Schiotz Tonometer  

In spite of the limitations of indentation tonometry, Schiotz tonometer is still in use in the primary eye care set up in developing countries.

The methods for sterilzation of such tonometers include:

1. 1. Heating the base of the instrument with the flame of a spirit lamp for 10 seconds and allowing sufficient time for cooling before use. Repeated heating may, however, distort the curvature of the foot plate and plunger, resulting in erroneous readings.

2. 2. Cleaning the foot plate with ether or alcohol swab (allowing sufficient time for drying of chemical).

3. 3. Ultraviolet rays.4. 4. Soaking the assembled foot plate in a bowl with 1:1000 merthiolate solution.5. 5. Use of tonofilm.

All these methods of sterlisation may be unsatisfactory because sterility is not achieved in all parts of the tonometer.

    Recommendations  

An ideal method for sterilisation of Schiotz tonometer is:

1. 1. Disassemble the tonometer between each use. Clean the barrel by inserting a white pipe cleaner saturated with alcohol, pulling back and forth several times and then inserting a second dry pipe cleaner.

2. 2. Then clean the foot plate and plunger with alcohol.3. 3. Clean the test cornea with alcohol swab.4. 4. Reassemble the instrument and wait for atleast 60 seconds (after cleaning with alcohol) before

placing the instrument on the cornea.

A more practical approach would involve keeping the base of the tonometer continuously dipped in a solution of 1:1000 merthiolate solution (Figure). Prior to use, the footplate can be rinsed in saline/distilled water. After usage it should be replaced in the merthiolate solution.

You will need (Figure 1)

Schiötz tonometer, weights, and scale card local anaesthetic drops clean cotton wool or gauze swabs isopropyl alcohol 70 per cent (methylated spirit) or impregnated ‘Mediswabs’.

Test the tonometer using the spherical mould in the box and the 5.5 g weight. The pointer should immediately reach the ‘O’ marking (see Figure 2).

Clean the plunger and disc of the tonometer with a gauze swab (or cotton wool) and the methylated spirit (or a Mediswab). Wipe dry with a clean dry gauze swab (or cotton wool).

Lie the patient flat with his or her head supported on a pillow.

Method

Wash and dry your hands. Position yourself correctly: stand upright, behind the head of the patient, with your hands

level with the patient's head. Note the health worker's good posture in Figure 3 and the awkward position of the health worker in Figure 4. Bad posture can affect the tonometry reading.

Instil local anaesthetic eye drops and wait about 30 seconds. Ask the patient to look at a fixed object (the patient's own thumb or finger held directly in

front of his or her eyes may work) and to keep absolutely still. With the thumb and index finger of one hand, gently hold open the patient's eyelids,

taking care not to put any pressure on the eye (see Figure 5). With the other hand, hold the tonometer (with the 5.5 g weight) between the thumb and

index finger and place the plunger on the central cornea (see Figure 5). Allow the disc to lower gently onto the corneal surface. Note the scale reading. If the scale reading is ‘2’ or less, remove the tonometer, replace the 5 g weight with the

7.5 g weight, and repeat the procedure. Note the scale reading again and remove the tonometer. Tell the patient not to rub the eye - the anaesthetic will last for about five minutes. Clean and dry the tonometer head. Repeat the whole procedure for the other eye. Clean and dry the tonometer again and store it safely in the box.

Using the scale card, convert the noted scale readings and record the IOP in the patient's records.

Most primary-care physicians use the Schiotz tonometer because of its ease of use and relative low cost; therefore, its use and characteristics are described in greatest detail.

Before measuring the intraocular pressure, the Schiotz tonometer needs to be calibrated and sterilized. Calibration can be simply done by placing the footplate of the instrument on the rounded metal stand (the artificial cornea) provided with the storage case. With the footplate resting on the stand, a correctly calibrated instrument will have a scale reading of zero. Following calibration, the footplate can

be sterilized with a flame, alcohol, or ether. Care must be taken to ensure that the footplate is cool and dry before placing on the cornea.

Following preparation of the instrument, the patient should be prepared. After a thorough explanation of the procedure, the patient is asked to lie on the examining table with eyes fixed upward on the ceiling. After applying a topical anesthetic to the cornea, such as 0.5% proparacaine, the examiner gently separates the eyelids with the thumb and index finger and applies the tonometer footplate directly on the cornea (Figure 118.1). The instrument must be held perpendicular to the eye to allow the plunger to move freely, indenting the cornea. The degree of indentation is measured by movement of a needle on a scale. Fine oscillations of the needle represent ocular pulsations, indicating free movement of the plunger and good technique. The midpoint of the needle excursion is taken as the pressure measurement.

Placing the Schiotz tonometer on a patient's eye.

The standard force on the plunger producing corneal indentation is a 5.5 g weight. Globes with elevated intraocular pressure will be resistant to denting by the plunger, resulting in inaccurate measurements. Three larger plunger weights are provided with the instrument and, when added to the standard 5.5 g weight, increase the total plunger weight to 7.5, 10, or 15 g. The extra weights should be used whenever the pressure reading on the instrument scale is 4 or less.

Since the Schiotz tonometer does not measure pressure directly, conversion tables, supplied with the instrument, are used to translate scale readings into estimates of intra-ocular pressure. Two conversion tables are available, published in 1948 and 1955. Studies have shown that the 1948 table more closely approximates pressures obtained with Goldmann applanation tonometry.

In patients with known or suspected ocular infection, trauma, or known sensitivity to the topical anesthetic, Schiotz tonometry should not be performed by a primary-care physician. The procedure is further contraindicated in patients who cannot inhibit their blinking, because of the increased risk of corneal abrasion. The actual complication rate of tonometry is quite small, estimated from large screening programs to be less than 1 %, and includes corneal abrasions, infections, and drug sensitivity.

Once the pressure readings have been taken, a standardized format for recording the data is strongly recommended, which includes the scale reading, tonometer weight, intraocular pressure, conversion table, and eye measured. A typical measurement would be recorded as follows: 7/5.5 = 12 mm Hg (1955), O.D.

Although technicians can be trained to use the Schiotz instrument, user inexperience is a considerable source of measurement error. Even with the experienced user, however, interinstrument and interexaminer errors, each of a magnitude of 2 mm Hg, are possible. Factors external to the instrument can be responsible for similarly large errors and are due to the force needed to overcome the natural resistance of the sclera independent of ocular pressure. For example, refractive errors including hyperopia and myopia increase and decrease scleral rigidity respectively. Corneal disease and past ocular operations alter the resistance to indentation and are an additional source of measurement error. The magnitude, and at times, unpredictable direction

of these measurement errors indicate that caution should be used when interpreting results. It may be prudent to assume that the Schiotz tonometer will indicate a probable range of intraocular pressures and, as with other tonometry measurements, is not sufficient in itself to make the diagnosis of glaucoma.

Schiøtz Tonometer

Schiøtz developed an excellent tonometer in 1905 and continued to refine it through 1927. His refined tonometer became the most widely used in the world and because of its simplicity, reliability, and relative accuracy, it is the only indentation tonometer in widespread use today. The tonometer has been modified only slightly since Schiøtz's time.1

In the Schiøtz tonometer, gravity provides a known force on a weighted metal plunger. The plunger rides inside a metal cylinder attached to a footplate curved to match the average human corneal curvature (Fig. 10). The top of the plunger rides along a curved lever that attaches to a pointer, which in turn rides along a scale. For each 0.05 mm that the plunger sinks below the level of the footplate, the pointer moves up 1 scale unit. Thus, the lower the intraocular pressure, the farther into the cornea the plunger sinks and the higher the scale reading. Scale readings can be converted to millimeters of mercury by conversion tables, based on the amount of weight placed on the plunger. Before each measurement, the tonometer is placed on a solid steel block, which should result in no plunger depression and a scale reading of zero.

Fig. 10. Schiøtz tonometer. Arrow points to scale. Each scale marking indicates 0.05 mm of plunger movement.

The scale measuring the amount of indentation is linear. The relation between the amount of indentation and intraocular pressure is not linear but about logarithmic, so that the higher intraocular pressures are compressed toward the lower end of the scale.152 Below the scale reading of 3, it is not possible to get an accurate pressure reading other than to know that the pressure is elevated above the “normal” range.2,4 Therefore, additional weights of 2, 4.5, and 9.5 g, respectively, may be added to the plunger to give effective plunger weights of 7.5, 10, and 15 g, respectively. The heavier weights cause the plunger to sink deeper for a given intraocular pressure and to give a higher scale reading. In effect, the heavier weights expand the lower end of the scale (Table 3).

 TABLE 7-3. Schiøtz Scale Readings: Intraocular Pressure (PO) Conversion Table (from 1955 revision): Assumes Average Ocular Rigidity (PO in mmHg)

  Plunger Load

Scale Reading 5.5 g 7.5 g 10 g 15 g

3.0 24.4 35.8 50.6 81.8

3.5 22.4 33.0 46.9 76.2

4.0 20.6 30.4 43.4 71.0

4.5 18.9 28.0 40.2 66.2

5.0 17.3 25.8 37.2 61.8

5.5 15.9 23.8 34.4 57.6

6.0 14.6 21.9 31.8 53.6

6.5 13.4 20.1 29.4 49.9

7.0 12.2 18.5 27.2 46.5

7.5 11.2 17.0 25.1 43.2

8.0 10.2 15.6 23.1 40.2

8.5 9.4 14.3 21.3 38.1

9.0 8.5 13.1 19.6 34.6

9.5 7.8 12.0 18.0 32.0

10.0 7.1 10.9 16.5 29.6

(Kolker AE, Hetherington J Jr: Becker-Shaffer's Diagnosis and Therapy of the Glaucomas. 4th ed. St. Louis: CV Mosby, 1976.)

Conversion tables to obtain Po (resting intraocular pressure) from Pt (pressure with the tonometer on the eye) were developed from studies done on cadaver eyes by Friedenwald.152 These values were basically confirmed by McBain153,154 in studies using an adjustable manometer. The scale reading of the tonometer with each plunger weight was recorded. The volume of aqueous humor displaced by the weight of the tonometer was also measured. These observations were recorded and plotted on a logarithmic scale to yield the Friedenwald nomogram (Fig. 11).

Fig. 11. Scleral rigidity from Friedenwald nomogram. Solid line (A) joins Po (determined by applanation tonometry) and Pt1 (Schiøtz scale reading with 5.5-g weight). Theoretically, the same line could be obtained by joining scale readings with any two Schiøtz weights indicated by dots. Slope of line is scleral rigidity, which can be obtained directly from nomogram by drawing parallel line through 20 on the pressure scale (dotted line B to scleral rigidity scale on top of

nomogram). In this case, scleral rigidity is 0.022.

SCLERAL RIGIDITY

When a significant external force is applied to the eye (Fig. 12), the intraocular pressure is raised, blood is squeezed out of internal blood vessels, the corneal and scleral coats are distended, and the volume of the eye expands slightly. The relative resistance an eye offers to expansion for a given rise in intraocular pressure is known as scleral rigidity. The reciprocal of rigidity is elasticity.

Fig. 12. Schiøtz tonometer on cornea. Footplate (A) rests on corneal surface supporting weight of tonometer, which raises intraocular pressure to Pt. The plunger (B) sinks into the cornea (C), displacing a volume of aqueous humor until the elasticity of the cornea and the intraocular pressure (P) push back with enough force to prevent further sinking of the plunger.

Scleral rigidity varies from individual to individual. Friedenwald's tables for conversion of Schiøtz scale readings to intraocular pressure are calculated based on an average scleral rigidity.155 In myopic eyes, scleral rigidity is lower than average, and the Schiøtz plunger sinks deeper into the cornea, compared with an eye with average scleral rigidity at the same intraocular pressure (Table 4). The scale reading will be higher and the intraocular pressure will be underestimated. A diagnosis of glaucoma may be missed. Conversely, hyperopes and patients

with scarred corneas have higher scleral rigidity, resulting in overestimation of their intraocular pressures.155

 

TABLE 7-4. Effect of Scleral Rigidity on Intraocular Pressure Measurement by Schiøtz Tonometry and Outflow Facility*

  A PO C

Average rigidity (0.0215) 19.5 19.0 0.18

Low rigidity (0.0135) 23.5 23.0 0.38

High rigidity (0.0315) 15.5 15.0 0.11

*Using a weight of 5.5 g, a scale reading of 4.5 on the Schi<aso>tz tonometer would indicate a pressure of 19 mmHg, using the 1955 revised conversion table (see Table 3). This table assumes an average ocular rigidity. Note that if the patient has a low ocular rigidity, the figure of 19 mmHg significantly underestimates the true intraocular pressure of 23.5 indicated in column A. The opposite is true for a patient with high ocular rigidity. An even larger error in outflow facility is seen with changes in the scleral (ocular) rigidity.A = applanation reading (mmHg); PO = intraocular pressure by Schi<aso>tz corrected for ocular rigidity at scale reading of 4.5 (mmHg); C = outflow facility corrected for ocular rigidity with final Schi<aso>tz scale reading 6.5.(Kolker AE, Hetherington J Jr: Becker-Shaffer's Diagnosis and Therapy of Glaucomas. 4th ed. St. Louis, CV Mosby, 1976.)

 

Scleral rigidity can be calculated from the Friedenwald nomogram as follows (see Fig. 11): Two different weights are used to obtain two tonometer scale readings (two different values for Pt). A more accurate method is to use the applanation tonometry value as Po and one Schiøtz reading as Pt. The slope of the line formed by joining these two points gives the scleral rigidity (mmHg change in pressure per cubic mm change in volume). In clinical practice, scleral rigidity measurements are not made as often as they should be. This introduces significant errors and makes dependence on Schiøtz tonometry hazardous.

Errors of Schiøtz Tonometry

In addition to the potentially large error that an abnormal scleral rigidity can produce in Schiøtz tonometry, other errors may arise. The footplate must be the right curvature and size, and the total instrument must be the correct weight for the Friedenwald tables and nomogram to apply.2 Poor attention to detail in manufacture can make a tonometer totally unreliable. The American Academy of Ophthalmology has established a Committee on Tonometer Standardization, which has set rigid criteria for Schiøtz tonometers. A tonometer certified by this committee or by a laboratory committed to the same standards can be expected to perform reliably.

Dirty tonometers are also potentially inaccurate. If tears inside the barrel are allowed to dry, the friction between the barrel and plunger is increased. A common error is to autoclave the tonometer without first carefully cleaning it. The secretions in the barrel harden, causing the plunger to stick. If the tonometer is not placed perpendicular to the corneal surface, additional friction is produced between plunger and barrel. A cornea that is scarred, edematous, or of abnormal curvature gives inaccurate readings with the Schiøtz.

A corneal abrasion can be caused by the plunger if the eye or tonometer moves during measurement. The Schiøtz tonometer must be used in the supine position or in the sitting position with the head back far enough to be horizontal. An initial blink or avoidance reaction may occur as the patient sees the tonometer descending toward his or her eye.

Despite the many potential sources of error, the Schiøtz tonometer has been a remarkably useful instrument for the past 75 years. Compared with most other methods of tonometry, it is inexpensive, simple, portable, and easily sterilized. Patients and their families can be taught how to use it for home tonometry. Although largely replaced in the United States by Goldmann or other types of tonometry, the Schiøtz tonometer still has a place in clinical practice, particularly if checked for accuracy against Goldmann tonometry in each patient.

TONOGRAPHY

Tonography is based on the observation that pressing on the eye causes an initial increase in intraocular pressure, then a decrease. The Schiøtz tonometer is used both to cause the initial rise to Pt and to measure the subsequent fall in pressure. The rate of decline in intraocular pressure is a measure of the ease or difficulty with which fluid can be forced out of the eye by the weight of the tonometer. This process is similar to forcing air out of an air mattress. Pressing on the mattress indents and deforms the mattress, displacing some of the air. The pressure inside the mattress is raised temporarily. This increases the rate of air loss through the open valve of the air mattress. By measuring the decreasing pressure in the air mattress and knowing the volume displaced by pressing on the mattress, it is possible to calculate the volume of air lost and more importantly, how easily the open valve allows air to escape. In tonography, the volume of fluid lost as the Schiøtz tonometer presses on the eye for a given time period is measured by how far the plunger sinks into the cornea; the pressure change is inferred from the Friedenwald relations. With a few assumptions and a formula, the outflow facility can be estimated.

That intraocular pressure seems to decrease after repeated Schiøtz tonometry had been long known when Grant178 first described clinical tonography in 1950. Tonography quantifies the change in pressure over time and became clinically feasible by the development of a Schiøtz tonometer that records the position of the plunger electronically. The recording is usually made over 4 minutes, a period found to give repeatable readings.178

When the tonometer is placed on the eye, the intraocular pressure is raised from Po to Pt. The degree of pressure rise depends on the plunger weight. As the tonometer stays on the eye, aqueous is forced out of the eye. The pressure declines to final value (Pf) at the end of the 4

minutes (and would continue to decline at a decreasing rate until a steady state was reached). The average pressure during tonography (Ptav) is assumed to be

It is assumed that the Ptav provides the pressure gradient that forces the fluid out of the eye. The volume change (ΔV) is assumed to be equivalent to the increased indentation of the tonometer plunger during the measurement. Note that the change in corneal indentation of the tonometer plunger gives both change in volume and—by reference to the Friedenwald data—change in pressure.5,177–179

Although tables have been constructed that allow easy calculation of outflow facility, these tables are based on the formula

The assumptions inherent in this formula (none of which are true) are that the ocular rigidity is average; that placing the tonometer on the eye does not alter intraocular blood volume, EVP, or aqueous secretion rate; and that the facility of outflow is not affected by the tonometer itself. The true topographic outflow facility should consider these factors, leading to the formula

where ΔVS is the change in the distention of the ocular coats, ΔVC the change in corneal indentation, ΔVB the change in ocular blood volume, FtT the change in aqueous secretion (Ft) over the time (T) of the tonogram, and Pvt the EVP with the tonometer on the eye.

The intraocular blood volume changed when the tonometer rests on the eye and squeezes blood out of the eye. This can cause large errors in the value of C but no practical way has been found to measure this in the living human eye.180 EVP is raised an average of 1.25 mm with the tonometer on the eye.2 Tonographic tables contain a correction factor for this, but it is not known how variable this factor is from eye to eye.181 A significant variation in ocular rigidity results in a dramatic error in calculated outflow facility (see Table 4). A technique called constant-pressure tonography has been developed that eliminates the problem of scleral rigidity.2 This method appears to be reliable but remains largely a laboratory procedure.