Although several risk factors are associated with the development and progression of glaucoma, IOP remains the only modifiable risk factor, making its monitoring crucial for staving off disease progression and, thus, vision loss. (See “IOP: An Overview,” p.24.)

There are several ways to measure IOP. This article describes the various types of tonometers, in alphabetical order, and their applicability for certain patient populations. This way, you can decide on the one or more tonometers best suited for your practice.

APPLANATION TONOMETER

An applanation tonometer is based on the Imbert-Fick principle, which states that the pressure inside a sphere is equal to the force needed to flatten its surface, divided by the area of flattening. An applanation tonometer measures the applanation force required to flatten a corneal diameter of 3.06 mm. At 3.06 mm, 1 gram of applanation force is equal to 10 mm Hg.¹ The diameter of 3.06 mm is reached when the edges of the inner mires touch.

An applanation tonometer requires a topical anesthetic, fluorescein dye, and a slit lamp. Capillary action of the tears pull the tonometer tip onto the cornea, which balances the corneal resistance. When the width of the tear meniscus is thick, too much fluorescein can overestimate the true IOP level, while mires that are too thin (too little fluorescein) may underestimate the true IOP level.

Central corneal thickness (CCT) can affect the accuracy of the IOP measurements, as thick and thin corneas lead to overestimation and underestimation, respectively. Studies measuring the CCT of different populations who do not have corneal pathology show mean CCT for specific populations range between 510 μm and 560 μm, with most closer to 530 μm to 550 μm.² CCT outside this range should be noted, and taken into account when considering an IOP measurement.

Additionally, applanation tonometry may inaccurately reflect the IOP if corneal irregularities, corneal scarring, and pressure applied on the eyelids during lid pinning occur.

A portable, handheld applanation tonometer that uses the same principle as Goldmann applanation tonometry and yields comparable IOP measurements to the Goldmann is available for patients unable to sit upright or position themselves in the slit lamp.

DYNAMIC CONTOUR TONOMETER

A dynamic contour tonometer (DCT) measures IOP independent of corneal properties. It is based on contour matching and uses a tonometer tip that has a concave surface. This allows the cornea to assume a shape without any external forces acting on the area of the cornea touching the tip. Therefore, the pressure is equal on the inside and outside, which is measured by a pressure sensor.

Because DCT is not affected by CCT, it can be suitable for patients who have thinner or thicker CCTs than normal, or patients who have had a history of LASIK, photorefractive keratectomy, and keratoconus, among other corneal conditions.

INDENTATION TONOMETER

An indentation tonometer is based on the principle that force will indent a soft eye, or one that has a lower IOP, further than a hard eye, or one that has a higher IOP.

One type of indentation tonometer uses a pneumatic pump to indent the cornea using a silicone tip. Once the cornea and silicone tip are both flat, the pressure pushing forward on the tip is equal to the IOP, and is displayed in mm Hg. This type of tonometer can be used on soft contact lens wearers sans lens removal. Specifically, IOP can be reliably measured with a pneumatic pump indentation tonometer over myopic and hyperopic soft contact lenses that have a mild lens power.³

Another type of indentation tonometer uses a combination of indentation and applanation tonometry. A plunger is connected to a strain gauge. As the small plunger on the tonometer contacts the cornea, there is resistance on the plunger, which increases force on the strain gauge. When the cornea is applanated, there is a momentary decrease in the increasing force, which is then converted to IOP in mm Hg.⁴

The last type of indentation tonometer uses a plunger attached to a weight. This tonometer is held vertically as patients lay in the supine position. It uses gravity to move the plunger down to indent the cornea. A conversion table is used to convert the reading into mm Hg. This tonometer is durable and does not have any electronic components. It is suitable for remote clinics.⁵

All three instruments are portable and require the cornea to be anesthetized.

NON-CONTACT TONOMETER

A non-contact tonometer (NCT), also known as air-puff tonometry, uses a column of air that increases in force to flatten the cornea. The force required to flatten the cornea is detected by sensors and is then converted to mm Hg.⁶

NCT does not require topical anesthesia or fluorescein dye, making it useful as a quick screening tool.

Various measurements can be obtained in one sitting, as NCT is available in combination with an autorefractor, auto-keratometer, and non-contact pachymeter. This maximizes efficiency, patient comfort, and frees up valuable office space for other technology.

Some patients are apprehensive about the “air puff,” which may result in an artificially high reading from a Valsalva maneuver.

REBOUND TONOMETER

A rebound tonometer propels a small, disposable plastic-tipped metal probe toward the cornea using electromagnetic fields. Its mechanism of action is based on the theory that eyes that have a higher IOP will decelerate the probe more rapidly than eyes that have a lower IOP. The speed of deceleration is then converted into mm Hg.^6,7 Since the probe is disposable, it decreases the risk of cross-infection and does not need disinfection between patient use.

A rebound tonometer is portable and battery-powered, making it a useful tool for house calls, nursing homes, and for patients to measure their IOP at home. Also, it does not require the cornea to be anesthetized, making it suitable for children and non-cooperative adults, who are drop-adverse.

DYNAMIC BI-DIRECTIONAL APPLANATION TONOMETER

This tonometer measures both corneal hysteresis and corneal compensated IOP. Hysteresis is derived from an Ancient Greek word meaning “lagging behind.” Corneal hysteresis is the difference in the ability of the cornea to deform and reform when a force is applied. It is a risk factor of glaucoma progression. Eyes that have lower corneal hysteresis values have been shown to have faster rates of visual field loss.^8-9

The American Academy of Ophthalmology recently released a report on corneal hysteresis, which appeared in a recent issue of Ophthalmology.¹⁰ Specifically, the AAO states that while corneal hysteresis is lower in glaucoma patients vs. those without the condition, a causal relationship must be demonstrated, so corneal hysteresis measurement “complements current structural and functional assessments in determining disease risk in glaucoma suspects and patients.”

The dynamic bi-directional applanation tonometer is based on similar principles to noncontact tonometry. Topical anesthesia and fluorescein are not required, resulting in a rapid measurement. The dynamic bi-directional applanation tonometer is useful as another data point in patients with glaucoma.

CHOICES ABOUND

Technological advances in tonometry, illustrated above, now afford optometrists the ability to choose the tonometer best suited for the patient based on their ocular history and personal characteristics. Which one or more devices will you employ this year? Send your answers or comments to Jennifer.Kirby@broadcastmed.com. OM

REFERENCES

1. Goldmann H, Schmidt T. Applanation Tonometry. Ophthalmologica. 1957;134(4):221-242. doi:10.1159/000303213.
2. Clement C, Parker D, Goldberg I. Intraocular pressure measurement in a patient with a thin, thick, or abnormal cornea. Open J Ophthalmol. 2016;10: 35-43. doi:10.2174/1874364101610010035
3. Schollmayer P, Hawlina M. Effect of soft contact lenses on the measurements of intraocular pressure with non-contact pneumotonometry. Klin Monbl Augenheilkd. 2003;220(12): 840-842. doi:10.1055/S-2003-812564.
4. Aziz K, Friedman DS. Tonometers — which one should I use? Eye (Lond). 2018;32(5):931-937. doi:10.1038/S41433-018-0040-4.
5. Cordero I. Understanding and caring for a Schiotz tonometer. Community Eye Health J. 2014;27(87): 57.
6. Farhood QK. Comparative evaluation of intraocular pressure with an air-puff tonometer versus a Goldmann applanation tonometer. Clin Ophthalmol. 2013;7(1):23-27. doi:10.2147/OPTH.S38418.
7. Liu J, de Francesco T, Schlenker M, Ahmed II. Icare Home tonometer: A review of characteristics and clinical utility. Clin Ophthalmol. 2020;14:4031-4045. doi:10.2147/OPTH.S284844.
8. Kaushik S, Pandav SS. Ocular Response Analyzer. J Curr Glaucoma Pract. 2012:6(1): 17-19. Doi:10.5005/jp-journals-10008-1103.
9. Medeiros FA, et al. Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study. Ophthalmology. 2013;120(8): 1533-1540. doi:10.1016/j.ophtha.2013.01.032.
10. Sit AJ, Chen TC, Takusagawa HL, et al. Corneal hysteresis for the diagnosis of glaucoma and assessment of progression risk: a report by the American Academy of Ophthalmology. Ophthalmology. 2022 16;S0161-6420(22)00901-0. doi:10.1016/j.ophtha.2022.11.009.
11. Machiele R, Motlagh M, Patel BC. Intraocular Pressure. Drug Discovery and Evaluation: Pharmacological Assay, Fourth Edition. Published online July 11, 2022:3749-3752. doi:10.1007/978-3-319-05392-9_85.
12. Sit AJ. Chen TC, Takusagawa HL, et al. Corneal Hysteresis for the Diagnosis of Glaucoma and Assessment of Progression Risk: A Report by the American Academy of Ophthalmology. Ophthalmology. 2022;S0161-6420(22)00901-0. doi: 10.1016/j.ophtha.2022.11.009.

DR. TOWNSEND is an optometrist at Bascom Palmer Eye Institute in Miami, Fla. She is the Director of Rapid Virtual Eye Care and Director of the Optometric Ocular Disease Residency Program. She provides care in the ocular emergency department, comprehensive optometry, virtual eye care, and sports vision

DR. TRAN is an Ocular Disease resident at Bascom Palmer Eye Institute in Miami, Fla. She graduated from the University of California School of Optometry in 2022. She has also completed her MS in Chemistry at the University of Toronto.