Tonometry

From Wikipedia, the free encyclopedia

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

Tonometry is the measurement of tension or pressure [1]. A tonometer is an instrument for measuring tension or pressure [2].

In ophthalmology, 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 ocular conditions such as glaucoma as well as conditions such as phthisis bulbi, and iritis. Most tonometers are calibrated to measure pressure in mmHg.

[edit] Methods of ophthalmotonometry

• Applanation tonometry measures intraocular pressure either by the force required to flatten a constant area of the cornea (e.g. Goldmann tonometry) or by the area flattened by a constant force.^[1]

• Goldmann tonometry is considered to be the gold standard in tonometry as it is the most widely accepted method of determining intraocular pressure.^[2][3] However, as James D. Brandt, MD, points out, Goldmann tonometry is an inherently imprecise measurement.

Applanation tonometry uses a special calibrated sterile probe attached to a slit lamp biomicroscope to flatten part of the cornea. Because the probe makes contact with the cornea, a topical anesthetic, such as oxybuprocaine, tetracaine, alcaine, proxymetacaine or proparacaine is introduced onto the surface of the eye in the form of one or a few eye drops. A yellow fluorescein dye is used in conjunction with a cobalt blue filter to aid the examiner in determining the IOP. 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.

• MacKay Marg

• Pneumotonometry, also known as non-contact tonometry or air-puff tonometry, uses an instrument that senses deflections of the cornea in reaction to a puff of pressurized air. [3] Although not considered to be the most accurate way to measure IOP, it is often used as a simple way to screen for high IOP. It is an easy way to test children. Because non-contact tonometry is accomplished without the instrument contacting the cornea the potential for disease transmission is reduced. Another interesting application of non-contact tonometers is demonstrated by this large case history where tens of thousands of intraocular pressure measurements have been recorded on a single individual under a wide variety of conditions. As part of this case history, as many as 200 intraocular pressure measurements are recorded in a single day -- something not possible with tonometry instruments that contact the cornea. This case history web site includes further discussion of non-contact tonometry vs. other forms.

• Electronic indentation tonometry. The Tono-Pen is a portable electronic, digital pen-like instrument that determines IOP by making contact with the cornea, afer use of topical anesthetic eye drops. This is especially useful for very young children, patients unable to reach a slit lamp due to disability, or patients who are uncooperative during applanation tonometry.

• Impression tonometry, also known as identation tonometry, measures the depth of the impression produced by a small plunger carrying a known weight.^[1] Intraocular pressure is determined by assessing the movement of the plunger to a calibrated scale.^[1]

Schiötz tonometry is a type of impression 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.

• Transpalpebral (Diaton) tonometry measures intraocular pressure through the eyelid. It is regarded as a simple and safe method of ophthalmotonometry. Transpalpebral tonometry requires no contact with the cornea, therefore sterilization of the device and topical anesthetic drops are not required and there is very little risk of infection. Compared with the current standard of Goldmann applanation tonometry, recent data suggests that transpalpebral tonometry via the Diaton tonometer is not as accurate in that it has been found to underestimate and show more variability in intraocular pressure values.^{[4] [5] [6] [7]} Although not advised for the clinical management of those with glaucoma ^[3], transpalpebral tonometery has been described as sufficient for clinical use and it may have applications in the home-monitoring of intraocular pressures.^[8] Use of transpalpebral tonometry may be indicated in those for whom Goldmann tonometry is not indicated^[9], such as in children, those with corneal pathology, or those who have had corneal surgery.^[2][10]

• Palpation, also known as digital tonometry, to measure intraocular pressure is performed by gently pressing the fingertips of both index fingers onto the upper part of the bulbus through the eyelid.^[10]

• Dynamic contour tonometry

[edit] References

[edit] 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.
• Tonometry- Transpalpebral Diaton Tonometry