Clinical Measurement of IOP
Tonometry is the noninvasive measurement of IOP. Many different methods of tonometry have been developed, and each has advantages and disadvantages. All currently used methods have sources of error, and no device can accurately measure intracameral pressure in all eyes.
Applanation tonometry, the most widely used method, is based on the Imbert-Fick principle, which states that the pressure inside an ideal dry, infinitely thin-walled sphere equals the force necessary to flatten its surface divided by the area of the flattening:
P = F/A
where P = pressure, F = force, and A = area. In applanation tonometry, the cornea is flattened, and IOP is determined by measuring the applanating force and the area flattened.
The Goldmann applanation tonometer (Fig 5-2) measures the force necessary to flatten an area of the cornea 3.06 mm in diameter. At this diameter, the material resistance of the cornea to flattening is counterbalanced by the capillary attraction of the tear film meniscus to the tonometer head. Furthermore, the IOP (in mm Hg) equals the flattening force (in gram-force) multiplied by 10. A split-image prism allows the examiner to determine the flattened area with great accuracy. Topical anesthetic and fluorescein dye are instilled in the tear film to outline the area of flattening. Fluorescein semicircles, or mires, visible through the split-image prism move with the ocular pulse, and the endpoint is reached when the inner edges of the semicircles touch each other at the midpoint of their excursion (Fig 6-2). By properly aligning the mires, the examiner can ensure the appropriate area of corneal applanation to obtain the most accurate IOP reading.
Figure 2-5 Goldmann-type applanation tonometry. A, Basic features of the tonometer, shown in contact with the patient’s cornea. B, The enlargement shows the tear film meniscus created by contact of the split-image prism and cornea. C, The view through the split-image prism (1) reveals a circular meniscus (2), which is converted into 2 semicircles (3) by the prisms.
(Redrawn with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Williams & Wilkins; 1992.)
The Perkins tonometer is a counterbalanced applanation tonometer that, like the Goldmann tonometer, uses a split-image prism and requires instillation of fluorescein dye in the tear film. It is portable and can be used with the patient either upright or supine.
Applanation measurements are safe, easy to perform, and relatively accurate in most clinical situations. Of the currently available devices, the Goldmann applanation tonometer is the most widely used in clinical practice and research. Because applanation does not displace much fluid (approximately 0.5 μL) or substantially increase the pressure in the eye, IOP measurement by this method is relatively unaffected by ocular rigidity, compared with indentation tonometry (see the section “Indentation tonometry”).
Figure 2-6 Mires viewed through the split-image prism of the Goldmann-type applanation tonometer. A, Proper width and position. The enlargement depicts excursions of the mires, which are caused by ocular pulsations. B, The mires are too wide. C, Improper vertical and horizontal alignment.
(Reproduced with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Williams & Wilkins; 1992.)
The accuracy of applanation tonometry can be reduced in certain situations (see Table 2-2, which lists possible sources of error in tonometry). An inadequate amount of fluorescein can lead to poor visualization of the mires and the measurement endpoint, resulting in inaccurate readings. Tear film thickness can also affect accuracy. A common misconception is that a thick tear film leads to erroneous readings because of excessively thick mires, which require greater force to reach the measurement endpoint. However, this is incorrect, as the inner edges of the mires will touch when an area 3.06 mm in diameter is applanated, regardless of the mire thickness. Rather, the error occurs because the surface tension changes with tear film thickness. Because the cornea is curved, an increasing tear film thickness will form a meniscus with a larger radius of curvature, which has a lower surface tension. The low surface tension inadequately counterbalances the corneal resistance, resulting in an artificially high IOP reading. Conversely, a very thin tear film has a small radius of curvature with a high surface tension that exceeds the corneal resistance, yielding a falsely low IOP reading.
Marked corneal astigmatism can also produce inaccuracy, as the fluorescein pattern seen by the clinician through the instrument appears elliptical, and the IOP may be artificially high or low. To obtain an accurate reading in an astigmatic eye, the clinician should rotate the prism so that the red mark on the prism holder is set at the least-curved meridian of the cornea (along the negative axis). Alternatively, 2 pressure readings taken 90° apart can be averaged. Corneal edema predisposes to inaccurately low readings, whereas pressure measurements taken over a corneal scar will be falsely high. Tonometry performed over a soft contact lens gives artificially low values. Central corneal thickness is another factor that can affect the accuracy of tonometry; see the following section.
Table 2-2 Possible Sources of Error in Tonometry
Tonometry and central corneal thickness
Measurements obtained with the most common types of tonometers are affected by central corneal thickness (CCT). Goldmann tonometer readings are most accurate when the CCT is 520 μm. Thicker corneas resist the deformation inherent in most methods of tonometry, resulting in an overestimation of IOP, while thinner corneas may give an artificially low reading. IOP may also be underestimated after laser or other types of keratorefractive surgery if these procedures resulted in a CCT significantly less than that assumed for Goldmann tonometry.
The relationship between measured IOP and CCT is not linear, so correction factors are only estimates at best. In addition, the biomechanical properties of individual corneas may vary, and the stiffness or elasticity of the cornea may affect IOP measurement. Currently, there is no validated correction factor for the effect of CCT on applanation tonometers; therefore, the correction methods proposed in the literature are not applicable to clinical use. A thin central cornea is a known risk factor for progression from ocular hypertension to glaucoma. However, it has not been determined whether this increased risk of glaucoma is attributable only to underestimation of IOP or whether a thin central cornea is a biomarker for another risk factor independent of IOP measurement (see Chapter 4).
Gordon MO, Beiser JA, Brandt JA, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002; 120(6):714–720.
Mackay-Marg–type tonometers use an annular ring to gently flatten a small area of the cornea. As the area of flattening increases, the pressure in the center of the ring increases as well and is measured with a transducer. The IOP is equivalent to the pressure when the center of the ring is just covered by the flattened cornea.
Portable electronic devices of the Mackay-Marg type (eg, Tono-Pen, Reichert Technologies; Fig 2-7A) contain a strain gauge to measure the pressure at the center of an annular ring placed on the cornea. The instrument tip is tapped against the surface of the cornea, obtaining several measurements that are then used by the device to provide an IOP measurement along with the coefficient of variation. These devices are particularly useful in patients with corneal scars or edema, as the measurement tip is small enough to be applied only on areas of normal cornea. It also can be used to obtain measurements regardless of the patient’s body position. However, some studies suggest that the Tono-Pen tends to overestimate low IOPs and underestimate high IOPs.
Eisenberg DL, Sherman BG, McKeown CA, Schuman JS. Tonometry in adults and children. A manometric evaluation of pneumatonometry, applanation, and TonoPen in vitro and in vivo. Ophthalmology. 1998;105(7):1173–1181.
Mackay RS, Marg E. Fast, automatic, electronic tonometers based on an exact theory. Acta Ophthalmol (Copenh). 1959;37:495–507.
The pneumatic tonometer, or pneumatonometer, is an applanation tonometer that shares some characteristics with the Mackay-Marg–type devices (Fig 2-7B). It has a cylindrical air-filled chamber and a probe tip covered with a flexible, inert silicone elastomer (Silastic membrane) diaphragm. Because of the constant flow of air through the chamber, there is a small gap between the diaphragm and the probe edge. As the probe tip touches and applanates the cornea, the air pressure increases until this gap is completely closed, at which point the IOP is equivalent to the air pressure. Similar to the Tono-Pen, this instrument makes contact with only a small area of the cornea and is especially useful in eyes with corneal scars or edema and can be used with the patient in sitting, lateral decubitus, or supine body positions. In addition, measurements obtained by placing the probe on the sclera appear to correlate well with those obtained with the probe placed on the cornea, suggesting that it can be used to monitor IOP in patients with keratoprostheses. The pneumatonometer can also record IOP continuously while the probe is on the eye and can be used for tonography.
Durham DG, Bigliano RP, Masino JA. Pneumatic applanation tonometer. Trans Am Acad Ophthalmol Otolaryngol. 1965;69(6):1029–1047.
Kapamajian MA, de la Cruz J, Hallak JA, Vajaranant TS. Correlation between corneal and scleral pneumatonometry: an alternative method for intraocular pressure measurement. Am J Ophthalmol. 2013;156(5):902–906.e1.
Noncontact (air-puff) tonometers determine IOP by measuring the force of air required to indent and flatten the cornea, thereby avoiding contact with the eye. IOP is measured when the amount of corneal indentation corresponds with the maximum amount of light reflection from the cornea. Readings obtained with these instruments vary widely, and IOP is often overestimated. Noncontact tonometers are often used in large-scale glaucoma-screening programs or by nonmedical health care providers.
The Ocular Response Analyzer (ORA; Reichert Technologies; Fig 2-7C) is a type of noncontact tonometer that uses correction algorithms so that its IOP readings more closely match those obtained with applanation techniques, and the effect of corneal biomechanical properties on pressure measurement is reduced. One study reported that the corneal compensated IOP (IOPcc) has a stronger correlation with glaucoma progression than does IOP measured with Goldmann or rebound tonometry. In addition, indicators of ocular biomechanical properties are calculated, including corneal hysteresis. During the measurement process, the cornea is indented beyond the IOP measurement point. Corneal hysteresis is the difference between IOP measured during the initial corneal indentation and IOP measured during corneal rebound. Reduced corneal hysteresis has been associated with an increased risk of developing visual field defects in glaucoma suspects and with disease progression in patients with confirmed glaucoma.
Figure 2-7 Nonapplanation tonometers in common use offer advantages and disadvantages compared with Goldmann applanation tonometry. A, Tono-Pen. B, Pneumatonometer. C, Ocular Response Analyzer. D, Rebound tonometer.
(Courtesy of Arthur J. Sit, MD.)
Medeiros FA, Meira-Freitas D, Lisboa R, Kuang TM, Zangwill LM, Weinreb RN. Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study. Ophthalmology. 2013;120(8):1533–1540.
Susanna BN, Ogata NG, Daga FB, Susanna CN, Diniz-Filho A, Medeiros FA. Association between rates of visual field progression and intraocular pressure measurements obtained by different tonometers. Ophthalmology. 2019;126(1):49–54.
Susanna CN, Diniz-Filho A, Daga FB, et al. A prospective longitudinal study to investigate corneal hysteresis as a risk factor for predicting development of glaucoma. Am J Ophthalmol. 2018;187:148–152.
Rebound tonometry determines IOP by measuring the speed at which a small probe propelled against the cornea decelerates and rebounds after impact (Fig 2-7D). Rebound tonometers are portable, and topical anesthesia is not required; these characteristics make them particularly suitable for use in pediatric populations and for home tonometry in some patients. However, rebound tonometry is strongly influenced by central corneal thickness, even when compared with applanation tonometry. Thus, care must be taken in the interpretation of IOP measurements in eyes with thick or thin corneas.
Kontiola AI. A new induction-based impact method for measuring intraocular pressure. Acta Ophthalmol Scand. 2000;78(2):142–145.
Rao A, Kumar M, Prakash B, Varshney G. Relationship of central corneal thickness and intraocular pressure by iCare rebound tonometer. J Glaucoma. 2014;23(6):380–384.
Dynamic contour tonometer
The dynamic contour tonometer, a newer type of nonapplanation contact tonometer, is based on the principle that when the surface of the cornea is aligned with the surface of the instrument tip, the pressure in the tear film between these surfaces is equal to the IOP and can be measured by a pressure transducer. The tip is similar in shape and size to an applanation tonometer tip, with a pressure transducer placed in the center, and enables measurement of ocular pulse amplitude in addition to IOP. Evidence suggests that IOP measurements obtained with dynamic contour tonometry may be more independent of corneal biomechanical properties and thickness than those obtained with applanation.
Schiøtz tonometry determines IOP by measuring the amount of corneal indentation produced by a known weight. Measurements are taken with the patient supine, and the amount of indentation is read on a linear scale on the instrument and converted to IOP by a calibration table. However, indentation of the cornea results in a significant volume change in the globe, the amount of which depends on the IOP and the ocular rigidity. The assumption of average ocular rigidity in the calibration tables makes the accuracy of Schiøtz tonometry highly dependent on ocular biomechanical properties. Although Schiøtz tonometry is now rarely used in the developed world, it remains the only form of tonometry that does not require electrical power.
It is possible to estimate IOP by digital pressure on the globe, referred to as tactile tension. This test may be useful in uncooperative patients; however, the results may be inaccurate even when the test is performed by very experienced clinicians. In general, tactile tensions are useful only for detecting large differences in IOP between a patient’s 2 eyes.
Infection control in clinical tonometry
Many infectious agents—including HIV, hepatitis C virus, and adenovirus—can be recovered from tears. Tonometers must be cleaned after each use to prevent the transfer of such pathogens. For Goldmann-type and Perkins tonometers, the tonometer tips (prisms) should be cleaned immediately after use. The ideal method for disinfection is controversial. Sodium hypochlorite (dilute bleach) offers effective disinfection against adenovirus and herpes simplex virus, the viruses commonly associated with nosocomial outbreaks in eye care. In contrast, alcohol wipes or soaks do not appear to be effective at eradicating all infectious virions. The efficacy of other disinfection protocols has not been adequately evaluated. The prism head should be rinsed with water and dried before reuse to prevent damage to the corneal epithelium. Single-use disposable applanation tonometer tips may be a useful alternative to cleaning. For cleaning other tonometers, refer to the manufacturer’s recommendations.
Junk AK, Chen PP, Lin SC, et al. Disinfection of tonometers: a report by the American Academy of Ophthalmology. Ophthalmology. 2017;124(12):1867–1875.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.