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An Update on Methods for Assessing Intraocular Pressure

Elliot M. Kirstein OD, FAAO

11304 Montgomery Road

Cincinnati, Ohio 45249-2313

 

Contents

 

Introduction

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.

 

180 Years of Evolution in Tonometry

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.

 

Fig. 6. Intraocular pressure pulsation.

 

Problem with the Applanation Standard

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?

 

OHTS, CCT, and GAT

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.