INSTRUMENTS FOR MEASURING INTRAOCULAR PRESSURE
It is possible to measure intraocular pressure (IOP) directly in a living eye using a manometric technique. For this approach a needle is inserted into the anterior chamber through a self-sealing, beveled corneal puncture. The needle is connected to a fluid-filled tubing, and the height of the fluid in the tubing corresponds to IOP. The tubing can also be connected to a fluid-filled reservoir that has a pressure-sensitive membrane. The movement of the membrane, recorded optically or electronically, is a measure of IOP. Although the direct method is perhaps the most accurate, its obvious clinical limitations necessitate alternative means for measuring pressure in patients.
Most techniques for measuring IOP in clinical use are indirect in that they are based on the eye’s response to an applied force. A good example of this process is palpation, during which the examiner estimates IOP by the response of the eye to digital pressure – that is, he or she determines whether the globe indents easily or whether it feels firm to the touch. Palpation should be used only in the most extraordinary circumstances because it is capable of detecting only gross alterations of IOP. Even in ideal circumstances palpation is notoriously inaccurate, and the examiner may over- or underestimate the IOP by large amounts. However, with practice, some doctors are able to get a reasonably accurate estimate of IOP which may be especially useful in patients with irregular corneas where applanation tonometry may not be possible.
Traditionally, tonometers could be divided into two major groups, referred to as applanation and indentation instruments. With applanation instruments, the clinician measures the force necessary to flatten a small, standard area of the cornea. With indentation instruments, the clinician measures the amount of deformation or indentation of the globe in response to a standard weight applied to the cornea. More recent tonometers work on different principles such as contour matching, transpalpebral phosphene induction, indentation/rebound and intraocular implantation of pressure sensors.
Because the Goldmann tonometer has been the international clinical standard for measuring IOP, it is appropriate to discuss this instrument at some length ( Fig. 4-1 ). The Goldmann tonometer determines the force necessary to flatten (or applanate) an area of the cornea 3.06 mm in diameter – a technique referred to as constant-area applanation. For this area of applanation and in a cornea of average thickness, the force required to bend or deform the cornea is approximately equal in magnitude and opposite in direction to the capillary attraction of the tear film for the tonometer head. Thus, under these conditions, these two forces cancel out one another. When the cornea is flattened, the force of the tonometer – supplied by a coiled spring or a weight – counterbalances and provides a measure of IOP. For this area of applanation, the IOP in millimeters of mercury is equal to the force of the tonometer in grams multiplied by 10 ( Fig. 4-2 ).
Applanation tonometry displaces only about 0.5 μl of aqueous humor, which raises IOP by about 3% – that is, P t (the pressure at that moment) is 3% greater than P O (the pressure in the undisturbed eye). Because the volume displaced is so small, ocular rigidity, or the ‘stretchability’ of the globe, has little effect on the pressure readings. In general, larger volumes are displaced with indentation tonometers, and a stretchable eye with low ocular rigidity may allow a greater degree of indentation per gram of force than the average eye, and thus indicates a falsely low pressure.
The degree of applanation is judged while viewing the cornea through a split prism device in the applanating head. To better distinguish the tear film and the cornea, which have similar refractive indexes, fluorescein is instilled in the anesthetized conjunctival cul-de-sac. When the front surface of the eye is illuminated with a cobalt blue filter, the fluorescein-stained tear film appears bright yellow-green. When the clinician looks through the split prism in contact with the eye, he or she sees a central blue circle, the flattened cornea, surrounded by two yellow-green semicircles. When the inner margins of the two semicircles are aligned in a smooth S curve at the midpoint of their pulsations, the proper degree of applanation has been achieved.
Goldmann tonometry is quite accurate and reproducible if the proper technique is used. Interobserver variability is in the range of 0–3 mmHg, which is less than the diurnal variation of IOP. The technique of Goldmann tonometry is as follows:
The patient is asked not to drink alcoholic beverages or large amounts of fluid (e.g., 500 ml or more) for 2 hours before the test, as the former will lower IOP and the latter may raise it.
The patient is told the purpose of the test and is reassured that the measurement is not painful. The patient is instructed to relax, maintain position, and hold the eyes open wide.
One drop of a topical anesthetic, such as 0.5% proparacaine, is placed in each eye, and the tip of a moistened fluorescein strip is touched to the tear layer on the inner surface of each lower lid. Alternatively, one drop of a combined anesthetic–fluorescein solution can be instilled in each eye. Contact lenses should be removed before the fluorescein is applied. Soft contact lenses can be irreversibly stained by fluorescein. Newer fluorescent solutions such as high molecular weight fluorescein or fluorexon do not stain soft contact lenses and may be used as substitutes in those patients.
The tonometer tip is cleaned with a sterilizing solution, and the tip and prism are set in correct position on the slit lamp. Care should be taken that the disinfecting solution is dry or wiped off the tip before applying the tip to the eye, as many of these solutions, especially alcohol-based ones, can be irritating to the eye or toxic to the epithelium and lead to a corneal abrasion. Sterile tonometer tip covers may be used rather than a disinfecting solution, if preferred. Disposable tonometer tips may also be used ( Fig. 4-3 ). When using disposable tips, they should each be examined to be sure of a smooth applanating surface. The acrylic disposable tips seem to be somewhat more accurate than the silicone ones. While disposable shields or tips may be safer than disinfecting solutions, they are not 100% protective against prion disease.
The tension knob is set at 1 g. If the knob is set at 0, the prism head may vibrate when it touches the eye and damage the corneal epithelium. The 1 g position is used before each measurement. As a rule, it is more accurate to measure IOP by increasing rather than decreasing the force of applanation.
The 0 graduation mark of the prism is set at the white line on the prism holder. If the patient has more than 3 D of corneal astigmatism, the area of contact between the cornea and the prism is elliptic rather than circular. In this situation the prism should be rotated to about 45° from the long axis of the ellipse – that is, the prism graduation corresponding to the least curved meridian of the cornea should be set at the red mark on the prism holder. An alternative approach is to average the IOP readings obtained with the axis of the prism horizontal and then vertical.
The cobalt filter is used with the slit beam opened maximally. The angle between the illumination and the microscope should be approximately 60°. The room illumination is reduced.
The patient is seated in a comfortable position on an adjustable stool or examining chair facing the slit lamp. The heights of the slit lamp, chair, and chin rest are adjusted until the patient is comfortable and in the correct position for the measurement. The patient’s chin is supported by the chin rest, and the forehead by the forehead bar. The forehead bar should be well above the patient’s eyebrows, so the frontalis muscle can be used to open the eyes wide. The patient’s collar should be loosened if necessary. The patient should breathe normally during the test to avoid Valsalva’s maneuver.
The palpebral fissure is a little wider if the patient looks up. However, the gaze should be no more than 15° above the horizontal to prevent an elevation of IOP that is especially marked in the presence of restrictive neuromuscular disease such as dysthyroid ophthalmopathy. A fixation light may be placed in front of the fellow eye. The patient should blink the eyes once or twice to spread the fluorescein-stained tear film over the cornea, and then should keep the eyes open wide. In some patients, it is necessary for the examiner to hold the eyelids open with the thumb and forefinger of one hand. Care must be taken not to place any pressure on the globe because this raises IOP. Resting the thumb and forefinger against the orbital rim while retracting the lids may help the examiner avoid putting pressure on the globe.
The operator sits opposite the patient, in position to look through the microscope, and moves the assembly toward the subject. When the black circle near the tip of the prism moves slightly, it indicates contact between the prism and the globe. Alternatively, the assembly is advanced toward the patient with the tester observing from the side until the limbal zone has a bluish hue. Yet another approach is to use the white-appearing rings seen through the prism just before contact with the cornea is made, and these can be used to align the prism so that adjustment, once contact is made, is minimized. The biprism should not touch the lids or lashes because this stimulates blinking and squeezing. If the tonometer tip touches the lids, the fluorescein rings will thicken, which may cause an overestimation of IOP.
The clinician observes the applanation through the biprism at low power. A monocular view is obtained of the central applanated zone and the surrounding fluorescein-stained tear film. Using the control stick, the observer raises, lowers, and centers the assembly until two equal semicircles are seen in the center of the field of view. If the two semicircles are not equal in size, IOP is overestimated. The clinician turns the tension knob in both directions to ensure that the instrument is in good position. If the semicircles cannot be made ‘too small,’ the instrument is too far forward. If the semicircles cannot be made ‘too large,’ the instrument is too far from the eye.
The fluorescein rings should be approximately 0.25–0.3 mm in thickness – or about one-tenth the diameter of the flattened area. If the rings are too narrow, the patient should blink two or three times to replenish the fluorescein; additional fluorescein may be added if necessary. If the fluorescein rings are too narrow, IOP is underestimated. If the fluorescein rings are too wide, the patient’s eyelids should be blotted carefully with a tissue, and the front surface of the prism should be dried with lint-free material. An excessively wide fluorescein ring is less of a problem than a very narrow ring, but can cause IOP to be overestimated.
The fluorescein rings normally undergo a rhythmic movement in response to the cardiac cycle. The tension knob is rotated until the inner borders of the fluorescein rings touch each other at the midpoint of their pulsations.
Intraocular pressure is measured in the right eye until three successive readings are within 1 mmHg. Intraocular pressure is then measured in the left eye.
The reading obtained in grams is multiplied by 10 to give the IOP in millimeters of mercury. This value is recorded along with the date, time of day, list of ocular medications, and time of last instillation of ocular medication.
It is possible to transfer bacteria, viruses, and other infectious agents with the tonometer head, including such potentially serious infections as epidemic keratoconjunctivitis, hepatitis B, Jacob-Kreutzfeld and, theoretically, acquired immunodeficiency syndrome. The biprism should be rinsed and dried immediately after use. Between uses, the prism head should be soaked in a solution such as diluted bleach or 3% hydrogen peroxide. Seventy per cent ethanol and 70% isopropanol are effective as sterilizing solutions but were shown in one study to cause mild damage to the tonometer tip after one month of immersion. Care must be taken to be sure any sterilizing solution has been completely rinsed off the tonometer tip, as some of these solutions may be toxic to the corneal epithelium, especially after LASIK or other corneal procedures. If the tonometer tip is not mechanically wiped after each use, epithelial cells may stick to the tip with the small but serious risk of transmitting Jacob-Kreutzfeld virus. Disposable tonometer tips may be an acceptable alternative to soaking in, and wiping with, antiseptic solutions.
The Goldmann tonometer should be calibrated at least once a month. If the Goldmann tonometer is not within 0.1 g (+1 mmHg) of the correct calibration, the instrument should be repaired; however, calibration errors of up to +2.5 mmHg may still be tolerated clinically. In one large clinic, approximately one-third of the tonometers were out of calibration at one month and one-half at four months. In addition, tonometer tips should be examined periodically under magnification as the antiseptic solutions and mechanical wiping may cause irregularities in the surface of the tip that can, in turn, injure the cornea.
Although the Goldmann tonometer is reliable and accurate through a wide range of IOPs, errors in measurement can arise from a number of factors, including those that follow ( Box 4-1 ):
Inadequate fluorescein staining of the tear film causes an underestimation of IOP. This commonly occurs when too much time elapses between the instillation of the fluorescein and the measurement of the pressure. To avoid this problem the IOP should be measured within the first minute or so after instilling the fluorescein.
Elevating the eyes more than 15° above the horizontal causes an overestimation of IOP.
Widening the lid fissure excessively causes an overestimation of IOP.
Repeated tonometry reduces IOP, causing an underestimation of the true level. This effect is greatest between the first and second readings, but the trend continues through a number of repetitions.
A scarred, irregular cornea distorts the fluorescein rings and makes it difficult to estimate IOP.
The thickness of the cornea affects IOP readings. If the cornea is thick because of edema, IOP is underestimated. If the cornea is thick because of additional tissue, IOP is overestimated. In thin corneas, the Goldmann tonometer will underestimate the IOP. Goldmann predicted that the tonometer would be inaccurate with thin and thick corneas, but failed to realize (since he measured corneal thickness in only a few citizens of Bern) the wide variation in corneal thickness seen in normal individuals. Some have suggested applying correction factors to the readings in corneas whose thickness is less than 545 microns or greater than 600. However, the errors are not linear and no formula has yet been derived that is accurate across the range of corneal thickness and intraocular pressures. It is probably best to use the corneal thickness as a rough guide to the direction and magnitude of the error but avoid the temptation to achieve a precision with a formula that does not match accuracy. The Goldmann tonometer is accurate after epikeratophakia. Central corneal pressures have been shown to be lower than peripheral corneal readings following photorefractive keratectomy and LASIK.
If the examiner presses on the globe, or if the patient squeezes his eyelids, IOP is overestimated. Taking time to reassure the patient and taking care to avoid causing pressure against the globe can help guard against these problems.
If corneal astigmatism is greater than 3 D, IOP is underestimated for with-the-rule astigmatism and overestimated for against-the-rule astigmatism. The IOP reading is inaccurate 1 mmHg for every 3 D of astigmatism.
A natural bias for even numbers may cause slight errors in readings.
Astigmatism >3 diopters
Too much fluorescein
Tonometer out of calibration
Elevating the eyes >15°
Pressing on the eyelids or globe
Squeezing of the eyelids
Observer bias (expectations and even numbers)
The Perkins tonometer is similar to the Goldmann tonometer, except that it is portable and counterbalanced, so it can be used in any position. This instrument is useful in a number of situations, including in the operating room, at the bedside, and with patients who are obese or for other reasons cannot be examined at the slit lamp. The light comes from batteries, and the force comes from a spring, varied manually by the operator. Because the Perkins tonometer is portable, it is useful in circumstances in which the patients or subjects do not have access to an examination room, such as in community or remote pressure screening sessions. This tonometer does seem to underestimate the IOP, at least in Chinese eyes in supine patients, and the underestimation increases as the true IOP increases.
The Draeger tonometer is similar to the Goldmann and Perkins tonometers, except that it uses a different biprism. The force for applanation is supplied by an electric motor. Like the Perkins instrument, the Draeger tonometer is portable and counterbalanced, so it can be used in a variety of positions and locations.
MacKay-Marg and Tono-Pen™ tonometers
The MacKay-Marg tonometer consists of a movable plunger, 1.5 mm in diameter, that protrudes slightly from a surrounding footplate or sleeve. The movements of the plunger are measured by a transducer and recorded on a paper strip. When the instrument touches the cornea, the plunger and its supporting spring are opposed by the IOP and the corneal bending pressure ( Fig. 4‑4A ). As the instrument is advanced to the point of applanation, the corneal bending pressure is transferred to the footplate, and a notch is seen in the pressure tracing ( Fig. 4.4B ). The height of the notch is the measure of IOP. When the instrument is advanced farther, the cornea is indented farther, and IOP rises ( Fig. 4.4C ). The transfer of the corneal bending force occurs at an applanation area 6 mm in diameter. Applanation over this area displaces approximately 8 μl of aqueous humor and raises IOP about 6–7 mmHg – P t is 6 or 7 mmHg higher than P O .
The MacKay-Marg tonometer measures IOP over a brief interval, so several readings should be averaged to reduce the effects of the cardiac and respiratory cycles. This instrument is useful for measuring IOP in eyes with scarred, irregular, or edematous corneas because the end point does not depend on the evaluation of a light reflex sensitive to optical irregularity, as does the Goldmann tonometer. The tip of the instrument is covered with a plastic film to prevent transfer of infection. The tonometer is calibrated by comparing the plunger displacement with gravity to a fixed number of units on the tonometer recording paper. The MacKay-Marg tonometer is also fairly accurate when used over therapeutic soft contact lenses.
A small portable applanation tonometer that works on the same principle as the MacKay-Marg tonometer is available ( Fig. 4-5 ) (Tono-Pen® XL, Medtronics, Minneapolis, MN). It appears to be accurate in common clinical situations but not quite as accurate as the Goldmann outside the physiologic range. The accuracy of the Tono-Pen can be improved by taking two readings and averaging them. Unfortunately, because it is an applanation device, the results of the Tono-Pen are affected by corneal thickness just like the Goldmann tonometer. We have found these devices particularly useful in community health fairs, on ward rounds, and in other circumstances in which rapid portable tonometry is indicated. The Tono-Pen (like the Perkins tonometer) tends to underestimate the true IOP in Chinese eyes in supine patients. The Tono-Pen may also be used in children as readings are obtained rapidly and the device gives an indication of the quality of the reading. However, the Tono-Pen tends to overestimate the IOP in infants so its usefulness in congenital glaucoma screening and monitoring is somewhat limited.
Because it depends on an electronic end point rather than an optical end point like the Goldmann, the Tono-Pen should theoretically be more accurate in corneas with irregular surfaces. However, in band keratopathy where the surface of the pathology is harder than normal cornea, the Tono-Pen tends to overestimate the IOP. The small applanating area also allows finding the smoothest part of the cornea. In a normal eye, there is little difference in IOP readings between applying the Tono-Pen to central or peripheral cornea; this allows reasonably accurate use of the Tono-Pen even if the central cornea is irregular or following photorefractive surgery. The Tono-Pen seems reasonably accurate even when measuring through an amniotic membrance patch graft. It has been suggested that the Tono-Pen could be used to read from the sclera rather than the cornea; however, one recent study suggested that such readings are highly inaccurate. A disposable latex cover which is discarded after each use provides infection control, although, in rare circumstances, the latex can cause an allergic reaction.
The pneumatic tonometer ( Fig. 4-6 ) has a sensing device that consists of a gas chamber covered by a polymeric silicone diaphragm. A transducer converts the gas pressure in the chamber into an electrical signal that is recorded on a paper strip. The gas in the chamber escapes through an exhaust vent between the diaphragm and the tip of the support nozzle. As the diaphragm touches the cornea, the gas vent is reduced in size, and the pressure in the chamber rises. Some models of this instrument use a digital display, and some a paper tracing, to record IOP. The instrument emits a whistling sound when it is placed properly on the cornea. The pneumatic tonometer was designed originally as an applanation instrument. However, as Moses and Grodzki have indicated, the device currently marketed has some properties that are more like an indentation tonometer.
The pneumatic tonometer is useful for measuring IOP in eyes with scarred, irregular, or edematous corneas. The small applanation tip makes the instrument useful in laboratory settings in which some other tonometers can prove unwieldy. The instrument provides a good measurement of IOP, although it overestimates pressure at low levels and underestimates pressure at high levels. Calibration of the instrument is empirical. The pneumatic tonometer can be used for tonography if it is fitted with weights and used in a continuous recording mode. The pneumatic tonometer is fairly accurate when used over therapeutic soft contact lenses. While the pneumotonometer is subject to errors related to corneal thickness, it seems less so than the Goldmann applanation tonometer. Yet, in another study of large numbers of patients, pneumotonometry seemed more susceptible to the effects of corneal thickness than the Goldmann applanation tonometer. In addition, the repeatability of pneumotonometry has been called into question. Like most tonometers, repeating readings with the pneumotonometer close together results in decreasing IOP readings; this effect seems to be lost after two minutes.
The non-contact tonometer applanates the cornea by a jet of air, so there is no direct contact between the device and the surface of the eye. This theoretically avoids the need to sterilize the instrument, but a recent study found the air puff produces a tear film aerosol that could potentially contain infectious material. The force of the air jet increases rapidly and linearly with time. The instrument also emits a collimated beam of light that is reflected from the central cornea and then received by a photocell. When an area of the cornea 3.6 mm in diameter is flattened, the light reflected to the photocell is at a maximum. The time required to produce the peak reflection is directly related to the force of the air jet and thus to the counterbalancing IOP.
The non-contact tonometer is useful for screening programs because it can be operated by non-medical personnel, it does not absolutely require topical anesthesia and there is no direct contact between the instrument and the eye. The IOP readings obtained with the non-contact tonometer correlate fairly well with readings taken by Goldmann tonometry, but differences of several millimeters of mercury are not unusual, particularly with pressures higher than the low 20s. The tonometer can be used without topical anesthesia, but it is more accurate with anesthesia. The patient should be warned that the air puff can be startling, even after topical anesthetic. The non-contact tonometer measures IOP over very short intervals, so it is important to average a series of readings. The instrument has an internal calibration system. Several newer iterations which have increased the popularity of this type of tonometry have appeared in recent years. The newer breed of units seem to be more comfortable for patients as well as improving the accuracy (at least as compared to Goldmann applanation tonometry (GAT)). One unit has software that allows indication and measurement of pulse amplitude.
However, not all studies have shown accuracy compared to Goldmann tonometry. In general, at least three but preferably four readings should be obtained on each eye. The accuracy of the non-contact tonometer in post-keratoplasty patients has been called into question.
One interesting adaptation of the non-contact tonometer is in the new Reichert Ocular Response Analyzer™ (Reichert Ophthalmic Instruments, Depew, NY, USA). This device is basically an air puff tonometer that directs the air jet against the cornea and measures not one but two pressures at which applanation occurs – when the air jet flattens the cornea as the cornea is bent inward and as the air jet lessens in force and the cornea recovers ( Fig. 4-7 ). The first is the resting intraocular pressure. The difference between the first and the second applanation pressure is called corneal hysteresis and is a measure of the viscous dampening and, hence, the biomechanical properties of the cornea. The biomechanical properties of the cornea are related to, but not the same as, corneal thickness and include elastic and viscous dampening attributes. It is thought that central corneal thickness is just one attribute that contributes to the biomechanical properties of the cornea.
Clinically, the IOPs as measured by the Ocular Response Analyzer (ORA) correlate well with Goldmann tonometry but, on average, measure a few millimeters higher since the device seems to be less dependent on central corneal thickness than the Goldmann applanation tonometer. Furthermore, while IOP varies over the 24-hour day, hysteresis seems to be stable. Congdon et al found that a ‘low’ hysteresis reading with the ORA correlates with progression of glaucoma, whereas thin central corneal thickness correlates with glaucoma damage. Not all studies have been impressed with the accuracy of this device. Whether the concept of corneal hysteresis, while showing promise based on early studies, will ultimately become of practical value in the management of glaucoma remains to be demonstrated.
The Ocuton™ tonometer
The Ocuton™ (Elektronik & Präzisionsbau Saalfeld GmbH, Jena, Germany) is a hand-held tonometer that works on the applanation principle using a probe that is so light that it is barely felt and, therefore, needs no anesthetic in most patients. It has been marketed in Europe for home tonometry ( Fig. 4-8 ). The device is comparable to Goldmann tonometry but tends to read higher than the Goldmann tonometer when the cornea is thicker, and its accuracy may be compromised by diurnal changes in corneal thickness. This device may be useful to get some idea of the relative diurnal variation in IOP if the patient or spouse (etc.) can learn to use it.
The Maklakow (also spelled Maklakov) tonometer differs from the other applanation instruments in that a known force is applied to the eye, and the area of applanation is measured – a technique known as constant-force rather than constant-area applanation. The instrument consists of a wire holder into which a flat-bottom weight, ranging from 5 to 15 g, is inserted. The surface of the weight is painted with a dye, such as mild silver protein (Argyrol) mixed with glycerin, and then the weight is lowered onto the cornea. During the procedure the patient is supine, and the cornea is anesthetized. The weight is lifted from the cornea, and the area of applanation is taken to be the area of missing dye, which is measured either directly or indirectly from an imprint on test paper. Intraocular pressure is inferred from the weight (W) and the diameter of the area of applanation (d) by using the following formula:
P t = W π ( d / 2 ) 2
Intraocular pressure is measured in grams per square centimeter and is converted to millimeters of mercury by dividing by 1.36.
The Maklakow tonometer is used widely in Russia and China but has never achieved great popularity in western Europe or the United States. This instrument displaces a greater volume of aqueous humor than the other applanation devices (but less than a Schiøtz tonometer), which means that the IOP readings are more influenced by ocular rigidity. Attempts have been made to overcome this problem by measuring IOP with two different weights. The Maklakow tonometer does not correct for corneal bending, capillary attraction, or tear encroachment on the layer of dye.
Many instruments similar to the Maklakow device have been described, including the Applanometer, Tonomat, Halberg tonometer, and GlaucoTest.
In indentation tonometry, a known weight is placed on the cornea, and the IOP is estimated by measuring the deformation or indentation of the globe. The Schiøtz tonometer is the prototype for this class of instruments.
The Schiøtz tonometer consists of a metal plunger that slides through a hole in a concave metal footplate ( Figs 4-9 and 4-10 ). The plunger supports a hammer device connected to a needle that crosses a scale. The plunger, hammer, and needle weigh 5.5 g. This can be increased to 7.5, 10, or 15 g by the addition of appropriate weights. The more the plunger indents the cornea, the higher the scale reading – that is, the lower the IOP, the higher the scale reading. Each scale unit represents a 0.05 mm protrusion of the plunger.
The technique of Schiøtz tonometry is summarized as follows:
The patient lies supine and fixates on an overhead target, such as a light or a mark on the ceiling. Alternatively, the examiner may place the patient’s thumb in the appropriate position to serve as a fixation target. This is useful in patients with limited vision in the fellow eye.
The examiner explains the nature of the test and reassures the patient that the measurement is painless. The patient is told to relax, breathe normally, fixate on the target, and open the eyes wide.
A drop of topical anesthetic, such as 0.5% proparacaine, is instilled in each eye.
The tonometer tip and footplate are wiped carefully with an alcohol swab and allowed to air dry. If the tonometer has been stored disassembled in its case, it should be wiped with the alcohol swab before assembly. The alcohol must be allowed to evaporate before the instrument touches the eye.
The examiner retracts the patient’s lids without placing tension on the globe. The tonometer is placed directly over the eye, and when the patient relaxes, it is lowered gently onto the cornea ( Fig. 4.10 ). The tonometer should be perpendicular to the corneal apex. The examiner must be careful not to press the tonometer against the globe.
The measurement is noted to the nearest 0.25 scale units. If a wide pulse pressure is present, the center point of the fluctuation is chosen as the end point. If the scale reading is less than 3 units, additional weight is added to the plunger.
The IOP measurement is repeated until three consecutive readings agree within 0.5 scale units.
The average scale reading is converted to IOP in millimeters of mercury using a conversion chart. The examiner records the scale reading, weight, converted IOP, time of day, ocular medications, and time since last instillation of ocular medication, as well as the conversion chart used.
The instrument is calibrated before each use by placing it on a polished metal sphere and checking to be sure that the scale reading is zero. If the reading is not zero, the instrument must be repaired.
After each use, the tonometer plunger and footplate should be rinsed with water, followed by alcohol, and then wiped dry with lint-free material. It is important to prevent foreign material from drying within the footplate because this affects the movement of the plunger. The most common ‘foreign material’ that finds its way onto the plunger tip is fluid from the patient’s tear film. The instrument can be sterilized with ultraviolet radiation, steam, ethylene oxide, or a variety of solutions that have been indicated for the Goldmann prism. As with other tonometer tips, the Schiøtz can be damaged by some disinfecting solutions such as hydrogen peroxide and bleach.
If the tonometer is not going to be used for a while, it is best to disassemble the unit, clean it, and store it in its case. Disassembly allows for better cleaning of the barrel and plunger apparatus, and case storage protects the instrument from becoming bent or otherwise damaged and thrown out of calibration.
The Schiøtz tonometer is portable, sturdy, relatively inexpensive, and easy to operate. The instrument is accurate over a wide range of IOPs, although pressures may vary from those obtained with GAT, particularly when relatively untrained examiners are administering the test. An important concern is that placing the heavy tonometer (total weight at least 16.5 g) on the eye raises IOP. The rise in pressure reflects the dispensability of the ocular coats, a property termed ocular rigidity. All of the tables that relate the change in volume to the IOP assume a normal ocular rigidity, and this introduces a substantial error for some measurements. Eyes with high ocular rigidity (e.g., high hyperopia or longstanding glaucoma ) give falsely high Schiøtz IOP readings, whereas eyes with low ocular rigidity (e.g., myopia, strong miotic therapy, retinal detachment surgery, or compressible gas ) give falsely low Schiøtz IOP readings. It is possible to estimate ocular rigidity by comparing applanation and Schiøtz measurements or by repeating the Schiøtz measurements with two or more weights using the Friedenwald nomogram. Recent data based on cadaver eye experiments suggest that the Friedenwald nomogram may have some errors and that there is a larger increment of volume change per unit pressure than was found by Friedenwald.
The Schiøtz tonometer may also affect the IOP estimation by altering the outflow facility, rate of aqueous humor formation, episcleral venous pressure, and blood volume of the eye. Although none of these alterations is as important during tonometry as it is during tonography (see Ch. 3 ), they add to the uncertainty of the measurement. The Schiøtz pressure reading is also influenced by the size of the footplate hole and the thickness and curvature of the cornea.
Electronic Schiøtz tonometer
The electronic Schiøtz tonometer has a continuous recording of IOP that is used for tonography. The scale is also magnified, which makes it easier to detect small changes in IOP.
A new and updated version of an indentation tonometer has been developed in which a very light, disposable, sterile probe is propelled forward into the cornea by a solenoid; the time taken for the probe to return to its resting position and the characteristics of the rebound motion are indicative of the IOP (and also the biomechanical properties of the cornea). The time taken for the probe to return to its resting position is longer in eyes with lower IOP and faster in eyes with higher IOP. The production model (ProTon, ICare) has been made portable ( Fig. 4-11 ). Because the probe is extremely light and its contact with the cornea is very short (like the air puff tonometer), this type of tonometer can be used without first anesthetizing the eye.
The impact–rebound tonometer has been shown to be comparable to the Goldmann in both normal and post-keratoplasty human eyes. While generally comparable to other clinically used tonometers, the impact–rebound tonometer does tend to read slightly higher than the Goldmann. Furthermore, based on its mechanism of action, it is not surprising that the accuracy falls off in scarred corneas (as does the Goldmann). The rebound tonometer does correlate, like the Goldmann, with central corneal thickness. In general, this tonometer can be used in screening situations, when patients are unable to be seated or measured at the slit lamp, or when topical anesthetics are not feasible or usable.
The impact–rebound tonometer has been found to be particularly useful in small animal eyes such as the mouse and rat where the traditional applanation tonometer tips are too large. The impact tonometer appears to be more accurate in rat eyes than the Tono-Pen and accurate in mouse eyes.
Since the identification of intraocular pressure as a risk factor for glaucomatous damage, attempts have been made to measure IOP through the eyelid, obviating the need for topical anesthetic and the risk of eye-to-eye transfer of pathogenic organisms. The simplest way to accomplish this is with the fingers, but, perhaps, with very few exceptions, digital (meaning with fingers) impressions are at best qualitative and at worst not correlative with real IOP. However, a reasonable qualitative (or semi-quantitative) assessment can be made in situations where other, more accurate, devices are not practical, such as in young children, demented patients and severely developmentally-challenged patients.
In addition to all the problems facing indentation tonometry, such as scleral rigidity, transpalpebral tonometry adds variables such as the thickness of the eyelids, orbicularis muscle tone and potential intrapalpebral scarring. Recently, two attempts have been made to develop more quantitative transpalpebral IOP measuring devices. The TGDc-01 (Envision Ophthalmic Instruments, Livonia, Michigan, USA) was developed in Russia and bases its measurement on a weight falling within the instrument onto the closed eyelid and the amount of indentation it causes. Initial studies suggested good correlation with Goldmann tonometry, but more rigorous, controlled studies suggest that, at least in a significant minority of patients not identifiable prospectively, the accuracy is limited. Furthermore, interobserver and intraobserver variability was large, making the readings unreliable for most clinical purposes.
Fresco had an ingenious idea – that pressure on the eyelid in most eyes produces retinal phosphenes. The pressure on the eyelid required to induce these phosphenes is proportional to the intraocular pressure. He then developed this into a usable transpalpebral tonometer – the Proview (Bausch & Lomb, Rochester, NY, USA) ( Fig. 4-12 ) – and found good correlation with GAT. Other studies raised the promise that patients could measure their own IOP at home, or wherever they were, and obtain information about their diurnal IOP variation that would be useful in managing their glaucoma.
Unfortunately, subsequent studies failed to confirm the accuracy of this device. The Proview could still be useful for diurnal IOP estimations by patients themselves if several validating measurements are made side-by-side with the Goldmann or other accurate transcorneal tonometer.
DYNAMIC CONTOUR TONOMETRY
Kanngiesser described a tonometer based on a totally different concept than either indentation or applanation tonometry. This tonometer is based on the principle that by surrounding and matching the contour of a sphere (or a portion thereof), the pressure on the outside equals the pressure on the inside. In the dynamic contour tonometer (DCT) (Pascal™, Zeimer, Zurich, Switzerland) ( Figs 4-13 and 4-14 ), the tip of the probe matches the contour of the cornea. A pressure transducer built into the center of the probe measures the outside pressure, which should equal the inside pressure, and the IOP is recorded digitally on the liquid crystal display (LCD). The concept developed from a previous contact lens tonometer called the ‘Smart Lens.’ The DCT was shown to be superior in accuracy to Goldmann tonometry and pneumotonometry in human cadaver eyes across the entire range of IOPs seen in clinical practice. In living human eyes, the DCT correlates well with Goldmann readings.
Unlike Goldmann and other tonometers, the DCT does not appear to be affected by corneal thickness in several studies. Also, unlike Goldmann tonometry, IOP as measured by DCT is not altered by corneal refractive surgery that thins the cornea.
Because the DCT measures IOP in real time, the actual measurement, like the IOP, is pulsed. The internal electronics ‘call’ the IOP as the bottom of the pulsed curve and indicate it digitally on the LCD ( Fig. 4-15 ). The reliability of the IOP measurement is also indicated on a five-point scale. Two readings are recommended. Certainly, any measurement with a poorer than average reliability reading should be repeated. One of the reasons that the IOP readings with the DCT are generally lower than GAT may be that GAT, when properly done, indicates the average difference between the maximum and minimum pressures whereas the DCT reads the minimum. The DCT also indicates the magnitude of the difference between maximum and minimum IOP as the ocular pulse amplitude. While several studies have suggested that the ocular pulse amplitude may be indicative of the status of ocular blood flow and be differentially affected in different types of glaucoma, we have found that the ocular pulse amplitude is increased over normals in most forms of glaucoma and may be related to the level of IOP.
In summary, the dynamic contour tonometer (Pascal) is a promising new technology that may give the clinician better information about the actual IOP. It is independent of corneal thickness and contour and may give more accurate readings than the Goldmann tonometer in those eyes with very thin corneas. Its practical value in terms of managing clinical glaucoma remains to be demonstrated. We have found that many of our patients whose optic nerves or visual fields are progressing, despite what appear to be satisfactory Goldmann readings, have 3–5 mmHg higher readings suggesting that more aggressive pressure-lowering therapy might be helpful.
CONTINUOUS MONITORING OF INTRAOCULAR PRESSURE
There have been a few attempts to monitor IOP continuously in animal and human eyes. The devices described consist of applanation instruments inside contact lenses or suction cups, or strain gauges in encircling bands that resemble scleral buckling elements. None of these instruments has achieved widespread use. One approach has used resonance applanation tonometry measuring the sonic resonance of the eye when a continuous force over a fixed area is applied. The latest iteration is independent of corneal position making a practical home tonometer using this principle at least achievable. Yet another innovative approach has been to use infrared spectroscopy to measure IOP. Infrared spectroscopic measurements correlate with IOP as measured by manometry in pig eyes. A newer approach has been to build a miniature pressure sensor that can reside inside the eye; one such device is part of an intraocular lens. The development of a device which can easily monitor IOP over a 24-hour period or longer would greatly aid our understanding of aqueous humor dynamics and glaucoma.
SUMMARY OF TONOMETRY
While IOP has been demoted from primacy in the diagnosis of glaucoma to just a risk factor, it still remains the single most important and only modifiable factor to assess the effectiveness of treatment. In both traditional primary open-angle glaucoma and normal-pressure glaucoma, the IOP level is still the most important risk factor for glaucoma damage. Furthermore, the Early Manifest Glaucoma Trial showed that for each mmHg lowering of IOP from baseline, there is a 10% decrease in the rate of progression. From only a few instruments just a few decades ago, there seems now to be a bewildering array of devices to measure IOP. The Goldmann tonometer has stood the test of time but is beginning to show some traces of gray. By now, everyone should recognize that thin corneas will cause the Goldmann to under read the IOP and a thick cornea will likely cause the Goldmann to over read. Because the relationship is not linear, no current formula can accurately convert Goldmann readings to ‘true’ IOP with any given corneal thickness.
Some newer tonometers have shown independence of corneal thickness and may, therefore, be more accurate in eyes on the thick and thin extremes of corneal thickness. These include the pneumotonometer, the dynamic contour tonometer and the hysteresis non-contact tonometer. However, it is still true that most of our current understanding of the treatment of glaucoma is based on Goldmann readings. Therefore, it may take some time for the newer, perhaps more accurate, devices to work their way into the diagnostic and treatment construct that has served so well for the past 60 years or so.
Other tonometers may be particularly useful in certain situations. For example, the pneumotonometer and the Tono-Pen may be more accurate than the Goldmann when the cornea is irregular or scarred. The pneumotonometer and the non-contact tonometermay be able to obtain reasonably accurate readings through a soft contact lens. While not very accurate for day-to-day use, the Proview tonometer, because it does not need anesthetic and is user friendly, may be helpful for home tonometry, in obtaining diurnal IOP estimations and in patients who cannot be tested with corneal contact instruments. The non-contact tonometers, pneumotonometers, and Tono-Pen may be useful for screening situations.
DISTRIBUTION OF INTRAOCULAR PRESSURE IN THE GENERAL POPULATION
There have been a number of studies on the distribution of IOP in the normal population ( Table 4-1 ). In a classic study, Leydhecker and co-workers performed Schiøtz tonometry on more than 10000 normal individuals. They found the mean (±SD) IOP to be 15.8 ± 2.6 mmHg. At first glance the pressure readings appeared to be distributed in a normal fashion (also referred to as a Gaussian or bell-shaped distribution). However, closer inspection of the data revealed that the distribution was not Gaussian, but rather skewed to the right. This distinction is important because it means we cannot define an upper limit for IOP by adding 2 or 3 standard deviations to the mean. This conclusion is supported by a number of studies, all of which have found a much higher prevalence of elevated IOP (e.g., >20 or 21 mmHg) than would be predicted by Gaussian statistics ( Table 4-2 ). Unfortunately, the skewed distribution also means that an abnormal IOP must be defined empirically – that is, an abnormal pressure is one that causes optic nerve damage in a particular eye. Because eyes differ markedly in their susceptibility to the effects of pressure, it is difficult to know a priori what level of IOP will be harmful to a given patient. Some individuals develop glaucomatous damage at IOPs near the population mean, whereas others maintain normal optic nerves and visual function for many years despite IOPs of 30 or even 40 mmHg.