, grade II is defined as a slight increase in the red reflex but no fundal detail visible, and grade I means a completely opaque vitreous.23 As grades V and VI are almost clear, patients who were diagnosed as grade V or VI were excluded.

The AL measurement was attempted with the OA-2000, IOLMaster (v5.4), LENSTAR (v2.1), and US (Axis-II, Quantel Medical). The 3 optical biometers were used in a random order. US was always performed last through the contact technique and by the same experienced examiner (Y.H.). At least 3 reliable measurements were acquired from every biometer.

Statistical Analysis

Statistics were calculated using the IBM SPSS Statistics for Windows software (version 21.0, IBM Corp.). A P value less than 0.05 was considered statistically significant. The Kolmogorov-Smirnov test was used to assess data for normality. For each biometer, the number of unobtainable AL measurements was recorded. The detection rate of AL measurements with each device was then grouped as per the Forrester grade and compared. Then, in each grade, the rate was subgrouped again by the severity of cataract using total scores. The Bland-Altman limits agreement (LoA) test was used to assess the agreement in AL measurements between the OA-2000 and other 3 techniques.24

RESULTS

In total, 38 patients (22 men) were recruited with a mean age of 59.35 years ± 11.52 (range 31 to 84 years). Forty eyes were scanned with each of the 4 biometers. The most common cause of vitreous hemorrhage was DR (27/40 eyes [67.5%]), and the rest were caused by other vascular ocular disease, sequenced as branch retinal vein occlusion (6 eyes), agnogenic vitreous hemorrhage (3 eyes), Eales disease (1 eye), age-related macular degeneration (1 eye), polypoidal choroidal vasculopathy (1 eye), and retinal macroaneurysm (1 eye). There were no cases of trauma. According to the duration, acute hemorrhage occurred within 2 weeks in 12 (30%) of 40 eyes. Vitreous hemorrhage occurred within 2 to 4 weeks in 13 (32.5%) of 40 eyes. Vitreous hemorrhage occurred within 1 to 3 months in 2 (5%) of 40 eyes. Vitreous hemorrhage occurred greater than 3 months in 13 (34.0%) of 40 eyes.

From the 40 scanned eyes, the overall measurement success rate was 62.5% (25 eyes) with the OA-2000, 15% (6 eyes) with the IOLMaster and LENSTAR, and 100% with the US. As grade I vitreous hemorrhage occurred in only 1 eye, it was combined with grade II. The AL measurement success rate and mean AL measurements are shown in Table 1. The measurement success rate for subgroups in relation to LOCS III cataract grading in each Forrester grade is shown in Table 2. In subgroups, as the lens opacity increased, the detection rates for AL measurements with the IOLMaster and LENSTAR reduced but remained stable with the OA-2000 (P < 0.05).

Table 1
Table 1:
Axial length measurement and measurement detection rates for different grades of vitreous hemorrhage.
Table 2
Table 2:
Axial length measurement detection rates for different grades and different severity of cataracts.

The Bland-Altman plots and 95% LoA (Figures 1 to 3) were used to assess the agreement in AL measurements between the OA-2000 and other 3 techniques. The results show that the mean difference was small, and the 95% LoA were narrow for all comparisons (OA-2000 vs LENSTAR [0.04 mm, −0.04 to 0.12 mm], OA-2000 vs IOLMaster [−0.05 mm, −0.15 to 0.05 mm], and OA-2000 vs US [0.13 mm, −0.3 to 0.56 mm]). Retaining only the eyes in which it was possible to acquire an AL measurement from the LENSTAR in comparison with the OA-2000 and US, the difference was smaller and the LoA is narrower ([0.00 mm, −0.3 to 0.29 mm], Figure 4). The results of consistency comparison indicated that these devices can be used interchangeably for AL measurements.

Figure 1
Figure 1:
Bland-Altman plot showing agreement in the axial length measurements between the swept-source optical coherence tomography–based biometer (OA-2000) and IOLMaster. Dotted gold line = zero line; thick solid blue line = mean difference (bias); dashed red lines = 95% limits of agreement.
Figure 2
Figure 2:
Bland-Altman plot showing agreement in the axial length measurements between the swept-source optical coherence tomography–based biometer (OA-2000) and LENSTAR. Dotted gold line = zero line; thick solid blue line = mean difference (bias); dashed red lines = 95% limits of agreement.
Figure 3
Figure 3:
Bland-Altman plot showing agreement in the axial length measurements between the swept-source optical coherence tomography–based biometer (OA-2000) and ultrasound. Dotted gold line = zero line; thick solid blue line = mean difference (bias); dashed red lines = 95% limits of agreement.
Figure 4
Figure 4:
Bland-Altman plot showing agreement in the axial length measurements between the swept-source optical coherence tomography–based biometer (OA-2000) and ultrasound for eyes in which the axial length could be acquired from the LENSTAR. Dotted gold line = zero line; thick solid blue line = mean difference (bias); dashed red lines = 95% limits of agreement.

DISCUSSION

The earliest and most widely used optical biometer is the IOLMaster based on PCI, with the LENSTAR based on OLCR also gaining popularity.25–28 These optical instruments are noncontact, thus avoiding iatrogenic injury to the cornea. With the point fixation, biometers can measure accurately along the optical axis with a resolution of up to 10 μm.13,29–31 A number of studies have compared the IOLMaster and LENSTAR for the general population, patients with cataract, and patients after corneal refractive surgery. The overall trend is that there is a good agreement, repeatability, and reproducibility. The present study included these 2 biometers with an SS-OCT device in the assessment of the AL in patients with vitreous hemorrhage. In the current study, the total detection rate with both the IOLMaster and LENSTAR was 15%, and in eyes with dense vitreous hemorrhage (Forrester grades I and II), this reduced to 3.57%. It shows hemorrhage greatly limits the application of these optical biometers. In such cases, an US measurement is typically used in the clinic.

The total detection rate of the SS-OCT biometer (OA-2000) was 62.5%, much higher than that of the other 2 optical biometers. In vitreous hemorrhage Forrester grade II, the detection rate of the OA-2000 is 14 times higher than that of the IOLMaster and LENSTAR. In grade III, detection rates are both 100%. With the use of a longer wavelength (1060 nm), there is reduced scattering and attenuation from ocular opacities compared with shorter wavelengths (780 nm for the IOLMaster and 840 nm for the LENSTAR).17,18 The study by McAlinden et al.21 also showed the OA-2000 had a higher detection rate (100%) than the IOLMaster (63.9%) or Aladdin (86.5%) in cataract. Several recent studies compared the SS-OCT–based IOLMaster 700 (1055 nm tunable laser source) with IOLMaster 500 (780 nm tunable laser source) in cataract and showed that the longer wavelength (1055 nm) had a better detection rate.32 An improved signal-to-noise ratio with SS-OCT manifests superior image quality and tissue penetration,16,19 which may explain the improved performance in eyes with severe vitreous hemorrhage.

As shown in Table 2, detection rates with the OA-2000 were similar to different subgroups, but rates decreased with the other 2 optical biometers as the LOCS III scores increased. In our previous study, the OA-2000 was compared with the IOLMaster and Aladdin in terms of AL measurements in eyes with cataract.21 The detection rate with each device was grouped according to the LOCS III scores for each type of cataract (nuclear color, cortical, and posterior subcapsular). It was found that the failure rate was 0% with the OA-2000 for all types and severity of cataracts, but the detection rate with the IOLMaster and Aladdin was more than 80% when the LOCS III scores were less than 2.5 and more than 60% when the LOCS III scores were less than 3.5. In the present study, 92.5% of participants had a cataract severity equal to or less than 3.5; hence, cataract was considered to have small influence on AL measurements in this study. Vitreous hemorrhage in eyes with Forrester grade I is completely opaque, even the red reflex cannot be seen from the fundoscope, and all 3 optical biometers failed to measure the AL in these cases. Nearly half of the grade II eyes failed to measure the AL with the OA-2000. By analyzing the B-scan of grade II eyes, we found that most of the failures had vitreous hemorrhage just anterior to the macula or along the optical axis. US was capable of measuring both grade I and II vitreous hemorrhage with a 100% detection rate. As US has a longer wavelength (0.19 mm) and lower frequency (8 MHz), it has better penetration than optical biometers. But the accuracy should be carefully considered. One reason is that vitreous hemorrhage will change the density of the vitreous with variability, causing the propagation velocity to be altered. Another is that US measures the AL between the anterior surface of the cornea and the inner limiting membrane, but the AL that is used in the IOL calculation formulas is between the anterior surface of the tears and the retinal pigment epithelium.

The Bland-Altman analysis with the 95% LoA showed a good agreement between the 4 biometers, and differences between the OA-2000 and the other 2 optical biometers are less than 0.1 mm. This corresponds to approximately a 0.25 to 0.30 D refractive error. So far, to our knowledge, there are only a few studies relating to the SS-OCT biometers. Huang et al. compared the OA-2000 with the IOLMaster (v5.4) in normal eyes; SS-OCT showed high intraobserver repeatability (coefficient of variation 0.10%) and interobserver reproducibility (coefficient of variation 0.04%), and the difference with the IOLMaster was low (0.01 mm, 95% LoA of −0.05 to 0.07 mm).33 McAlinden et al. compared the OA-2000 with the IOLMaster (v5.4) and the Aladdin; the results of the Bland-Altman analysis also showed high agreement (OA-2000 vs IOLMaster [−0.01 mm, 95% LoA −0.09 to 0.08 mm], OA-2000 vs Aladdin [−0.00 mm, 95% LoA −0.05 to 0.04 mm]).21 Goebels et al. studied the same 3 optical biometers as in the present study; their difference in AL measurements was statistically significant but not clinically significant (the mean difference of 0.01/0.06 mm).29 Kongsap et al. compared the OA-2000 with the IOLMaster; they found the OA-2000 had a higher detection rate than the IOLMaster (98.53%/79.41%) and obtained excellent correlations between the 2 instruments.34

There are some limitations to our study. First, the influence of cataract is not ruled out. However, most cases of vitreous hemorrhage patients have an accompanying cataract, and perhaps, the AL measurement is more meaningful in eyes with cataract than without. Second, more than half of the vitreous hemorrhage included in our study was caused by DR. Spontaneous causes and ocular trauma were not included.

In conclusion, the findings of this study have significant implications. The SS-OCT biometer (OA-2000) has outperformed both the IOLMaster PCI and LENSTAR OLCR biometers in severe vitreous hemorrhage; it has good agreement with the IOLMaster, LENSTAR, and US measurements. The SS-OCT–based biometer could overcome some weaknesses of the existing optical and US instruments; it may become a better choice than US instruments for the measurement of the AL in dense vitreous hemorrhage.

WHAT WAS KNOWN

  • The OA-2000 is a swept-source optical coherence tomography–based biometer that has a high signal-to-noise ratio and excellent penetration. Previous studies demonstrated that the OA-2000 has a higher detection rate than other commonly used biometers but it had not been studied in patients with vitreous hemorrhage.

WHAT THIS PAPER ADDS

  • Swept-source optical coherence tomography–based biometers can overcome the weaknesses of the existing optical and ultrasound instruments and are particularly useful in eyes with dense media opacities.

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