Figure 1. Maklakoff’s original tonometer, circa 1885.
By Elliot Kirstein, OD, FAAO
NAVIGATION GUIDE: Use this outline to navigate through the course. To return to the top of this page, use your browser's back button.
This course presents the historic and scientific background of tonometry followed by a review of recent technologic breakthroughs in intraocular (IOP) measurement. Included are case studies and citations of research papers that compare IOP measurements using traditional versus new technology. Implications of correct IOP measurement for understanding, diagnosing, and managing glaucoma are also discussed.
Digital palpation tonometry
The technology used to estimate intraocular pressure has evolved tremendously since Sir William Bowman emphasized the importance of ocular tension measurements. In an address delivered at the 1826 meeting of the British Medical Association, Sir William underscored the critical role that digital estimation of ocular tension played in his practice. (In this case the term "digital" refers to palpation of the eyes using the fingers – the digits.) In his address, Sir William stated:
“…it is now my constant practice, where defective vision is complained of, to ascertain almost at the first instant the state of tension in the eye.… It is easy enough to estimate the tension of the eye, though there is a right and a wrong way of doing even so simple a thing…. With medical men, the touch is already an educated sense, and a very little practice should suffice to apply it successfully to the eye.” (1)
Soon afterwards, digital palpation tonometry became an essential clinical skill to be mastered by all ophthalmologists. When mechanical tonometry was first introduced in the late 1800s, many ophthalmologists felt so confident with their ability to estimate IOP by palpation that they viewed the new technology as inferior. Schnabel, in a 1908 address to the Vienna Ophthalmological Society, stated that although he did not object in principle to mechanical tonometry, he expected “…very little from this test since digital tonometry by an expert is a much more accurate test.” (2)
Ironically, at the World Glaucoma Congress in Vienna in July 2005, a contemporary version of that same theme was debated. (3) Great confidence in Goldmann applanation tonometry has been established by eye doctors around the world for the past 50 years. How much benefit could be achieved by use of newer tonometers now being introduced?
Impression tonometry
Although Albrecht von Graefe is credited with the first attempts to create instruments that mechanically measured IOP in the early 1860s, his proposed instruments were neither designed nor built. Rather, it was Donders who designed the first instrument capable of estimating IOP – albeit not accurately – in the mid 1860s. The principle behind Donders instrument was to displace intraocular fluid by contact with the sclera. Ophthalmologists first measured the curvature of the sclera at the site of contact and then used the measurement as a reference plane to measure the depth of indentation produced by the tonometer.
Smith and Lazerat refined this technology in the 1880s, and the discovery of cocaine by Carl Koller in 1884 led the way to corneal impression tonometry. Using corneal anesthesia, corneal tonometry became the definitive choice for IOP measurement because it offered a well-defined and uniform site of impression.
The major shortcoming of impression tonometry was that it displaced so much fluid upon contact with the eye that the measured readings were highly variable and mostly inaccurate. What was needed was a way to displace a minimal amount of fluid to record IOP.
This breakthrough came in 1867 when Adolf Weber designed the first applanation tonometer that gave a highly defined applanation point without indentation. After two decades of skepticism, the value of applanation tonometry was re-discovered when Alexei Maklakoff and others introduced new versions of applanation tonometers.
In the early 20th century, there were about 15 tonometer models in use. However, digital palpation tonometry remained the “gold standard” among most ophthalmologists during the early 1900s.
Figure 1. Maklakoff’s original tonometer, circa 1885.
The first commonly used mechanical tonometer was designed and introduced by Hjalmar Schiotz in the early 1900s. The instrument was simple, easy to use, and relatively precise. It was quickly accepted and became the new gold standard beginning the 1910s. Innovations in calibration led to its increased use, and a tremendous amount of knowledge about the normal and glaucomatous eye was quickly acquired.
Goldmann introduced an adjustment for ocular rigidity in the 1950s, which led to the development of the Goldmann applanation tonometer. (4) The Goldmann tonometer displaces so little fluid that variations in ocular rigidity were then thought to be mostly negligible.
Fig 2. Maurice applanation apparatus, circa 1951.
Today, digital palpation tonometry has largely been replaced by more sophisticated technologies used to estimate IOP. Today’s instruments are far more accurate and easier to use. Yet, sometimes, there is no good substitute for palpation tonometry. For example, some optometrists and ophthalmologists may still have to rely on digital palpation to estimate IOP in patients who are uncooperative. (3)
Indentation (Schiotz) tonometry
This type of tonometry uses a plunger to indent the cornea. IOP is determined by measuring how much the cornea is indented by a given weight. The test is less accurate than applanation tonometry and is not commonly used today by ophthalmologists and optometrists. However, some family medicine or urgent care doctors still use the Schiotz tonometer.
Fig 3. Schiotz Tonometer.
Goldmann applanation tonometry
The Goldmann applanation tonometer (GAT) is a variable force tonometer, which makes a static measurement of the force required to flatten a fixed area of the cornea. For the past fifty years, it has been considered to be the clinical gold standard in IOP measurement.
When Hans Goldmann designed the tonometer, he recognized that certain corneal effects (e.g., resistance to deformation) would influence pressure measurements. (5) Therefore he based his calculations on the resistance to deformation of an average corneal thickness (520 microns) and estimated that the resistance to deformation would be cancelled by the surface tension generated by the pre-corneal tear film when the area applanated had a diameter of 3.06 mm.
Fig 4. Dr. Hans Goldmann and the Goldmann applanation tonometer.
Non-contact Tonometry (NCT)
Non-contact (also called air-puff) tonometers do not touch the eye because they use a puff of air to flatten (applanate) the cornea. Once initiated, the puff force increases until the cornea is applanated by a predetermined amount. The tonometer then translates this force into a measure of IOP.
Fig 5. The original AO (Reichert) non-contact tonometer.
Because the air puff tonometer relies on corneal applanation, it is subject to the same potential measurement errors induced by variations in corneal properties, as is the Goldmann tonometer.
An additional source of error in NCT measurements is that IOP is determined at a single very brief instant in time and IOP can pulsate considerably over time as the choroid fills with blood and then empties in concert with the cardiac cycle. This phenomenon can be directly observed by viewing pulsation of mires during Goldmann tonometry. (To some degree, Goldmann takes this pressure variation into account because measurements are made when the inner aspects of the pulsating mires just touch.)
In some individuals, IOP can vary as much as 5 or 6 mm Hg within one second while the choroid fills and empties. The NCT has no ability to determine at what point in an individual's intraocular pressure cycle the IOP was measured.
Until the late 1990’s, the Goldmann applanation tonometer enjoyed an unchallenged 45-year reign as the “gold standard.” However, two thought provoking events caused many to begin questioning whether GAT was measuring true IOP in a variety of situations: the acceptance and use of refractive surgery, and publication of the Ocular Hypertensive Treatment Study (OHTS) results.
Refractive surgery and applanation error
As soon as radial keratotomy (RK) became commonplace, eye doctors observed differences in pre- and post-operative Goldmann IOPs. Commonly, IOP was found to decrease by 3 to 5 mm Hg after surgery. Similar observations were made with newer refractive technologies such as Photorefractive Keratectomy (PRK) and Laser Assisted In-Situ Keratomeilusis (LASIK).
Some observers accounted for this apparent pressure decrease exclusively in terms of the decrease in central corneal thickness caused by the PRK and LASIK surgery. (6,7,8) However, with the case of radial keratotomy (RK), a decrease in CCT could not explain the IOP changes because RK causes no decrease in CCT. Indeed, one could argue that post-RK corneas often show increased CCT resulting from varying degrees of corneal edema.
Water provocative testing
To investigate the change of IOP during PRK, a group of investigators measured applanation IOPs in subjects during a water provocative test. For those who may not recall this test, the water provocative test was designed to measure facility of aqueous outflow. It is performed by measuring IOP, having the patient drinking one liter of water, and then re-measuring IOP at 10-minute intervals during the next hour. The supposition was that due to compromised outflow, glaucomatous individuals would have greater and longer IOP increases than would normal subjects.
The water provocative test was performed on a group before and after PRK surgery. Before surgery, water drinking caused IOP to increase in the group by about 3 mm Hg. After surgery, the same group showed only a 1.5 mm Hg increase.
Assuming that the rise in measured IOP in the water provocative test is the result of a rise in pressure in the eye’s anterior chamber and that this should not be affected by corneal surgery, what does change to account for the IOP difference?
One explanation is that refractive surgery decreased the CCT so that the measuring device failed to accurately report the true pressure in the anterior chamber. The real pressure increased 3 mm Hg and the applanator measured a rise of only 1.5 mm Hg.
Fig 7. Water provocative test results pre- and post-PRK.
Researchers have designed various algorithms based on the PRK-produced decrease in CCT to somehow compensate for the apparent downward shift in measured IOP after surgery. (6,7,8) Although these formulas seemed to alleviate some discomfort with the confusing post-operative GAT IOP measurements, the drop in IOP seen after RK surgery could still not be explained in these terms. After all, RK patients have no decrease in CCT and, paradoxically, may have higher CCT post-operatively due to corneal swelling. Was there an important piece missing from this puzzle?
Although the central theme of the well-known Ocular Hypertensive Treatment Study was an analysis of the tendency for ocular hypertensives to convert to primary open angle glaucoma (POAG) over time (with or without treatment), it was also an opportunity to observe the effect of variables other than IOP in this tendency. (9) CCT was one variable measured in OHTS subjects. (10) Among the results of this portion of the study, the investigators reported that they had observed an increased propensity to convert from ocular hypertension to POAG in those individuals who had comparative low CCT (under 545 microns). They suggested that an error in GAT imposed by variability in CCT might be cause an under- or over-estimation of IOP when measured with Goldmann.
Two lines of research evolved out of the startling revelations of OHTS. One has been in the direction of ocular structural dynamics and the pathogenesis of glaucoma, and the other has been in the technology of IOP measurement.
Given the compelling results of the OHTS, it seems quite natural that investigators would start to look closely at the true impact of CCT on the Goldmann measurement.
Ironically, in the early 1950’s, Hans Goldmann revealed in his renowned (but seldom read) publications that IOP measurements with his tonometer could be seriously flawed if the subject’s corneal biomechanics did not fulfill certain stringent criteria. CCT was one of the significant criteria that Goldmann discussed.
A commonly used CCT correction formula was published by Ehlers. In fact, many clinicians carry an Ehlers CCT correction chart in their clinic coat pockets. The fundamental supporting concept for this correction formula is that as corneas get thinner, GAT reads too low. If CCT is “average," GAT is essentially correct. And, if the cornea is thicker than average, GAT overvalues true manometric IOP. Although Ehlers' formula was actually based on manometric data, a weakness of the formula arises from the low number of study subjects used to determine the formula and the high degree of variability between the subjects.
Ehler's data convinces us that there is a tendency for Goldmann IOP to increase in concordance with increasing CCT. But a close look at his data shows that there are many subjects who clearly defy the trend. Goldmann readings were too low in some subjects with thick corneas and too high in some with thin corneas. IOP to CCT correlations for Ehlers’ data, as well as for data from similar studies, are too low to allow one to confidently make clinical decisions based on these formulae. (11)
Table 1. Correction values for IOPs based on CCT. Corrections derived from data from Ehlers, et al., (1975), Stodtmeister (1998), and Doughtry and Zaman (2000).
|
CCT in microns
|
IOP correction in mm Hg
|
|
445
|
7
|
|
455
|
6
|
|
465
|
6
|
|
475
|
5
|
|
485
|
4
|
|
495
|
4
|
|
505
|
3
|
|
515
|
2
|
|
525
|
1
|
|
535
|
1
|
|
545
|
0
|
|
555
|
-1
|
|
565
|
-1
|
|
575
|
-2
|
|
585
|
-3
|
|
595
|
-4
|
|
605
|
-4
|
|
615
|
-5
|
|
625
|
-6
|
|
635
|
-6
|
|
645
|
-7
|
Other IOP correction formulae beyond Ehlers' formula have also been developed. Below, is a simplified version of the Orssengo-Pye formula that has been advocated by James Tsai and Stephen Trokel at Columbia University. (12, 13, 14)
Corrected IOP = Measured IOP – (CCT-545)/50 x 2.5 mm Hg
This simplified formula instructs the clinician to correct IOP by 1.0 mm Hg for every 20 microns of CCT variation from the 545 standard. For example, a patient with a 645-micron cornea has a 5 mm Hg Goldmann overestimation and a patient with a 445-micron cornea has a 5 mm Hg underestimation.
However, as is usually the case with most biological functions or processes, things are not so simple as these formulae and correction tables suggest. Newer investigations have shown that the formulae represented an incorrect and sometimes dangerous oversimplification of the complex relationship between corneal biomechanics and IOP.
In addition to CCT, Goldmann also observed that a range of other corneal properties could cause GAT to be inaccurate. (4,5) Current investigations have validated Goldmann’s observations and have suggested that corneal elasticity rather than CCT seems to be a principle cause of GAT error.
Perhaps an easy way to think of corneal elasticity is in terms of relative corneal rigidity or softness. Reliable GAT measurements rely on average corneal rigidity. When the cornea is more rigid than average, GAT reads too high, and, when the cornea is softer than average, GAT reads too low.
What can make a cornea too hard or too soft? Some corneal scars, high CCT (without edema or refractive surgery), and microcornea can cause a cornea to be unusually rigid. The circumstances that can cause a cornea to be unusually soft seem to be more common. Here are some examples:
To support this notion, Roberts at The Ohio State University, Department of Ophthalmology, has reported that certain corneal properties (such as rigidity) can be as much as ten times more influential than CCT alone in affecting GAT. (15)
Additional information about the effect of corneal rigidity on Goldmann measurements was gained by looking at the IOP before and after refractive procedures in selected subjects.
Most corneal refractive procedures cause some degree of corneal thinning. All of them cause corneal softening (i.e., a reduction in relative elasticity). For example, LASIK obviously causes a restructuring of corneal architecture. The naturally interwoven collagen fibrils in the anterior cornea, which significantly contribute to corneal strength and elasticity, are dissected when the LASIK flap is created. After LASIK, it is unlikely that the cornea can ever behave mechanically as it did pre-operatively.
A study we conducted on tonometry and LASIK surgery was focused on an unusual group of LASIK patients; those with unusually high post-operative CCT values (over 545 microns). These patients are unusual because most post-operative LASIK patients have CCTs that are relatively thin. Our hypothesis was that if we could observe a group of LASIK patients who had thicker than average corneas, their average Goldmann IOP readings should be higher than average un-operated eyes (i.e., over 15 mm Hg). This would hold true if the CCT-IOP correction formula was valid and actually worked for LASIK patients.
We found that the post -LASIK group had very low GAT IOP readings even though they had relatively thick corneas. The uncorrected mean Goldmann IOP in the group after surgery was only 11.4 mm Hg. This was far less than the above average of 16 to 19 mm Hg that we had expected with a mean post-operative CCT of 559.4 microns. Even more interesting was that when we used the formula to “correct” for CCT, the mean apparent IOP in the group dropped even further to 10.7 mm Hg.
Table 2. Mean results from the study of pre- and post-LASIK IOPs for 28 eyes with thick corneas. The PASCAL® data come from a new tonometer to be discussed later in the course.
|
Measurement
|
Pre-operative
|
Post-operative
|
|
Goldmann IOP
|
16.5 mm Hg
|
11.4 mm Hg
|
|
Pachymetry
|
590.5 microns
|
559.4 microns
|
|
Goldmann IOP (CCT corrected)
|
14.3 mm Hg
|
10.7 mm Hg
|
|
PASCAL® IOP
|
Not available
|
17.5 mm Hg
|
Based on this small study, we concluded that IOP had decreased from LASIK as the result of significant changes in corneal properties other than CCT alone. LASIK may make the cornea “soft,” even if it is still relatively thick after treatment. Refractive procedures weaken or soften the cornea in a way that makes it less elastic (rigid). When elasticity decreases, Goldmann reads too low. (16)
Can we do better than the 50 year old “Gold Standard?”
In view of recent investigations, interest in tonometry has increased and research engineers were charged with the mission of developing a better understanding of various corneal properties and their respective influences on GAT measurements, as well as developing new techniques to more accurately determine true IOP. With these goals in mind, the technology has taken two different directions: the Reichert Ocular Response Analyzer® and The PASCAL® Dynamic Contour Tonometer (Ziemer Ophthalmic Systems, AG, Switzerland) have been developed.
The Reichert Ocular Response Analyzer® utilizes a "dynamic bi-directional applanation process" to measure both the biomechanical properties of the cornea and the IOP. The basic output is a Goldmann-correlated applanation pressure measurement (IOPG) and a measure of corneal tissue properties called corneal hysteresis (CH), which is related to viscous damping in the corneal tissue.
The CH measurement also provides a basis for two additional parameters measured by the ORA: the corneal-compensated intraocular pressure (IOPCC) and the corneal resistance factor (CRF).
IOPCC is an IOP measurement designed to be less affected by corneal properties than is IOP measured by Goldmann or NCT. IOPCC has essentially a zero correlation with CCT in normal eyes and stays relatively constant pre- versus post-LASIK.
CRF appears to be an indicator of the overall “resistance” of the cornea to applanation and is significantly correlated with CCT and GAT, but not with IOPCC.
Understanding hysteresis: elastic, viscous, and visco-elastic materials
In order to understand the Ocular Response Analyzer, a brief discussion of properties of viso-elastic materials will be presented.
Elastic materials are those for which strain (deformation) is directly proportional to stress (applied force) independent of the length of time or the rate at which the force is applied. Therefore, if the elastic modulus of a structure (e.g., a steel beam) is known, one can easily predict the amount of force required to bend it a specific amount.
Viscous materials are those for which the relationship between strain and stress depends on time or rate of force application. Think of pushing a spoon into a jar of honey. The resistance to the applied force depends primarily on the speed at which the force is applied (greater speed equals greater resistance).
Structures that are said to be “visco-elastic” possess characteristics of both types of material. A simple example is an automotive suspension strut assembly. There is a component of static resistance (a coil spring) and a component of dynamic resistance (a shock absorber - also known as a damper). Together, the two parts make up a visco-elastic system.
The response of such a system to an applied load depends upon the material properties, the magnitude of the force, and the rate at which the force is applied. The human cornea is a complex visco-elastic structure.
The corneal hysteresis measurement is an indication of viscous damping in the cornea. In other words, it is related to the ability of the cornea to absorb and dissipate energy. Subjects whose corneas exhibit low CH can be thought of in simple terms as having a “soft” cornea.
Operation of the Ocular Response Analyzer®
The ORA utilizes an air pulse to apply force to the cornea and an advanced electro-optical system to monitor the resultant corneal deformation. Alignment to the patient’s eye is fully automated.
Fig 8. Reichert Ocular Response Analyzer®
A precisely metered, collimated air pulse causes the cornea to move inwards, past applanation, and into a slightly concave shape. Milliseconds after applanation, the air pump shuts off and the pressure declines in a smooth fashion. As the pressure decreases, the cornea begins to return to its normal configuration and once again passes through the applanated state. An applanation detection system monitors the cornea throughout the entire process and pressure values are recorded for the inward and outward applanation events.
One might initially expect these two pressure values to be the same. However, viscous damping in the cornea causes delays in the inward and outward applanation events, resulting in two different pressure values. The average of these two pressure values provides a repeatable, Goldmann-correlated IOP value (IOPG). The difference between these two pressure values is corneal hysteresis (CH). The ability to measure this effect is the key to understanding the biomechanical properties of the cornea and their influence on the IOP measurement process.
Fig 9. The difference between "inward" applanation pressure and "outward" applanation pressures defines corneal hysteresis.
CH, CRF, and IOPCC: New Ocular Parameters
Ongoing clinical studies over the past three years have shown that CH is a function of corneal properties and not an artifact of any other variable. Corneal hysteresis is a phenomenon that results from the dynamic nature of the air pulse and the viscous damping inherent in the cornea.
The corneal resistance factor is also derived from this response. CRF is a measurement of the cumulative effects of both the viscous and elastic resistance encountered by the air pulse while deforming the cornea. CRF exhibits the expected property of increasing at significantly elevated pressures.
Although CH and CRF are related, in some instances they are significantly different, and each provides distinct information about the cornea. Corneal-compensated IOP is a pressure measurement that utilizes information provided by the corneal hysteresis measurement to provide an IOP value that is less affected by corneal properties.
Although the manufacturer of the ORA cannot yet claim to be measuring “true intraocular pressure,” early investigations have demonstrated that IOPCC is a better indicator of the real IOP than traditional NCT or GAT can provide. (17)
Dynamic contour tonometry (DCT) is a novel measuring technique using the principle of contour matching instead of applanation to eliminate the systematic errors inherent in previous tonometers. These factors include the influence of corneal thickness, rigidity, curvature, and elastic properties.
The PASCAL® is a relatively new device that uses DCT to measure IOP. Although this device is similar in appearance to a Goldmann, the PASCAL® it is unlike Goldmann applanation in that it is not a variable force tonometer.
PASCAL® uses a miniature pressure sensor embedded within a tonometer tip contour-matched to the shape of the cornea. The tonometer tip rests on the cornea with a constant appositional force of one gram. This is an important difference from all forms of applanation tonometry in which the probe force is variable.
When the sensor is subjected to a change in pressure, the electrical resistance is altered and the PASCAL's computer calculates a change in pressure in concordance with the change in resistance.
The contour matched tip has a concave surface of radius 10.5 mm, which 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 neutralize the effect of intra-individual variation in corneal properties. (18-21)
Fig 10. Juxtaposition of cornea and PASCAL tip.
Once a portion of the central cornea has taken up the shape of the tip, the integrated pressure sensor begins to acquire data, measuring IOP 100 times per second. A complete measurement cycle requires about 8 seconds of contact time. During the measurement cycle, audio feedback is generated, which helps the clinician insure proper contact with the cornea.
Fig 11. The PASCAL® contoured piezoelectric sensor tip.
Fig 12. The PASCAL® device.
Fig 13. The PASCAL® can be mounted on any biomicroscope.
IN VITRO COMPARISON OF IOP MEASURED WITH THE PASCAL® DCT AND GAT
Robert Stamper, MD, University of California, San Francisco, performed in vitro comparisons between PASCAL® DCT and GAT versus an intracameral, manometric reference in 16 freshly enucleated human cadaver eyes. Along with comparisons at different pressure levels, additional comparisons were made over a wide range of corneal thicknesses (corneas were hydrated to achieve CCT values between 400 and 800 microns). Results of these comparisons indicate that:
The close adherence of DCT IOPs to actual manometric values shown in this study leads to the conclusion that DCT provides values that are extremely close to true manometric IOPs, even when the corneal properties were significantly altered during the experiment. (20)
Fig 14. Measuring PASCAL® IOP values against manometric values in a cadaver eye.
IN VIVO POPULATION STUDY OF IOP MEASURED WITH THE PASCAL® DCT AND GAT
Kaufmann, et al., published the first detailed comparison of the PASCAL® Dynamic Contour Tonometer (DCT) with Goldmann applanation tonometry (GAT). The study analyzed IOP and biometric measurements taken from a large population of healthy volunteers and featured 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. (21)
Unlike many other comparisons, these authors took care to take pressure readings three times per device per patient and to analyze intra- and inter-observer variability.
Results:
Conclusions:
Clearly, some of the most compelling evidence that supports the notion that DCT is not influenced by corneal properties is the lack of a “LASIK effect.” Kaufmann, et al., compared GAT and DCT values for a group of subjects before and after LASIK surgery. (19) Their results showed that the GAT values in the 62 LASIK eyes decreased by about 3.8 mm Hg. The DCT pressures changed by an increase of only 0.6 mm Hg. Irrefutably, corneal thickness and corneal properties are altered by LASIK, yet DCT pressures were relatively unaffected by the surgery.
Although difficult to perform, in vivo intracameral measurements of IOP can provide the ultimate validation of tonometric accuracy. Studies using the PASCAL® are currently being performed on live patients in Europe and the United States. Results from the first of these studies have been submitted for publication. (Boehm, et al.) (18)
In vivo manometric studies can be done during the initial phase of cataract surgery by inserting a cannula into the anterior chamber. With the cannula in place, true manometric IOP can be monitored and anterior chamber pressure can be altered as desired. This technique gives researchers the opportunity to compare GAT and PASCAL® measurements to actual manometric IOPs. These tests are performed at different pressure levels and on different subjects with various corneal thicknesses and properties. It is encouraging that the results of these challenging experiments seem to be consistent with previous studies performed on cadaver eyes.
Fig 15. Schematic for setup for in vivo cannulation.
In an in vivo cannulation study by Weber, Boehm, Spoerl, and Pillunat, the following methods were used:
Fig 16. The yellow curve is the manometric reference line (true IOP). The blue curve is the reported IOP from the PASCAL®. When the PASCAL® is on the cornea, the two curves are almost identical.
Results of the study include the following:
The cannulation study has shown that there is an effect of CCT and corneal curvature on applanation tonometry. However, CCT and corneal curvature appear to have little effect on PASCAL® IOPs. DCT seems to be a good alternative to GAT if corneal thickness or curvature factors are important considerations.
Fig 17. At The University of Dresden, Germany, Andreas Boehm, MD, performs a PASCAL® IOP measurement while a cannula is in the anterior chamber for pressure control and reference measurement.
Fig 18. Before phacoemulsification, a cannula is inserted into the anterior chamber to control IOP and to precisely determine manometric IOP.
The current literature on DCT tells us that the PASCAL® tends to read higher than Goldmann. There are two reasons for this. First, the PASCAL® is calibrated to actual manometric values and Goldmann calibrated his thickness-dependant instrument to a relatively thin 520 micron cornea. Second, many people have low corneal elasticity and the GAT underestimates the IOP in these individuals. Kaufmann, et al., showed that the normative curve for the DCT is about 1.7 mm Hg higher than for GAT. If we consider 12 to 21 mm Hg to be the normal range of IOPs for GAT, then we should consider 14 to 23 mm Hg to represent the normal range for DCT IOPs. This does not mean that all individuals with 21mm GAT IOPs are likely to have glaucoma, but it does mean that when using the PASCAL® one must become comfortable with a new and different normal IOP range.
Glaucoma is a complex disease with a multivariate etiology. Commonly known risk factors are heredity, age, systemic health, optic nerve cupping, angle anomalies, Drance hemorrhage, decreased optic nerve head perfusion, and elevated IOP. Most definitely, all patients with similar IOP values do not progress in the same way or at the same rate. Indeed, one patient with 25 mm Hg IOPs might have substantial visual field loss whereas another of the same age with similar IOPs might never progress at all.
Glaucoma is a disease of the optic nerve and elevated IOP is only one of several salient risk factors that contribute to glaucomatous axonal demise. It is the change in the retinal nerve fiber layer and subsequent change in visual fields that mark glaucomatous progression; not elevated IOP. So why is accurate IOP measurement so important?
The first answer to this question is the most obvious. If we are measuring something, why not do it as accurately as possible as long as the measurement can be made done practically? If we choose to use a yardstick, why not select one that is straight? Secondly, we must accept that elevated IOP is the most common parameter within the limits of a typical, routine eye examination that mandates a more detailed work-up including threshold visual fields, gonioscopy, and optic nerve imaging and analysis. Some might argue that elevated IOP rather than observable nerve changes represent over 95% of reasons for conducting these additional tests.
Goldmann is a poor screening test. As stated earlier, ideal screening tests show no false negatives and only a small number of false positives. This minimizes to possibility of people going undiagnosed. Current research has shown that the most common errors in applanation tonometry are in undervaluing true IOP. (17) The false negatives represent individuals who can easily slip through the cracks because of their erroneously low GAT IOP measurements.
In the recent Latino Eye Study, Varma, et al., measured GAT and Pascal (DCT) IOP for over 4000 eyes. In the subset of individuals with Goldmann pressures between 18 and 21 mm Hg, 2.5% had DCT pressures greater than 6 mm Hg higher than comparable GAT values. Independent of other risk factors, these patients (between 24 and 27 mm Hg, DCT) are far more likely to be referred for glaucoma workup. Conservatively, this can be interpreted to mean that based on GAT alone as many as 2 to 4 patients per week per doctor in a general practice could be missed and appropriate follow-up care would not be provided.
In addition to the information provided by the Latino Eye study, we must consider the large number of people who have had refractive surgery. It has been clearly demonstrated that GAT is unreliable in this group of patients. In the United States alone, by the end of 2006 nearly 10,000,000 people will have had corneal refractive surgery. As this large group of patients moves on into their 60’s and 70’s, it will be valuable to be able to measure their IOPs with confidence. (22)
To summarize, it is fair to say that although IOP is certainly not the only predictor of glaucoma, it is the only variable that can currently be treated and it remains the variable that most often causes additional testing. It is, therefore, reasonable to measure IOP as accurately as possible.
The OHTS was one of the most the most influential glaucoma research studies of our generation, however we are still inclined to ask, “How could we have done it better?” The obvious question relative to this course is how the results might have been influenced by using the PASCAL® instead of GAT?
Although we can only speculate, in view of what we have already learned about DCT, it seems likely that OHTS results would be significantly different had the PASCAL® been used in the study.
Here’s how some think that the results might have be affected:
Four cases reports are presented to illustrate some of the concepts discussed in this course.
Case 1. 48 year-old male who had LASIK OS
12 months after LASIK on left eye only
Case 1 Discussion:
With monocular LASIK, the patient is a clear example of the apparent effect of LASIK on GAT IOP, CCT, and he demonstrates the difference between GAT and DCT following LASIK. Although the un-operated right eye showed similar IOP findings with GAT and DCT, the left (operated) eye showed a significant shift in the GAT reading, but only a small change in the DCT measurement. The 5 mm Hg drop in GAT IOP cannot be explained by the 50-micron reduction in CCT (approximately a 2.5 mm Hg correction would be expected according to the Ehlers' table).
Case 2. 35 year-old male
15 months post bilateral LASIK
Case 2 Discussion:
This case is unusual for two reasons; first his pre- and post-operative pachymetric values are unusually high. Although we intuitively expect post-LASIK corneas to be thinner than average, this statistical outlier has pre- and post-operative values that remain above average. The compelling point is that when we impose an often-used thickness IOP compensation formula (20 microns = 1 mm Hg) to this patient’s GAT values, his “corrected IOPs” are only about 6 mm Hg OU. Not only the amount, but even the sign of the suggested correction is obviously wrong.
This case begs the question of whether the patient's thick corneas are actually behaving “as though they were thin,” deceptively showing artificially low GAT values. Research has demonstrated that CCT may not be the only variable which leads to misleading GAT results.
Second, if this patient's pre-op GAT readings are over-estimated due to his thick corneas, according to Ehlers’ correction formula the true pre-op IOP was probably closer to 15 mm Hg rather than the 18 to 19 mmHg found by GAT (DCT was not performed pre-operatively). This appears to be consistent with the post-op DCT measurement.
Case 3. Hyperopic LASIK patient
One-year follow-up
Case 3 discussion:
One of the differences between LASIK procedures for hyperopic and myopic patients is that only about 15 microns of CCT is lost for a typical hyperopic procedure compared with as much as 100 microns lost during myopic treatment. Even so, hyperopic LASIK decreases corneal elasticity by inducing a hinging effect. The resultant thinning in the cornea's mid-periphery weakens corneal structure.
Even with a relatively small reduction in CCT, case 3 shows a substantial (7 mm Hg) reduction in GAT. This finding exemplifies the lack of linear relationship between CCT reduction and change in IOP after LASIK. Unlike GAT, DCT post-LASIK IOPs remain similar to the pre-op GAT values.
Commonalities for cases 1, 2, and 3
Although these three cases represent significantly different individual situations, they exhibit an interesting common feature: all of the post-LASIK eyes seem to yield very similar post-operative GAT readings ranging from 10 to 12 mm Hg. This is irrespective of pre- or post-operative CCT.
This finding repeats itself throughout almost all the cases encountered in the author’s practice as well as in those reported in the literature. It strongly suggests that post-LASIK GAT readings may be mere artifacts and relatively useless for diagnostic purposes.
Case 4. Glaucoma suspect with borderline findings
Case 4 discussion:
This case illustrates a situation in which the nerves are suspect, CCT is slightly low, and there is a possible family history of glaucoma. Without knowing the DCT values, the decision to treat or not would be somewhat difficult. However, given the DCT values of 26 and 25mm Hg, one is more likely to follow this case closely or possibly consider pressure lowering treatment options.
The case highlights one of the most significant examples of the impact of DCT measurements in a general clinical setting. Preliminary comparison data, comparing DCT to GAT, (Varma, et al., – Latino Eye Study) have shown that over 10% of DCT values are at least 3 mm Hg higher than GAT values that have been adjusted for 1.8 mm Hg difference in normative curves.
Although at very high or very low pressure ranges, 3 mm Hg differences may have limited clinical relevance, 3 mm Hg differences at pressures around 20 mm Hg tend to make a big difference with regard to how individuals are managed. For example, a patient with 21 mm Hg pressures who has borderline non-IOP findings would likely be managed more aggressively if his true pressures would be known to be 24 or 25mm Hg.
1. Bowman, W. Br Med J, 1852; 377-382
2. Schnabel I., Klin Montasbl Augenh 1908; 48:318
3. Kalayoglu, M V. http://www.medcompare.com/spotlight.asp?spotlightid=185&headerid=0
4. Goldmann H, Schmidt T. Über Applanationstonometrie. Ophthalmologica, 1957;134:221-242.
5. Goldmann H, Schmidt T. Weiterer Beitrag zur Applanationstonometrie. Ophthalmologica 1961;141:441-456.
6. Munger R, Hodge WG, Mintsioulis G, Agapitos PJ, Jackson WB, Damji KF. Correction of intraocular pressure for changes in central corneal thickness following photorefractive keratectomy Can J Ophthalmol. 1998 Apr;33(3):159-65
7. Fournier AV, Podtetenev M, Lemire J, et al. Intraocular pressure change measured by Goldmann tonometry after laser in situ keratomileusis. J Cataract Refract Surg. 1998;24:905–910.
8. Gimeno JA, Munoz LA, Valenzuela LA, Molto FJ, Rahhal MS. Influence of refraction on tonometric readings after photorefractive keratectomy and laser assisted in situ keratomileusis. Cornea. 2000;19:512–516.
9. Kass MA, Heuer DK, Higginbotham EJ, et al. The Ocular Hypertension Treatment Study. Arch Ophthalmol 2002;120:701-703.
10. Brandt J, Beisser J, Gordon M. Central corneal thickness in Ocular Hypertension treatment study (OHTS). Ophthalmology. 2001;108(10):1779-88
11. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975;53:34-43
12. Orssengo GJ, Pye DC. Determination of the true intraocular pressure and modulus of elasticity of the human cornea in vivo. Bull Mathematical Biol 1999;61:551-72.
13. Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol 1993;115:592-596
14. Carolyn Y. Shih, MD; Joshua S. Graff Zivin, PhD; Stephen L. Trokel, MD; James C. Tsai, MD - Clinical Significance of Central Corneal Thickness in the Management of Glaucoma Arch Ophthalmol. 2004;122:1270-1275
15. Liu and Roberts, JCRS, JCRS 31, Issue 1, p 146-155 (January 2005)
16. Kirstein E, Huesler A. Evaluation of the Orssengo-Pye IOP Corrective Algorithm in LASIK Patients with Thick Corneas. Optometry – 9/05
17. Luce D, Taylor D. Reichert Ocular Response Analyzer Measures Corneal Biomechanical Properties and IOP Provides New Indicators for Corneal Specialties and Glaucoma Management
www.ocularresponseanalyzer.com/ Ocular%20Response%20Analyzer%20White%20Paper.pdf
18. Mueller-Holz MF, Spanier J, Schmidt E, Boehm AG, Pillunat LE, Dynamic Contour Tonometry vs. Applanation Tonometry – Comparison of IOP - Measurements, Dept. of Ophthalmology, University of Dresden, Dresden, Germany, ARVO 2006
19. Kaufmann C, Bachmann LM, Thiel MA Intraocular Pressure Measurements Using Dynamic Contour Tonometry after Laser In Situ Keratomileusis Investigative Ophthalmology and Visual Science. 2003;44:3790-3794.) © 2003 DOI: 10.1167/iovs.02-0946
20. Kniestedt C, Nee M, Stamper RL - Dynamic Contour Tonometry A Comparative Study on Human Cadaver Eyes Arch Ophthalmol. 2004;122:1287-1293
21. Kaufmann C, Bachmann LM, Thiel MA, Comparison of Dynamic Contour Tonometry and Goldmann Applanation Tonometry. Invest Ophthal & Vis Sci, Vol 45,
Sept 2004, pp. 3118-3121.
22. Chopra V, Wu J, Doshi V, Francis BA, Azen S, Varma R, the LALES Group. Diabetes Mellitis, Hypertension and the Prevalence of Open-Angle Glaucoma in Adult Latinos: The Los Angeles Latino Eye Study (LALES). ARVO 2006
OTHER REFERENCES
Ciro Tamburrelli, Andrea Giudiceandrea, Agostino Salvatore Vaiano, Carmela Grazia Caputo, Francesca Gulla`, and Tommaso Salgarello. Underestimate of Tonometric Readings after Photorefractive Keratectomy Increases at Higher Intraocular Pressure Levels. Investigative Ophthalmology & Visual Science, September 2005, Vol. 46, No. 9
Madjlessi F, Marx W, Reinhard T, Althaus C The influence of corneal thickness on applanation error in eyes with pathologic corneas, 98th Annual Meeting DOG 2000
Hansen F, Ehlers N. Elevated tonometer readings caused by a thick cornea. Acta Ophthalmol (Copenh) 1971;9:775-778.
Doughty, M.J. and Zaman, M.L., (2000) Human Corneal Thickness and Its Impact on Intraocular Pressure Measures. A Review and Meta-analysis Approach. Surv Ophthalmol. 44. 367-408.
Rehany U, Bersudsky V, Rumelt S, - Paradoxical hypotony after laser in situ keratomileusis, Journal of Cataract & Refractive Surgery, Volume 26, Number 12, 12/2000
Wolfs RC, Klaver CC, Vingerling JR, et al. Distribution of central corneal thickness and its association with intraocular pressure: The Rotterdam Study. Am J Ophthalmology 1997;123(6):767-72.
Pan X, Roberts CJ, Shah S,.Laiquzzaman M. Mantry S, Cunliffe I. Comparing LASIK and LASEK, ARVO 2006 Poster 559/B327
Sanderson JP, Qazi1 MA, Roberts CJ, Pepose JS. IOP and Corneal Biomechanical Metrics in Eyes With Keratoconus and Fuchs’Dystrophy Poster Compared to Pachymetry-Matched Controls ARVO 2006 Poster 2267/B1010
Milla E,.Duch S, Sacoto M,.Masuet C. (Barcelona, Spain) Correlation Between Pascal and Goldmann Tonometry in Eyes With Different Pachymetry Values ARVO 2006 Poster 4446/B205
Schneider ED, Grehn F. (Wuerzburg University Hospital, Germany)
Intraocular Pressure Measurement - Comparison of Dynamic Contour Tonometry and Goldmann Applanation Tonometry J Glaucoma 2006; 15:2-6
Contact this author:
Elliot M. Kirstein OD, FAAO 11304 Montgomery Road Cincinnati, Ohio 45249-2313 513-530-0440Conflict of interest: Elliot M. Kirstein, OD, is a consultant and research coordinator for Ziemer Ophthalmic Systems AG, Port, Switzerland, which is the company that markets and sells the PASCAL® Dynamic Contour Tonometer. In addition to his work in his private optometric practice, he has been affiliated with Ziemer Ophthalmics since September 2003.
Pacific University College of Optometry provides On-Line CE courses as a service to optometrists. The college does not endorse or recommend any products, equipment, or services that might be discussed in the courses. Courses are prepared by individuals believed to be experts in their areas of specialization who are compensated for their efforts. The College relies on their expertise to produce accurate and timely courses. Questions or concerns about courses should be directed to the individual authors and/or the Continuing Education Department at the College of Optometry at kundart@pacificu.edu.
© Copyright 2006,
