Ocular tonometry

Last updated
Tonometry
Patient and tonometer.jpg
A patient in front of a tonometer
Specialty optometry
ICD-9-CM 89.11
MeSH D014065
MedlinePlus 003447

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. [1] Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg), with the normal eye pressure range between 10 and 21 mmHg (13–28 hPa).

Contents

Methods

The slit lamp of a Goldmann tonometer Slit lamp.jpg
The slit lamp of a Goldmann tonometer
Semicircles seen during Goldmann tonometry through slit lamp Goldmann mires.jpg
Semicircles seen during Goldmann tonometry through slit lamp

Applanation tonometry

In applanation tonometry the intraocular pressure (IOP) is inferred from the force required to flatten (applanate) a constant area of the cornea, for the Imbert-Fick law. [2] The Maklakoff tonometer was an early example of this method, while the Goldmann tonometer is the most widely used version in current practice. [3] 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 drop.[ citation needed ]

Goldmann tonometry

Goldmann tonometry is considered to be the gold standard IOP test and is the most widely accepted method. [4] [5] 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 semicircles. 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 the area of a circle with diameter 3.06 mm (0.120 in) 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 and may need to be adjusted. [6] [7]

Perkins tonometer

The Perkins tonometer is a type of portable applanation tonometer, which may be useful in children, anesthetised patients who need to lie flat, or patients unable to co-operate with a sitting slit lamp examination, that yields clinical results comparable to the Goldmann. [8]

Dynamic contour tonometry

The PASCAL dynamic contour tonometer PASCAL tonometer.jpg
The PASCAL dynamic contour tonometer

Dynamic contour tonometry (DCT) 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. [9]

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 tonometer's computer calculates a change in pressure according to the change in resistance. A complete measurement cycle requires about eight seconds of contact time. The device also measures the variation in pressure that occurs with the cardiac cycle. [10] [11] [12]

Electronic indentation tonometry

Electronic indentation tonometers 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 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 into 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. [13]

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 (0.20 in) diameter) fenestrated membrane at one end. This membrane 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.[ citation needed ]

Impression tonometry

Schiotz tonometer Schiotz Tonometer.jpg
Schiøtz tonometer

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. [14] The movement of the plunger is measured using a calibrated scale. [14] The Schiøtz tonometer is the most common device to use this principle.

Non-corneal and transpalpebral tonometry

Diaton transpalpebral tonometer TONOMETER DIATON 2011.jpg
Diaton transpalpebral tonometer

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. [15] Non-corneal and transpalpebral tonometry does not involve contact with the cornea and does not require topical anesthetic during routine use. Transpalpebral tonometry may be useful for measuring postsurgery IOP after myopic LASIK ablation because this technique is not influenced by the treatment. [16] The Diaton tonometer still requires further evaluation and is not a substitute or alternative for more established methods. [17] The Diaton tonometer has a large margin of error compared with commonly used tonometers (e.g., GAT) in most patients (including those with ocular hypertension, glaucoma, and glaucoma tube shunts). [18]

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. [19] 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.[ citation needed ]

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 into 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. [20] In a population based study in healthy children that compared non-contact IOP measuring tonometer, including ORA and CORVIS with a contact tonometer, GAT, which is a routine instrument for IOP measurement. It was firmly evident that due to significantly low positive or negligible correlation, none of these 2 non-contact tonometers can replace the GAT. [21]

Palpation

Palpation (also known as digital tonometry) is the method of estimating intraocular pressure by gently pressing the index finger against the cornea of a closed eye. This method is notoriously unreliable. [22]

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 overestimated (and a thin cornea of an IOP being underestimated), but the extent of measurement error in individual patients cannot be ascertained from the CCT alone. [23] The Ocular Response Analyzer and Pascal DCT Tonometers are less affected by CCT than the Goldmann tonometer. Conversely, non-contact and rebound tonometers are more affected. [23] [24] [25] Corneal thickness varies among individuals as well as with age and race. It is reduced in certain disease and following LASIK surgery.

Related Research Articles

<span class="mw-page-title-main">Glaucoma</span> Group of eye diseases

Glaucoma is a group of eye diseases that lead to damage of the optic nerve, which transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of sight" because the loss of vision usually occurs slowly over a long period of time. A major risk factor for glaucoma is increased pressure within the eye, known as intraocular pressure (IOP). It is associated with old age, a family history of glaucoma, and certain medical conditions or medications. The word glaucoma comes from the Ancient Greek word γλαυκóς, meaning 'gleaming, blue-green, gray'.

<span class="mw-page-title-main">Intraocular pressure</span> Fluid pressure inside the eye

Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg).

<span class="mw-page-title-main">Eye injury</span> Physical or chemical injuries of the eye

Physical or chemical injuries of the eye can be a serious threat to vision if not treated appropriately and in a timely fashion. The most obvious presentation of ocular (eye) injuries is redness and pain of the affected eyes. This is not, however, universally true, as tiny metallic projectiles may cause neither symptom. Tiny metallic projectiles should be suspected when a patient reports metal on metal contact, such as with hammering a metal surface. Corneal foreign body is one of the most common preventable occupational hazard. Intraocular foreign bodies do not cause pain because of the lack of nerve endings in the vitreous humour and retina that can transmit pain sensations. As such, general or emergency department doctors should refer cases involving the posterior segment of the eye or intraocular foreign bodies to an ophthalmologist. Ideally, ointment would not be used when referring to an ophthalmologist, since it diminishes the ability to carry out a thorough eye examination.

<span class="mw-page-title-main">Red eye (medicine)</span> Eye that appears red due to illness or injury

A red eye is an eye that appears red due to illness or injury. It is usually injection and prominence of the superficial blood vessels of the conjunctiva, which may be caused by disorders of these or adjacent structures. Conjunctivitis and subconjunctival hemorrhage are two of the less serious but more common causes.

<span class="mw-page-title-main">Latanoprost</span> Chemical compound

Latanoprost, sold under the brand name Xalatan among others, is a medication used to treat increased pressure inside the eye. This includes ocular hypertension and open-angle glaucoma. Latanaprost is applied as eye drops to the eyes. Onset of effects is usually within four hours, and they last for up to a day.

Ocular hypertension is the presence of elevated fluid pressure inside the eye, usually with no optic nerve damage or visual field loss.

<span class="mw-page-title-main">Gonioscopy</span> Measurement of the angle between an eyes iris and cornea

In ophthalmology, gonioscopy is a routine procedure that measures the angle between the iris and the cornea, using a goniolens together with a slit lamp or operating microscope. Its use is important in diagnosing and monitoring various eye conditions associated with glaucoma.

<span class="mw-page-title-main">Corneal topography</span> Medical imaging technique

Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the anterior curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for some 70% of the eye's refractive power, its topography is of critical importance in determining the quality of vision and corneal health.

A glaucoma valve is a medical shunt used in the treatment of glaucoma to reduce the eye's intraocular pressure (IOP).

Iridocorneal endothelial (ICE) syndromes are a spectrum of diseases characterized by slowly progressive abnormalities of the corneal endothelium and features including corneal edema, iris distortion, and secondary angle-closure glaucoma. ICE syndromes are predominantly unilateral and nonhereditary. The condition occurs in predominantly middle-aged women.Iridocorneal Endothelial (ICE) syndrome presents a unique set of challenges for both patients and ophthalmologists, and effective treatment of this group of rare ocular diseases requires a combination of diagnostic and therapeutic complexity. It's important to understand.

<span class="mw-page-title-main">Corneal pachymetry</span>

Corneal pachymetry is the process of measuring the thickness of the cornea. A pachymeter is a medical device used to measure the thickness of the eye's cornea. It is used to perform corneal pachymetry prior to refractive surgery, for Keratoconus screening, LRI surgery and is useful in screening for patients suspected of developing glaucoma among other uses.

Armand Imbert (1850-1922) and Adolf Fick (1829-1901) both demonstrated, independently of each other, that in ocular tonometry the tension of the wall can be neutralized when the application of the tonometer produces a flat surface instead of a convex one, and the reading of the tonometer (P) then equals (T) the IOP," whence all forces cancel each other.

<span class="mw-page-title-main">Boston keratoprosthesis</span> Prosthetic cornea

Boston keratoprosthesis is a collar button design keratoprosthesis or artificial cornea. It is composed of a front plate with a stem, which houses the optical portion of the device, a back plate and a titanium locking c-ring. It is available in type I and type II formats. The type I design is used much more frequently than the type II which is reserved for severe end stage dry eye conditions and is similar to the type I except it has a 2 mm anterior nub designed to penetrate through a tarsorrhaphy. The type I format will be discussed here as it is more commonly used.

The Trabectome is a surgical device that can be used for ab interno trabeculotomy, a minimally invasive glaucoma surgery for the surgical management of adult, juvenile, and infantile glaucoma. The trabecular meshwork is a major site of resistance to aqueous humor outflow. As angle surgeries such as Trabectome follow the physiologic outflow pathway, the risk of complications is significantly lower than filtering surgeries. Hypotony with damage to the macula, can occur with pressures below 5 mmHg, for instance, after traditional trabeculectomy, because of the episcleral venous pressure limit. The Trabectome handpiece is inserted into the anterior chamber, its tip positioned into Schlemm's canal, and advanced to the left and to the right. Different from cautery, the tip generates plasma to molecularize the trabecular meshwork and remove it drag-free and with minimal thermal effect. Active irrigation of the trabectome surgery system helps to keep the anterior chamber formed during the procedure and precludes the need for ophthalmic viscoelastic devices. Viscoelastic devices tend to trap debris or gas bubbles and diminish visualization. The Trabectome decreases the intra-ocular pressure typically to a mid-teen range and reduces the patient's requirement to take glaucoma eye drops and glaucoma medications. The theoretically lowest pressure that can be achieved is equal to 8 mmHg in the episcleral veins. This procedure is performed through a small incision and can be done on an outpatient basis.

The aim of an accurate intraocular lens power calculation is to provide an intraocular lens (IOL) that fits the specific needs and desires of the individual patient. The development of better instrumentation for measuring the eye's axial length (AL) and the use of more precise mathematical formulas to perform the appropriate calculations have significantly improved the accuracy with which the surgeon determines the IOL power.

<span class="mw-page-title-main">Schiøtz tonometer</span>

Schiøtz tonometer is an indentation tonometer, used to measure the intraocular pressure (IOP) by measuring the depth produced on the surface of the cornea by a load of a known weight. The indentation of corneal surface is related to the IOP.

<span class="mw-page-title-main">Primary juvenile glaucoma</span> Medical condition

Primary juvenile glaucoma is a subtype of primary congenital glaucoma that develops due to ocular hypertension and is diagnosed between three years of age and early adulthood. It is caused due to abnormalities in the anterior chamber angle development that obstruct aqueous outflow in the absence of systemic anomalies or other ocular malformation.

<span class="mw-page-title-main">Goldmann Applanation Tonometer</span>

Goldmann Applanation Tonometer is an instrument that is based on Imbert-Fick law. It is considered to be the gold standard instrument for measurement of Intraocular pressure (IOP).

<span class="mw-page-title-main">Phacolytic glaucoma</span> Medical condition

Phacolytic glaucoma (PG) is a form of glaucoma which is caused due to a leaking mature or immature cataract. Inflammatory glaucoma which occurs in phacolysis is a condition which is a result of the leakage of protein within the lens into the capsule of a mature or hyper mature cataract and involves a simple procedure to be cured that is referred to as cataract extraction.

<span class="mw-page-title-main">Uveitic glaucoma</span> Glaucoma caused by uveitis or its treatments

Uveitic glaucoma is most commonly a progression stage of noninfectious anterior uveitis or iritis.

References

  1. ↑ Farandos, NM; Yetisen, AK; Monteiro, MJ; Lowe, CR; Yun, SH (2014). "Contact Lens Sensors in Ocular Diagnostics". Advanced Healthcare Materials. 4 (6): 792–810. doi:10.1002/adhm.201400504. PMID   25400274. S2CID   35508652.
  2. ↑ Goldman, H; Schmidt, TH (1957). "Uber Applanations-tonometrie". Ophthalmologica. 134 (4): 221–242. doi:10.1159/000303213. PMID   13484216.
  3. ↑ Okafor KC, Brandt JD (Mar 2015). "Measuring intraocular pressure". Curr Opin Ophthalmol. 26 (2): 103–9. doi:10.1097/ICU.0000000000000129. PMID   25594767. S2CID   9760687.
  4. ↑ Amm M, Hedderich J (Jan 2005). "[Transpalpebral tonometry with a digital tonometer in healthy eyes and after penetrating keratoplasty.]". Ophthalmologe. 102 (1): 70–6. doi:10.1007/s00347-004-1082-5. PMID   15322801. S2CID   35088823.
  5. ↑ Schlote, T; Landenberger, H (Feb 2005). "[Intraocular pressure difference in Goldmann applanation tonometry versus a transpalpebral tonometer TGDc-01'PRA' in glaucoma patients]". Klin Monatsbl Augenheilkd. 222 (2): 123–31. doi:10.1055/s-2005-857881. PMID   15719316. S2CID   260207933.
  6. ↑ Groves, N (15 September 2006). "Should IOP be adjusted for corneal thickness alone?". Ophthalmology Times. Archived from the original on 30 October 2006. Retrieved 29 December 2006.
  7. ↑ Chihara, E. (May–Jun 2008). "Assessment of true intraocular pressure: the gap between theory and practical data". Surv Ophthalmol. 53 (3): 203–18. doi:10.1016/j.survophthal.2008.02.005. PMID   18501267.
  8. ↑ Arora, R; Bellamy, H; Austin, M. (March 2014). "Applanation tonometry: a comparison of the Perkins handheld and Goldmann slit lamp-mounted methods". Clin. Ophthalmol. 26 (8): 605–610. doi: 10.2147/OPTH.S53544 . PMC   3971938 . PMID   24707165.
  9. ↑ Francis, BA; Hsieh, A; Lai, MY; Chopra, V; Pena, F; Azen, S; Varma; R (Jan 2007). "Effects of corneal thickness, corneal curvature, and intraocular pressure level on Goldmann applanation tonometry and dynamic contour tonometry". Ophthalmology. 114 (1): 20–6. doi:10.1016/j.ophtha.2006.06.047. PMID   17070592.
  10. ↑ Kaufmann, C; Bachmann, LM; Thiel, M (2004). "Comparison of Dynamic Contour Tonometry with Goldmann Applanation Tonometry". Investigative Ophthalmology & Visual Science. 45 (9): 3118–3121. doi:10.1167/iovs.04-0018. PMID   15326129.
  11. ↑ Kaufmann, C; Bachmann, LM; Thiel, MA (2003). "Intraocular Pressure Measurements Using Dynamic Contour Tonometry after Laser In Situ Keratomileusis". Investigative Ophthalmology & Visual Science. 44 (9): 3790–3794. doi:10.1167/iovs.02-0946. PMID   12939293.
  12. ↑ Kniestedt, C; Nee, M; Stamper, RL (2004). "Dynamic Contour Tonometry. A Comparative Study on Human Cadaver Eyes". Arch Ophthalmol. 122 (9): 1287–1293. doi:10.1001/archopht.122.9.1287. PMID   15364707.
  13. ↑ Schreiber, W; Vorwerk, CK; Langenbucher, A; Behrens-Baumann, W; Viestenz, A. (April 2007). "A comparison of rebound tonometry (ICare) with TonoPenXL and Goldmann applanation tonometry". Ophthalmologe. 104 (4): 299–304. doi:10.1007/s00347-007-1487-z. PMID   17333073. S2CID   22409644.
  14. 1 2 Hofstetter, Cline D; Griffin Jr., HW (1997). Dictionary of Visual Science (4th ed.). Boston: Butterworth-Heinemann. ISBN   978-0-7506-9895-5.
  15. ↑ 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
  16. ↑ Cacho, I; Sanchez-Naves, J; Batres, L; Pintor, J; Carracedo, G (2015). "Comparison of Intraocular Pressure before and after Laser In Situ Keratomileusis Refractive Surgery Measured with Perkins Tonometry, Noncontact Tonometry, and Transpalpebral Tonometry". J Ophthalmol. 2015: 683895. doi: 10.1155/2015/683895 . PMC   4475733 . PMID   26167293.
  17. ↑ Li, Y; Shi, J; Duan, X; Fan, F (December 2010). "Transpalpebral measurement of intraocular pressure using the Diaton tonometer versus standard Goldmann applanation tonometry". Graefes Arch Clin Exp Ophthalmol. 248 (12): 1765–70. doi:10.1007/s00417-009-1243-y. PMID   20495818. S2CID   20523926.
  18. ↑ Kwon, YH (4 March 2016). "The Utility of Diaton Tonometer Measurements in Patients With Ocular Hypertension, Glaucoma, and Glaucoma Tube Shunts: A Preliminary Study for its Potential Use in Keratoprosthesis Patients". J Glaucoma. 25 (8): 643–7. doi:10.1097/IJG.0000000000000394. PMID   26950582. S2CID   31305299.
  19. ↑ http://www.mercksource.com Archived May 9, 2006, at the Wayback Machine
  20. ↑ Medeiros, Felipe A.; Weinreb, Robert N. (October 2006). "Evaluation of the Influence of Corneal Biomechanical Properties on Intraocular Pressure Measurements Using the Ocular Response Analyzer". J Glaucoma. 15 (5): 364–370. doi:10.1097/01.ijg.0000212268.42606.97. PMID   16988597. S2CID   7069001.
  21. ↑ Salouti, Ramin; Alishiri, Ali Agha; Gharebaghi, Reza; Naderi, Mostafa; Jadidi, Khosrow; Shojaei-Baghini, Ahmad; Talebnejad, Mohammadreza; Nasiri, Zahra; Hosseini, Seyedmorteza; Heidary, Fatemeh (2018). "Comparison among Ocular Response Analyzer, Corvis ST and Goldmann applanation tonometry in healthy children". International Journal of Ophthalmology. 11 (8): 1330–1336. doi:10.18240/ijo.2018.08.13. ISSN   2222-3959. PMC   6090110 . PMID   30140637.
  22. ↑ Troost, A; Yun, SH; Specht, K; Krummenauer, F; Schwenn, O (Mar 2005). "Transpalpebral tonometry: reliability and comparison with Goldmann applanation tonometry and palpation in healthy volunteers". Br J Ophthalmol. 89 (3): 280–3. doi:10.1136/bjo.2004.050211. PMC   1772547 . PMID   15722303.
  23. 1 2 Robert N. Weinreb, James D. Brandt, David Garway-Heath and Felipe A. Medeiros 2007 "4th Consensus Meeting:Intra-Ocular Pressure" Archived 2011-06-10 at the Wayback Machine
  24. ↑ Boehm, AG; Weber, A; Pillunat, LE; Koch, R (June 2008). "Dynamic Contour Tonometry in Comparison to Intracameral IOP Measurements". Investigative Ophthalmology & Visual Science. 49 (6): 2472–7. doi:10.1167/iovs.07-1366. PMID   18316699.
  25. ↑ Kirstein E, An Update on Methods for Assessing Intraocular Pressure. online CE, 2006
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