Intraocular pressure

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.1 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 overor underestimate the IOP by large amounts.2 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.3

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.

Applanation Instruments

Goldmann tonometer

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

Fig. 4-1  Goldmann tonometer on the eye.

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  l of aqueous humor, which raises IOP by about 3% – that is, Pt (the pressure

Fig. 4-2  Goldmann applanation tonometer. Dial indicates force applied to applanate cornea; this number multiplied by 10 equals intraocular pressure in millimeters of mercury.

at that moment) is 3% greater than PO (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.4,5 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,6,7 which is less than the diurnal variation of IOP.The technique of Goldmann tonometry is as follows:

 1. 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.

 2. 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.

 3.  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.8

Fig. 4-3  Disposable tonometer tip designed to fit over Goldmann prism. (Tonosafe, Clement Clarke, UK)

  4. The tonometer tip is cleaned with a sterilizing solution,2,9–12 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 solu-

tion, if preferred.13 Disposable tonometer tips may also be used (Fig. 4-3).14,15 When using disposable tips, they should each be exam-

ined to be sure of a smooth applanating surface.16 The acrylic disposable tips seem to be somewhat more accurate than the silicone ones.17 While disposable shields or tips may be safer than disinfecting solutions, they are not 100% protective against prion disease.18

 5. 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.

 6. 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.19 An alternative approach is to average the IOP

readings obtained with the axis of the prism horizontal and then vertical.20,21

 7. 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.

 8. The patient is seated in a comfortable position on an adjust-

able 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.

  9. 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.22,23 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.

10. 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.24 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.

11. 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.

12. 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.

13. 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.

14.  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.

15. 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.

16.  It is possible to transfer bacteria, viruses, and other infectious agents with the tonometer head,25 including such potentially serious infections as epidemic keratoconjunctivitis, hepatitis B, Jacob-Kreutzfeld and, theoretically, acquired immunodeficiency

Box 4-1  Potential errors of applanation tonometry

Thin cornea

Thick cornea

Astigmatism 3 diopters

Inadequate fluorescein

Too much fluorescein

Irregular cornea

Tonometer out of calibration

Elevating the eyes 15°

Repeated tonometry

Pressing on the eyelids or globe

Squeezing of the eyelids

Observer bias (expectations and even numbers)

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.26 Seventy per cent ethanol and 70% isopropanol are effective as sterilizing solu-

tions but were shown in one study to cause mild damage to the tonometer tip after one month of immersion.27,28 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.29 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.30 Disposable tonometer tips may be an acceptable alternative to soaking in, and wiping with, antiseptic solutions.31

17. 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 onehalf at four months.32 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.33

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):

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.

2.  Elevating the eyes more than 15° above the horizontal causes an overestimation of IOP.

3. Widening the lid fissure excessively causes an overestimation of IOP.34

4.  Repeated tonometry reduces IOP, causing an underestimation of the true level.35,36 This effect is greatest between the first

and second readings, but the trend continues through a number of repetitions.7

5.  A scarred, irregular cornea distorts the fluorescein rings and makes it difficult to estimate IOP.

6. The thickness of the cornea affects IOP readings.37 If the cornea is thick because of edema, IOP is underestimated.37 If the cornea is thick because of additional tissue, IOP is overestimated.37,38

In thin corneas, the Goldmann tonometer will underestimate the IOP.39–41 Goldmann predicted that the tonometer would be inac-

curate 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 600.42 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.43 Central corneal pressures have been shown to be lower than peripheral corneal readings following photorefractive keratectomy and LASIK.44–47

7.  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.

8.  If corneal astigmatism is greater than 3 D, IOP is underestimated for with-the-rule astigmatism and overestimated for against- the-rule astigmatism.20 The IOP reading is inaccurate 1 mmHg for every 3 D of astigmatism.48

9.  A natural bias for even numbers may cause slight errors in readings.49

Perkins tonometer

The Perkins tonometer is similar to the Goldmann tonometer, except that it is portable and counterbalanced, so it can be used in any position.50 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.51

Draeger tonometer

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.52,53 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).54–56 The

transfer of the corneal bending force occurs at an applanation area 6 mm in diameter. Applanation over this area displaces approximately 8  l of aqueous humor and raises IOP about 6–7 mmHg

– Pt is 6 or 7 mmHg higher than PO.

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 MacKayMarg tonometer is also fairly accurate when used over therapeutic soft contact lenses.57

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.58,59 The accuracy of the Tono-Pen can be improved by taking two readings and averaging them.60 Unfortunately, because it is an applanation device, the results of the Tono-Pen are affected by corneal thickness just like the Goldmann tonometer.61 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.51 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.62 However, the Tono-Pen tends to overestimate the IOP in infants so its usefulness in congenital glaucoma screening and monitoring is somewhat limited.63

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.61 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.64–66 The Tono-Pen seems reasonably accurate even when measuring through an amniotic membrance patch graft.67 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.68 A disposable latex cover which is discarded after each use provides infection control, although, in rare circumstances, the latex can cause an allergic reaction.69

Pneumatic tonometer

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

Fig. 4-5  Tono-Pen™.

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.70 72

Fig. 4-4  Intraocular pressure (IOP) tracing with MacKay-Marg tonometer. (A) Advancing plunger is opposed by IOP and corneal

bending pressure. (B) Notch indicates corneal bending pressure has been transferred to footplate. Height of notch corresponds to IOP. (C) With continued advancement of plunger, cornea is indented, and IOP rises.

(Modified from Moses RA: Tonometry. In: Cairns JE, editor: Glaucoma, vol 1, London, Grune & Stratton, 1986.)

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,73 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.74 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.75,76 While

the pneumotonometer is subject to errors related to corneal thickness, it seems less so than the Goldmann applanation tonometer.77 Yet, in another study of large numbers of patients, pneumotonometry seemed more susceptible to the effects of corneal thickness than the Goldmann applanation tonometer.78 In addition, the repeatability of pneumotonometry has been called into question.79 Like most tonometers, repeating readings with the pneumotonometer