214      Vol. 29, No. 4, Oct – Dec, 2013 Pakistan Journal of Ophthalmology 

Original Article 

 

Correlation between Central Corneal 
Thickness Measurements Using Two 
Different Ultrasonic Pachymeters 
 
Sharmeen Akram, Zarksis H. Anklesaria, Khabir Ahmad 

 
Pak J Ophthalmol 2013, Vol. 29 No. 4 

 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . .  
See end of article for 
authors affiliations 
 
…..……………………….. 
 
Correspondence to: 
Sharmeen Akram 
Section of Ophthalmology,  
Department of Surgery,  
Aga Khan University, 
Stadium Road, P.O. Box 3500, 
Karachi 74800,  
sharmeen.akram@aku.edu 

 
 
 
 

…..……………………….. 

Purpose: To assess correlation between central corneal thickness 
measurements using two different ultrasonic pachymeters. 

Material and Methods: This prospective study involved normal subjects aged 
16 to 45 years. Central corneal thickness was measured in 47 eyes by two 
ultrasonic pachymeters – Tomey SP – 100 and Sonomed 300 AP. Correlations 
between CCT measurements assessed by the two pachymeters were tested by 
Pearson correlation. 

Results: Forty seven eyes were included in the study. The mean (± SD) age of 
the subjects was 27.79 years (± 6.88).The mean (± SD) Tomey Pachymeter 
CCT was 536.45 m (34.37) and the mean (SD) Sonomed CCT was 540.64 m 
(33.48).CCT measurements by the two modalities were very strongly correlated 
(r = 0.98; P <0.0001). 

Conclusions: In healthy individuals, Tomey pachymeter measurements of 
corneal thickness were highly correlated with those obtained using Sonomed 
pachymeter, and hence the two may be used interchangeably. 

 
pplanation ultrasound (US) pachymetry is 
the gold standard for corneal thickness (CCT) 
measurement, which is an important step in 

ophthalmic evaluations prior to refractive procedures 
such as laser in situ keratomelisus (LASIK)1. This 
approach uses the ultrasonic principle to determine 
CCT and requires both topical anesthesia and contact 
of the probe with the cornea. 

In the literature, a variety of methods of 
measuring CCT have been described2. These include 
contact methods, such as ultrasound and confocal 
microscopy, or noncontact methods such as optical 
pachymetry with Scheimpflug cameras, optical 
coherence tomography and optical coherence 
pachymetry. In this study, we aimed to assess the 
correlation between CCT measurements using two 
different ultrasonic pachymeters (Sonomed pachy-
meter 300 AP and Tomey SP-100 Handy Pachymeter) 
in normal subjects. 

MATERIAL AND METHODS 

This prospective study was conducted at Laser Vision 
Center, Karachi during June 2012 to 30 Jan 2013. 
Healthy individuals aged 16 to 45 years with refractive 
errors were included in the study. Those with corneal 
abnormalities like corneal scars were excluded. After 
informed consent, CCT was measured. All readings 
were taken with Tomey SP-100 Handy Pachymeter 
first followed by Sonomed pachymeter 300 AP with an 
interval of 24 hours. The specifications of both 
pachymeters are shown in Table 1. For both machines 
measurements, the cornea was anesthetized with 
topical proparacaine hydrochloride 1% (Alcaine). The 
calibrated US probe was used to obtain 5 
measurements from the central cornea. The highest 
and the lowest values were excluded, and the mean of 
the remaining 3 was used for analysis. 

Data were entered and analyzed using SPSS v.19 
(IBM Corp, Armonk, NY). Correlations between 

A 



CORRELATION BETWEEN CENTRAL CORNEAL THICKNESS MEASUREMENTS USING TWO DIFFERENT PACHYMETERS 

Pakistan Journal of Ophthalmology Vol. 29, No. 4, Oct – Dec, 2013      215 

Sonomed pachymeter 300 AP and Tomey SP – 100 
Handy Pachymeter CCT measurements were tested by 
Pearson correlation. To assess if the two methods may 
be used interchangeably, Bland-Altman analysis was 
performed (Figure 2). A p value < 0.05 was considered 
statistically significant. 

 

 

 
RESULTS 

Forty seven eyes (20 male eye and 27 female eyes) of 
24 patients were included in the study. The mean (± 
SD) age of the subjects was 27.79 years (± 6.88). Of the 
selected eyes, 23 (48.9%) were right and 24 (51.1%) 
were left. 

The mean (± SD) Tomey Pachymeter CCT was 
536.45μm (34.37) and the mean (SD) Sonomed CCT 
was 540.64μm (33.48). As shown in Table 2, CCT 
measurements by the two pachymeters were very 
strongly correlated (r = 0.98, 95% CI 0.97, 0.9; P 
<0.0001). Bland-Altman plot showed that the average 
discrepancy between the two pachymeters was not 
large enough to be important. 

 

Tomey

S
o

n
o

m
e
d

400 450 500 550 600 650
450

500

550

600

650

 
 

Fig. 1: Scatter plot of CCT measurements between 
``Sonomed 300 AP and Tomey SP-100 

 
 

500 550 600 650

-20

-10

0

10

20

Tomey-Sonomed average (µ)

T
o

m
e
y
-S

o
n
o

m
e
d

d
if

fe
re

n
c

e
(µ

)

 
 

Fig. 2: Bland – Altman plot of the difference in CCT 
measurements versus average CCT measure-
ments 

 
DISCUSSION 

To the best of our knowledge, this is the first study to 
assess the correlation between CCT measurements 
using two different ultrasonic pachymeters (Sonomed 
pachymeter 300 AP and Tomey SP-100 Handy 
Pachymeter) in normal subjects in a Pakistani popu-
lation. Our study showed that both the measurements 
were highly correlated and hence the two pachymeters 
may be used interchangeably. Comparative data is 
limited as most previous studies have compared 
pachymeters that use different principles (ultrasonic 
and optical) to use to measure CCT. 

Accurate measurement of CCT is important for 
detection, evaluation, and treatment of many eye 
conditions. Key diagnostic and therapeutic decisions 
are made based on these readings. Accurate 



SHARMEEN AKRAM, et al 

216      Vol. 29, No. 4, Oct – Dec, 2013 Pakistan Journal of Ophthalmology 

measurement of CCT is also important before 
refractive procedures to minimize the risk of 
iatrogenic keratectasia which is one of the most 
dreaded complications of LASIK procedure3. As 
mentioned earlier both contact and non-contact 
methods of measuring CCT have been described 
including ultrasonic pachymetry, optical pachymetry 
by rotating Scheimpflug camera, corneal con focal 
microscopy, and OCT4,5. Studies have shown 
difference in measurements between optical and 
ultrasonic pachymetry which is currently considered 
the gold standard6-8. However, this kind of contact 
examination still has some problems, including the 
need to anesthetize the cornea, corneal indentation 
during measurement, and corneal epithelial damage 
and cross infections9. Despite these problems, the 
measurements of CCT by means of ultrasonic 
pachymetry are very accurate and highly 
reproducible, with a low intra-observer and inter-
observer and variability. Hence it is still the most 
common method for measuring corneal thickness. 

Although several different models of ultrasonic 
pachymeters are available, they all work on the same 
principle, are inexpensive and easy to use. The two 
most commonly used such pachymeters in our setting 
are Tomey and Sonomed and we conducted this study 
to see if the two could be used interchangeably. Our 
results showed that readings of the two pachymeters 
were highly correlated. 

 
CONCLUSION 

In healthy individuals, there is a high co-relation 
between CCT measurements of Tomey SP-100 and 
Sonomed 300 AP pachymeters, and hence the two may 
be used interchangeably. 

 
Author’s Affiliation 

Dr. Sharmeen Akram 

Section of Ophthalmology, Aga Khan University 
Department of Surgery, Karachi 

Dr. Zarksis H. Anklesaria 

Laser Vision Center 
Dr. S. D. Anklesaria Eye Clinic and Hospital, Karachi 

Dr. Khabir Ahmad 
Section of Ophthalmology, Aga Khan University 
Department of Surgery, Karachi 

 
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