Download Central Corneal Thickness and Relating Factors

June 2009
A. Elias et al. - CCT
149
ORIGINAL
ARTICLE
Central Corneal Thickness and
Relating Factors - A Prospective
Observational Study
Dr. Anna Elias DNB, Dr. A. Giridhar MS DO, Dr. Mahesh G. MS DO DNB FRCS(Ed),
Dr. Savita Bhat MS DO DNB MNAMS, Dr. Ramkumar DO MS
Abstract
Aim: To investigate the association between central corneal thickness and degree of myopia, corneal
curvature, axial length and age.
Methods: It was a prospective observational study. 53 emmetropic subjects and 53 myopes were
studied. Central corneal thickness was measured in all patients with an ultrasonic pachymeter.
Axial length was measured using an A-scan and corneal curvature was measured using a Keratometer.
CCT was correlated with degree of myopia, axial length, corneal curvature and age using the Karl
Pearson’s Correlation Coefficient.
Results: 106 patients (203 eyes) were recruited for the study. The age of patients ranged from
18 to 45 years, with a mean of 32 years. Myopia ranged from –0.5D to –17.5D with a mean of
–2.8D. The mean CCT was 543.11 microns. The mean axial length was 23.69 mm and the mean
corneal curvature was 43.68D.
Karl Person’s Correlation Coefficient was as follows:
CCT & degree of myopia : r = 0.11
p = 0.122
CCT & corneal curvature : r = - 0.23
p = 0.001
CCT & axial length : r = 0.038
p = 0.944
CCT & age : r = 0.0048
p = 0.696
Conclusion: There was no correlation between CCT & degree of myopia. There was a significant
negative correlation between CCT & corneal curvature. There was no correlation between CCT &
age or axial length.
Key Words: Myopia, CCT
Introduction
Although the true etiology of myopia is still unknown,
the cornea is responsible for approximately two-thirds
Giridhar Eye Institute, Ponneth Temple Road, Kadavanthara, Cochin – 682 020,
Email: [email protected]
of optical refraction and its role in myopia has
consequently been studied intensely over the years.
Most of the changes in the myopic eye are located in
the posterior segment namely, thinning of the retina
and sclera, posterior staphyloma, choroidal atrophy and
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Kerala Journal of Ophthalmology
a higher incidence of retinal detachment. Changes in
the anterior segment associated with myopia are still
under debate. The myopic eye is known to be longer
than the normal emmetropic eye. If this is the result of
general growth, one might expect the cornea to have
grown thicker than normal. If instead, the myopic eye
is larger due to a mechanism similar to that of a balloon
being inflated, one would expect the cornea to be
thinner than normal according to a simple stretching
theory. An emmotropic eye could then be compared to
a sphere and a myopic eye to a prolate spheroid 1.
Myopia is increasing in prevalence among the
populations of East Asian origin. Estimates of the
proportion of myopia in the young population of South
East Asian countries range from 30% to 60%. 2 With
increasing rates of myopia, refractive surgery such as
laser in situ Keratomileusis (LASIK) has become popular
in Asia. When undertaking such surgery to correct
myopia, central corneal thickness (CCT) is an important
consideration to prevent the cornea from becoming too
thin after treatment.
Studies that have attempted to investigate the effect of
refractive errors on CCT have reported conflicting
results. Some studies have reported no correlation
between corneal thickness and level of myopia 3,
whereas some studies have found the cornea to be
thinner in more myopic eyes 4.
Central corneal thickness (CCT) indicates corneal
physiologic health and affects the measurement of intra
ocular pressure (IOP). A thin central cornea is a risk factor
for development of glaucoma in patients with ocular
hypertension 5. A thin scleral bed in deeply excavated
optic nerves has been the quintessential finding in
advanced glaucomatous eyes. 6 This has further led to
a hypothesis that thinness of the corneas may be an
indication of generalized weakness of the ocular integument
associated with longer eyes with a thin scleral bed of
lamina cribrosa. Studies have tried to determine if thin
corneas are associated with longer globes. 7
Aim
The purpose of the study was to determine the
correlation between CCT and 4 variables, namely,
degree of myopia, corneal curvature, axial length and
age of the patient.
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Materials & Methods
A prospective observational study was done. 106
consecutive patients were enrolled in the study; 53
patients were myopes and 53 patients were
emmetropes. Patients were in the age group 18 years
to 45 years. Subjects with previous ocular surgery,
glaucoma or any disease affecting the corneal thickness
were excluded.
All patients underwent a complete ophthalmic
evaluation. Central corneal thickness was measured
with an ultrasonic pachymeter (Pacscan 300p, digital
biometric ruler, Sonomed). Axial length was measured
using an A Scan (Echorule2, Biomedix). Corneal
curvature was measured using a Keratometer.
Statistical Analysis was done using SPSS V.11 and
Microsoft Excel. Correlation between CCT and four
factors namely, degree of myopia, axial length , corneal
curvature and age of patient was studied using Karl
Pearson’s correlation co-efficient. A probability of 0.05
was considered statistically significant.
Results
106 patients (203 eyes) were recruited for the study.
53 patients were myopes and 53 patients were
Table 1 Demographic Features of Study Patients
No. of patients
No. of Eyes
No. of Myopes
No. of Emmetropes
Age:
Range (years)
Mean (SD)
Median
Sex:
Females
Males
106
203
53
53
18 – 45
32 (8.17)
32
66
40
Table 2. Demographic Features of Myopia & central corneal
thickness – split in groups
Myopes (Diopters)
0–3
3–6
More than 6.0
CCT (Microns)
< 500
500 – 550
550 – 600
> 600
No. of patients
68
21
10
11
51
34
3
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A. Elias et al. - CCT
Table 3. Characteristics of central corneal thickness & Myopia
in patients central corneal thickness (Microns)
Range
Mean (SD)
Median
Myopia (Diopters)
Range
Mean (SD)
Median
453- 620
543.11 (30.61)
541
17
- 2.8 (2.65)
-2
Table 4. Characteristics of Corneal Curvature & Axial Length
of patients.
Corneal Curvature: (Diopters)
Range
Mean (SD)
Median
Axial Length (MM)
Range
Mean (SD)
Median
39.37 – 47.50
43.68
(1.41)
43.62
22.03 – 30.88
23.69 (1.31)
23.48
Table 5. Correlation between central corneal thickness &
relating Factors.
Karl Pearson’s
Correlation
Correlation Analysis
CCT & degree of myopia
CCT & corneal curvature
CCT & axial length
CCT & age
Co-efficient
0.11
-0.23
0.0048
0.038
P value
0.122
0.001
0.944
0.696
emmetropes. There were 66 female patients and
40 male patients. Patients in the age group 18 – 45
years were included in the study. Mean age was
32 yrs. (SD 8.17, median 32 yrs.)(Table 1).
Myopia and CCT were split into groups (Table 2) . Most
of the patients were in the 0-3.0 diopters group (Fig2).
51 of the 99 myopic eyes had a range of 500-550
microns.(Fig1). Table 3 sumarises the features of central
corneal thickness and myopia in study patients. The
mean central corneal thickness was 543.11 μm(SD
151
30.61, range 453 – 620 μm, median 541μm). The mean
myopia was –2.8 diopters (SD 2.65, range –0.5D to –
17.5D, median – 2D).
Table 4 shows the features of corneal curvature and
axial length in study patients. The mean corneal
curvature was 43.68 diopters (SD 1.41, range 39.37D
– 47.50D, median 43.62D) . The mean axial length was
23.69 mm (SD 1.31, range 22.03mm – 30.88 mm,
median 23.48).
Correlation between CCT and 4 variables was analysed
using the Karl Pearson’s Correlation Coefficient. There
was a statistically significant negative correlation
between CCT and corneal curvature, r = -0.23,
P=0.001. (Fig.5). There was no significant correlation
between CCT and degree of myopia, axial length or
age. (Table 5) (Fig. 3, 4, 6).
Discussion
The study attempted to determine the correlation
between CCT and four variables, namely degree of
myopia, corneal curvature, axial length and age of the
patient.
The mean (SD) CCT was 543.11 (30.61) microns. This
is comparable with similar studies. Fam et al 2 in a study
on 714 Chinese patients had a mean of 534.5 microns.
Chang’s 8 series had a mean of 533 microns, whereas
Vijaya et al 9 reported the mean (SD) CCT in a normal
rural South Indian population to be 505.9
(31.10)microns.
This study showed no statistically significant difference
between CCT in myopes and emmetropes. There was
no correlation between degree of myopia and CCT
(Pearson r = 0.11, P=0.122). This result is in
Table 6. Results of Studies showing correlation between central corneal thickness & Myopia
Authors
Year
Country
Equipment
Results (CCT & Myopia)
Liu & Pflugfelder
Cho & Lam
Price et al
Ehlers & Hansen
Touzeau et al
Srivannaboon
Alsbirk
Von Bahr
Kunert et al
2000
1999
1999
1976
2003
2002
1978
1956
2003
China
China
USA
Denmark
France
Thailand
Greenland
Sweden
India
Orbscan
Ultrasound
Ultrasound
Optical
Orbscan
Orbscan
Optical
Optical
Ultrasound
No correlation
No correlation
No correlation
No correlation
Thinner CCT when Myopic
Thinner CCT in high Myopes
Thinner CCT when Myopic
Thinner CCT when Myopic
Thinner CCT in high myopia
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Kerala Journal of Ophthalmology
Fig. 1. Histogram showing the distribution of CCT in the
study population.
Fig. 2. Histogram showing the distribution of myopia in the
study population.
Fig. 3. Scatter plot showing the association between CCT and
degree of myopia.
agreement with majority of previous studies (1). Von
Bahr 4 first generated interest in the correlation between
myopia and CCT in 1956, when he reported thinner
corneas in myopia less than –4D. Similar findings were
also reported by Tokoro et al 10, Chang et al 8, Touzeau
et al, Srivannaboon and Alsbirk from Greenland.
However other studies by Liu Z and Pflugfelder 11, Price
et al 12 and Pedersen et al 1 could find no correlation
between CCT and degree of myopia. Table 6 gives an
Vol. XXI, No. 2
Fig. 4. Scatter plot showing the association between CCT and
age.
Fig. 5. Scatter plot showing the association between CCT and
Corneal curvature.
Fig. 6. Scatter plot showing the association between CCT and
axial length.
overview of published papers with information on
myopia and central corneal thickness.
There was a negative correlation (Pearson r= -0.23)
(P=0.001) between CCT and corneal curvature. The
correlation was significant at the 0.01 level (2-tailed).
Similar results were reported by Atsuo Tomidokoro
et al in the Tajimi Study from Japan 13. A study done
on Singaporean children 14 showed that the radius
June 2009
A. Elias et al. - CCT
of corneal curvature correlated with CCT (Pearson
r = 0.19, P< 0.001) significantly.
There was no correlation between CCT and axial length.
Mitsugu Shimmyo et al 7 in their study, corneal thickness
and axial length, studied ocular parameters of 1084
eyes. They found no statistically significant association
between CCT and axial length. Subgroup analysis by
age, gender and race failed to show an association.
There was no statistically significant association
between CCT and age. (Pearson r = 0.038, P = 0.696).
Other studies have shown CCT to be inversely related
to age. In the study by Eun Suk Lee et al, CCT of Korean
patients with glaucoma showed an inverse relationship
between age and CCT (Pearson r = -0.12; P < 0.0001).
CCT decreased by 2.8 microns per decade. This relation
was also significant in the multivariate model (P=0.01).
Myopia is increasing in prevalence and may be a growing
problem in the future. Investigations in Denmark,
Iceland, Japan and North American aboriginal
populations have indicated the increasing prevalence
of myopia. Consequently there is a higher rate of
refractive surgeries to correct myopia. With Lasik there
is a general concern that one should not thin the cornea
further than a given amount. It is therefore prudent to
measure central corneal thickness prior to surgery.
A thin central cornea is a risk factor for the development
of glaucoma in patients with ocular hypertension.
In the Ocular Hypertension Treatment Study (OHTS),
a multivariate model that included IOP, CCT was the
most powerful component of the predictive model. CCT
is the most heritable aspect of ocular structure (more
than refraction, axial length or optic disc size),
suggesting it is under exquisite genetic control.
Conclusions
This study has shown that CCT has no correlation with
degree of myopia. CCT was significantly associated with
corneal curvature. There was no association between
CCT and axial length or age.
153
References
1.
Pederson L, HjortdalJ, Ehlers N. Central corneal
thickness n high myopia. Acta Ophthalmol. Scand.
2005; 83:539-42.
2.
H-B Fam, ACS How, M Bhaskaran, K-L Lim,
Y-H Chan, T Aung. Central Corneal thickness and
its relationship to myopia in Chinese adults.
Br J Ophthalmol 2006; 90:1451-1453.
3.
Wensor M, McCarty CA, Taylor HR. Prevalence and risk
factors of myopia in Victoria, Australia. Arch
Ophthalmol 1999; 117: 658-63.
4.
Von Bahr G. Corneal thickness: its measurement and
changes. AM J Ophthalmol 1956; 42:251-66.
5.
Gordon MD, Beiser JA, Brandt JD etal. The ocular
hypertension study : baseline factors that predict the
onset of primary open angle glaucoma. Arch
Ophthalmol 2002; 12:714-720
6.
Quigley HA, Hahman RM, Addicks EM, Massof RW,
Green WR. Morphologic changes in the lamina cribrosa
correlated with neural loss in open angle glaucoma.
Am J Ophthalmol 1983; 95:673-691.
7.
Mitsugu Shimmyo, Pasul N. Orloff. Corneal Thickness
and Axial Length. Am J Ophthalmol 2005;139:
553-554.
8.
Chang SW, Tsai IL, HuFR, etal. The cornea in young
myopic adults. Br J Ophthalmol 2001; 85:916-20.
9.
Vijaya L, George R, Paul PG, etal. Prevalence of open
angle glaucoma in a rural South Indian population.
Invest Ophthalmol Vis Sci 2005; 46:4461-7.
10.
Tokoro T, Hayashi K, Muto M, et al. central corneal
thickness in high myopia. Folia Ophthalmol Jpn 1976;
27:610.
11.
Liu Z, Pflugfelder Sc. The effects of long term contact
lens wear on corneal thickness, curvature and surface
regularity. Ophthalmology 2000; 107:105-11.
12.
Price FW Jr, Koller DL, Price MO> Central corneal
Pachymetry in patients undergoing laser in situ
Keratomileusis. Ophthalmology 1999; 106: 2216-20.
13.
Atsuo Tomidokoro, Makoto Araie, Aiko Iwase &
the Tajimi Study Group. Corneal Thickness and Relating
Factors in a Population – Based study in Japan:
The Tajimi Study. AmJ Ophthalmol 2007; 144:
152-154.
14.
Tong L, Saw Seang Mei, Siak Jykuen, Gazzard G,
Tan D. Corneal thickness determination and correlates
in Singaporean school children. Investigative
Ophthalmology and amp; Visual Science, 2004;
45:4004-9.