CHANGES IN APICAL CORNEAL CURVATURE IN UNILATERAL PRIMARY PTERYGIUM AND NORMAL ADULTS USING SIMULATED-K AND CORNEAL IRREGULARITY MEASUREMENT

Abstract

Introduction: This paper aimed to describe variation in apical corneal curvature between unilateral primary pterygium and normal adults utilizing simulated-K and corneal irregularity measurement corneal indices. Methods: A total of 100 participants comprise 50 unilateral primary pterygium eyes from 50 patients and 50 normal adults were recruited in this study. Diagnosis and classification of primary pterygium were done by a consultant ophthalmologist (KMK). Standard optometric examinations were performed in all participants. Simulated-K (SimK) and corneal irregularity measurement (CIM) was objectively measured using a corneal topographer. Three measurements based on best image quality for SimK and CIM were taken by single operator in a same visit. Difference for both SimK and CIM parameters between primary pterygium and normal groups were determined via independent T-test. Results: Overall mean and standard deviation (n = 120) of SimK and CIM were found higher in primary pterygium group (9.06 ± 4.49 D and 11.48 ± 3.12) compared to normal (1.63 ± 0.67 D and 0.62 ± 0.24) respectively. Independent T-test results showed significance difference in SimK and CIM values between primary pterygium groups and normal (both P< 0.001). Conclusions: Both SimK and CIM corneal indices can be an important tool in describing and predicting changes on the corneal curvature due to pterygium progression. However, it is worth to note that the detectability of changes in anterior corneal curvature is limited to 5 mm of central corneal curvature.

Full text

CHANGES IN SIMULATED-K AND CORNEAL IRREGULARITY MEASUREMENT…
INTERNATIONAL JOURNAL OF ALLIED HEALTH SCIENCES, 3(2), 588-594
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CHANGES IN APICAL CORNEAL CURVATURE IN UNILATERAL PRIMARY
PTERYGIUM AND NORMAL ADULTS USING SIMULATED-K AND CORNEAL
IRREGULARITY MEASUREMENT
MOHD RADZI HILMI, PhD (CORRESPONDING AUTHOR)
DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
mohdradzihilmi@iium.edu.my
NUR HIDAYAH MUSA, B.OPTOM (HONS.)
DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
nurhidayah_musa@ymail.com
KHAIRIDZAN MOHD KAMAL, MS (OPHTHAL)
DEPARTMENT OF OPHTHALMOLOGY, KULLIYYAH OF MEDICINE, INTERNATIONAL
ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN AHMAD SHAH, BANDAR INDERA
MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
Khairidzan@gmail.com
MOHD ZULFAEZAL CHE AZEMIN, PhD
DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
zulfaezal@iium.edu.my
NUR NABILAH MARUZIKI, BOPTOM (HONS.)
DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
nsolehah94@gmail.com
NUR AIN NORAZMAR, BOPTOM (HONS.)
DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
ainazmariium@gmail.com
MARDHIAH SYAZWANI NASIR, BOPTOM (HONS.)

CHANGES IN SIMULATED-K AND CORNEAL IRREGULARITY MEASUREMENT…
INTERNATIONAL JOURNAL OF ALLIED HEALTH SCIENCES, 3(2), 588-594
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DEPARTMENT OF OPTOMETRY AND VISUAL SCIENCE, KULLIYYAH OF ALLIED HEALTH
SCIENCES, INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA (IIUM), JALAN SULTAN
AHMAD SHAH, BANDAR INDERA MAHKOTA, 25200 KUANTAN, PAHANG, MALAYSIA
marsyadhiani_2210@yahoo.com
ABSTRACT
Introduction: This paper aimed to describe variation in apical corneal curvature between unilateral
primary pterygium and normal adults utilizing simulated-K and corneal irregularity measurement
corneal indices. Methods: A total of 100 participants comprise 50 unilateral primary pterygium eyes
from 50 patients and 50 normal adults were recruited in this study. Diagnosis and classification of
primary pterygium were done by a consultant ophthalmologist (KMK). Standard optometric
examinations were performed in all participants. Simulated-K (SimK) and corneal irregularity
measurement (CIM) was objectively measured using a corneal topographer. Three measurements
based on best image quality for SimK and CIM were taken by single operator in a same visit.
Difference for both SimK and CIM parameters between primary pterygium and normal groups were
determined via independent T-test. Results: Overall mean and standard deviation (n = 120) of SimK
and CIM were found higher in primary pterygium group (9.06 ± 4.49 D and 11.48 ± 3.12) compared to
normal (1.63 ± 0.67 D and 0.62 ± 0.24) respectively. Independent T-test results showed significance
difference in SimK and CIM values between primary pterygium groups and normal (both P< 0.001).
Conclusions: Both SimK and CIM corneal indices can be an important tool in describing and
predicting changes on the corneal curvature due to pterygium progression. However, it is worth to
note that the detectability of changes in anterior corneal curvature is limited to 5 mm of central
corneal curvature.
Keywords: pterygium; morphology; Simulated-K; corneal irregularity measurement; corneal
curvature
INTRODUCTION
Pterygium is defined as an abnormal fibrovascular lesion which originates from the bulbar
conjunctiva and progresses towards central cornea (Ang, Chua and Tan, 2007). Prevalence of
pterygium has been closely associated with chronic ultraviolet (UV) ray exposures (Liu et al., 2013)
and limbal stem-cell alteration at corneo-limbal junction (Chui et al., 2011). It is an established fact
that pterygium is a significant factors which contribute to induced corneal astigmatism which closely
related to its physical properties such as its horizontal width or its total area (Mohammad-Salih and
Sharif, 2008). However, it is worth to note that clinically not all large size of pterygium induced
significant astigmatism, as based on our clinical observation small pterygium size could also give
similar effects.
Tan et al., (1997) has proposed classification of pterygium based on its clinical appearance.
This classification is based on three (3) types or grades known as type I - atrophy, type II -
intermediate and type III - fleshy. The classification framework was based on loss of translucency of
pterygium tissue which relates to increased fleshiness that signifies abnormal fibrovascular growth of
pterygium. Apart from this grading, there are several clinical grading has been suggested in
evaluating pterygium which based on its morphologies as shown in Table 1 below.
Several clinical grading’s has been suggested in evaluating pterygium. Pterygium can be
assessed based on several methods such as via its morphology (Mohd Radzi et al., 2017), extension or
length (Chui et al., 2011; Farhood and Kareem, 2012; Kheirkhah et al., 2012;), its size (Mohammad-

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Salihand Sharif, 2008; Altan-Yaycioglu et al., 2013; Vives et al., 2013) and based on its encroachment
relative to the corneal size (Mohammad-Salih and Sharif, 2008; Mohd Radzi et al., 2017).
Table 1: Current available clinical pterygium grading based on its morphology
Morphology
Evidence from literatures
Fleshiness
Tan et al., 1997; Mohd Radzi et al., 2017
Extension or length
Chui et al., 2011; Farhood and Kareem, 2012; Kheirkhah et al., 2012
Size or total area
Mohammad-Salih and Sharif, 2008; Altan-Yaycioglu et al., 2013; Vives
et al., 2013
Encroachment relative to
the corneal size
Mohammad-Salih and Sharif, 2008; Mohd Radzi et al., 2017
Apical corneal curvature changes are commonly measured using Simulated-K (SimK)
(Farhood and Kareem 2012; Kheirkhah et al., 2012; Kheirkhah et al., 2012; Altan-yaycioglu et al., 2013;
Viveset al., 2013). SimK is an index which characterizes estimation of total corneal astigmatism based
on measurement of anterior corneal curvature (Eom et al., 2014). Simulated-K index characterise
corneal curvatures in the central 3-mm optical zone of cornea. The steep simulated K-reading is the
steepest meridian of the cornea, using only the points along the central pupil area with 3-mm
diameter. The flat simulated K-reading is the flattest meridian of the cornea and is, by definition, 90°
apart, with the normal value of SimK is approximately 43.00 ± 2.00 Dioptres (D). These readings gave
an idea about the central corneal curvature that is frequently visually most significant.
In contrast, corneal irregularity measurement (CIM) is an index which signifies the
probability of irregular anterior corneal surface. CIM value indicates the regularity of the corneal
surface, with normal CIM values are between 0.03μm to 0.68μm, whereas 0.69μm to 1.0μm are
considered as borderline and abnormal values are between 1.1μm to 5.0μm. A higher CIM values
would indicates higher probability of irregular anterior corneal surface and ocular pathologies. In a
simple term, CIM is a measurement in describing changes of the corneal regularity in comparison
with the normal corneal shape. However, CIM is rarely addressed as a clinical parameter in
describing peripheral corneal lesion such as pterygium. Nonetheless, CIM has been used in
describing other corneal pathologies such as keratoconus and pellucid marginal degeneration (PMD).
Although numerous works (Ozdemir and Cinal, 2005; Yagmur et al., 2005; Maheshwari, 2007;
Mohammad-Salih and Sharif, 2008; Farhood and Kareem, 2012; Kheirkhah et al., 2012; Kheirkhah et
al., 2012; Altan-yaycioglu et al., 2013; Vives et al., 2013) had proven that progression of pterygium
does induce changes on the anterior corneal curvature, based on our literature search, lack of
evidence found which employs CIM in describing effects of pterygium on anterior corneal curvature.
Hence, this study aims to evaluate the changes in apical corneal curvature in primary pterygium
utilizing two (2) corneal indices (SimK and CIM).
METHODS
A total of 100 participants comprise of 50 unilateral primary pterygium eyes from 50 patients and 50
normal adults were recruited in this study who visits a tertiary ophthalmic centre in East Coast of
Malaysia in order to display a wide range of severity of pterygium patients. All participants in this
study were selected based on specific criteria. Inclusion criteria include established diagnosis of
primary pterygium, both genders were included with age ranges from 20 to 70 years and free from

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any history of ocular trauma, ocular surgery, contact lens wear, and any ocular anterior segment
disease other than pterygium which may affect vision as previously described (Mohd Radzi et al.,
2017; CheAzemin et al., 2015; Azemin, Hilmi and Kamal, 2014; CheAzemin et al., 2014). Diagnosis of
primary pterygium was performed by a consultant ophthalmologist (KMK). The study was
conducted according to recommendation of the tenets of Declaration of Helsinki and approved by the
International Islamic University Malaysia (IIUM) research ethical committee (IREC)
(IIUM/310/G13/4/4-125). Written and informed consent was obtained from all participants prior
any procedures performed.
All participants undergo standard optometric examination comprises dry refraction, slit-lamp
examination and fundus examination. Then, each participant’s average central corneal curvature
(SimK) and corneal irregularity measurement (CIM) was objectively measured using Zeiss ATLAS™
995 corneal topographer (Zeiss Meditec, Inc, Dublin, USA). Three measurements were taken and the
measurement with the best image quality was taken as the SimK and CIM value. These
measurements were done by single operator and performed on the same visit. All data were then
been exported to statistical software.
Statistical analyses were performed using IBM SPSS (Predictive analytics software) (Version
19, SPSS Inc., Chicago, IL, USA). Independent T-test was employed to evaluate the difference between
both primary pterygium and normal groups for both SimK and CIM parameters. A significance level
of P< 0.05 was set as the confidence level.
RESULTS
The analysis include 100 participants, with 53% (n = 53) were men. Normality testing was evaluated
using ratio of skewness and kurtosis (George and Mallery, 2010), with ± 2.50 was taken as normal
distribution. Normality testing showed normal data distribution for both groups.
The mean of SimK and CIM for primary pterygium group were 9.06 ± 4.49 D and 11.48 ± 3.12
respectively. In contrast, normal group showed lower values of SimK and CIM with 1.63 ± 0.67 D and
0.62 ± 0.24 respectively. Independent T-test results revealed that there were significance differences
between normal and primary pterygium groups for both parameters (both P< 0.05). All results were
summarized in Table 2.
Table 2 Comparison of SimK and CIM values between primary pterygium and normal group (n =
100)
Corneal
Index
Group
P-value*
Primary pterygium (Mean ± SD)
Normal (Mean ± SD)
SimK (D)
9.06 ± 4.49
1.63 ± 0.67
P < 0.001
CIM
11.48 ± 3.12
0.62 ± 0.24
P < 0.001
SD: Standard Deviation
D: Dioptres
SimK: Simulated-K
CIM: Corneal Irregularity Measurement
*: Independent T-test (Significance level set at 0.05)
DISCUSSION
This study aims to evaluate the difference in apical corneal curvature utilizing two (2) corneal indices
(SimK and CIM). Hence, this paper aim to demonstrate the usability of both indices in describing the
changes in central 3-mm curvature of cornea (which also known as apical) between both normal and
primary pterygium eyes. In this study, equal number of participants in both normal and primary

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pterygium group (both n = 30) were recruited to evaluate the difference in SimK and CIM relative to
pterygium types in comparison with normal eyes.
Increase of SimK values indicates higher changes in anterior corneal curvature which
indirectly signify induced-astigmatism. Simulated K (SimK) values simulate the traditional
keratometer readings by expressing the curvature in two orthogonal axes (90° apart) in the central
approximately 3 mm area of the apical cornea. SimK would display the average power and axis of
both corneal meridians. Normally, SimK would give values which resemble the topographic image of
cornea which provides information on the degree of severity and angle of astigmatism. With regards
to pterygium patients, SimK would provide information on the progression of the disease, gross
prediction of visual performance and also the needs of surgical intervention. Based on our findings,
Pterygium group revealed significantly higher SimK compared to normal (P< 0.05). We postulate that
pterygium could induce changes in corneal astigmatism in form of higher value of SimK due to its
fleshiness appearance (Tan et al., 1997; Mohd Radzi et al., 2017; Mohd Radzi et al., 2018) which
indirectly induce higher corneal toricity. The fleshiness appearance of pterygium also could give rise
to obscured episcleral vessels which could signify presence of fibrovascular tissue due to excessive
proliferative disorders (Touhami et al., 2005; Ribatti et al., 2007).
This study also found higher CIM values in pterygium group compared to normal (P< 0.05).
Higher corneal irregularity measurement (CIM) indicates the anterior corneal curvature change
towards irregular shape which does not resemble the ‘normal’ corneal shape known as prolate. With
regards to pterygium, this finding showed that pterygium progression caused the corneal surface
become irregular, which gives rise to unwanted corneal astigmatism (Roh et al., 2015). To the best of
our knowledge, information on CIM related to pterygium is scarce. However, we suggest the
irregularity of the corneal curvature could be due to compression of corneal curvature due to
pterygium progression, which indirectly inducing corneal flattening on the pterygium region
(Maheshwari, 2007). Corneal compression could be due to increase mechanical traction of pterygium
tissue on the corneal surface and its tissue weight. Although these findings look promising, we need
to highlight that both SimK and CIM only measures changes on the central cornea, approximately 5 -
6 mm centrally. Thus, the overall effect of pterygium progression is still unknown as it progresses
from peripheral cornea.
CONCLUSION
Both SimK and CIM corneal indices can be an important tool in describing and predicting changes on
the corneal curvature due to pterygium progression. However, it is worth to note that the
detectability of changes in anterior corneal curvature is limited to 5 mm of central corneal curvature.
ACKNOWLEDGEMENT
This research is financially supported by International Islamic University Malaysia (IIUM) under
Research Initiative Grant Scheme (RIGS) RIGS17-148-0723.
DECLARATION OF INTEREST
The authors report no conflicts of interest.
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