Pressure /signal amplitude

0

‘Out’ signal peak

‘In’ signal peak

Applanation pressure 1

compromised by diurnal changes in corneal thickness.105–107 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.

Maklakow tonometer

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.108 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:

W

Pt 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.109 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,110 and GlaucoTest.111

Fig. 4-8  The Ocuton™ tonometer.

Indentation instruments

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.

Schiøtz tonometer

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.112

The technique of Schiøtz tonometry is summarized as follows:

1. 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.

Fig. 4-9  Shiøtz tonometer.

Fig. 4-10  Shiøtz tonometry technique.

  2. 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.

 3.  A drop of topical anesthetic, such as 0.5% proparacaine, is instilled in each eye.

 4. 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.

  5. 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.

  6. 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.

 7. The IOP measurement is repeated until three consecutive readings agree within 0.5 scale units.

 8. 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.

 9. 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.

10.  After each use, the tonometer plunger and footplate should be rinsed with water, followed by alcohol, and then wiped dry with lintfree 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.113

11.  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.114–116 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 hyperopia117 or longstanding glaucoma109) give falsely high Schiøtz IOP readings, whereas eyes

with low ocular rigidity (e.g., myopia,117 strong miotic therapy,117 retinal detachment surgery,118,119 or compressible gas120) give

falsely low Schiøtz IOP readings. It is possible to estimate ocular rigidity by comparing applanation and Schiøtz measurements5 or by repeating the Schiøtz measurements with two or more weights using the Friedenwald nomogram.109 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.121

Fig. 4-11  The Impact–Rebound Tonometer (ICare).

(Courtesy of Tiolat, Oy, Helsinki, Finland.)

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.122 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.123

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.

Impact–rebound tonometer

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).124 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.125

The impact–rebound tonometer has been shown to be comparable to the Goldmann in both normal and post-keratoplasty human eyes.126 While generally comparable to other clinically

used tonometers, the impact–rebound tonometer does tend to read slightly higher than the Goldmann.127–129 Furthermore, based on

its mechanism of action, it is not surprising that the accuracy falls off in scarred corneas (as does the Goldmann).130 The rebound

tonometer does correlate, like the Goldmann, with central corneal thickness.131,132 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.