|Year : 2016 | Volume
| Issue : 3 | Page : 597-600
Use of ultrasonic pachymetry to compare the changes in postoperative corneal thickness in extracapsular cataract extraction and in phacoemulsification
Mayada H Shebl1, Ahmed A El-Hagaa2, Mostafa K Nassar2
1 Department of Ophthalmology, Shebin El-koum Ophthalmology Hospital, Shebin El-koum, Egypt
2 Ophthalmology Department, Faculty of Medicine, Menoufia, Egypt
|Date of Submission||05-Jan-2015|
|Date of Acceptance||18-Feb-2015|
|Date of Web Publication||23-Jan-2017|
Mayada H Shebl
2nd Melig Road, 32511, Shebin El-Koum
Source of Support: None, Conflict of Interest: None
The aim of the present study was to evaluate and compare the changes in corneal thickness after extracapsular cataract extraction (ECCE) and phacoemulsification using ultrasonic pachymetry.
During cataract surgery, several mechanisms may lead to corneal endothelial injury and consequent edema. Ultrasonic pachymetry is the gold standard technique to evaluate corneal thickness changes following different types of cataract surgeries.
Patients and methods
Of the 50 patients included, 25 patients underwent phacoemulsification (group 1) and 25 patients underwent ECCE (group 2). The inclusion criteria were corneal clarity, absence of lens subluxation, axial lengths of 21-24 mm, and fully dilatable pupils. The exclusion criteria were pre-existing corneal disease, lens subluxation, pseudo-exfoliation syndrome, coexisting eye disease, or a complicated cataract surgery. Corneal pachymetry was carried out preoperatively and on postoperative days 1, 7, 14, and 21.
The mean preoperative central corneal thickness (CCT) was 546.64 ± 49.95 in group 1 and 537.12 ± 41.01 in group 2 (P > 0.05). The postoperative day 1 CCT was 627.52 ± 66.93 in group 1 and 682.88 ± 68.85 in group 2 (P < 0.05). The postoperative week 1 CCT was 558.92 ± 52.97 in group 1 and 602.68 ± 71.50 in group 2 (P < 0.05). The postoperative week 2 CCT was 548.36 ± 51.99 in group 1 and 585.96 ± 49.37 in group 2 (P < 0.05). The postoperative week 3 CCT was 547.56 ± 50.01 in group 1 and 538.20 ± 41.13 in group 2 (P > 0.05).
Ultrasonic pachymeter is a reliable instrument that can be used to obtain rapid, accurate, and reproducible measurements of corneal thickness. In short-term follow up, the postoperative corneal edema after phacoemulsification cataract surgery offers faster recovery compared with that after ECCE.
Keywords: cataract, corneal edema, pachymetry
|How to cite this article:|
Shebl MH, El-Hagaa AA, Nassar MK. Use of ultrasonic pachymetry to compare the changes in postoperative corneal thickness in extracapsular cataract extraction and in phacoemulsification. Menoufia Med J 2016;29:597-600
|How to cite this URL:|
Shebl MH, El-Hagaa AA, Nassar MK. Use of ultrasonic pachymetry to compare the changes in postoperative corneal thickness in extracapsular cataract extraction and in phacoemulsification. Menoufia Med J [serial online] 2016 [cited 2020 Apr 3];29:597-600. Available from: http://www.mmj.eg.net/text.asp?2016/29/3/597/198725
| Background|| |
During cataract surgery, several mechanisms may lead to corneal endothelial injury. These mechanisms include direct iatrogenic trauma, ultrasound energy from phacoemulsification, and irrigation fluid turbulence. Despite the use of viscoelastics, corneal edema is one of the commonest complications after cataract extraction, affecting ~10% of the patients  . The endothelial insult caused by cataract surgery is best studied by a specular microscope, which delineates the extent of morphological and numerical changes in the endothelial cells  . However, in clinical settings, corneal thickness varies in direct proportion to the endothelial cell function  . Pachymetry is defined as the measurement of corneal thickness  . The instrument used to measure corneal thickness is referred to as a pachymeter  . Central corneal thickness (CCT) can be assessed by means of many instruments, including specular microscopy, confocal microscopy, ultrasound pachymetry, ultrasound biomicroscopy, slit-scanning corneal topography (Orbscan), the rotating Scheimpflug camera (Pentacam), optical biometry, and spectral optical coherence tomography  ; yet, ultrasound technology is one of the most commonly accepted in terms of accuracy  . Its advantages include ease of use, portability, and low cost. Its disadvantages include being a contact technique and the need for topical anesthesia, which can modify CCT values  .
| Patients and methods|| |
This prospective study was carried out to evaluate the use of ultrasonic pachymetry to compare the changes in CCT in patients undergoing cataract surgery. The study included 50 patients undergoing cataract surgery at the Department of Ophthalmology, Menoufia University Hospital. The study was approved by the scientific ethical committee of Menoufia medical school, and the patients signed written consent after they were informed about the pachymetry procedures in detail. Of the 50 patients included, 25 patients underwent phacoemulsification (group 1) and 25 patients underwent extracapsular cataract extraction (ECCE) (group 2). The inclusion criteria were corneal clarity, absence of lens subluxation, axial lengths (21-24 mm), and fully dilatable pupils. The exclusion criteria were pre-existing corneal disease, lens subluxation, pseudo-exfoliation syndrome, coexisting eye disease, or complicated cataract surgery.
The patients underwent thorough history taking, general examination, and routine laboratory investigations. The preoperative ocular examination included measurement of uncorrected visual acuity and best corrected visual acuity (using Landolt Broken Ring Chart, then converted to log MAR fraction), slit lamp examination of the anterior segment, intraocular pressure measurement by Goldman's applanation tonometer, fundus examination (direct and indirect), ophthalmic ultrasonography (to assess the retina, the posterior vitreous face, and the vitreous), biometry, and keratometry. CCT was measured by using an ultrasonic pachymeter (PACSCAN, Sonomed, NY, USA); Sonomed Inc.). The patients were examined in the sitting position with a fixed target for the patient to look at to help in the better alignment of the probe. Before taking the measurement, the ultrasonic pachymeter was properly calibrated to ensure accurate measurement and the ultrasound velocity was set at 1640 m/s. The details of the pachymetry process were explained to the patient; a drop of topical anesthetic agent (benoxinate HCl 0.4%) was applied to the eye to be examined. The clean dry probe tip was gently placed perpendicularly on the cornea at the optical center to take the CCT measurement. The process was repeated to take three additional measures and the final measurement was the average of the four measurements. This gives a mean ultrasound pachymetry reading with an SD. The reading was accepted when the SD was less than 2.0 mm. The cooperative younger patients with low to moderate nucleus density were chosen to undergo phacoemulsification, whereas the older patients with higher nucleus density were chosen to undergo ECCE. All the surgeries were carried out by surgeons of the same clinical and surgical experience. The average operative time for phacoemulsification surgeries was 30 min, whereas in ECCE surgeries the average time was 37.5 min. In phacoemulsification, the foldable acrylic hydrophilic intraocular lens was used (diameter: 12.5 mm, optic: 6 mm, A-constant: 118.2, angle: 5°). In ECCE, the single piece polymethylmethacrylate intraocular lens was used (diameter: 18 mm, optic: 6.5 mm, A-constant: 118.2, angle: 10°). The phacoemulsification machine used was megaTRON s4-Geuder® (Megatron S4, Geuder, Heidelberg, Germany). The first postoperative CCT measurement was taken before discharging the patient from the hospital and three follow-up visits were scheduled (day 7, day 14, and day 21). The Student t-test was used to measure the significance of CCT changes.
| Results|| |
There was a statistically significant difference between the two groups regarding age and preoperative visual acuity (P < 0.05). There was no statistically significant difference between the two groups regarding sex, preoperative intraocular pressure readings, and preoperative CCT (P > 0.05). The preoperative CCT mean ± SD in the phaco group (group 1) was 546.64 ± 49.95 μm, whereas it was 537.12 ± 41.01 μm in the ECCE group (group 2). The phaco group (group 1) patients had postoperative corneal edema, which was evident by the CCT that increased from 546.64 ± 49.95 μm preoperatively to 627.52 ± 66.93 at postoperative day 1 (P < 0.05). The edema resolved and the CCT returned to the preoperative range after 1 week (558.92 ± 52.97, P < 0.05). The week 2 and week 3 measurements carried no statistically significant difference from the preoperative values ([Table 1]). The ECCE group (group 2) patients also had postoperative corneal edema, which was evident by the CCT that increased from 537.12 ± 41.01 μm preoperatively to 682.88 ± 68.85 at postoperative day 1 (P < 0.05). The edema took longer time to resolve compared with the phaco group and the CCT remained significantly higher than the preoperative levels at week 1 (602.68 ± 71.50, P < 0.05) and week 2 (585.96 ± 49.37, P < 0.05) measurements to - finally - reach the preoperative range after 3 weeks (538.20 ± 41.13, P > 0.05) ([Table 2]). There was no statistically significant difference between group 1 and group 2 regarding the preoperative and week 3 CCT values (P > 0.05). There was a statistically significant difference between group 1 and group 2 regarding the CCT values at day 1, week 1, and week 2 (P < 0.05) ([Table 3]).
| Discussion|| |
Corneal endothelial decompensation after cataract extraction is a well-known complication of all types of cataract surgeries. The overall incidence is less than 1%  . In severe cases, the resultant corneal edema can negatively impact patients' postoperative visual outcomes  . Corneal thickness is an important factor to evaluate corneal barrier and endothelial pump function and in the diagnosis of corneal diseases  . Ultrasound pachymetry has been the gold standard for estimating the corneal thickness for the past few decades because of its high degree of interobserver, intraobserver, and interinstrument reproducibility  . In this study, the authors evaluated the role of ultrasonic pachymetry in assessing the postoperative corneal thickness changes in ECCE and phacoemulsification. Regarding the patient's age mean and SD, for group 1 the mean ± SD was 58.08 ± 6.41 years, whereas for group 2 the mean ± SD was 66.64 ± 6.82 years. In a similar study, the mean age for the ECCE patients was 57.0 ± 6.6 years  ; in another study on patients undergoing phacoemulsification, it was 71 ± 15 years  . The preoperative CCT mean ± SD in the phaco group was 546.64 ± 49.95 μm, whereas it was 537.12 ± 41.01 μm in the ECCE group (no significant difference between the two groups, P > 0.05). This was in contrast to the results of Sati et al.  who found that the preoperative CCT mean ± SD for the phaco group was 524.65 ± 32.67 and 525.55 ± 33.03 μm for the ECCE group. In our study, we found a statistically significant increase in CCT in the day 1 measurement in both groups, but the following measurements showed a significant difference in the resolution of corneal edema between the two groups. In group 1, the edema resolved and the CCT returned to the preoperative range after 1 week only (558.92 ± 52.97, P < 0.05), whereas in group 2, the edema took longer time to resolve compared with the phaco group, and also the CCT values did not return to the preoperative levels except after 3 weeks postoperatively (538.20 ± 41.13, P > 0.05). These results are comparable to those of the study by Sati and colleagues in which the preoperative value of corneal thickness was achieved on day 30 in ECCE, day 15 in small incision cataract surgery (SICS), and day 7 in phacoemulsification. On day 7, a highly significant (P ≤ 0.01) difference was observed in corneal thickness among all the three techniques. However, no significant (P ≥ 0.05) difference in corneal thickness was observed on day 15 between phacoemulsification and SICS. This means, endothelial damage at central cornea was the least in phacoemulsification, followed by SICS and then ECCE  . In addition, in the study of Ravalico and colleagues, the coefficient of variation in size, corneal thickness, and endothelial permeability were significantly increased 7 days postoperatively in both groups and 30 days postoperatively in the ECCE group. The differences between the two groups were statistically significant. The endothelial pump function was significantly increased after 7 days in the phacoemulsification group only  . Stumpf and Nosι  found no significant difference in the pachymetric indices between the phaco group and the ECCE group. Sood et al.  found that ECCE group CCT increased from 520 ± 20 μm preoperatively to 600 ± 70 μm at week 1 follow-up, and then the values declined steadily to a value of 540 ± 40 μm at week 6 follow-up.
The most important features of a measuring technique are accuracy and precision. Because the true value of the corneal thickness of a particular eye is unknown, it is impossible to calculate the accuracy of measurements. Therefore, there should be an agreement between new devices and an instrument considered to be the gold standard that should be used. The precision of measurements may be stratified into repeatability and reproducibility. Repeatability is defined as the variability of results obtained from one object in the same measurement conditions (time, instrument, technique, place, and operator); on the other hand, reproducibility is the variability of results obtained from one object using the same device in different measurement conditions  .
In their study on endothelial permeability, Dνaz-Valle et al.  reported that corneal pachymetry is not useful for assessing postoperative endothelial changes. Leung et al.  and colleagues found that CCT measurements by optical coherence tomography and ultrasound pachymetry are highly correlated but they warned of the potential ultrasound pachymetry inaccuracy caused by its need to touch the cornea, the need for topical anesthesia, and the uncertainty about the perpendicularity of the probe over the corneal surface. Piotrowiak et al.  found that the mean CCT measurements from the ultrasound pachymetry are 9.8 μm higher than those from the Pentacam, and 17.82 μm higher than those from the optical coherence tomography; the differences are statistically significant and they reported that different technologies cannot be used interchangeably without using correcting coefficients. Another study found that noncontact specular microscope is a more precise and reproducible instrument for the measurement of CCT than the ultrasonic pachymeter, but both are reliable, useful instruments for measuring CCT  . Three different studies found that CCT the measurements by Orbscan II and by ultrasonic pachymetry are not significantly different ,, .
| Conclusion|| |
Corneal edema is one of the commonest complications after cataract extraction. CCT is an important parameter in evaluating postoperative corneal edema. Ultrasonic pachymeter is a reliable portable instrument that can be used to obtain rapid, accurate, and reproducible measurements of corneal thickness at a reasonable cost. In short-term follow up, postoperative corneal edema after phacoemulsification cataract surgery offers faster recovery compared with that after ECCE.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]