|Year : 2019 | Volume
| Issue : 2 | Page : 39-42
Comparison between corrected intraocular pressure using contact versus noncontact methods in glaucoma patients
Tamer Badran, Radwa El-Shereif
Department of Ophthalmology, Faculty of Medicine, Ain Shams University, Cairo, Egypt
|Date of Submission||01-Feb-2019|
|Date of Acceptance||26-Mar-2019|
|Date of Web Publication||19-Jul-2019|
Department of Ophthalmology, Faculty of Medicine, Ain Shams University, Cairo
Source of Support: None, Conflict of Interest: None
Background To determine the difference between corrected intraocular pressure (IOP), based on the central corneal thickness (CCT), measured by the noncontact anterior segment anterior segment optical coherence tomography (AS-OCT), versus contact ultrasound pachymetry (USP) in the management of glaucoma patients.
Patients and methods Ophthalmology Department, Ain Shams University, Cairo, Egypt. The study is a prospective, nonrandomized case series. Fifty patients were diagnosed with open-angle glaucoma and controlled with medical treatment with prostaglandin analogs and β-blockers fixed combination, controlled under medical treatment. CCT is measured by two methods, the AS-OCT and by the USP of ocular response analyzer. IOP was measured by applanation tonometry. Corrected IOP was calculated adjustment factor using Ehler’s correction nomogram, statistical analysis of the adjusted IOP with the contact and noncontact methods was done in relation to the CCT measurements.
Results The mean age was 60.5±5.24 years, 54% of the studied patients were men while 46% were women. The mean CCT using AS-OCT for the right eye was 529.86±43.52 μm and for the left eye was 530.28±43.57 μm, while CCT using USP for the right eye was 534.88±43.59 μm and for the left eye was 533.34±44.33 μm. Highly significant correlation between CCT was measured by AS-OCT and ultrasound; 0.94 (P<0.01). Mean adjusted IOP by AS-OCT CCT was 15.56±2.83 mmHg (11–22) and IOP adjusted by USP was 15.32±2.78 mmHg (10–21), with no significant differences (P=0.29) and highly significant correlation; 0.96 (P<0.01).
Conclusion Although USP has slightly higher measurements of CCT in comparison with measurements with anterior segment OCT, it has no significance on the adjusted IOP in controlled glaucoma patients, making the two methods equally accurate in the management and follow-up of these patients.
Keywords: agreement, corneal thickness, optical coherence tomography, ultrasound pachymetry
|How to cite this article:|
Badran T, El-Shereif R. Comparison between corrected intraocular pressure using contact versus noncontact methods in glaucoma patients. J Egypt Ophthalmol Soc 2019;112:39-42
|How to cite this URL:|
Badran T, El-Shereif R. Comparison between corrected intraocular pressure using contact versus noncontact methods in glaucoma patients. J Egypt Ophthalmol Soc [serial online] 2019 [cited 2019 Aug 18];112:39-42. Available from: http://www.jeos.eg.net/text.asp?2019/112/2/39/263009
| Introduction|| |
A series of published reports including the ocular hypertension treatment study demonstrated a strong correlation between central corneal thickness (CCT) with the development and progression of primary open-angle glaucoma (POAG) from ocular hypertension . Recent studies have suggested that CCT may be an independent risk factor for progression in patients with POAG and suggested that single measurement of CCT is not sufficient for proper glaucoma management because measurements vary on separate occasions . The corrected value of intraocular pressure (IOP) based on the CCT is considered essential for ensuring accuracy of management and follow-up of glaucoma cases .
The use of topical antiglaucoma medications could be associated with changes in the corneal structure, and prostaglandin analogs, for example, may decrease CCT values , while some studies realize that β-blockers may result in a reversible increase in CCT . It is important to determine the CCT values serially during the follow-up of glaucoma patients due to the longitudinal changes that might occur owing to the medication .
Corneal thickness measurements can be performed using ultrasonic-based or optic-based techniques. The most commonly used method is applanation ultrasound pachymetry (USP). However, USP has several possible sources of error such as probe misplacement, lack of fixation light for gaze control, oblique positioning of the probe in relation to the cornea, corneal compression during measurement, and sound transmission variability due to dryness .
The optical technique is done using ophthalmic optical coherence tomography (OCT) with a wave length of 1310 nm; it is capable of obtaining high-definition cross-sectional images of cornea by adjusting a corneal adaptor module, and this provides both central and regional pachymetry . The anterior segment optical coherence tomography (AS-OCT) image represents the differential backscattering contrast between different tissue types on a micron scale. It is a gray scale or false color two-dimensional representation of backscattered light intensity in a cross-sectional plane; AS-OCT provides a useful tool for the assessment of corneal thickness.
| Patients and methods|| |
The study included the eyes of 50 patients of the outpatients’ clinic at the Department of Ophthalmology, Ain Shams University. All patients were diagnosed with POAG, a typical glaucomatous optic disk with focal or diffuse structural damage, glaucomatous visual field defect in at least two consecutive reliable automated visual field examinations that are at least 3 months apart and an IOP equal to 21 mmHg or more, with no limitation to age or sex. Exclusion criteria included patients with any corneal diseases including dry eye, past history of ocular trauma, any intraocular surgery, recent contact lens wear and diabetes mellitus.
All patients received a combination of topical antiglaucoma medications: prostaglandin analogs and β-blockers. All patients underwent: visual acuity assessment, complete slit-lamp examination, and IOP measurements were done using contact Goldmann applanation tonometry. Two measurements were recorded at 10 a.m. and 8 p.m., and measures were discarded if a difference of more than 2 mmHg is recorded. CCT measurements before both IOP measurements using anterior segment OCT were done first, using Cirrus HD-OCT model 5000 (Carl Zeiss AG, Oberkochen, Germany) ([Figure 1]), followed by USP to preserve intact corneal epithelium, using an ocular response analyzer (Reichert Inc., New York, USA). If the patient moved and the probe decentered, then the measurements were repeated. During the AS-OCT, the central cornea was identified from the peak of the reflectivity profile on the horizontal axis. The calipers were then aligned on the peak reflections at the anterior and posterior boundaries of the cornea, in the axis of the corneal apex. Corneal anesthesia is given using drops of 4% benoxinate hydrochloride (BENOX; E.I.P.I.Co., Cairo, Egypt).
|Figure 1 Anterior segment optical coherence tomography for one of our cases on topical antiglaucoma medications.|
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Experienced independent operator completed all the measurements of the CCT for each eye within 3–4 min apart to ensure the same possible diurnal variations.
Measured IOP is corrected with regard to the CCT measured by both AS-OCT and USP, using the equation
Corrected IOP = Goldmann applanation IOP − (CCT − 545)/50 × (X mmHG)
All patients signed an informed consent under the tenets of the Helsinki guidelines and all the aspects of the study were explained and statistical analysis and reporting were done by SPSS version 21, using t-test by (IBM, Armonk, New York, USA).
| Results|| |
The mean age of the included patients was 60.5±5.24 years; 54% of the patients were men while 46% were women. There were no significant difference between sex groups in all variants ([Table 1]).
|Table 1 Sex difference in central corneal thickness measurements, with both anterior segment optical coherence tomography and ultrasound pachymetry|
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Mean CCT measured using AS-OCT for all right eyes was 529.86±43.52 μm, while the mean CCT measured AS-OCT for all left eyes was 530.28±43.57 μm. Mean CCT measured using USP for all right eyes was 534.8843.59 μm, while the mean CCT measured using USP for all left eyes was 533.34±44.33 μm. Statistical analysis showed a highly significant correlation between CCT measured by AS-OCT and ultrasound; 0.94 (P<0.01).
The mean corrected IOP for all eyes using CCT measured by anterior segment OCT was 15.56±2.83 mmHg with a range of 11–22. The mean corrected IOP for all eyes using CCT measured by USP was 15.32±2.78 mmHg, with the range 10–21. Statistical analysis showed no significant differences between corrected IOP when using CCT measured by AS-OCT versus USP (P=0.29); also, there was highly significant correlation between the corrected IOP measured by the two methods; 0.96 (P<0.01).
| Discussion|| |
The measurement of CCT is influenced by factors such as the type of the device used, the measuring operator, age, sex, daily variation, and history of intraocular surgery. We were able to minimize most of the variants that may affect the measurement of CCT, by excluding any corneal comorbidities, and by assigning an experienced independent operator, with minimal SD in both age and sex of the study sample.
While the average mean CCT in our normal population is considered to be about 545 μm, our results showed a relatively reduced mean CCT, measured by both AS-OCT and USP, irrelative to sex or age. This can be attributed to the use of antiglaucomatous medications especially prostaglandins, which was reported to decrease the mean CCT with prolonged treatment .
Many authors reported the comparison of the CCT measures obtained from AS-OCT versus that of the USP. Bayhan et al.  reported a sample with a mean age of 29.78±4.9 years, in which he found the mean CCT measured by AS-OCT was 525±34 μm in comparison to 543±37 μm measured by USP. Kim et al.  also reported a sample of 57±12 years. They reported a mean difference of 26.3 μm between AS-OCT and USP measurements. Chen et al.  sample was 23.9±2.7 years; the mean difference in CCT was 34 μm between USP and AS-OCT. All authors reported a noticed underestimation of CCT when measured by AS-OCT in comparison with that measured by USP ,,. This is coherent with the results of our study sample as there was a statistically insignificant underestimation of CCT measured by AS-OCT as compared with USP; mean CCT measured using AS-OCT for all right eyes was 529.86±43.52 μm while the mean CCT measured AS-OCT for all left eyes was 530.28±43.57 μm. Mean CCT measured using USP for all right eyes was 534.88±43.59 μm, while the mean CCT measured using USP for all left eyes was 533.34±44.33 μm, still with high correlation between measurements of both methods.
This underestimation of CCT measured by AS-OCT can be attributed to decentration of the eye, or the ultrasound probe to the cornea, or the effect of topical anesthesia, because of the variability of ultrasound speed in tissues of different hydration. On the other hand, the small calibration error in AS-OCT may play a role. Another point to be considered is the differences in analysis software between the two techniques, while using slightly different methodologies ,,.
Many authors have confirmed the importance of the correlation between CCT and measured IOP, for accurate screening, diagnosis, and management of glaucomatous patients. And many formulas have been suggested for the correction of the measured IOP in relation to the CCT , for that it is important to confirm that the difference between the CCT measurements obtained by different methods will not significantly influence the corrected IOP, hence the accuracy of the glaucoma management.
In our sample, we found that the corrected IOP using AS-OCT CCT was slightly higher than that obtained by USP CCT with a mean difference of 0.24 mmHg. This was statistically insignificant with high correlation between the two methods.
| Conclusion|| |
Finally, we can conclude that both methods, AS-OCT and USP when executed in an expert way, will be equally accurate in the measurement of CCT, and will not affect the final result of corrected IOP, ensuring better accuracy in monitoring glaucoma patients.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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