Short-term variation of central corneal thickness measurements in keratoconus using the Tono-Pachymeter NT530P (Tonopachy™)

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S Afr Optom 2012 71(3) 102-108
The South African Optometrist ISSN 0378-9411
102
Short-term variation of central corneal thickness
measurements in keratoconus using the Tono-
Pachymeter NT530P (Tonopachy™)
KP Mashigea, OA Oduntanb and NM Gcabashec
a, b, c Discipline of Optometry, School of Health Sciences, University of KwaZulu-Natal, Westville
Campus, Private Bag X54001, Durban, 4000 South Africa
<mashigek@ukzn.ac.za>
Received 13 March 2012; revised version accepted 31 August 2012
a BSc BOptom CAS MOptom
b BSc(Hons)Optom PhD
c BOptom
Introduction
Keratoconus is a disease of unknown etiology which
is characterized by thinning and protrusion of the central
or paracentral cornea1, 2. This corneal thinning can be
measured easily in humans by pachymetry and this
measurement is used for diagnosis and management
of keratoconus and other ectasias. Corneal thickness
measurement in the human eye is also important in
monitoring and planning of corneal graft3, accurately
measuring eye pressure4-7, and as an indicator of corneal
tissue hydration in the eye8. Corneal thickness can be
evaluated with a variety of optical or ultrasound based
methods such as Scheimpug imaging, ultrasound and
optical coherence tomography9, 10. One of the most
important issues relates to whether the measurements
by an instrument are valid, reliable and repeatable.
Over the years, several studies10-13 have investigated
the reliability and repeatability of various pachymeters.
Such studies have shown that pachymeters provide
reasonably accurate and repeatable measures of corneal
thickness in normal subjects10-13.
Although ultrasound pachymetry is often regarded as
the gold standard for measurement of corneal thickness
Abstract
Repeatability of values is a crucial reliability in-
dex in any instrument used to measure ocular param-
eters. The purpose of this study was to investigate
short-term variability of central corneal thickness
(CCT) measurements in keratoconic eyes. The To-
no-Pachymeter NT530P (Tonopachy™) was used
to take fty successive automatic measurements of
CCT of the right eyes of ten keratoconic subjects
(six females and four males), aged 19 to 41 years
(mean = 30.4 ± 6.8 years). The means, standard
deviations and variances of the measurements
showed little variation with good repeatability. The
inter-subject or overall mean values and standard
deviations of the measurements were 452.1 ± 30.3
μm. The Kolmogorov-Smirnov (K-S), Lilliefors
and Shapiro-Wilks (S-W) tests showed that gener-
ally the data was normally distributed, with a few
exceptions. The results of this study suggest that
over short periods of time, the Tonopachy™ pro-
vides repeatable measures of CCT in keratoconic
corneas. (S Afr Optom 2012 71(3) 102-108)
Key words: Tonopachy™, central corneal thick-
ness, keratoconus, repeatability, univariate nor-
mality.

S Afr Optom 2012 71(3) 102-108 KP Mashige, OA Oduntan and NM Gcabashe -Short-term variation of central corneal thickness... (Tonopachy™)
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due to its high degree of intra-examiner, inter-examiner
and inter-instrument reproducibility10-14 the procedure is
invasive and shows “variability of measurements caused
by probe misalignment or decentering and changes in the
speed of sound in corneal tissues with different degrees
of hydration”14. Several non-contact methods have
been introduced in order to overcome the limitations
of ultrasound pachymetry. Also, some studies15, 16 have
examined the repeatability and agreement of several
non-contact methods for the measurement of CCT in
normal and post LASIK subjects. Among these, the
Tonopachy™ (Nidek, Japan) was introduced recently
which simultaneously measures CCT and IOP values.
A recent study17 compared the CCT values measured
with Tonopachy™ to those measured with Pentacam
and ultrasound pachymeters. The results showed that
values from ultrasound pachymetry were lowest (mean
= 541.7 ± 30.6 µm) whereas those by Tonopachy™
and Pentacam were similar (means = 557.3 ± 34.3
and 558.0 ± 33.7 µm respectively). Tonopachy™
overestimated CCT by 13.9 µm when compared
with those of ultrasound pachymetry in 104 subjects
with normal corneas17. The authors17 concluded that
Tonopachy™ is a reliable instrument for evaluating
CCT, however, careful attention should be paid when
comparing Tonopachy™ CCT measurement with
ultrasound pachymetry as the values may not be
interchangeable. Lomoriello et al18 and Mashige et
al19 reported that the Tonopachy™ provides accurate
and reliable CCT measurements in healthy corneas.
However, its reliability in assessing CCT in eyes with
corneal diseases such as keratoconus has not been
established. The purpose of this study therefore, was to
evaluate short-term variability in CCT measurements in
keratoconic eyes using the Tono-Pachymeter NT530P
(Tonopachy™).
Methods
The study was performed in accordance with the
ethical standards stated in the Declaration of Helsinki
and the protocol was approved by the Research and
Ethics Committee of the University of KwaZulu-
Natal. Written informed consent was obtained from
all the participants after the nature and intent of the
study had been fully explained to them. Ten subjects
with keratoconus (varying from early to advanced
keratoconus) as veried with refraction, keratometry,
characteristic topographic patterns and slit-lamp
examination participated in the study. The Bausch and
Lomb keratometer™ has a range20 of 36.00 D to 52.00
D, measured in increments of 0.12 D. The advanced form
of keratoconus (>52.00 D) exceeded the keratometer’s
measuring ability. Therefore, a +1.25 D lens was placed
over the keratometer aperture to extend20 the range
by approximately 9.00 D. Slit-lamp ndings included
one or more of the following signs: conical protrusion,
Fleischer’s ring, Vogt’s striae or anterior stromal
thinning. All subjects reported excellent general health
and apart from keratoconus, none of participants had
any other ocular abnormalities, history of eye disease
or prior refractive surgery. To minimize possible contact
lens-induced swelling, the participants were instructed
not to wear their contact lenses from the evening before
the data collection day until after the data collection.
Fifty CCT automatic measurements of the right eye
of each subject were taken by one examiner between
12 pm and 2 pm. The measurements were obtained
in approximately 10 to 15 minutes for each eye due
to the auto-alignment function of the instrument and
the measurements were printed out for recording. The
testing room was set up in one of the Optometry clinics
at the University of KwaZulu-Natal and all the tests
were performed by the same operator with the eyes in
an undilated state.
The Statistica Ver8 software programme was used
for all statistical analysis. The distributions of the CCT
measurements were plotted using box and whisker plots,
histograms and normal probability plots. The ranges,
means and standard deviations were determined. The
Kolmogorov-Smirnov (K-S), Lilliefors and the Shapiro-
Wilks (S-W) tests were done to explore univariate
normality of the data and p-values were obtained to
determine whether samples were normally distributed
and whether parametric statistical tests were appropriate.
Results
The subjects included seven Indians and three Blacks,
aged between 19 and 41 years (mean = 30.4 ± 6.8
years). The mean keratometric values obtained with the
Bausch and Lomb keratometer ranged from 47 to 61 D
(mean = 50 ± 3.50 D) and corneal astigmatism ranged
from 3.00 to 7.50 D (mean = 4.50 ± 1.50 D). Table 1
shows the descriptive statistics such as means, standard
deviations minimum and maximum values for the CCT

measurements for all the subjects.
Figure 1 shows the box and whisker plots for the CCT
(μm) values of the right eyes of the 10 subjects. The
CCT means (see Table 1) are indicated by lines through
the middle of the box part of the box and whisker plot
while the ends of the box depict the standard deviations
of the distribution. The whiskers represent the minimum
and maximum values for each subject.
Figure 1: The box and whisker plots for CCT measurements in
micrometers (y-axis) for the right eyes of the 10 subjects.
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S Afr Optom 2012 71(3) 102-108 KP Mashige, OA Oduntan and NM Gcabashe -Short-term variation of central corneal thickness... (Tonopachy™)
Table 1: Means, standard deviations, maxima and minima for CCT measurements in micrometres (µm) for the 10 subjects are given. A
negative sign indicates negative skewing or platykurtosis. The last two columns are unit-less.
Figure 2: Proles of short-term variation of the 50 measurements
of the CCT for 10 subjects are indicated. The CCT prole of subject
7 (bottom light blue colour) appears to be more variable (401 to 426
µm) than the others.
The temporal proles for the samples of CCT values
for all subjects (N = 50 per sample) are shown in Figure
2 to allow the reader to inspect the repeatability and
variation of the measurements of each subject. Some but
not all of the proles appear relatively at.
The CCT samples for all participants except subjects
3 and 8 were normally distributed. Figures 3 and 4 show
histograms and normal probability plots for the samples
of CCT for subjects 5 and 6, which were randomly
selected from the eight CCT samples that were normally
distributed. Normality of the samples was also tested
using the K-S, Lilliefors and S-W tests (see the relevant
gure captions for such results).

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Figure 3: Histograms (a) and normal probability (b) plots for the CCT sample of subject 5. The samples were normally distributed (the
p-values for the K-S, Lilliefors and S-W tests were all greater than 0.05).
Figure 4: Histograms and normal probability plots for subject 6. The histogram shows the frequency distribution for the CCT measurements.
a) b)
Figure 5: Histograms and normal probability plots for the CCT sample of subject 3. The sample for CCT was not normally distributed
(the p-values for the K-S, Lilliefors and S-W tests were < 0.05).

S Afr Optom 2012 71(3) 102-108 KP Mashige, OA Oduntan and NM Gcabashe -Short-term variation of central corneal thickness... (Tonopachy™)
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Figure 6: Histogram and normal probability plot for the CCT of subject 8. The samples were not normally distributed according to the
p-values for the K-S, Lilliefors and S-W tests. The p-values for all the three tests were < 0.05, suggesting rejection of the null hypothesis
that the sample is from a normally distributed population.
The data for subjects 3 and 8 demonstrated departure
from normality. Figures 5 and 6 include histograms
and normal probability plot for subjects 3 and 8 that
demonstrated departure from normality for the CCT.
Discussion
The measurement of CCT is useful in identifying
corneal thinning disorders such as keratoconus1, 2.
Ultrasound biometry has been the most common
method for measuring CCT10-13. A major advantage of
ultrasound biometry is that it requires minimal observer
judgment and is therefore consistent and repeatable
between observers10-13. However, the use of a topical
anesthetic agent, its invasive nature (which may
increase the risk for infection), exact alignment at the
centre of the cornea and proper handling of the probe
perpendicular to the corneal surface are some of the
limitations of this technique10-13. It is also not advised
to rely exclusively on ultrasonic pachymetry to exclude
or diagnose keratoconus10-13. These potential limitations
stimulated the development of possibly more reliable,
repeatable and operator independent non-contact ocular
biometry techniques including the Tonopachy™,
Pentacam, Orbscan, optical coherence tomography
and other optical methods for imaging and measuring
the CCT10. This study is the rst to investigate short-
term variation of CCT in keratoconic eyes using the
Tonopachy™.
Although the sample size in this study was not as large
as that used in recent studies18, 19 with the Tonopachy™ it
was similar to that of previous studies with Scheimpug
photography via the Oculus Pentacam10. However,
the 50 consecutive measurements taken and analysed
per eye seemed sufcient to determine the short-term
variation of the Tonopachy™ in keratoconic eyes. The
measured keratometric and CCT values indicate that the
eyes studied had keratoconus that varied from moderate
to severe1, therefore covering a wide range of corneal
thicknesses.
In cases of keratoconus, several devices are
used to assess the CCT and the repeatability of the
measurements is very important for diagnosis of
the disease and monitoring of its progression. The
Tonopachy™ allows fast, non-contact measurement
of the CCT of the individual while providing
good comfort and avoiding application of topical
anesthesia18, 21. The instrument provides the CCT via
Scheimpug camera principles18, 21. An additional
advantage is that this instrument also provides automatic
and simultaneous measurements of the individual’s
intra-ocular pressure18, 21.
The box and whisker plots for the CCT (see Figure
1) showed that the standard deviations were small
suggesting that multiple measurements of CCT in
keratoconus with the Tonopachy™ were similar for

S Afr Optom 2012 71(3) 102-108 KP Mashige, OA Oduntan and NM Gcabashe -Short-term variation of central corneal thickness... (Tonopachy™)
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each of the subjects. The line plots of the short-term
variation shown in Figure 2 are also mainly relatively
at, further suggesting that there is minimal variation in
CCT measurements of the keratoconic eyes. Therefore,
the CCT measured with Tonopachy™ are largely
repeatable within individuals. This suggests that the
instrument is a potentially useful clinical and research-
oriented device. However, some CCT proles were
more variable than others. For instance, CCT of subject
7 ranged from 401 to 426 µm with a mean of 411.7 µm
and a standard deviation of 5.7 µm. This subject had
severe keratoconus (61 D). It is therefore possible that
variations in CCT in the short-term may occur in severe
keratoconus. Further investigation is recommended
wherein repeated CCT measures are undertaken on
multiple subjects with severe keratoconus. All samples
were also investigated for normality using the K-S,
Lilliefors and S-W tests. Probabilities were used in
determining whether these statistics were signicant and
whether there was departure from univariate normality.
Probabilities were greater than 0.05 for most, but not all,
indicating that most samples were essentially normal
distributions22.
Positive values for the skewness indicate data that are
positively skewed towards larger values and negative
values for the skewness indicate data that are negatively
skewed. Similarly, positive values for kurtosis indicate
leptokurtosis and a negative value indicates platykurtosis.
As shown in Table 1, in the 10 eyes studied, CCT
measurements showed minimal variation. No sample
showed profound skewness and only subject 9 showed
some leptokurtosis (2.8) while subject 4 demonstrated a
statistical measure that indicates a distribution that has
some platykurtosis (_1.2).
In conclusion, Tonopachy™ provides measurements
of CCT that are repeatable and it is therefore suitable
to aid in the diagnosis and monitor the progression of
keratoconus when measurements are repeated over
a short time. A possible limitation of this study is the
limited number of cases and therefore, future studies
including repeated measures on a large population
sample are recommended.
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