Ocular tonometry

This article is about Ocular tonometry. For Gastric Tonometry, see Gastric tonometry.

Ocular tonometry
Intervention
A patient in front of a tonometer
ICD-9-CM	89.11
MeSH	D014065

Tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg).

Methods

The slit lamp of a Goldmann tonometer

Semicircles seen during Goldmann tonometry through slit lamp

The PASCAL Dynamic Contour Tonometer

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Diaton Transpalpebral Tonometer

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Applanation tonometry

In applanation tonometry the intraocular pressure is inferred from the force required to flatten (applanate) a constant area of the cornea, as per the Imbert-Fick law.^[1] The Maklakoff was an early example of this method, while the Goldmann tonometer is the most widely used version in current practice. Because the probe makes contact with the cornea, a topical anesthetic, such as proxymetacaine is introduced on to the surface of the eye in the form of an eye drops.

Goldmann tonometry

Goldmann tonometry is considered to be the gold standard test and is the most widely accepted method.^[2][3] A special disinfected prism is mounted on the tonometer head and then placed against the cornea. The examiner then uses a cobalt blue filter to view two green semi circles. The force applied to the tonometer head is then adjusted using a dial connected to a variable tension spring until the inner edges of the green semicircles in the viewfinder meet. When an area of 3.06mm has been flattened, the opposing forces of corneal rigidity and the tear film are roughly approximate and cancel each other out allowing the pressure in the eye to be determined from the force applied. Like all non-invasive methods, it is inherently imprecise.^[4]

Perkins tonometer

The Perkins is a type of portable applanation tonometer, useful in children, patients unable to co-operate with a sitting slit lamp examination or in anesthetised patients who need to lie flat.

Dynamic Contour tonometry

Dynamic contour tonometry (DCT)(PASCAL Tonometer)uses the principle of contour matching instead of applanation. The tip contains a hollow the same shape as the cornea with a miniature pressure sensor in its centre. In contrast to applanation tonometry it is designed to avoid deforming the cornea during measurement and is therefore thought to be less influenced by corneal thickness and other biomechanical properties of the cornea than other methods but because the tip shape is designed for the shape of a normal cornea, it is more influenced by corneal curvature.^[5]

The probe is placed on the pre-corneal tear film on the central cornea (see gallery) and the integrated piezoresistive pressure sensor automatically begins to acquire data, measuring IOP 100 times per second. The tonometer tip rests on the cornea with a constant appositional force of one gram. When the sensor is subjected to a change in pressure, the electrical resistance is altered and the PASCAL's computer calculates a change in pressure according to the change in resistance. A complete measurement cycle requires about 8 seconds of contact time. The device also measures the variation in pressure that occurs with the cardiac cycle.^[6][7][8]

Non-contact tonometry

Non-contact tonometry (or air-puff tonometry) is different from pneumatonometry and was invented by Bernard Grolman of Reichert, Inc (formerly American Optical). It uses a rapid air pulse to applanate (flatten) the cornea. Corneal applanation is detected via an electro-optical system. Intraocular pressure is estimated by detecting the force of the air jet at the instance of applanation.^[9] Historically, Non-contact tonometers were not considered to be an accurate way to measure IOP but instead a fast and simple way to screen for high IOP. However, modern non-contact tonometers have been shown to correlate well with Goldmann tonometry measurements and are particularly useful for measuring IOP in children and other non-compliant patient groups.

Ocular response analyzer

The ocular response analyser (ORA) is a non-contact (air puff) tonometer that does not require topical anaesthesia and provides additional information on the biomechanical properties of the cornea. It uses an air pulse to deform the cornea in to a slight concavity. The difference between the pressures at which the cornea flattens inward and outward is measured by the machine and termed corneal hysteresis (CH). The machine uses this value to correct for the effects of the cornea on measurement.^[10]

Electronic indentation tonometry

The Tono-Pen (Reichert, Inc) and AccuPen (Accutome, Inc) are modified Mackay-Marg tonometers that use a free floating transducer to detect the transmitted pressure. The transducer is surrounded by an outer ring that flattens the adjacent cornea reducing its influence on measurement. Because the device touches the cornea, Topical anesthetic eye drops are used to numb the eye but as with non-contact tonometry, these devices are often used in children and non-cooperative patients because of their portability and ease of use. Portable electronic tonometers also play a major role in veterinary tonometry.

Rebound tonometry

Rebound tonometers (Icare Tonometer ^[11]) determine intraocular pressure by bouncing a small plastic tipped metal probe against the cornea. The device uses an induction coil to magnetise the probe and fire it against the cornea. As the probe bounces against the cornea and back in to the device, it creates an induction current from which the intraocular pressure is calculated. The device is simple and easy to use and self use versions are available. It is portable, does not require the use of eye drops and is particularly suitable for children and non-cooperative patients.

Pneumatonometry

A pneumatonometer utilizes a pneumatic sensor (consisting of a piston floating on an air bearing). Filtered air is pumped into the piston and travels through a small (5-mm dia.) fenestrated membrane at one end. This membrance is placed against the cornea. The balance between the flow of air from the machine and the resistance to flow from the cornea affect the movement of the piston and this movement is used to calculate the intra-ocular pressure.

Impression tonometry

Impression tonometry (also known as indentation tonometry) measures the depth of corneal indentation made by a small plunger carrying a known weight. The higher the intraocular pressure, the harder it is to push against and indent the cornea. For very high levels of IOP, extra weights can be added to make the plunger push harder.^[12] The movement of the plunger is measured using a calibrated scale.^[12] The Schiötz tonometry is the most common device to use this principle.

Non-corneal and Transpalpebral tonometry

Diaton tonometer^[13] (BiCOM, Inc) Transpalpebral tonometry refers to methods of measuring intraocular pressure through the Eyelid. The Diaton non-corneal tonometer calculates pressure by measuring the response of a free falling rod, as it rebounds against the tarsal plate of the eyelid through the sclera. The patient is positioned so that the tip of the device and lid are overlying sclera.^[14] Non-corneal and transpalpebral tonometry does not involve contact with the cornea and does not require topical anesthetic during routine use. The tonometer is not a substitute or alternative for more established methods.^[15]

Palpation

Palpation (also known as digital tonometry) is the method of estimating intraocular pressure by gently pressing your index finger against the cornea of a closed eye. This method is notoriously unreliable.^[16]

OCT Tonometry

Non-contact tonometry using optical coherence tomography (OCT) is currently under development. Like many other forms of tonometry, this methodology relies on a force being applied to the cornea and simultaneous measurement of the corneal reaction. In the case of OCT tonometry, the force applied to the cornea can be air pressure in the form of a high pressure jet (like air-puff tonometry), a shock or acoustic wave, or low pressure air using air pumped into a sealed chamber around the eye (like a snorkeling or scuba mask). The OCT device is used to measure changes in the curvature of the cornea or movement of the apical corneal interface. This deflection can be compared to the pressure measured around the eye and corrected for corneal thickness and potentially, corneal hysteresis.

Influencing factors

Central Corneal Thickness (CCT)

The thickness of the cornea affects most non-invasive methods by varying resistance to the tonometer probe. A thick cornea gives rise to a greater probability of an IOP being over-estimated (and a thin cornea of an IOP being under-estimated), but the extent of measurement error in individual patients cannot be ascertained from the CCT alone.^[17] The Ocular Response Analyzer and Pascal DCT Tonometers are less affected by CCT than the Goldmann tonometer. Conversley, Non-contact and rebound tonometers are more affected.^[18][17][19] Corneal thickness varies among individuals as well as with age and race. It is reduced in certain disease and following LASIK surgery.

Gallery

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  A Goldmann tonometer

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  PASCAL Dynamic Contour Tonometer

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  SensorTip of a PASCAL tonometer in contact with patient's cornea

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  Diaton Tonometry through the Eyelid‎

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  Position of the Diaton Tonometer Tip on the Eyelid with No Contact with Cornea or use of Anesthesia

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  Hand-held Diaton tonometer with Test and Training Eye case

References

1. ^ Goldman H, Schmidt TH.Uber Applanations-tonometrie. Ophthalmologica. 1957; 134: 221-242
2. ^ Amm M, Hedderich J. "[Transpalpebral tonometry with a digital tonometer in healthy eyes and after penetrating keratoplasty.]" Ophthalmologe. 2005 Jan;102(1):70-6. PMID 15322801.
3. ^ Schlote T, Landenberger H. "[Intraocular pressure difference in Goldmann applanation tonometry versus a transpalpebral tonometer TGDc-01'PRA' in glaucoma patients]." Klin Monatsbl Augenheilkd. 2005 Feb;222(2):123-31. PMID 15719316.
4. ^ Groves N (15 September 2006). "Should IOP be adjusted for corneal thickness alone?". Ophthalmology Times. http://www.ophthalmologytimes.com/ophthalmologytimes/article/articleDetail.jsp?id=379903. 
5. ^ Francis BA, Hsieh A, Lai MY, Chopra V, Pena F, Azen S, Varma R; Los Angeles Latino Eye Study Group. Effects of corneal thickness, corneal curvature, and intraocular pressure level on Goldmann applanation tonometry and dynamic contour tonometry. Ophthalmology. 2007 Jan;114(1):20-6.
6. ^ Kaufmann C, Bachmann LM, Thiel M, Comparison of Dynamic Contour Tonometry with Goldmann Applanation Tonometry. Invest Ophthalmol and Vis Sci 2004, Vol.45, No.9, 3118-3121. PMID 15326129
7. ^ Kaufmann C, Bachmann LM, Thiel MA, Intraocular Pressure Measurements Using Dynamic Contour Tonometry after Laser In Situ Keratomileusis. Invest Ophthalmol and Vis Sci 2003;44:3790-3794. PMID 12939293
8. ^ Kniestedt C, Nee M, Stamper RL, Dynamic Contour Tonometry. A Comparative Study on Human Cadaver Eyes. Arch Ophthalmol. 2004; 122:1287–1293. PMID 15364707
9. ^ http://www.mercksource.com^{[dead link]}
10. ^ Felipe A. Medeiros, MD and Robert N. Weinreb, MD. "[Evaluation of the Influence of Corneal Biomechanical Properties on Intraocular Pressure Measurements Using the Ocular Response Analyzer]." J Glaucoma Volume 15, Number 5, October 2006.
11. ^ Icare revolutionarizes IOP control http://www.icaretonometer.com
12. ^ ^{a b} Cline D; Hofstetter HW; Griffin JR. Dictionary of Visual Science. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0
13. ^ http://www.TonometerDiaton.com
14. ^ Dr. Shaun Maria Dacosta, Dr. Babu Rajendran, Dr. Janakiraman P. "Comparison of Diaton Tonometry and Non Contact Tonometry in Indian Subjects" AIOC PROCEEDINGS 2008 Dec; 260
15. ^ Li Y, Shi J, Duan X, Fan F. Transpalpebral measurement of intraocular pressure using the Diaton tonometer versus standard Goldmann applanation tonometry. Graefes Arch Clin Exp Ophthalmol. 2010 Dec;248(12):1765-70. Epub 2010 May 22.PubMed PMID: 20495818.
16. ^ Troost A, Yun SH, Specht K, Krummenauer F, Schwenn O. "Transpalpebral tonometry: reliability and comparison with Goldmann applanation tonometry and palpation in healthy volunteers." Br J Ophthalmol. 2005 Mar;89(3):280-3. PMID 15722303.
17. ^ ^{a b} Robert N. Weinreb, James D. Brandt, David Garway-Heath and Felipe A. Medeiros 2007 "4th Consensus Meeting:Intra-Ocular Pressure"
18. ^ Boehm AG, Weber A, Pillunat LE, Koch R, Spoerl (IOVS) 2008 Jun;49(6):2472-7. Epub (3/2008)"Dynamic Contour Tonometry in Comparison to Intracameral IOP Measurements"
19. ^ Kirstein E, An Update on Methods for Assessing Intraocular Pressure. http://www.pacificu.edu/optometry/ce/courses/16282/interocprespg1.cfm . online CE, 2006

External links

• Tonometry - WebMD
• Tonometry - The Johns Hopkins Consumer Guide to Medical Tests
• Kirstein, E. "An Update on Methods for Assessing Intraocular Pressure." Retrieved June 7, 2006.
• Rebound Tonometer from Icare Finland
• Trans-Scleral Tonometry
• Tonometers in the market
• Tonometer from Keeler Ltd, UK based Company
• Dynamic Contour Tonometry Pascal