Comparison of central corneal thickness in corneal edema by ultrasound pachymetry, specular microscopy, and anterior segment optical coherence tomography : Saudi Journal of Ophthalmology

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Original Article

Comparison of central corneal thickness in corneal edema by ultrasound pachymetry, specular microscopy, and anterior segment optical coherence tomography

Artuç, Tuncay1,; Batur, Muhammed2

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Saudi Journal of Ophthalmology 37(1):p 1-5, Jan–Mar 2023. | DOI: 10.4103/sjopt.sjopt_63_20
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Abstract

INTRODUCTION

Accurate measurement of corneal thickness continues to be important in the diagnosis, evaluation, and management of corneal diseases, with accurate detection of intraocular pressure and refractive and nonrefractive corneal surgical procedures. Although there are many studies that compare the reliability, correlation, and agreement of measurement devices with healthy corneas, a very few studies involving pathological corneas, especially cases of corneal edema, have been conducted.

In a large study on healthy corneas, a strong correlation was shown between ultrasonic pachymetry (UP), specular microscopy (SM), and anterior segment optical coherence tomography (ASOCT).[1] UP, which has been the gold standard in corneal thickness measurement for many years, is a more user-dependent method and has to compete with automated noncontact devices that have found a widespread clinical use. UP requires direct corneal contact with the ultrasonic probe, therefore topical anesthesia is required and contamination is a concern. Unlike UP, some noncontact devices are capable of mapping the entire cornea. For these reasons, UP has some disadvantages compared to noncontact devices. Nevertheless, UP is still the most widely used method for central corneal thickness (CCT) measurement due to its low cost and ease of use. SM acquires images using specular reflections from the smooth interface of two environments with different refractive indices. Finally, ASOCT acquires real-time in vivo images from different tissue layers using the low-coherence interferometry method.[2] In this way, it provides a detailed visualization of all layers forming the cornea.[3]

In this study, our goal was to compare the CCT values measured with UP, SM, and ASOCT in patients who experienced corneal edema after routine cataract surgery and to determine the correlation and agreement among those methods.

METHODS

Preoperative pachymetric measurements were performed in all patients undergoing cataract surgery. Sixty-seven eyes of 67 patients who underwent uncomplicated phacoemulsification and foldable intraocular lens implantation to treat senile cataracts, and who experienced corneal edema, were included in this prospective study. A signed informed consent form was obtained from each patient. The study commenced after receiving approval from the Van Yüzüncü Yıl Clinical Research Ethics Committee and was carried out in accordance with the Declaration of Helsinki. Criteria for exclusion from this study included corneal pathology (such as scars or dystrophy) and systemic collagen tissue disease resulting in possible predisposition to corneal anomalies; suspected or obvious corneal ectasia (such as keratoconus or pellucid marginal degeneration); active or chronic uveitis and corneal endothelial disease (such as Fuchs' dystrophy); combined surgery (such as phacoemulsification and trabeculectomy); and surgical complications (such as posterior capsule tear, vitreous loss, or Descemet membrane detachment).

Measurements were taken first with ASOCT, followed by SM, and then UP uniformly for all patients. Each of the ASOCT and SM measurements was performed by an experienced technician, while the UP measurements were performed by a single doctor (T. A.). Three reliable measurements were taken with ASOCT and SM, and an average value was obtained, whereas the average of 10 reliable values in UP was taken. Patients, whose heads were held in fixed position in the ASOCT (Spectralis® Optical coherence tomography, Heidelberg Engineering, Heidelberg, Germany) and SM (Tomey, EM-3000, Japan) devices, were asked to look straight ahead at a distant target while serial measurements were carried out. For UP (Quantel Aviso, Quantel Medical, Axis Nano™ France) measurements, patients were placed in a comfortable seat and asked to look straight ahead at a distant target 1 min after topical proparacaine (Alcaine, Alcon Lab, USA) was administered. Serial measurements were carried out by touching the probe tip vertically to the center of the corneal apex without pressing on the cornea. Between measurements, patients were asked to blink to ensure the redistribution of the tear film layer. The probe tip was disinfected between each patient.

The paired samples t-test was used to determine mean, minimum and maximum CCT, and standard deviation. Correlation between the devices was determined by the intraclass correlation coefficient (ICC), and the agreement was illustrated by Bland–Altman plots and limits of agreement (LoA) values. IBM SPSS (SPSS Statistics for Windows, Version 17.0. Chicago: SPSS Inc.) was used for statistical analysis.

RESULTS

The mean age of the 67 patients was 70.99 ± 9.56 (36–92) years. The demographic characteristics of the patients are given in Table 1. While all 67 patients were examined preoperatively, and on the 1st and 7th days after surgery, only 52 (77.6%) patients returned for examination on the 14th day. Measurements were taken with all three devices in all eyes before surgery. On the 1st, 7th, and 14th days after the surgery, measurements were made with ASOCT in all eyes. Percentages (numbers) of patients whose eyes were measured with UP and SM on postoperative day 1, day 7, and day 14, respectively, were: 11.7% (24) versus 16.1% (27); 82.1% (55) versus 77.6% (52); and 88.8% (46) versus 86.9% (45) [Figure 1 and Table 2].

T1-1
Table 1:
Demographic characteristics of the patients
T2-1
Table 2:
Preoperative and postoperative mean central corneal thickness
F1-1
Figure 1:
The number (percent) of patients with which postoperative devices could measure (Postoperative 1st, 7th, and 14th days, respectively)

Preoperative ICC values among each of the devices (UP vs. SM, UP vs. ASOCT, SM versus ASOCT) were determined to be >0.90 [Table 3]. This correlation continued between UP and SM, and between SM and ASOCT on postoperative days 1 and 7, while it was below 0.90 between UP and ASOCT (0.86 and 0.85, respectively) [Table 3]. On the 14th day, ICC > 0.90 was detected among all devices again [Table 3]. Based on the width of the LoA values, the preoperative best fit was between UP and ASCOT, whereas it was the best between UP and SM on postoperative days 1, 7, and 14 [Table 3]. Considering the mean difference values, the smallest difference was between preoperative and postoperative measurements on the 14th day between UP and ASOCT, and on the 1st and 7th days postoperatively between UP and SM [Figure 2]. Due to the limitations of UP and SM, a total of only 98 measurements could be made of eyes with CCT ≥600 μm. Of these, 42 (42.8%) and 38 (38.7%) were made with UP and SM, respectively.

T3-1
Table 3:
Comparison of the correlation and agreement between the devices
F2-1
Figure 2:
Pre- and postoperative Bland-Altman plots (preop: Postoperative, postop: Postoperative, UP: Ultrasonic pachymetry, OCT: Optical coherence tomography, SM: Specular microscopy)

DISCUSSION

Cataract and glaucoma surgeries, vitrectomy, keratoplasty, all intraocular surgeries, endothelial dystrophies, and corneal hydrops are some common causes of corneal edema in clinical practice. Although there are minor differences among studies in which CCT measurements are performed with different methods on healthy corneas, there is a general consensus on the high correlation and reproducibility between UP and noncontact devices.[4,5,6,7] In addition, a good correlation between UP and noncontact measurements continues to be reported in eyes that have undergone photorefractive keratectomy[8] and in myopic eyes.[9] However, there are a very few studies on CCT measurement in patients with corneal edema.[10,11,12,13] Techniques that are used in practice to evaluate corneal edema include slit-lamp biomicroscopy, UP, SM, ASOCT, corneal tomography (Orbscan II, Sirius, Pentacam, etc.), and in vivo confocal microscopy.[14,15]

There are fewer publications on the use of SM in corneal edema. López-Miguel et al. compared CCT in patients with postoperative corneal edema using UP and SM similar to our study.[12] They stated that SM-measured mean CCT, both preoperative and postoperative, was lower than that measured with UP, and they also reported comparable LoA width and mean difference.[12] In our work, when the agreement was compared based on the width of LoA values, the best preoperative result was between UP and ASOCT, while the best fit was found between UP and SM on the 1st, 7th, and 14th days after surgery. The stronger agreement between UP and SM after surgery may be due to the fact that these devices could be used to measure eyes with relatively lower CCT [Table 2]. While there was a significant difference between UP and SM before surgery and on the 7th and 14th days after surgery, there was no significant difference on the 1st postoperative day (P = 0.147). This may be due to the small number of patients (roughly same patients) that could be measured with both SM and UP (n = 18), and because these patients had relatively low CCT [Tables 2 and 3].

A strong correlation has been reported between UP and ASOCT in terms of CCT in postoperative corneal edema.[11,13] CCT measurements made with Visante and Cirrus ASOCT devices and UP from patients with corneal edema and in these eyes, which were affected by corneal decompensation and dystrophies, were compared with the two-way random, absolute agreement, single-measure model, and high ICC values were reported between the ASOCT and UP measurements[13]. In our study, the correlation between UP and ASOCT in the measurements performed with ICC two-way random, absolute agreement, single-measure model was relatively lower on postoperative days 1 and 7 (preoperative 0.89, 1st postoperative day 0.71, 7th postoperative day 0.71, and 14th postoperative day 0.85). The reason for this may be that the eyes in our study had a more acute corneal edema picture than those in the study by Wongchaisuwat et al.[13] It is known that UP is not a suitable method in cases of severe corneal edema,[11,13] and the data in our study showed that the reliability of UP decreased even more significantly in such cases. By the 2nd week, our correlation values between UP and ASOCT were similar to those reported by Wongchaisuwat et al. and approached preoperative values. In addition, when the mean differences were compared according to the Bland–Altman plots, the best values were between UP and ASOCT before surgery and on the 14th day after surgery. In the same article, the authors also stated that the high correlation between ASOCT and UP measurements continued in eyes with CCT of at least 650 μm.[13] In our study, the number of eyes that UP and SM could measure was very limited when ASOCT-measured CCT was 650 μm or greater [Table 4]. For this reason, eyes with CCT ≥600 μm as measured with at least two of the three devices at any time postoperatively were considered (if any device measured below 600 μm, that eye was not included). There was a poor correlation between UP and ASOCT (ICC: 0.23) and between UP and SM (ICC: 0.41), but there was a good correlation between SM and ASOCT (ICC: 0.83) in these eyes.

T4-1
Table 4:
The number and ratios of the eyes that the ultrasound pachymetry and specular microscopy could measure in eyes with central corneal thickness values 600 μm ≤ and 650 μm ≤ measured by anterior segment optical coherence tomography

ASOCT can reveal the two- and three-dimensional structure of the cornea, displaying the true state of the tissue with accurate structural measurements[15,16,17] and provides near-histologic resolution.[18] Unlike retinal imaging, it has the ability to penetrate deeper structures (to a depth of about 6 mm, e.g., to the scleral spur) since longer wavelength (around 1300 nm) light sources can be used in ASOCT.[2] SM requires two smooth surfaces to make accurate measurements. The anterior and especially posterior corneal surfaces that are impaired in acute corneal edema due to postoperative endothelial losses restrict the use of SM. However, when SM was able to measure eyes with relatively high CCT (600 μm and above, n = 27), had a good correlation with ASOCT (r = 0.83).

CONCLUSION

According to our literature review, this is the first study in which CCT is evaluated progressively over short intervals using different devices in eyes with acute corneal edema after uncomplicated phacoemulsification. As a result, UP, SM, and ASOCT show high correlation in cases with corneal edema. However, the ability of UP and SM to measure in cases where the corneal thickness reaches high values (over 600 μm) was decreased. The reliability of UP in eyes with acute corneal edema and corneal thickness over 600 μm is quite low, whereas noncontact devices (SM and ASOCT) are more reliable. The advantage of ASOCT over UP and SM is that it can measure all the cases with corneal edema and perform three-dimensional corneal imaging.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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Keywords:

Anterior segment optical coherence tomography; cataract surgery; central corneal thickness; corneal edema; specular microscopy; ultrasound pachymetry

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