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Clinical science
Influence of cataract surgery on repeatability and measurements of spectral domain optical coherence tomography
  1. Maria P Bambo1,2,
  2. Elena Garcia-Martin1,2,
  3. Sofia Otin1,2,
  4. Eva Sancho1,
  5. Isabel Fuertes1,2,
  6. Raquel Herrero1,2,
  7. Maria Satue1,2,
  8. Luis Pablo1,2
  1. 1Department of Ophthalmology, Miguel Servet University Hospital, Zaragoza, Spain
  2. 2Aragones Institute of Health Sciences, Zaragoza, Spain
  1. Correspondence to Dr Elena Garcia-Martin, C/Padre Arrupe, Ophthalmology Department, Miguel Servet University Hospital, Zaragoza 50009, Spain; egmvivax{at}yahoo.com

Abstract

Backgrounds/aims To evaluate the effect of uncomplicated cataract phacoemulsification on the measurements of macular and retinal nerve fibre layer (RNFL) in healthy subjects using two spectral domain (SD) optical coherence tomography (OCT) instruments—Cirrus OCT (Zeiss) and Spectralis OCT (Heidelberg)—and to assess the reliability of the measurements obtained with these two devices before and after cataract surgery.

Methods The study included 60 eyes of 60 healthy subjects (22 men and 38 women, 54–88 years of age) who underwent cataract phacoemulsification. One month before and one month after surgery, three repetitions of scans were performed using the RNFL and macular analysis protocols of the Cirrus and Spectralis OCT instruments. The differences between RNFL and macular thickness measurements obtained in the two visits were analysed. Repeatability was evaluated by calculating the coefficient of variation (COV) for each of the parameters recorded and for each visit.

Results The RNFL measurements obtained with the Cirrus and Spectralis OCT differed before and after surgery, and most of these differences were statistically significant (p<0.05). Macular thickness measurements using the Spectralis OCT were not significantly different between the two visits, whereas the differences found with the Cirrus OCT were statistically significant. The repeatability was better after surgical removal of the cataract, and the differences between COV in the two visits were significant with the Cirrus OCT.

Conclusions The presence of cataracts affects RNFL and macular measurements performed with SD-OCT. The repeatability of the images significantly improved after cataract phacoemulsification when using the Cirrus OCT.

https://doi.org/10.1136/bjophthalmol-2013-303752

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    Introduction

    Optical coherence tomography (OCT) allows cross-sectional imaging of the retina and optic disc based on interference patterns produced by low-coherence light reflected from the retinal tissues. The clinical utility of OCT for the study of several retina and optic nerve diseases has been established by numerous studies.1–3 The OCT data can be significantly affected by the OCT image quality. Factors such as the presence of cataracts, the extent of pupillary dilation and corneal dryness affect image quality and the repeatability of subsequent measurements.4–6

    Classic OCTs, such as the Stratus instrument, use a time domain (TD) technique that requires long acquisition times and provides axial and lateral resolutions of the order of 15 μm. Spectral domain OCT (SD-OCT) is a new system for obtaining high-resolution cross-sectional image and quantitative assessment of the retina and optic nerve. This technology is fast and is capable of producing three-dimensional volumetric measurements. The main difference between TD-OCT and SD-OCT (also called Fourier domain OCT) is the manner in which information is processed. TD-OCT uses a point detector or a photodetector in the detector arm, whereas SD-OCT uses a spectrometer that comprises a transmission grating and an air-spaced focusing lens. In this device, depth information is acquired by analysing the interference patterns in a spectrum of mixed reflected lights. To achieve ultra-high-resolution images, TD-OCT requires increased acquisition times, whereas SD-OCT obtains 2–3 µm axial resolution images, increasing the acquisition speed without a reduction in image quality.7 ,8

    OCT is an important ophthalmic diagnostic tool, particularly in macular diseases affecting the aging population, such as age-related macular degeneration and diabetic retinopathy. OCT imaging relies on near-infrared light. Similar to fundus photography and scanning laser ophthalmoscopy, during OCT a cataract is likely to increase light scattering and degrade the image quality. Other authors have reported variations in OCT measurements before and after uncomplicated cataract surgery but only for a few case series. These studies were particularly focused on a slight increase in macular thickness after surgery9–18 and on an apparent increase in retinal nerve fibre layer (RNFL) thickness due to the improved image quality.11 ,19–22 To the best of our knowledge, no studies have compared the repeatability before and after cataract surgery using the Spectralis OCT. Most of the previous reports analysed the influence of cataract surgery on measurements obtained with Stratus TD and Cirrus OCTs.9–23

    The aim of this study was to assess the influence of cataracts on SD-OCT scans, to determine the repeatability before and after surgery, and to evaluate preoperative and postoperative macular and RNFL thickness measurements in healthy subjects undergoing cataract surgery.

    Materials and methods

    This was an observational, prospective, longitudinal study. Sixty patients with a diagnosis of cataract (22 men and 38 women, 54–88 years of age) were enrolled in the study. Exclusion criteria were the presence of significant refractive errors (more than 5 dioptres of spherical equivalent refraction or 3 dioptres of astigmatism), intraocular pressure of 21 mm Hg or higher and a history of ocular pathologies (optic neuritis, retinal disease, etc.).

    All procedures adhered to the tenets of the Declaration of Helsinki, and the experimental protocol was approved by the local Ethics Committee. All subjects provided informed consent to participate in the study and underwent a complete ophthalmological evaluation that included pupillary, anterior segment and fundoscopic examinations; assessment of best-corrected visual acuity relative to the Snellen scale; and three repetitions of scans using RNFL and macular cube 512×128 analysis protocols of the Cirrus OCT and the RNFL protocol of the Glaucoma application and Fast Retinal protocol of the Retinal application of the Spectralis OCT. All measurements were performed 1 month before and 1 month after cataract surgery. Cataract surgery was carried out by the same surgeon using an Alcon Infiniti Phaco System with OZil available and an AcrySof intraocular lens (IOL) with blue-light-filtering technology because this type of IOL reduces the transmission of blue light to the retina in the same way as a yellow crystalline lens.20 Each eye was considered separately, and only one eye of each subject was included in the study. The indication for cataract surgery was reduced visual acuity below 0.6 on the Snellen scale or low enough to interfere with the patient’s life and daily activities or when no satisfactory visual function can be obtained with glasses, contact lenses and other optical aids. The study was conducted in the Miguel Servet University Hospital’s Ophthalmology Department in Zaragoza (Spain).

    The OCT tests were performed to obtain measurements of peripapillary RNFL using the Cirrus and Spectralis OCT devices, both of which were used in random order to prevent any effect of fatigue bias. All scans were performed by the same experienced operator. Between scan acquisitions, there was a time delay and subject position and focus were randomly disrupted, meaning that alignment parameters had to be newly adjusted at the start of each image acquisition. No manual correction was applied to the OCT output. An internal fixation target was used because it provides the highest reproducibility.24 The quality of the scans was assessed prior to the analysis, and poor quality scans were rejected. The Cirrus OCT determines the quality of images using a signal strength measurement that combines the signal-to-noise ratio with the uniformity of the signal within a scan and is measured on a scale of 1–10, where 1 is categorised as poor image quality and 10 as excellent image quality. Only images with a score higher than 7 were evaluated in our study. The Spectralis OCT uses a blue quality bar in the image to indicate the signal strength. The quality score range is 0 (poor quality) to 40 (excellent quality). Only images with a score higher than 25 were analysed. Only two patients were excluded because a centred scan could not be acquired due to poor fixation. Fifteen images with artefacts, missing parts or showing seemingly distorted anatomy were excluded.24 To obtain good quality and centred images, five eyes required repeat scan acquisition using the Cirrus OCT and three eyes using the Spectralis OCT.

    Three repetitions of optic disc cube 200×200 and macular cube 512×128 scans in each eye were performed using the Cirrus OCT. Following the recommended procedure for scan acquisition, the subject's pupil was first centred and focused in an Iris Viewing camera on the system data acquisition screen, and then the system's line-scanning ophthalmoscope was used to optimise the view of the retina.25 The OCT scan was aligned to the proper depth and patient fixation, and system polarisation was optimised to maximise the OCT signal. The Cirrus OCT optic disc protocol generates 200×200 cube images with 200 lineal scans that are performed by 200 A-scans. This option analyses an area of 6 mm3 around the optic nerve. In each series of scans, the mean RNFL thickness and quadrant RNFL thickness (superior, inferior, temporal and nasal) were analysed.

    Three image acquisitions with the RNFL protocol of the classic Glaucoma application and Fast Retina protocol of the Retinal application using Spectralis OCT were performed for all subjects using TruTrack eye-tracking technology. The Spectralis OCT system simultaneously captures infrared fundus and SD-OCT images at 40 000 A-scans per second. A real-time eye-tracking system measures eye movements and provides feedback to the scanning mechanism to stabilise the retinal position of the B-scan. This system thus enables sweep averaging at each B-scan location to reduce speckle noise. The mean number of scans to produce each circular B-scan was 9.

    Retinal thickness values were calculated with Cirrus and Spectralis OCT for the nine areas corresponding to the Early Treatment Diabetic Retinopathy Study (ETDRS).26 The ETDRS areas include a central 1 mm circle, representing the foveal area, and inner and outer rings of 3 and 6 mm diameter, respectively. The inner and outer rings are divided into four quadrants: superior, nasal, inferior and temporal. Central foveal thickness was also calculated.

    Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS V.19.0, SPSS Inc., Chicago, Illinois, USA). The Kolmogorov–Smirnov test was used to assess sample distribution. RNFL and macular thicknesses were compared before and after cataract surgery using a Student t test for paired data. p Values less than 0.05 were considered indicative of statistically significant differences.

    For each parameter, the coefficient of variation (COV) was calculated as the SD divided by the mean of the measurement value and expressed as a percentage. Most investigators consider that devices with a COV less than 10% have high repeatability, while a COV less than 5% indicates very high repeatability.7 Bland–Altman plots were used to assess agreement.

    Results

    In total, 60 eyes from 60 subjects (22 men and 38 women) were examined. The epidemiologic characteristics of the subjects are shown in table 1. The age range was from 54 to 88 years, with a mean of 73.9 years. Best-corrected visual acuity was 0.37±0.14 before cataract surgery and 0.84±0.18 after the surgery.

    View this table:
    Table 1

    Epidemiologic characteristics of the 60 subjects with cataract

    RNFL thickness comparison before and after cataract surgery

    We compared RNFL parameters obtained 1 month before and 1 month after cataract surgery using Cirrus and Spectralis OCTs. The first of the three RNFL thickness measurements in each visit was used for the analysis (table 2). Both devices detected significant differences in RNFL thickness between before and after surgery. Using the Cirrus OCT, statistical differences were observed in mean thickness and superior and temporal quadrants (p<0.05). The greatest difference was in the superior quadrant (5 µm higher after surgery, p=0.026). With the Glaucoma application of the Spectralis OCT, the mean thickness and the superior, inferotemporal, temporal and superotemporal RNFL sectors were significantly thicker after surgery (p<0.05). The largest difference was in the inferotemporal sector (10 µm thicker after surgery, p=0.029).

    View this table:
    Table 2

    First of the three RNFL thickness measurements of 60 subjects using Cirrus and Spectralis OCT (glaucoma application) before and after cataract surgery and statistical significance (p)

    Figure 1 shows the RNFL thickness differences before and after surgery for the mean thickness and the four quadrants measured by the Cirrus OCT (figure 1A) and the Glaucoma application of the Spectralis OCT (figure 1B). A slight increase in RNFL thickness after surgery was observed for all measurements.

    Figure 1
    Figure 1

    Representation of retinal nerve fibre layer thicknesses using Cirrus (A) and Glaucoma application of Spectralis optical coherence tomography (B) in 60 eyes of healthy subjects before and after surgery. Measurements were higher after cataract surgery.

    Macular thickness comparison before and after cataract surgery

    We compared all structural macular parameters obtained by the Cirrus and Spectralis OCTs in the two visits: 1 month before and 1 month after cataract surgery (table 3). The first of the three macular thickness measurements of each visit was used for the analysis. All macular parameters were significantly increased after surgery with Cirrus OCT (figure 2A). The largest difference was in the fovea thickness (60 µm higher after surgery, p=0.006). Using the Spectralis OCT, however, the macular thickness measurements were not significantly different after surgery (figure 2B).

    View this table:
    Table 3

    First of the three macular structural measurements of the 60 subjects using Cirrus and Spectralis OCT before and after cataract surgery and statistical significance (p) of comparison between both visits

    Figure 2
    Figure 2

    Representation of the nine Early Treatment Diabetic Retinopathy Study (ETDRS) areas macular thicknesses using Cirrus (A) and Spectralis optical coherence tomography (B) in 60 eyes of healthy subjects before and after surgery. Measurements were higher after cataract surgery.

    Repeatability of Cirrus and Spectralis OCT before and after cataract surgery

    RNFL and macular thickness measurements had lower COVs after surgery (table 4). The RNFL measurements obtained with Cirrus OCT were highly reproducible after surgery for all quadrants and sectors: the mean COV before surgery was 7.92±8.72% and after surgery 2.15±2.15% (p=0.005). The mean thickness after surgery had the lowest variability (COV=1.51%). Only the mean thickness and temporal quadrant COV values were significantly different between before and after cataract surgery (p=0.001 and 0.040, respectively; table 4).

    View this table:
    Table 4

    COV in % for repeated RNFL thickness measurements and repeated macular structural measurements and statistical significance (p) with Cirrus OCT and Spectralis OCT in the 60 subjects before and after surgery

    Measurements performed using the Glaucoma application of the Spectralis OCT had better repeatability after surgery, but the COVs were not significantly different between before and after surgery. The mean COV using this application before surgery was 4.86±5.10% and after surgery 3.13±3.70% (p=0.179). The mean thickness after surgery had the lowest variability (COV=1.92%). Figure 3 shows the Bland–Altman plots of RNFL mean thickness repeatability before and after cataract surgery using the Cirrus OCT and Glaucoma application of the Spectralis OCT. Macular thickness measurements had a lower COV after surgery with both devices. The COVs were significantly different before and after surgery for the fovea, superior inner, nasal inner, temporal inner and superior outer macular thicknesses using the Cirrus OCT, and in the fovea, superior inner, nasal inner and temporal outer macular thicknesses using the Spectralis OCT (table 4). The superior inner macular thickness after surgery had the lowest variability using either the Cirrus OCT (COV=0.83%) or the Spectralis OCT (COV=0.61%). Figure 4 shows the Bland–Altman plots of foveal thickness repeatability before and after cataract surgery using Cirrus and Spectralis OCTs.

    Figure 3
    Figure 3

    Graph of the agreement in retinal nerve fibre layer (RNFL) mean thickness using Cirrus optical coherence tomography (OCT) (A, before surgery; B after, surgery) and Glaucoma application of Spectralis OCT (C, before surgery; D, after surgery) in 60 eyes of subjects with cataract. Bland– Altman plots represent the difference (mean thickness measurement 1–2) against the mean of the three measurements of mean RNFL thickness. Measurements with both devices had decreased variability after surgery.

    Figure 4
    Figure 4

    Graph of the agreement in fovea thickness using Cirrus optical coherence tomography (OCT) (A, before surgery; B, after surgery) and Spectralis OCT (C, before surgery; D, after surgery) in 60 eyes of subjects with cataract. The difference (mean thickness measurement 1–2) is represented against the mean of the three measurements of mean thickness. Measurements with both devices had decreased variability after surgery.

    Discussion

    Despite the fact that OCT is widely used by ophthalmologists to diagnose and monitor patients, the influence of cataract on OCT measurements has been scarcely investigated. Several studies suggest that cataracts reduce image quality. This influence seems to change with the type of cataract, and nuclear cataracts have less influence on OCT image quality relative to cortical and posterior cataracts.4–6 Some studies indicate that RNFL and macular thicknesses are increased after surgery in healthy subjects.9–12 ,15 ,19 ,24 Most of these studies compared TD-OCT images or SD-OCT images before and after cataract surgery but did not evaluate the Spectralis OCT. For example, a recent study by Kim et al11 analysed differences in RNFL thickness measurements before and 8 weeks after surgery using TD and SD-OCT. The differences found were more prominent and frequent with SD-OCT, and change in the signal strength before and after surgery was statistically significant only in the case of SD-OCT. The number and moment of postoperative visits varied in the different studies, but all of them found differences (occasionally statistically significant) in RNFL or macular thicknesses measurements before and after cataract removal. Therefore, while Cagini et al9 conducted an assessment of the macular thickness measurements at 3, 6, 12, 20 and 28 weeks after surgery, others such as von Jagow et al12 considered the first day, 1 and 6 weeks after cataract phacoemulsification.

    To our knowledge, studies comparing cataract influence on Cirrus and Spectralis OCTs (both SD-OCT) or studies comparing repeatability of OCT measurements have not been published. Our study evaluated changes in RNFL and macular thicknesses after uncomplicated cataract phacoemulsification and also compared repeatability before and after surgery using the Cirrus and Spectralis OCTs. Our findings demonstrated that postoperative OCT scans showed a considerable increase in RNFL and macular thicknesses and improved repeatability after cataract removal.

    OCT instruments have recently become very useful for analysing optic disc and macular disorders.1 ,3 The presence of a cataract is common in elderly people and causes light scattering on OCT acquisition, which produces distortions that affect the OCT quality and measurements. In the present study, the OCT measurements were statistically different before and after cataract phacoemulsification, where the influence of cataracts should be considered when following up changes in a patient because an increase or decrease in RNFL or retinal thickness may be due to the presence or absence of the cataract rather than an actual pathologic change.

    In the case of retinal thickness measured by OCT devices, we found a significant increase of eight ETDRS areas using the Cirrus OCT, but not when using the Spectralis OCT. This observation is particularly interesting in patients with macular pathology whose progression is monitored by OCT (age-related macular degeneration, diabetic macular oedema, etc.). In these cases, if the follow-up after cataract surgery is performed using a Cirrus OCT, the ophthalmologist must consider that the observed changes may be due to the variability of the measurements associated with the cataract removal. Using the Spectralis OCT, however, the changes in macular thickness detected after surgery should be due solely to the patient’s pathology.

    Our study demonstrated that the Cirrus and Spectralis SD-OCTs have better repeatability after cataract removal. The clinical implication of these results is important because the measurements obtained with an OCT in patients with cataract should be interpreted cautiously: The changes observed in macular or RNFL thicknesses may be caused by the variability associated with the presence of cataract and not by an actual change in the patient’s pathology. Furthermore, our results show that there are some parameters whose variability improved more than others after cataract extraction: The repeatability of the mean and temporal quadrant thicknesses using a Cirrus OCT showed a statistically significant improvement after surgery. It is noteworthy that in case of a discrepancy in the measurements the most reliable parameters for the ophthalmologist (because they were the least variable) are the RNFL mean thickness and macular superior inner area thickness with both devices (Cirrus and Spectralis OCT) after surgery. These two parameters are reported to be less variable in other studies evaluating both healthy and pathological eyes.3

    Previous investigators have described the nasal RNFL thicknesses as being the most variable of the OCT parameters and the RNFL mean thickness as being the least variable OCT parameter.3 Our results confirmed these findings in both visits (before and after surgery) and with both OCT devices (Cirrus and Spectralis). The higher variability in RNFL nasal sectors could be explained in terms of it being harder to use the measurement algorithm to calculate RNFL thickness in the nasal quadrant.

    We observed lower variability using the Spectralis OCT (with Tru track technology) compared with the Cirrus OCT device before surgery. The Tru tracking technology seems to improve measurement repeatability because this system locks on to and follows the patient's retina and optic nerve during the scanning, independent of eye fixation, which improves repeatability. This might explain the decreased differences in the COVs observed before and after cataract surgery when using the Spectralis OCT. For this reason, we recommend using the Spectralis OCT in patients with cataract. After cataract removal, however, both devices have very good repeatability.

    Longer prospective studies using SD-OCT to analyse the changes on RNFL and macular thickness measurements after cataract surgery are needed. It is also important to implement studies with larger samples because most of the published articles only included a small series of cases.

    In conclusion, cataracts influence OCT measurements and this needs to be taken into account when interpreting OCT scans in elderly patients. Even in the presence of cataract the OCT scans of individual patients remain reliable for clinical interpretation of gross retinal pathology, although it should be noted that RNFL and macular thickness values may be lower due to the presence of the cataract.

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    Footnotes

    • Contributors EG-M confirm that I had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis as well as the decision to submit for publication. The principal author takes full responsibility for the data, the analyses and interpretation, and the conduct of the research. The author has full access to all of the data and has the right to publish any and all data separate and apart from any sponsor. This manuscript has not been evaluated in any form by another journal. MPB, EG-M, SO, ES, IF, RH, MS and LP have made a substantive intellectual contribution to the submitted manuscript (design and conceptualisation of the study and manuscript, analysis and interpretation of the data, and revising the manuscript). They have also given final approval of the version to be published.

    • .

    • Competing interests None.

    • Ethics approval Ethics Committee of the Miguel Servet Hospital.

    • Provenance and peer review Not commissioned; externally peer reviewed.