Original Article
Difference in Central Corneal
Thickness between Applanation Ultrasound and Oculus Wave Light Occulyzer II
Munira Shakir, Ronak Afza
Memon, Sahira Wasim, Shakir Zafar
Pak J Ophthalmol 2019, Vol. 35, No. 3
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See end of article for authors affiliations …..……………………….. Correspondence to: Munira
Shakir Department of
Ophthalmology, Liaquat National Hospital Karachi Email: dr_munirasz@yahoo.com |
Purpose: To determine the mean
difference of central corneal thickness measurements by using ultrasound
Pachymetry and oculus Wave light Occulyzer II. Study Design: Cross-sectional comparative
study using non-probability consecutive sampling. Place and Duration of Study: This
study was conducted at Department of Ophthalmology, Liaquat National Hospital
Karachi from November 2018 to June 2019. Material & Methods: After approval
from ethical committee, patients were included in our study on the basis of
inclusion & exclusion criteria. Central corneal thickness measurements
were taken by using ultrasound Pachymetry & oculus Wave light Occulyzer
II. All the data was collected by single researcher. The results were
plotted, compared & analyzed. Paired t-test was used for the comparison
of quantitative variables. Results: There were 130 patients
included in the study out of which 73 (56.2%) were males and 57 (43.8%) were
females. Mean age of these patients was Conclusion: There is a high correlation of
central corneal thickness between the readings obtained from ultrasound and
optical pachymetry machines therefore oculus Wave light Occulyzer II can be
used as an alternative technique to ultrasound pachymetry while assessing CCT
in clinical settings. Key Words: Central corneal thickness,
Ultrasound Pachymetry, Optical Pachymetry. |
Cornea is the main refractive surface of
human eye and along with sclera it forms the outer fibrous layer of eyeball. It
is transparent and avascular, with normal diameters of about 11-12 mm & 9-11
mm horizontal & vertical respectively1.
Corneal thickness is a precise indicator of
corneal hydration & normal functioning of endothelial pump2. CCT
has also got an important role in determining the flap thickness along with
residual stromal thickness before refractive surgeries3. Besides, central
corneal thickness, which measures about the central 3 mm of cornea, is an
independent risk factor for the development and progression of glaucoma4.
There are various methods for assessing the
central corneal thickness. The most common is ultrasound pachymetry, which
measures CCT by estimating the time difference between echoes of ultrasound
waves reflected from anterior and posterior surface of cornea. But this method
carries more chances of errors due to misalignment of probe or obliquely placed
probe in relation to cornea, lack of light fixation, excessive indentation
during procedure & dryness related problems which include variability in
sound transmission5. Another method is Corneal topography that uses
newer technique known as Scheimpflug imaging, in which a rotating camera is
used to photograph corneal cross-sections illuminated by slit beams at
different angles (Pentacam). It provides information about anterior segment
including iris, angle and cillary body. From these details, the corneal maps
give an idea about corneal thickness at different points along with the thinnest
point6.
The rationale of our
study is to find pachymetry techniques with reliable results. Internationally
available research also shows wide variability. As central corneal thickness
has impact on measuring the intraocular pressure which in turns helps in early
detection and management of glaucoma and will help in reducing the overall
burden of blindness caused by glaucoma. The purpose of our study is to measure
the central corneal thickness using contact (ultrasound) technique &
non-contact (oculus Wave light Occulyzer II) technique and compare the results
of two methods in our population.
MATERIAL AND METHODS
This study was conducted from Nov 2018 to June
2019 at the department of Ophthalmology, Liaquat National Hospital Karachi
after approval from ethical committee. The total sample size of 130 patients
was calculated using WHO software taking 95% confidence level. One eye of each of
130 patients, aged between 20-50 years was assessed. Thorough slit lamp examination
was performed.
Patients with corneal problems (e.g. scar,
keratoconus, keratoglobus), those who were using contact lenses, those who had
previous history of refractive or any ocular surgery, those with history of
ocular trauma or using ophthalmic drops were not included in this study.
Central corneal
thickness was firstly measured on oculus Wave light Occulyzer II (Am Wolfmantel
91058 Erlangen, German). Local anesthetic drops were then instilled and central
corneal thickness was measured by ultrasound pachymetry (Sonomed model 300 AP+)
by placing the probe perpendicular to the cornea. Five consecutive readings were
taken. All measurements of central corneal thickness were conducted by a single researcher.
Patient’s data was compiled and
analyzed through Statistical Package for Social Sciences (
Mean ± SD was calculated for age and
corneal thickness as quantitative variables. Sample size was calculated by taking Mean ± SD of the thinnest corneal thickness of 538.7 ±
0.29 μm5 with oculus Wave light Occulyzer II using margin of
error (d) = 5%. Paired t-test
was used for the comparison of quantitative variables. P ≤ 0.05 was considered
as significant.
RESULTS
There were 130 patients attending
ophthalmology department and fulfilling the inclusion criteria. One eye of each
patient was measured (65 were right eyes and 65 were left eyes). 73 (56.2%)
were males and 57 (43.8%) were females (table 1). Mean age of these patients
was 33.9 ± 8.9 years. About 78 patients were less than 35 years & 52 were
greater than 35 years (table 1).
In our study, we found
that the mean thinnest Pentacam measurement was 538.615 ± 23.4677 μm and
ranged between 476.0 and 619.0 μm, whereas the mean thinnest ultrasound Pachymetry
measurement was 535.1 ± 21.816 μm and ranged between 482 and 601 μm.
(Table 2).
Table 1: Descriptive Statistics.
Characteristics |
Mean ± SD |
Age |
33.93 ± 8.91 |
Age Group |
|
≤ 35 years |
27.6 ± 4.26 |
>35 years |
43.42 ± 4.52 |
There was highly significant correlation of
central corneal thickness between both the instruments. (r = 0.96, p <
0.001).
According to the results
of our study, there were no differences of CCT readings measured by two devices
i.e. oculus wave light occulyzer and ultrasound Pachymeter.
Table 2: Comparison of central corneal thickness between
Ultrasound pachymeter and topography.
|
n(%) |
Ulrrasound Pachymeter |
Topography |
Correlation R |
P-value |
Corneal Thickness |
130 (100 |
535.10
± 21.8 |
538.61 ± 23.4 |
0.96 |
< 0.001** |
Age |
|
|
|
|
|
≤ 35 years |
78 (60 |
535.29
± 22.65 |
537.59 ± 23.48 |
0.97 |
< 0.001** |
>
35 years |
52 (40) |
534.82
± 20.71 |
537.59 ± 23.48 |
0.95 |
0.01** |
Gender |
|
|
|
|
|
Male |
73 (56.2) |
532.28
± 20.26 |
536.64 ± 21.55 |
0.96 |
< 0.001** |
Female |
57 (43.8) |
538.71
± 23.34 |
541.14 ± 25.68 |
0.95 |
0.017** |
Eyes |
|
|
|
|
|
Left |
65 (50) |
536.29
± 22.12 |
539.29 ± 24.16 |
0.96 |
0.001** |
Right |
65 (50) |
533.92
± 21.60 |
537.95 ± 22.91 |
0.96 |
< 0.001** |
Paired t-test is applied.
*Significant at p-value < 0.05
**Insignificant at p-value > 0.05
DISCUSSION
Central corneal thickness measurement has a
major role in both diagnostic and therapeutic aspects7. Accurate
assessment of central corneal thickness is necessary for various concerns such
as used preoperatively to prevent corneal ectasia prior to refractive surgery8.
CCT represents the physiologic function of corneal endothelium9,10.
It is also useful for diagnosis of some corneal diseases like keratoconus,
which is a progressive disease having four stages (1-4), causing thinning and steepening
of central cornea11. CCT also helps to decide the management options
for Keratoconus, which include corneal crosslinking (CXL) and corneal
transplant. CXL basically stabilizes the disease process and requires at least
400-450 μm of central corneal thickness. CCT helps in cases of Fuch’s
dystrophy12.
Glaucoma is one of the main causes of
blindness these days. Intraocular pressure (IOP) being an important and
modifiable risk factor in diagnosis and management of glaucoma has a correlation
with central corneal thickness measurement13.
There are various methods for CCT
measurement, which include both contact and non-contact methods. An ideal
method should be accurate, safe, easy, and less time consuming. Ultrasound
pachymetry is considered as gold standard for CCT measurement14,15.
This method requires contact with central cornea. UP has got the disadvantage
of using topical anesthetic agents which can affect the thickness. Fixation
along with proper position of ultrasound probe has got a major role in CCT
measurement. Errors can also occur due to excessive indentation and dryness. On
the other hand, pentacam overcomes all the above problems with UP16.
Pentacam uses the rotating Scheimpflug principle, which obtains about
25,000 data points for assessing not only CCT but also corneal curvature and
anterior chamber details17.
Our study compared the central corneal
thickness by applanation ultrasound and oculus Wave light Occulyzer II. Most of
the previous literature review showed greater CCT with oculus Wave light Occulyzer
II as compared to ultrasound pachymetry (UP). The reason behind this could be
the displacement of pre-corneal tear film, which is about 7-30 μm. In addition,
the compression by ultrasound probe over the epithelium can give thinner CCT measurements
by UP18.
Study conducted by Khater et al also compared
the mean thinnest corneal thickness with oculus Wave light Occulyzer II 538.7 ±
29.35 μm and with Quantel Pocket II UP 527.6 ± 28.04 μm. Their study
showed that both devices are highly correlated & Wave light Occulyzer II
can be used as a substitute for UP19.
Zlatanović et al reported the mean CCT
with Occulyzer as 552.94 μm ± 22.88 μm and with ultrasound Pachymetry
as 559.46 ± 26.0 μm. There were no statistically significant differences
among both devices20. Piotrowiak et al conducted a study which
showed higher value of CCT with ultrasound Pachymetry (555 μm) as compared
with pentacam (545 μm)21. Tai et al showed closest agreement
for Len Star–UP, followed by Len Star–Pentacam and Pentacam–UP22.
Limitation of our study was the small
sample size and single center for data collection. Multicenter studies are
required for further analysis.
CONCLUSION
According to the results
of our study, there is a high correlation of central corneal thickness found between
the readings obtained from both UP and oculus Wave light Occulyzer II. So we
concluded that oculus Wave light Occulyzer II can be used as an alternative technique
to ultrasound pachymetry, while assessing CCT in clinical settings to decrease
the risk of procedure associated problems with UP like epithelial trauma and
infection, to decrease the frequent use of topical anesthetic agents, for early
detection and management of glaucoma & in anxious patients as well.
Disclaimer
None.
Conflict of Interest
None.
Source of Funding
None.
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Author’s Affiliation
Dr. Munira Shakir
FCPS, FRCS
Department of
Ophthalmology, Liaquat National Hospital, Karachi
Dr. Ronak Afza Memon
MBBS, R4
Department of
Ophthalmology, Liaquat National Hospital, Karachi
Dr. Sahira Wasim
MBBS, R4
Department of
Ophthalmology, Liaquat National Hospital, Karachi
Dr. Shakir Zafar
FCPS, Department of
Ophthalmology, Liaquat National Hospital, Karachi
Author’s Contribution
Dr. Munira Shakir
Study Design, Manuscript
writing, critical analysis.
Dr. Ronak Afza Memon
Researcher, data
collection and manuscript writing.
Dr. Sahira Wasim
Data analysis,
Manuscript writing.
Dr. Shakir Zafar
Statistical analysis and
critical review.