IOP: Instruments to Measure IOP

Core Messages

››No single tonometer is accurate or usable in all situations.

››Goldmann applanation tonometry is still the most commonly used tonometer for diagnosing and managing glaucoma.

››Goldmann tonometry becomes less accurate when the cornea is significantly thinner or thicker than average, highly astigmatic, irregular, edematous, or scarred.

››Dynamic contour tonometry is the most accurate tonometer and maintains its accuracy even when the cornea is very thin, edemetous, or of irregular curvature.

››Pneumotonometry is somewhat less influenced by corneal thickness than Goldmann tonometry. It is useful in the operating room, in supine position as well as upright, and on irregular corneas and provides a real-time paper tracing that helps establish reliability.

››The Tonopen is portable, battery powered, usable in any position and may be especially useful in irregular and scarred corneas or in screening situations where electricity is unavailable or portability trumps accuracy.

R.Stamper

Department of Ophthalmology, University of California at San Francisco, 10 Koret Way, Room K301,

San Francisco, CA 94143, USA e-mail: stamperr@vision.ucsf.edu

9.1  What is the Brief History of IOP Measurement?

Elevated intraocular pressure was first noted to be associated with blindness in the tenth century by an Arabic physician, At-Tabari, and was redescribed in 1622 as being associated with what came to be known as glaucoma by Richard Bannister [1]. Until the late nineteenth century, pressure was estimated by palpation through the eyelid – at best, a way to determine if the intraocular pressure (IOP) is low, medium, high, or very high. Obviously, a way to actually measure IOP was needed if glaucomatous conditions were to be diagnosed and treated effectively. This is especially true now that efficacy of lowering IOP in both ocular hypertension and glaucoma has been firmly established by large, randomized studies [2–7].

The most accurate way to measure IOP is to cannulate the eye and directly measure IOP by water column or pressure gauge. For obvious reasons, this approach is not practical for everyday management of glaucoma. All other methods of IOP measurement are indirect. An instrument that purports to measure IOP is called a tonometer. There are basically three different types of tonometers – indentation, applanation, and contour matching. Each of these types of tonometers has some source of error and no single tonometer is good for every situation.

Summary for the Clinician

››The most accurate measure of IOP is through direct cannulation of the eye, which is not clinically practical.

››All other methods of IOP measurement indirectly measure IOP.

››Three types of tonometers exist: indentation, applanation, and contour matching; each has its own set of advantages, disadvantages, and sources of errors.

9.2  What Instrument(s) Most

Accurately Measures IOP?

9.2.1  Maklakov Tonometer

The first practical tonometer was the Maklakov tonometer which was an applanation tonometer. The theory of applanation tonometry comes from the Imbert-Fick law which states that the internal pressure of a very thinwalled sphere can be obtained by knowing the force required to flatten (applanate) a known area of the sphere. The Maklakov tonometer has a fixed force (i.e., a weight) and a flat bottom that was smeared with ink; when the tonometer first touched and then flattened the cornea, the ink was transferred to the cornea. The tonometer was then “printed” onto a piece of paper. The area (as determined by the diameter) in the center of the inkblot that was devoid of ink was proportional to the IOP. If the eye moved during the time the tonometer was on the eye, more ink was transferred to the cornea than was necessary due to applanation alone and the IOP was underestimated. Furthermore, for the same reasons that the Goldmann tonometry is inaccurate in thin and thick corneas (see below), the Maklakov suffers from the same source of error. Eyes with corneal irregularity, scarring, edema, or high astigmatism, spread the ink irregularly making it difficult to read the area of applanation. This tonometer was widely used in Europe throughout the last century and still enjoys popularity in Russia and other Eastern European countries.

9.2.2  Shiøtz Tonometry

In 1905 Schiøtz introduced an indentation tonometer in which a plunger with a weight on top was allowed to indent the cornea through a footplate [8]. The depth of indentation gave a good indication of the IOP although low or high scleral rigidity outside the average range introduced a significant error (Fig. 9.1a). A proper tonometer was calibrated at the factory and needed to be returned if the calibration was altered (Fig. 9.1b). The patient had to be in the supine position for the measurement. If the eye moved during the measurement process, corneal abrasion was a possible side effect. These tonometers tended to underestimate the IOP in myopic and young eyes. Any corneal scarring or irregularity invalidated the reading.

9.2.3  Goldmann Tonometry

Goldmann revolutionized tonometry in 1948 with the invention of a variable force, fixed area applanation tonometer whose accuracy surpassed any of the devices preceding it. He predicted that the tonometer’s accuracy would be affected by thick or thin corneas but incorrectly concluded that these would be rare based on his studies of a relatively few eyes in Bern [9, 10]. Because of its presumed accuracy and ease of use, the Goldmann tonometer became the world standard for tonometry within two decades of its introduction. While the Goldmann tonometer requires mounting on a slit lamp biomicroscope and an upright subject, modifications by Perkins and Draeger allowed the same principle to be transferred to a portable unit that could be used both in upright and supine positions. Unfortunately, IOP is underestimated significantly by this device (or its modifications) when the cornea is thin, thick, or edematous. Scarring or other surface irregularity prevents a crisp optical endpoint, so accurate IOP determination is difficult if not impossible in such eyes.

9.2.4  McKay-Marg and Tonopen

The next breakthrough in tonometry came from a new hybrid applanation and indentation tonometer devel-

oped by McKay and Marg in the late 1950s [11]. In this tonometer, a tiny plunger indents the cornea until the footplate takes the strain off the plunger (the point of applanation). At that point, the gradually increasing force generated by the plunger as read by a strain guage shows a momentary flattening or reduction. This momentary flattening represents the force necessary to flatten the cornea; this force divided by the area of applanation gives the IOP. Since the endpoint is determined electronically (rather than optically as in the Goldmann), a more precise and presumably accurate reading than with the Goldmann instrument [12] can be obtained in scarred, edematous, and irregular corneas A portable version using a “smart” chip to calculate the endpoint, the Tonopen, soon followed. The chip also calculates the standard deviation of multiple readings which serves as a rough measure of consistency and presumed accuracy. The Tonopen can be used in any position unlike the Schiotz and Goldmann tonometers which can only be used in the supine or

upright positions respectively. The Tonopen relates well to Goldmann readings in eyes with “normal” corneas and in the “physiologic” range of IOP’s. However there are some outliers where the Tonopen may differ from the Goldmann by as much as 8 mm Hg and accuracy appears to fall off both below 10 mm Hg and above 20 mm Hg [13, 14]. Being portable and relatively easy to use, the Tonopen can be used for home tonometry to obtain some idea of diurnal or even longer-term fluctuation in IOP.

9.2.5  Air-Puff Tonometry

Air-puff tonometers were originally developed to provide a way to measure IOP without the need for topical anesthetic. They are basically applanation tonometers where a column of air of known area is generated at increasing force aimed at the cornea

over a very short time (measured in milliseconds) until the cornea is flattened. When the cornea is flattened a light beam is reflected into a sensor which stops the generation of air and records the force at the moment of applanation. The force divided by the area of applanation is the IOP. It is the same principle as the Goldmann and Maklakov tonometers. Airpuff tonometers are reasonably accurate over the range of “physiologic” pressures and compare well with Goldmann tonometry [15, 16]. They are particularly useful as screening devices since they do not normally require anesthetic and cannot serve as carriers of pathogens as can tonometers that actually touch the cornea. On the other hand, they are large, not very portable, expensive, and require frequent calibration. Furthermore they become inaccurate in eyes whose corneas are irregular, scarred, edematous, or astigmatic.

A recent innovation is the Reichert Ocular Response Analyzer (Reichert, Rochester, NY). This is a variant on the air-puff tonometer which reads the point of applanation like other air-puff tonometers, but in addition further indents the cornea with air pressure and measures the point at which the cornea recovers to applanation. The difference between the first applanation point and the point of recovery is a measure of the biomechanical properties (hysteresis) of the cornea [17]. Abnormal biomechanical properties of the cornea have been associated with both initial and progression of optic nerve damage in glaucoma [18]. Exactly how this can or should be incorporated into the diagnosis and/or management of glaucoma is not clear at this time.

9.2.6  Dynamic Contour Tonometry

A major revolution in the methodology of measuring IOP occurred with the invention by Kanngeisser of the dynamic contour tonometer (DCT). Unlike any of its predecessors, the DCT (Pascal – Zeimer MicroTechnology, Switzerland) does not depend on flattening or indenting the cornea. The basic concept is that the curved tip of the sensor duplicates the curvature of the cornea and the strain gauge located in its center measures a pressure on the outside of the cornea that very closely replicates the pressure on the inside [19] (Fig. 9.2). Accordingly, the device is insignificantly affected by corneal thickness, curvature, optical

Fig. 9.2  Dynamic contour tonometer mounted on slit lamp. Note digital readout that includes intraocular pressure in mm Hg, an indicator of quality (Q) with 3 or more indicating a satisfactory reading and the ocular pulse pressure (OPA) in mm Hg (the difference between diastolic and systolic intraocular pressure)

aberrations, or surface irregularity i.e., it is independent of the errors of applanation and indentation tonometry [20–25]. The DCT appears significantly more accurate both in cadaver and in living eyes than Goldmann tonometry and Pneumatonometry when compared with manometery, even when the cornea is edematous or surgically thinned by LASIK [26–28].

Based on recent evidence, it is clear that the DCT is the most accurate instrument for measuring IOP without significant effect from corneal properties or abnormalities. However the device is expensive, as it is now constituted, can only measure IOP in the upright position, requires topical anesthetic (although no fluorescein is needed), and necessitates a slit lamp to operate. Therefore it cannot be used in the operating room or at the bedside. It probably is not as useful as some of the other devices in screening situations. Finally, the DCT has not been validated when the corneal curvature is significantly different from physiologic, such as in the early postoperative period for corneal transplants, cornea plana, or buphthalmos.