Open Journal of Ophthalmology, 2012, 2, 78-82 Published Online August 2012 (
History and Clinical Validation of the PASCAL® Dynamic
Contour Tonometer
Elliot M. Kirstein
Harper’s Point Eye Associates, Cincinnati, USA.
Received April 16th, 2012; revised May 28th, 2012; accepted June 15, 2012
Will Goldmann applanation tonometry, the age old “gold standard” of intraocular pressure (IOP) measurement, be re-
placed with newer methods? One of the newer methods of IOP measurement is PASCAL® dynamic contour tonometry
(DCT). A review of the history, scientific principles, clinical validation and clinical utility o f DCT is presented.
Keywords: Cornea; Ocular Surface; External Disease Glaucoma Ophthalmic Pathology
1. Introduction
The technology used to estimate intraocular pressure
(IOP) has evolved tremendously since, in the late 1800’s,
Sir William Bowman emphasized the importance of ocu-
lar tension measurements. While, in the context of glau-
coma diagnosis, the relative importance of IOP meas-
urement seems to have waxed and waned, few would
argue that IOP reduction remains our fundamental tool in
glaucoma management [1]. If we could measure IOP
more precisely, we could then increase our understanding
of the role of IOP in the pathogenesis of glaucoma as
well as its diagnosis and management. This notion seems
to explain why we have been driven to develop tech-
nologies which can most precisely estimate that value.
In Bowman’s time, digital palpation tonometry be-
came the clinical standard. Since then, there has been a
milieu of devices which aimed to increase precision and
clinical utility. Since the 1950’s, the Goldmann applana-
tion tonometer (GAT) has clearly been the standard in
clinic and research [1].
During the past two decades, knowledge has been
gained from studies such as the Ocular Hypertensive
Treatment Study (OHTS) regarding meaningful system-
atic errors in GAT measurement [2]. With this in mind,
researchers and clinicians have been prompted to seek
out methods of correcting GAT’s systematic errors and
to develop enhanced practical methods of IOP measure-
ment [3-8].
PASCAL® Dynamic Contour Tonometer (DCT)
The PASCAL® Dynamic Contour Tonometer (DCT) is a
novel measuring technique, using the principle of con-
tour matching instead of applanation. Approved for dis-
tribution in the United States in 2003, the PASCAL ® was
designed to eliminate or significantly reduce the system-
atic errors inherent in all previous tonometers, such as
the influence of corneal thickness, rigidity, curvature or
elastic properties. Initial in vitro studies with cadaver
eyes and later in vivo cannulatio n studies on live patients
have demonstrated a nearly linear relationship of DCT to
true manometric IOP. Additionally, DCT values have
been shown to remain unchanged in individuals, before
and after LASIK surgery. The absence of change in mea-
sured IOP after LASIK is compelling repeatable evi-
dence that relative corneal properties seem not to have
any effect on its ability to measure IOP.
While this device is similar in app earance to GAT, the
DCT it is unlike Goldmann applan ation in that it is not a
variable force tonometer. DCT implements a miniature
piezo-resistive pressure sensor, which is imbedded within
a contour-matched tonometer tip. When an electric cur-
rent passes through it, the sens or vibrates at a predictable
rate. When the sensor is subjected to a change in pressu re,
the vibration rate and resistance ar e altered and the PAS-
CAL’s computer calculates a change in pressure in con-
cordance with the change. The tonometer tip rests on the
cornea with a constant appositional force of one gram.
This is an important distinction from all forms of ap-
planation tonometry, wherein the probe force is variable.
The contour matched tip has a concave surface of ra-
dius 10.5 m. This curvature approximates the cornea’s
shape when the pressures on both sides of it are equal.
This is the key to the PASCAL’s ability to neu tralize the
effect of inta-individual variation in corneal properties,
which have significant influence on applanation meas-
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History and Clinical Validation of the PASCAL® Dynamic Contour Tonometer 79
Once a portion of the central cornea has taken up the
shape of the tip, the integrated pressure sensor begins to
acquire its value, measuring IOP 100 times per second.
Complete measurements require about 8 seconds of con-
tact time. During the measurement, an audio feedback is
generated, which helps the clinician insure proper contact
2. Clinical Validation—Initia l Chal lenge s
Beyond the premarket studies, showing the scientific
basis for the function of the DCT along with demon-
strating reasonable concordance to GAT, the challenge of
the research community has been to scientifically estab-
lish superior precision of DCT, compared with the GAT
standard and, ultimately, to weigh the relative clinical
value of such (more precise) data. Early investigators
such as Lachkar et al. in 2005 [21], attempted, with no
success, to compare DCT and GAT measurements in the
context of widely accepted central corneal thickness cor-
rection algorithms (Ehlers et al.) [3]. These types of in-
vestigations seemed only to validate the idea that there
was no reliable mathematical link between the two IOP
values. It soon became clear that the differences in
measured IOP values between traditional GAT and DCT
could not be explained in terms of individual variability
of central corneal thickness (CCT) measurements and
other corneal properties must play a significant role [6,
2.1. In Vitro Comparison between the PASCAL®
In 2004, Robert Stamper, MD at University of California,
San Francisco performed in vitro comparisons intracam-
eral (manometric) between the PASCAL®, DCT and
GAT in sixteen freshly enucleated human cadaver eyes.
The study revealed a close adherence of DCT to actual
manometric values, which seemed to remain relatively
unchanged even where the corneal properties were sig-
nificantly altered during the experiment [13].
2.2. Corneal Properties
Physical models proposed by Roberts et al., in 2005,
showed that non CCT corneal properties appeared to
account for the majority of factors that accounted for
these differences. Roberts showed that, while CCT vari-
ability may account for little more than 2 mm Hg error in
GAT, variability in corneal properties in total may ac-
count of over 10 mm Hg. [6]
2.3. LASIK and DCT
One of the most perplexing challenges of managing the
diagnosis and care of glaucoma patients in the context of
kerato refractive surgery was the apparent downward
shift in GAT in virtually all subjects post operatively.
Initially, it was assumed that the mere reduction in CCT
caused by LASIK or PRK justified the decrease. Pepose
showed that similar GAT decreases occurred in subjects
where LASIK flaps were made with no ablation . In these
subjects, the CCT decrease from the LASIK microkera-
tome was between 10-15 microns, compared with CCT
decreases of over 100 microns typically seen in LASIK
with ablation. Without significant CCT decrease, the
only thing that could explain the IOP decrease that Pe-
pose observed in these non-LASIK subjects would be the
apparent alteration in corneal properties caused by the
creation of the LASIK flap. In simpler terms, while not
significantly decreasing CCT, the creation of the LASIK
flap alone made the cornea significantly less rigid [17].
In 2003, Kaufman n et al. observed 62 patients pre and
post LASIK. GAT measurements decreased 3.0 +/– 1.9
mm Hg (p = 0.001). In contrast, no significant change in
IOP readings was recorded by DCT (0.2 mm Hg +/– 1.5
mm Hg, p = 0.30). There was no change in IOP in the
untreated control eyes as measured by GAT [1 0] .
Kaufmann’s observations were particularly meaning-
ful because he demonstrated that, in the context of sig-
nificant changes in corneal CCT and properties caused
by LASIK, DCT measurements were not significantly
affected [10].
2.4. DCT Population Study
In 2004, Kaufmann et al. performed a detailed compari-
son of the PASCAL® Dynamic Contour Tonometer
(DCT) with Goldmann applanation tonometry (GAT).
The study analyzes IOP measurements and biometric
measurements which were taken from a large population
of healthy volunteers, and features a careful statistical
analysis to determine any influence of corneal thickness,
axial length, corneal curvature, and anterior chamber
depth on either of the two types of tonometers. Unlike
many other comparisons, these authors acquired pressure
readings three times per device per patient to analyze
intra- and inter-observ er variability.
Kaufmann’s results showed that DCT readings have a
high concordance with GAT readings and that DCT were
not significantly influenced by corneal thickness or cur-
vature, axial length, or anterior chamber depth or varia-
tions in central corneal thickness. Additionally, they ob-
served that DCT readings are on average 1.7 mmHg
higher than GAT readings and that intra-observer vari-
ability (reading error made by same observer) in repeated
measurements is higher (approximately 2 times higher)
with GAT than with DCT [9].
2.5. In Vivo Manometric Comparison
Traditionally, the ultimate method for verifying the va-
Copyright © 2012 SciRes. OJOph
History and Clinical Validation of the PASCAL® Dynamic Contour Tonometer
lidity of a clinical tonometer is in vivo comparison to a
manometric standard. Manometry (measurement of IOP
within the anterior chamber) requires that an IOP meas-
uring device has a direct connection, via intracameral
cannulation, to the anterior chamber. Boehm et al. pub-
lished the results of their in vivo DCT manometric com-
parison in 2006.
While difficult to perform, in vivo intracameral meas-
urement and comparison between IOP measuring devises
is the ultimate validation of tonometric accuracy. During
the initial phase of cataract surg ery, a cannula is inserted
in the anterior chamber. With the cannula in place, true
manometric IOP can be monitored and anterior chamber
pressure and be altered to desired levels. This technique
gives the investigator the opportunity to compare the
measurements of the Goldmann and the PASCAL to ac-
tual manometric IOP. These tests are performed at dif-
ferent pressure levels and on different subjects with dif-
ferent corneal thicknesses and properties. It is encourag-
ing that the results of these challenging experiments
seem to be consistent with previous studies performed on
cadaveric eyes, showing that DCT measurements are
highly concordant to actual anterior chamber measure-
ments [23].
2.6. Tonometry Practicality and Precision
Schneider et al., in 2006 compared DCT and GAT ma-
thematically and in terms of clinical practicality, showing
that DCT seems to be a reliable method for intraocular
pressure measurement which, unlike Goldmann applana-
tion tonometry, is not influenced by central corneal thick-
ness. In clinical practice, advantages from DCT can be
expected for cooperative patients, outpatients, and pa-
tients with sufficient bilateral ocular fixation, whereas
Goldmann applanation tonometry measurements are more
reliable in case of patients with inadequate cooperation,
poor vision, or ny st agmus [24].
In 2010, Kotecha et al. compared the repeatability and
reproducibility of the GAT, DCT, and Reichert Ocular
Response Analyzer (ORA) and agreement between tono-
Their results showed that the DCT shows excellent
measurement precision, displaying the best repeatability
and reproducibility of the 3 tonometers. On average,
GAT under-read both DCT and ORA IOP measurements
by approximately 2 mm Hg. Corneal stiffness, as defined
using corneal response factor CRF, was associated sig-
nificantly with agreement between devices. The IOP
measurements with each device are not interchangeable
3. PASCAL® in Clinical Practice
As our understanding of the function of the DCT became
more clear along with a more robust knowledge of the
more complex range of influences that might influence
GAT readings, researchers began to take a closer look at
the real clinical value of this new, and presumably more
precise, clinical data.
3.1. Comparing GAT and DCT in Patients with
In a novel 2007 study comparing the relationships be-
tween glaucomatous visual field loss and IOP as meas-
ured by both PASCAL® DCT and GAT, Sullivan-Mee et
al. suggest that DCT-IOP is correlated with glaucoma-
tous damage, and moreover, DCT-IOP is more closely
related to extent of glaucoma damage than is GAT-IOP.
The most likely explanation for these results is that
GAT-IOP systematically underestimates IOP compared
with DCT- IOP. Their findings also support the hypoth e-
sis that corneal biomechanical factors other than CCT are
major confounders of applanation tonometry measure-
ments [18].
3.2. A Comparison with African Americans
In 2007, Madeiros et al. evaluated the relationship be-
tween IOP measurements obtained by DCT and GAT in
African Americans and assessed whether these measures
were influenced by ocular parameters including corneal
thickness, corneal curvature, and axial length. Their
findings indicate that DCT measurements in African
Americans seem to provide an estimate of IOP that is
less influenced by corneal properties than those provided
by GAT. [16]
3.3. An Insight on DCT’s Clinical Value
A review of the abundant literature that now exists seems
to point to the conclusion that DCT measurements aver-
age slightly less than 2 mm HG above GAT and are rela-
tively uninfluenced by the variations in corneal proper-
ties that appear to cause GAT to under read IOP in cer-
tain patients. The anecdotal conclusion that one might
reach is that one measuring with DCT is less apt to over-
look a significant number of patients with increased IOP
or early glaucoma. As shown in post LASIK patients, the
systematic errors in GAT cause it to read low in a clearly
unpredictable manner. Therefore, some patients found to
have increased IOP with DCT may be underdiagnosed or
diagnosed late when measured with GAT [1,4,9,16].
4. Discussion
As time and technologies progress, doctors are charged
with the task of seeking various testing modalities that
have demonstrated increased precision and specificity.
The apparent goal in this quest is to seek out earlier and
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History and Clinical Validation of the PASCAL® Dynamic Contour Tonometer 81
more precise diagnosis. In the case of glaucoma, eye
doctors seem to have embraced the dramatic evolution of
imaging, where, in the 1950’s the standard of care was
manual disc drawing, to current automated OCT devices
which have retinal and optic nerve resolution of only a
few microns. In the case of visual field measurement, the
1950’s standards were the Tangent Screen and Goldmann
Arc Perimeters. Today, automated devices such as the
Octopus (Haag-Streit) and Humphrey (Carl Zeiss) ana-
lyzers have understandably become clinical standards.
One must ask why, in this world where we have
evolved from the slide rule to sophisticated microproc-
essors which are expected to soon outpace the human
mind, we adhere to the Goldmann “gold standard” in IOP
measurement. Consider that the Goldmann tonometer
was developed in the 1950’s and has well documented
clinically significant systematic errors. While a more pre-
cise technology seems to exist, why has its acceptance
been slow? The answer is probably more economic and
practical then it is scientific. Goldmann tonometers are
relatively inexpensive and tend to be very durable. Addi-
tionally, tonometry as a procedure lacks specific finan-
cial reimbursement in the American insurance system.
While the financial incentive for newer imaging, visual
field tests has helped propel the acceptance of these new
technologies, the financial incentiv e is lacking in the case
of tonometry. It has also been apparent that Ziemer
Group, AG, Switzerland, the manufacturer of the PAS-
CAL®, is a relatively small and unknown to the ophthal-
mic community. This reality has seemed to heighten the
strategic challenges that one would expect in replacing
an age old “gold standard” such as Goldmann applana-
tion tonometry. Given this history, one would hope that a
more precise and reliable technology like the PASCAL®,
with better precision and reliability, will soon become the
new clinical standard in IOP measurement.
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