View source for Tonometry

[[Image:Patient and tonometer.jpg|thumb|250px|A patient in front of a tonometer]]
'''Tonometry''' is the measurement of tension or pressure <ref>http://www.mercksource.org/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_t_13zPzhtm</ref>. A '''tonometer''' is an instrument for measuring tension or pressure <ref>http://www.mercksource.org/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_t_13zPzhtm</ref>.

In music, a tonometer is an instrument used to determine the pitch or vibration rate of tones, such as a [[tuning fork]].

In [[ophthalmology]], tonometry is the procedure [[eye care professional]]s perform to determine the [[intraocular pressure]] (IOP), the fluid [[pressure]] inside the [[eye]]. It is an important test in the evaluation of patients with [[glaucoma]]. Most tonometers are calibrated to measure pressure in millimeters of mercury ([[mmHg]]).

==Methods ==
[[Image:slit lamp.jpg|thumb|250px|The slit lamp of a Goldmann tonometer]]
[[Image:Goldmann mires.jpg|thumb|250px|Semicircles seen during Goldmann tonometry through slit lamp]]
[[Image:PASCAL tonometer.jpg|thumb|250px|The PASCAL Dynamic Contour Tonometer]]‎ 
[[Image:Diaton Tonometer Pen Doctor Patient Positioning Eyelid.jpg|thumb|250px|Non-corneal Diaton Tonometer Pen, tonometry over Sclera and through the Eyelid]]

;Applanation tonometry
Applanation tonometry is the method that infers the intraocular pressure from the force required to flatten (applanate) a constant area of the cornea, as per the  [[Imbert-Fick law]].<ref name="Goldmann">Goldman H, Schmidt TH.Uber Applanations-tonometrie. Ophthalmologica. 1957; 134: 221-242</ref>. The Maklakoff  was an early example and the Goldmann tonometer is the most widely used version in current practice. 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 the dial until the inner edges of these green semicircles meet. Because the probe makes contact with the cornea, a [[topical anesthetic]], such as [[oxybuprocaine]], [[tetracaine]], [[proparacaine]] (alcaine) or [[proxymetacaine]] is introduced onto the surface of the eye in the form of an [[eye drop]]s. 

;Goldmann tonometry
Goldmann tonometry is considered to be the [[gold standard (test)|gold standard]] in tonometry and is the most widely accepted method of determining intraocular pressure.<ref name="Amm">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.</ref><ref name="Schlote">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.</ref>  By flattening an area of 3.06mm so that the meniscal foces of the tear film become equivalent to those of corneal rigidity and the intraocular pressure can be determined from the force applied. Like all non-invasive methods, it is inherently imprecise<ref>http://www.ophthalmologytimes.com/ophthalmologytimes/article/articleDetail.jsp?id=379903</ref>

;Perkins tonometer
Perkins tonometer is a special type of portable applanation tonometer, which allows measurement of IOP in children, patients unable to cooperate for slit lamp exam, and in anesthetised patients who may be in a supine position. 

;Dynamic contour tonometry
Dynamic contour tonometry (DCT) is a novel method which uses principle of '''contour matching''' instead of applanation. This is designed to reduce the influence of biomechanical properties of the cornea on measurement. These include corneal thickness, rigidity, curvature, and elastic properties.  It is less influenced by corneal thickness but more influenced by corneal curvature than the Goldmann tonometer.<ref name="pascal">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.</ref>

The PASCAL tonometer is currently the only commercial DCT tonometer available. It uses a miniature [[pressure sensor]] embedded within a tonometer tip contour-matched to the shape of the cornea. 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 in concordance with the change in resistance. 

The contour matched tip has a [[wiktionary:Concave|concave]] surface of radius 10.5&nbsp;mm, which approximates to the shape of a normal cornea when the pressure on both sides is equal. 
The probe is placed adjacent to the central cornea (see gallery) and the integrated [[piezoresistive effect|piezoresistive]] [[pressure sensor]] automatically begins to acquire data, measuring IOP 100 times per second. A complete measurement cycle requires about 8 seconds of contact time. During the measurement cycle, audio feedback is generated, which helps the clinician maintain proper contact with the cornea. The device also measures the variation in pressure that occurs with the cardiac cycle. Literature references: <ref name="Kaufmann1">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</ref><ref name="Kaufmann2">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 </ref><ref name="Kniestedt1">Kniestedt C, Nee M, Stamper RL, ''Dynamic Contour Tonometry. A Comparative Study on Human Cadaver Eyes''. Arch Ophthalmol. 2004; 122:1287–1293. PMID 15364707 </ref> 

;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 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<ref>http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_t_13zPzhtm</ref>. 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 tonomtery measurements and are particularly useful for measuring IOP in children and other non-compliant patient groups. '''non-contact tonometry''' is accomplished without the instrument contacting the cornea which reduces the potential for disease transmission.

;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 <ref name="Medeiros">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.</ref>

;Electronic indentation tonometry
The '''Tono-Pen''' (Reichert, Inc) is a essentially a portable Mackay marg tonometer.[[Topical anesthetic]] [[eye drop]]s are used to numb the eye and the device is placed against the cornea. An outer ring flattens the cornea adjacent to the area of measurement allowing a free floating transducer to detect the transmitted pressure. The device is often used in children and those unable to reach a slit lamp due to disability or poor cooperation. It is also useful in patients with cornea disease in whom applanation tonometer cannot be reliably performed.

;Pneumatonometry
A pneumatonometer utilizes a pneumatic sensor (consisting of a piston floating on an air bearing).  It is touched to the anesthetized cornea.  A precisely regulated flow of filtered air (from an internal air pump) enters the piston.  A small (5-mm dia.) fenestrated membrane at the end of the piston reacts to both the force of the air blowing through it and to the force represented by the pressure behind the cornea, against which it is being pressed. The precise balance between these two forces represents the precise intra-ocular pressure (I.O.P.)

;Impression tonometry
Impression tonometry (also known as indentation tonometry) measures the depth of the impression produced by a small plunger carrying a known weight.<ref name="Cline">Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0</ref> Intraocular pressure is determined by assessing the movement of the plunger to a calibrated scale.<ref name="Cline"/> Schiötz tonometry is a type of indentation tonometery, historically was used to determine IOP. This type of tonometry makes use of a plunger to indent the cornea. The IOP is determined by correlation of scale reading using a [[nomogram]], with additional small metal weights added for higher levels of IOP. 

;Rebound tonometry
Rebound tonometers (Icare Tonometer <ref>http://www.icaretonometer.com</ref>) 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, cheap and easy to use. It is portable, does not require the use of eye drops and is particularly suitable for children.

;Non-corneal and Transpalpebral tonometry
Diaton tonometer<ref>http://www.tonometerDiaton.com</ref> (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 and underlying [[sclera]]. The patient is positioned so that the tip of the device and lid are overlying sclera <ref name="<ref name="Dacosta"> Dr. Shaun Maria Dacosta, Dr. Babu Rajendran, Dr. Janakiraman P. [http://www.aios.org/proceed08/papers/GLU-II/Glu-II5.pdf "Comparison of Diaton Tonometry and Non Contact Tonometry in Indian Subjects"] ''AIOC PROCEEDINGS'' 2008 Dec; 260 </ref>. Non-corneal transpalpebral tonometry does not involve contact with the [[cornea]] or require [[topical anesthetic]] during routine use. Non-corneal readings using other devices have been shown to be of little value <ref name="<ref name=" Kolin">Kolin T, Wedemeyer LL, Kolin E, Braun Y. Comparison of scleral and corneal Tono-Pen readings.J AAPOS. 2003;7: 291-292</ref> <ref name="<ref name="Breitfeller">Breitfeller JM, Krohn DL. Limbal pneumatonometry. Am J Ophthalmol. 1980; 89: 344-352</ref> <ref name="<ref name="Poostchi">Poostchi A, Mitchell R, Nicholas S, Purdie G, Wells A.The iCare rebound tonometer: comparisons with Goldmann tonometry, and influence of central corneal thickness.Clin Experiment Ophthalmol. 2009 Sep;37(7):687-91</ref> and Diaton measurements show only moderate correlation with those provided by applanation tonometry <ref name="<ref name="Toker"> Toker MI, Vural A, Erdogan H, Topalkara A, Arici MK. Central corneal thickness and Diaton transpalpebral tonometry. Graefes Arch Clin Exp Ophthalmol. 2008 Jun;246(6):881-9. Epub 2008 Feb 20 PMID: 18286299</ref>. Despite avoiding the cornea, the device also seems to be more affected by the corneal thickness than Goldmann tonometry<ref name="Toker"/>. The Diaton tonometer was preceded by the TGDc-01 which was discontinued in 2001.

;Palpation
Palpation (also known as digital tonometry) is the method of estimating intraocular pressure by pressing gently against the cornea of a closed eye with your index finger.<ref name="Troost">Troost A, Yun SH, Specht K, Krummenauer F, Schwenn O. [http://bjo.bmjjournals.com/cgi/content/full/89/3/280 "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.</ref>

;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. 

==Gallery==
<gallery>
Image:Tonometer.jpg|A Goldmann tonometer
Image:Pascal Tonometer.jpg|PASCAL Dynamic Contour Tonometer
Image:PASCAL_SensorTip_on_eye.jpg|SensorTip of a PASCAL tonometer in contact with patient's cornea
Image:Diaton_tonometty_through_eyelid.jpg|Diaton Tonometry through the Eyelid‎ 
Image:Diaton_tonometer_IOP_through_Eyelid.gif|Position of the Diaton Tonometer Tip on the Eyelid with No Contact with Cornea or use of Anesthesia 
Image:Diaton_tonometer_BiCOM.jpg|Hand-held Diaton tonometer with Test and Training Eye case
</gallery>

==References==
{{reflist}}

==External links==
*[http://www.webmd.com/hw/lab_tests/hw201323.asp Tonometry - WebMD]
*[http://www.hopkinsafter50.com/html/silos/vision/viLIB_JHMT_Tonometry.php Tonometry - ''The Johns Hopkins Consumer Guide to Medical Tests'']
*Kirstein, E. [http://www.opt.pacificu.edu/ce/catalog/16282-GL/UpdateIOP.html#180%20YEARS%20OF%20EVOLUTION%20IN%20TONOMETRY "An Update on Methods for Assessing Intraocular Pressure."] Retrieved June 7, 2006.
*[http://www.tonometerdiaton.com/index.php?do=home.description Tonometry- ''Transpalpebral Diaton Tonometry'']
*[http://www.icaretonometer.com Rebound Tonometer from Icare Finland]
*[http://www.tonometerdiaton.com Trans-Scleral Tonometry]
*[http://www.optivision2020.com/tonometer.html Tonometers in the market]
*[http://www.keeler.co.uk/ Tonometer from Keeler Ltd, UK based Company]
*[http://www.tonometer.net Through Eyelid Tonometer from BiCOM Inc.]

{{Eye procedures}}
[[Category:Medical equipment]]
[[Category:Ophthalmology]]
[[Category:Ophthalmic equipment]]

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