W. Morton Grant, MD; Jeremy B. Wingard, MD; and Joel S. Schuman, MD, FACS
TONOMETRY
The Goldmann applanation tonometer (Haag-Streit) remains a standard of reference in clinical tonometry (Figure 6-1). The principle of applanation tonometry is old. It is based on the physical relationship that applies to the flat end of a piston at rest: The pressure against the piston is calculated from the force applied to the piston divided by the area of the face of the piston. This principle has been used in the Russian Maklakov applanation tonometer since the 19th century, but it was never applied with great precision until the advent of the Goldmann applanation tonometer. In the Goldmann instrument, the force is measured with a sensitive spring or counterpoise balance, and the area is precisely established by an accurate split-field de vice.
Simple as the principle of applanation tonometry may appear to be, seeming only to require measurements of the applied force and of the resulting flattened area of cornea for determination of the pressure, in actual development of a practical and accurate instrument, a number of additional complicating factors had to be considered. These factors include the effect of capillary attraction between the face of the applanation tonometer and the cornea, the stiffness of the cornea itself, the translation of the force and area on the outer surface of the cornea to intraocular pressure (IOP) at the inner surface of the cornea, the influence of varying corneal curvatures, the influence of corneal astigmatism, the influence of varying amounts of fluid on the cornea, and the influence of varying scleral rigidity. The Goldmann applanation tonometer actually permits measurement of IOP with very little disturbance of this pressure and with insignificant error from variation of scleral rigidity. It also has a well-defined range of insensitivity to the other factors listed.1
In the Goldmann applanation tonometer, the endpoint diameter of applanation is fixed by the construction of the plastic prisms within the instrument to be suitable, especially for the human eye. The diameter of applanation has been selected to give a balance between the capillary attractive force of the tear film and the opposing resistive force of the cornea, while producing essentially the same area of flattening of the endothelial surface of the cornea. For the human eye, the diameter of flattening employed is 3.06 mm. The same diameter is not suitable for use on animal eyes, presumably due to differences in structures of the corneas. For each species, Goldmann found that it is necessary to use a different, specific diameter of flattening, usually larger than that which is best for the human eye.
A portable adaptation of the spring-type Goldmann applanation tonometer, available as the Perkins applanation tonometer (Haag-Streit; Figure 6-2), is well suited to measurements on patients in either the seated or the supine position. The Perkins applanation tonometer is particularly useful in measuring the IOP in young children, permitting accurate measurement without having to position the child at a slit lamp, which can be both inconvenient and alarming to the child. The Perkins applanation tonometer, in addition to several other devices that are discussed in this chapter, has made tonometry on awake children so much easier than formerly that the need for general anesthesia has been reduced; however, this tonometer is also convenient and well suited for measurements on children in the supine position under general anesthesia.
Figure 6-1. The Goldmann applanation tonometer, the gold standard instrument for the measurement of IOP. (Reprinted with permission from Haag-Streit.)
The Perkins applanation tonometer allows the accurate measurement of IOP in individuals in whom the Goldmann tonometer produces a falsely elevated IOP. The IOP can be measured more accurately with the Perkins than the Goldmann tonometer in people who hold their breath during Goldmann tonometry, individuals with obesity, patients with tight collars, or patients with great anxiety at the slit lamp. By permitting applanation tonometry outside the confines of the slit lamp, the Perkins tonometer enables precise assessment of the IOP in a more relaxed setting for the patient.
For topical anesthesia for Goldmann applanation tonometry, use is made of a drop of a preparation (Fluress, Akorn, Inc) containing both a topical anesthetic (benoxinate) and 0.25% fluorescein stabilized by polyvinylpyrrolidone, or of proparacaine 0.5% and a fluoresceinated paper strip. The resulting fluorescent tear film is more suitable than a colorless film for observing and measuring flattening of the cornea by the applanation device. To enhance fluorescence, a blue light is used to illuminate the eye and its tear film. Looking with a slit-lamp microscope, one sees at the moment of contact of the tonometer with the eye a bright yellow-green spot that breaks into 2 bright yellow-green semicircular arcs as the tonometer is moved slightly farther forward. The arcs should then be seen in sharp focus. By suitable adjustment of a graduated drum, varying the force applied to the cornea, the arcs can be made to overlap so that the inner edge of the upper can be aligned with the inner edge of the lower. This is the desired endpoint at which a reading is taken from the graduated drum. The numerical reading on the drum indicates grams of force applied by the tonometer and equivalent mm Hg IOP. Thus, number 1 on the drum is equal to 10 mm Hg, and 2 is equal to 20 mm Hg.
Figure 6-2. The Perkins applanation tonometer is portable and allows accurate IOP measurement without the use of a slit lamp. It is useful in applanation on children and older patients. Occasionally, patients (particularly with obesity) demonstrate breath holding and presumed increased venous pressure when IOP is measured with the slit lamp, with false elevation of IOP; these elevations are not confirmed when the Perkins instrument is used. (Reprinted with permission from Haag-Streit.)
The precision with which the inner edges of the overlapping upper and lower arcs can be lined up is limited be cause of the ocular pulse, causing the edges to swing back and forth. The best that can be done is to adjust the alignment to a mean position in which there is equal swing to either side of the aligned position.
Measurement is made first on one eye and then the other, alternating back and forth, repeating measurements until reasonably constant values are obtained. This is important because, particularly with apprehensive patients, the initial reading may be several mm Hg higher than the readings obtained when the patient has become relaxed and accustomed to the procedure. Usually, 3 measurements on each eye is sufficient, but if the last 2 measurements do not agree within 1 mm Hg, more measurements should be made. Presumably, when initial measurement is higher than the later steady state measurement, the initial measurement is erroneous.
The significance of readings obtained with the Goldmann applanation tonometer when the patient is relaxed may be generalized as follows: reliable and repeated measurements of 22 mm Hg pressure or higher suggest that the eye is abnormal.
In practically all eyes that have a definite pressure of 22 mm Hg or higher, the tonographic PO/C ratio (steady state pressure to facility of aqueous outflow) is 100 or higher, indicating possible glaucoma. The higher the pressure, the more definite is the indication. How dangerous a given pressure may be to a given eye depends on many additional factors, such as the nature of the optic nerve head, height and duration of IOP elevation, and possibly blood pressure, all of which are discussed throughout this book.
An IOP of 21 mm Hg by applanation should be regarded as borderline. IOPs from 18 to 20 mm Hg does not rule out glaucoma if these are single samplings at a single time of day. It is not uncommon for the IOP in eyes having definite glaucoma to vary during the course of 24 hours from the high-teens to the high 20s, 30s, or even 40s. Thus, a single measurement, be it ever so accurate at the moment, may give a completely misleading impression if by chance it happens to be obtained at the low point of the daily fluctuation.
The Goldmann applanation tonometer can be modified for use on the eyes of infants with small palpebral fissures as well as on eyes of adult patients who have had penetrating keratoplasty and are suspected of developing glaucoma. The end of the cylindrical plastic cone of the tonometer can be reduced in diameter by turning down in a lathe, leaving just enough of the flat applanating surface to still give accurate pressure measurements.2,3
In most of the developed world, the Schiotz tonometer is a seldom-used instrument for estimation of IOP, but it offers a reasonable combination of convenience and reliability. It is subject to more error than the Goldmann applanation tonometer, but the Schiotz tonometer has advantages in portability, availability, ease of use, and lower price.
In the customary manner of using the Schiotz tonometer, the patient is recumbent with gaze vertical and corneas anesthetized by a drop of topical anesthetic. The tonometer is allowed to rest on the patient’s cornea, and the extent to which the weighted plunger of the tonometer indents the cornea is shown on a scale by a simple lever-arm indicator. The lever-arm system magnifies the motion of the plunger 20-fold so that a 0.05-mm movement of the plunger is represented by a 1-mm space between units on the tonometer scale. Thus, the scale readings merely indicate the depth to which the plunger indents the eye. The softer the eye, the greater is the depth of the indentation. A reference point is provided by a convex test block resembling the cornea but made of steel, plastic, or glass, which will not indent. On a suitable test block of this sort, the tonometer should give a reading of zero.
A conversion table or graph is required to convert from scale readings obtained when the Schiotz tonometer is resting on the cornea to the corresponding mm Hg IOP. The precision with which the scale reading can be ascertained from the tonometer is limited by oscillation of the indicator needle caused by the ocular pulse and by other physiologic moment-to-moment variations in IOP. The accuracy with which IOP can be estimated is further limited by variability of elastic properties of the eye and by variation in curvature of the cornea. In common clinical practice, the mechanical Schiotz tonometer (Biro) is read to the nearest quarter-scale unit, but it is sometimes difficult to read to the nearest half-scale unit. More precise readings, estimating to the nearest tenth of a scale unit, can be made with electronic Schiotz tonometers (Mueller), in which the scale of the instrument is expanded by the use of electronic amplification in place of the simple lever arm.
Figure 6-3. The pneumotonometer is useful in measuring IOP in patients with abnormal corneas in whom regular mires with the Goldmann applanation tonometer cannot be obtained, or in children or patients with limited ability to cooperate for Goldmann applanation tonometry. Shown is a newer version with a touch-screen interface. Tonography is also possible with this device. (Reprinted with permission from Reichert Technologies.)
In interpreting the scale readings obtained when the Schiotz tonometer is applied to the patient’s eye, it can be said that most normal eyes give readings from 5 to 8 units on the scale and that when readings of 4 or less units on the scale are obtained, the eye should be suspected of being glaucomatous and should be subjected to additional testing.
If the IOPs indicated by the Schiotz tonometer on any type of eye are near borderline between normal and glaucomatous, the IOP should also be measured with the Goldmann applanation tonometer. In the myopic eye in particular, the IOP should be checked by an applanation tonometer, even though it is apparently normal by Schiotz tonometry. There is little need to resort to the applanation tonometer for cases in which the Schiotz tonometer gives measurements well in the glaucoma range in eyes of ordinary size and shape. There is, however, an advantage to comparing the Schiotz and applanation tonometers to deter mine whether the values indicated by the Schiotz tonometer are accurately applicable to a given eye or whether a correction factor is needed.
Patients with abnormal corneas may present difficulty in measurement of IOP. After penetrating keratoplasty, if irregular mires are observed, Goldmann applanation tonometry should be performed. It is often useful to rotate the applanation prism into more than one meridian to check the reliability of the measurement. If distorted mires are observed due to the presence of an abnormal cornea, a pneumotonometer (Reichert Technologies; Figure 6-3) or Mackay-Marg tonometer have proven useful in estimating IOP.
The pneumotonometer measures IOP by calculating the force required to flatten an area of cornea using an elastic membrane inflated with gas or air. The current models use compressed air rather than fluorocarbon gas. This device is extremely useful and accurate, especially in the setting of an irregular corneal surface. Additionally, the pneumotonometer can be used against the sclera near the limbus, with IOP results nearly identical to those achieved by contact with the cornea. Pneumotonometry also produces a tracing of the IOP during measurement; this gives the examiner the ability to determine the quality of the examination. A good measurement will produce a flat tracing, with the presence of ocular pulsations evident. During the examination, there is a high-pitched, whining noise. Absence of the pulsations or of this sound indicates an error in measurement. The pneumotonometer hand piece has 2 lines, 1 black and 1 red, on the metal rod, which moves within the handpiece and to which the elastic membrane is attached. For proper measurement, when the handpiece is placed on the cornea, the red line should just be visible, but the black line should be hidden within the housing of the handpiece.
Figure 6-4. The Tono-Pen is a small, portable device that can be used with ease. This is an earlier model (Tono-Pen XL) with a linear design, showing a disposable tip cover in place. (Reprinted with permission from Reichert Technologies.)
Figure 6-5. This is another, newer version of the Tono-Pen (AVIA), slightly more ergonomic and with longer battery life. (Reprinted with permission from Reichert Technologies.)
Figure 6-6. Pascal Dynamic Contour Tonometer applied to a patient’s eye. This instrument measures IOP by inducing conformation of the patient’s cornea to the concave instrument tip while applying 1 g of force. A sensor in the instrument tip measures the IOP. Correlation with Goldmann applanation is good and with less confounding by corneal thickness. (Reprinted with permission from Ziemer Ophthalmic Systems AG.)
The Tono-Pen (Reichert Technologies) can be used for IOP measurement both as a screening tool and in eyes with irregular surfaces (Figures 6-4 and 6-5).4–6 This tonometer, how ever, may overestimate the IOP in eyes with low IOP and underestimate the IOP in eyes with high IOP.6–8 The Tono-Pen also has poor reproducibility of measurements.9
Despite the problems with early models of the instrument, the Tono-Pen has improved over the years and is in frequent use, especially by nonglaucoma specialists. The Tono-Pen’s ease of use and the fact that it can measure IOP even in the presence of corneal irregularity have aided technicians and other support personnel in the evaluation of patients by recording IOP taken with this device. Additionally, its portability allows the use of the Tono-Pen in examination of IOP in remote locations and screenings. Nevertheless, the Goldmann tonometer remains the gold standard and, if available, is preferred to the Tono-Pen in the measurement of IOP.
A relatively new device is the dynamic contour tonometer (Pascal DCT device, Ziemer Ophthalmic Systems AG; Figure 6-6). This instrument measures IOP by inducing conformation of the patient’s cornea to the concave instrument tip while applying 1 g of force. A sensor in the instrument tip measures IOP. IOP measurements are similar to Goldmann applanation tonometry, but with less confounding by corneal thickness; however, corneal biomechanical properties still affect IOP assessment with this device.10
Noncontact air-puff tonometers have been used for screening of IOPs, especially in optometry (Figure 6-7). No anesthesia is required for measurement with this device. Unfortunately, reproducibility of measurements is poor.9 Additionally, a microaerosolized mist of tear film is created on use of air-puff tonometers, introducing the risk of dispersion of infectious material, particularly viral particles.11
The Ocular Response Analyzer (ORA; Reichert Technologies; Figure 6-8) is a noncontact tonometer designed to address measurement of corneal biomechanical properties while assessing IOP. The ORA measures corneal hysteresis, or the response of the cornea to bursts of air blown by the device. The change in shape of the cornea and the pressure at which the cornea is flattened by the air jets permit calculation of IOP and corneal biomechanical properties of hysteresis.12 Variability has been an issue with this device, but future iterations of the technology may permit more accurate and reproducible measurements.
Impact tonometry, also called rebound tonometry, in which a lightweight magnetic probe in an enclosed shaft is propelled toward the cornea and the rebound characteristics of the probe are measured, has shown promise. The first version of this device (Icare TAO1i tonometer, Icare Finland Oy; Figure 6-9) requires patients to be completely vertical, but as anesthesia is unnecessary due to the very brief impact time of the probe, it has become useful for pediatric exams in the clinic. Sterile probes are available for each measurement, minimizing the risk of infectious spread. This new technology has been developed for use in humans, large animals, and laboratory rodents.13
The estimation of IOP by palpation of the globe is notoriously inaccurate. This technique can be relied on only to determine whether the eye is hard or soft and can be de pended on to distinguish an IOP greater than 30 mm Hg.14
The following section deals with tonography, a technique by which the actual facility of aqueous outflow is measured in living patients. While it is the only clinical measure of trabecular meshwork (TM) function available to ophthalmologists, it is exceedingly difficult to perform and interpret, requiring the resources of a skilled technician to perform the test and a trained ophthalmologist to interpret it. Additionally, results can be variable, as well as technician dependent. For these reasons, tonography is relegated to the status of research tool. We find that this is a valuable clinical research resource and recognize the difficulty involved in bringing this technology to the community.
Figure 6-7. The noncontact air-puff tonometer has the advantage of decreasing the risk of transmission of communicable disease, but suffers from lack of accuracy. (Reprinted with permission from Reichert Technologies.)
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