History and Clinical Validation of the PASCAL<sup>®</sup> Dynamic Contour Tonometer

doi:10.4236/ojoph.2012.23016

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Open Journal of Ophthalmology, 2012, 2, 78-82

http://dx.doi.org/10.4236/ojoph.2012.23016 Published Online August 2012 (http://www.SciRP.org/journal/ojoph)

History and Clinical Validation of the PASCAL® Dynamic

Contour Tonometer

Elliot M. Kirstein

Harper’s Point Eye Associates, Cincinnati, USA.

Email: [email protected]

Received April 16th, 2012; revised May 28th, 2012; accepted June 15, 2012

ABSTRACT

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-

History and Clinical Validation of the PASCAL® Dynamic Contour Tonometer 79

urements.

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

[8-20].

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,

22].

2.1. In Vitro Comparison between the PASCAL®

DCT and GAT

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-

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-

meters.

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

[15].

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

Glaucoma

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

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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