Ординатура / Офтальмология / Английские материалы / Shields Textbook of Glaucoma, 6th edition_Allingham, Damji, Freedman_2010

CCT of 520 µ m. Deviations from the average CCT are a source of error with cornea edema underestimating the true IOP, whereas variations of CCT in normal corneas can lead to falsely higher pressure readings with thicker corneas and falsely lower ones with thinner corneas (168). After refractive surgery, the IOP is lower due

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to a thinner cornea as a result of laser-assisted in situ keratomileusis (LASIK) (169).

Figure 2.4 Goldmann-type applanation tonometry. A: Basic features of tonometer, shown in contact with patient's cornea. B: Enlargement shows tear film meniscus created by contact of biprism and cornea. C: View through biprism (1) reveals circular meniscus (2), which is converted into semicircles (3) by prisms.

These latter observations have been evaluated to address the variance of CCTs in general populations and subgroups, including various glaucoma groups and the effect of refractive surgery influence the IOP measurements (170). From 300 datasets involving healthy eyes, the group-averaged CCT was 534 µ m.

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From 230 datasets in which interindividual variance was reported, the group-averaged CCT (±SD) was 536 + 31 µm. There are ethnoracial differences, wit h thinner mean CCTs of 530 to 531 µm in one African-American population and 495 to 514 µm in a Mongolian population (171, 172). A study in Japan revealed a mean of 552µm among healthy person s (173). Individuals in the Ocular Hypertension Treatment Study (OHTS) had a mean CCT of 573.0 ± 39.0 µm, and 24% of the OHTS cohort had a CCT greater than 600 µm (174). Patients with normal -tension glaucoma have thinner mean CCTs of 514 to 521 µm(175).

Figure 2.5 Technique of applanation tonometry with Goldmann tonometer.

This variance of CCT and its effect on the accuracy of IOP measurements raised questions as to what correction factor for the adjusted IOP measurement should be used when the CCT deviates from the assumed average, 520 µm. Ehlers and colleagues have published a table in which the average error is 0.7 mm Hgper 10 µ of deviation from the mean of 520 µ ( 168). Another study, however, revealed a smaller error, of 0.19 mm Hg per 10 µ (176), which is consi stent with findings of a direct cannulation study (177). IOP measurements with the Tono-Pen are also affected by CCT, with reported errors of 0.29 mm Hg per

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10 µ in men and 0.12 mm Hg per 10 µ in women (178). However, there is a lack of general agreement on the correction factor that should be used for adjusting the IOP measured by Goldmann tonometry, when the CCT deviates from the norm (179).

Figure 2.6 Semicircles of Goldmann-type applanation tonometry. A: Slitlamp view of Goldmann mires. B: Proper width and position. Enlargement (B, at right) depicts excursions of semicircles caused by ocular pulsations. C: Semicircles are too wide. D: Improper vertical and horizontal alignment.

Deviations of corneal curvature also influence IOP measurements, with an increase of approximately 1 mm Hg for every 3 diopters (D) of increase in corneal power (180). Marked corneal astigmatism produces an elliptical area of corneal contact. When the biprism is in the usual orientation, with the mires displaced horizontally, the IOP is underestimated for with-therule and overestimated for against- the-rule astigmatism, with approximately 1 mm Hg of error for every 4 D of astigmatism (181). To minimize this error, the biprism may be rotated until the dividing line between the prisms is 45 degrees to the major axis of the ellipse, or an average may be taken of horizontal and vertical readings. An irregular cornea distorts the semicircles and interferes with the accuracy of the IOP estimates. Prolonged contact of the biprism with the cornea leads to corneal injury, as manifested by staining, which makes multiple readings unsatisfactory. In addition, prolonged contact causes a decrease in IOP over a period of minutes, which is less pronounced in eyes with carotid occlusive disease, suggesting that it may be related to intraocular blood (182).

The Goldmann tonometer must be calibrated at least monthly. Instructions for quick, simple calibration come with the instrument. If the tonometer does not meet calibration specifications, it must be returned to the manufacturer or distributor for recalibration or repair.

Disinfection of Goldmann (and Other) Tonometers

With all tonometers that contact the eye, there is the risk of transmitting infection, such as the adenovirus of epidemic keratoconjunctivitis and herpes simplex virus type 1. In addition, there is the potential for transmitting more serious diseases, such as hepatitis and acquired immunodeficiency syndrome (AIDS) (183, 184), although there is no evidence to suggest transmission of HIV by contact with tears.

Various techniques have been described for disinfecting tonometer tips (185, 186). Adenovirus type 8 was removed or inactivated by soaking the applanation tip for 5 to 15 minutes in diluted sodium hypochlorite (1:10 household bleach), 3% hydrogen peroxide, or 70% isopropyl alcohol, or by wiping with alcohol, hydrogen peroxide, iodophor (povidone-iodine), or 1:1000 Merthiolate (187). Herpes simplex virus type 1 was eliminated by swabbing the applanation head with 70% isopropyl alcohol (188). Ten minutes of continuous rinsing in running tap water was reported to remove all detectable hepatitis B virus (HBV) surface antigen from contaminated tonometers (183), although another study showed that soap-and-water wash was the only disinfection method that removed all HBV DNA (189). Wiping with 3% hydrogen peroxide or 70% isopropyl alcohol swabs completely disinfected tonometer tips contaminated with HIV-1 (190).

The American Academy of Ophthalmology Clinical statement on infection prevention in eye care services and operating areas and operating rooms (http://one.aao.org/CE/PracticeGuidelines/ClinicalStatements_Content.aspx?cid=bfa87dce-adc9-4450- 94a2-e49493154238) references the guidelines of the U.S. Centers for Disease Control and Prevention (186). With any technique, it is important to carefully remove the disinfectant from the contact surface before the next use, because alcohol and hydrogen peroxide each cause transient corneal defects. Other Applanation Tonometers with Variable Force

The Maklakoff applanation tonometer was once popular in Russia and consisted of a dumbbell-shaped metal cylinder; it had a 10-mm diameter flat endplate of polished glass on either

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end. A set of four such instruments were available, weighing 5, 7.5, 10, and 15 g. A dye suspension of Argyrol, glycerin, and water was applied to either endplate and, with the patient in a supine position and the cornea anesthetized, the instrument rested vertically on the cornea for 1 second. The resultant circular white imprint on the endplate corresponded to the area of cornea that was flattened. The diameter of the white area is measured with a transparent plastic measuring scale to 0.1 mm, and the IOP is read from a conversion table in the column corresponding to the weight used (191).

Figure 2.7 Applanation tonometry using the Perkins tonometer.

Although not commonly used now, the Perkins applanation tonometer uses the same biprism as the Goldmann applanation tonometer (192). The light source is powered by a battery and the force is varied manually. A counter balance makes it possible to use the instrument in either the vertical or horizontal position (Fig. 2.7). The Draeger applanation tonometer is similar to the Perkins tonometer, but uses a different biprism and has an electric motor that varies the force (193).

The original Mackay-Marg tonometer, which is no longer available, had a plate diameter of 1.5 mm surrounded by a rubber sleeve. The force required to keep the plate flush with the sleeve was electronically monitored and recorded on a paper strip (194). The most commonly used Mackay-Marg- type tonometer today is the Tono-Pen, a handheld instrument with a strain gauge that creates an electrical signal as the footplate flattens the cornea (195) (Fig. 2.8). A built-in single-chip microprocessor senses the proper force curves and averages 4 to 10 readings to give a final digital readout. It also provides the percentage of variability between the lowest and highest acceptable readings from 5% to 20%.

The pneumotonometer is similar to the Mackay-Marg in that a central sensing device measures the IOP, while the force required to bend the cornea is transferred to a surrounding structure. The sensor in this case, however, is air pressure, rather than an electronically controlled plunger (196). At one end of a pencil-like holder is a sensing nozzle, which has a 0.25-inch outer diameter and a 2.0-mm central chamber. The nozzle is covered with a Silastic diaphragm, and pressurized air in the central chamber exhausts at the face of the nozzle between the orifice of the central chamber and the diaphragm. As the sensing nozzle touches the cornea and when the area of contact equals that of the central chamber, an initial inflection is recorded, which represents the IOP and the force required to bend the cornea (Fig. 2.9). With further enlargement of the corneal contact, the bending force is transferred to the face of the nozzle, which is interpreted as the actual IOP.

Figure 2.8 Technique of measuring IOP with handheld Tono-Pen.

A newer applanation tonometer with a disposable cover, called the PASCAL tonometer, is available (Fig. 2.10). It repeatedly samples IOP 100 times per second in addition to ocular pulse amplitude and the systemic pulse rate (197). This portable slitlamp mounted device provides a digital output of the IOP and a graphic output of the ocular pressure pulse.

The noncontact tonometer was introduced by Grolman (198) and has the advantage over other tonometers of not touching the eye, other than with a puff of air. This instrument should not be confused with the pneumatic tonometers discussed earlier that require eye contact. After proper alignment of the patient, a puff of room air creates a constant force that momentarily deforms the central cornea, which is detected by an optoelectronic system of a transmitter, which directs a

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collimated beam of light at the corneal vertex, and a receiver and detector, which accepts only parallel, coaxial rays reflected from the cornea. At the moment that the central cornea is flattened, the greatest number of reflected light rays are received, which is recorded as the peak intensity of light detected. The time from an internal reference point to the moment of maximum light detection is converted to IOP. With the newer instrument, additional data is provided on cornea hysteresis, which may be an indication of elasticity (199). The time interval for an average noncontact tonometer measurement is 1 to 3 milliseconds (1/500th of the cardiac cycle) and is random with respect to the phase of the cardiac cycle so that the ocular pulse becomes a significant variable—that is, unlike with some tonometers, it canno t be averaged. The probability that an instantaneous pressure measurement will lie within a given range of mean IOP increases as the number of tonometric measurements, averaged together, increases (200). For this reason, it is recommended that a minimum of three readings within 3 mm Hg be taken and averaged as the IOP.

Figure 2.9 IOP measurement using a pneumotonometer.

Figure 2.10 Measurement of IOP using the Pascal Dynamic Contour Tonometer. Schiötz Indentation Tonometry

The prototype indentation tonometer is the Schiötz tonometer, which consists of a footplate that rests on the cornea and a weighted plunger that moves freely (except for the effect of friction) within a shaft in the footplate with the degree to which it indents the cornea is indicated by the movement of a needle on a scale. A 5.5-g weight is permanently fixed to the plunger, which can be increased to 7.5,10, or 15 g by adding additional weights (Fig. 2.11). When the plunger indents the cornea, the baseline or resting pressure (P0) is artificially raised to a new value (Pt). The change in pressure from P0 to Ptis an

expression of the resistance an eye offers to the displacement of a volume of fluid (Vc). Because the tonometer actually measures Pt, it is necessary to estimate P0 for each scale reading and weight. Schiötz estimated P0 by experiments in which a manometer was attached to enucleated eyes by a cannula inserted through the optic nerve.

Figure 2.11 Technique of IOP measurement using Schiötz indentation tonometer.

In the early days of indentation tonometry, the IOP values that were considered to be normal were considerably higher than today's accepted range, and it was not until Friedenwald's work that indentation tonometry acquired a mathematical basis (201). The formula has a single numerical constant, the coefficient of ocular rigidity (K), which is roughly an expression of the distensibility of the eye. He developed a nomogram for estimating K on the basis of two tonometric readings with different weights,

and subsequent studies using applanation tonometry with different sized applanating areas have supported the accuracy of his formulations (202). On the basis of this formula and additional experiments, Friedenwald developed a set of conversion tables, referred to as the 1948 and 1955 tables for IOP. Subsequent studies indicated that the 1948 tables agree more closely with measurements by Goldmann applanation tonometry (203, 204).

The basic technique involves positioning the patient in a supine position with a fixation target just overhead. The examiner separates the eyelids and gently rests the tonometer footplate on the anesthetized cornea in a position that allows free vertical movement of the plunger. When the tonometer is properly positioned, the examiner observes a fine movement of the indicator needle on the scale in response to the ocular pulsations. The scale reading should be taken as the average between the extremes of these excursions. It is customary to start with the fixed 5.5-g weight. However, if the scale reading is 4 or less, additional weight should be added to the plunger. A conversion table is then used to derive the IOP in mm Hg from the scale reading and plunger weight. Grant combined the concept of Schiötz tonometry with continuous electronic monitoring of the pressure for use in tonography (discussed in Chapter 3).

It is important to be aware of the potential sources of error with indentation tonometry. The accuracy depends on the assumption that all eyes respond the same way to the external

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force of indentation, which is not the case. Because conversion tables were based on an “average” coefficient of ocular rigidity (K), eyes that deviate significantly from this K value give false IOP measurements. The technique for determining K is based on the concept of differential tonometry, using two indentation tonometric readings with different weights, and the Friedenwald nomogram, as previously discussed. Another variable that affects accuracy is expulsion of intraocular blood during indentation tonometry (205). In addition, a relatively steep or thick cornea causes an increased displacement of fluid during indentation tonometry, which leads to a falsely high IOP reading (206). Miscellaneous Tonometers

Rebound Tonometer

A new handheld tonometer, the Icare tonometer (Icare Finland, Helsinki) is able to measure IOP without the use of topical anesthetic (Fig. 2.12). IOP is determined by measuring the force produced by a small plastic probe as it rebounds from the cornea. This device has been assessed for use in children and adults. The rebound tonometer has been shown to have similar accuracy to the Tono-Pen, and it is comparable with Goldmann tonometry for IOPs over a reasonable range in adults. Icare was reported to be comfortable and highly reproducible for tonometry in healthy school-aged children (207). The Icare tonometer has already proven valuable as a screening tool in children (see Chapter 13). The ability to evaluate IOP without the use of topical anesthesia potentially provides the opportunity to monitor IOP at home.

IOP Monitoring Devices

In the diagnosis and management of glaucoma, there is need for an IOP telemetry device without artificially altering the pressure (208, 209). Several prototypes—based on a contact lens, an implanta ble device, or a scleral band device (210, 211)—have be en developed. Such a lens will help us monitor and manage individuals who are susceptible to wide IOP fluctuations, who have poor adherence to medical therapy, who perhaps are “poor responders” to medic al therapy, and who have wide IOP fluctuations in the postoperative period (212).