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 . A tonometer is an instrument for measuring tension or pressure .
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.
Methods of ophthalmotonometry
Applanation tonometry measures approximate 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.
In Applanation tonometry, a special calibrated disinfected probe attached to a slit lamp biomicroscope is used to flatten the central cornea a fixed amount. 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 who may be in a supine position.
MacKay Marg tonometer
Goldmann tonometry is considered to be the gold standard in tonometry as it is the most widely accepted method of determining approximate intraocular pressure. However, as James D. Brandt, MD, points out, Goldmann tonometry is an inherently imprecise measurement.
Dynamic Contour Tonometry
Dynamic contour tonometry (DCT) is a novel measuring technique using the principle of contour matching instead of applanation to eliminate the systematic errors inherent in previous tonometers. These factors include the influence of corneal thickness, rigidity, curvature, and elastic properties. It is not influenced by mechanical changes, such as those seen in refractive surgery that would otherwise cause error in applanation tonometers.
The PASCAL® Dynamic Contour Tonometer (Ziemer Ophthalmics) is a relatively new device that uses DCT to measure IOP. Although this device is similar in appearance to a Goldmann, the PASCAL® it is unlike Goldmann applanation in that it is not a variable force tonometer.
PASCAL® 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. This is an important difference from all forms of applanation tonometry in which the probe force is variable.
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 concave surface of radius 10.5 mm, which approximates the cornea’s shape when the pressures on both sides of it are equal. This is the key to the PASCAL’s ability to neutralize the effect of intra-individual variation in corneal properties.
Once a portion of the central cornea has taken up the shape of the tip, the integrated piezoresistive pressure sensor 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 insure proper contact with the cornea. Literature references: 
Transpalpebral (Diaton) Tonometry
Diaton tonometer 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 and Tono-Pen® (Reichert, Inc), recent data suggests that transpalpebral tonometry via the Diaton tonometer is clinically useful device for measuring IOP in routine eye exams.  Although requiring practice of the technique , transpalpebral tonometery has been described as sufficient for clinical use and it may have applications in the home-monitoring of intraocular pressures. Use of transpalpebral tonometry may be indicated in those for whom Goldmann tonometry is not indicated, such as in children, those with corneal pathology, or those who have had corneal surgery.
Thus, corneal thickness is an important factor of IOP evaluation and monitoring, it requires the necessity of including corneal pachymetry in the program of examination the patients with suspicion of glaucoma and hypertension, especially after various keratorefractive surgeries while using the traditional corneal methods of ophthalmotonometry. At the same time clinical application of transpalpebral scleral Diaton tonometer makes it possible to evaluate IOP using only one device, the procedure being efficient, economical, simple in performance and requiring no additional instrumental examination.
The images to the right and below show painlessness of the Diaton tonometry procedure and its proper positioning on the Eyelid, where the edge of the lid coincides with the edge of the limbus. Since the screening occurs above the cornea, contact lenses are not being taken out.
non-contact tonometry or air-puff tonometry (NOT the same thing as pneumatonometry) invented by Bernard Grolman of American Optical(now Reichert, Inc), 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. Historically, Non-contact tonometers were not considered to be the most accurate way to measure IOP. They have generally been considered a fast and simple way to screen for high IOP. However, modern-day non-contact tonometers have been shown to correlate very well with goldmann tonomtery measurements. 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® (Reichert, Inc) 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, patients who are uncooperative during applanation tonometry, or patients with cornea disease in whom contact tonometer cannot be accurately 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, also known as indentation tonometry, measures the depth of the impression produced by a small plunger carrying a known weight. Intraocular pressure is determined by assessing the movement of the plunger to a calibrated scale.
Rebound tonometry (such as iCare Tonometer) is a new method for measuring intra-ocular pressure. Unique and carefully studied and tested measurement technique which requires no local anaesthesia. An ultralight, sensitive probe touches the eye for a moment, and does not even always cause, for example, the corneal reflex (blinking). The disposable probe is also safe for the eye in microbiological terms. The accurate measurement device is primarily intended for professional use by general practitioners, opticians, nurses.
It is the only portable device of its kind that does not require anaesthesia. Compact and easy to use, it is suited to
the investigation of patients such as children. Made of durable plastic and anodized aluminium, the device is light and small. Comfort and ease of use in different positions have been taken into account. The device employs an electromagnetic field, and the disposable probe is easy to change. It is also sturdy and the possible movement of the patient during investigation will not affect it.
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.
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.
Ocular Response Analyzer The Ocular Response Analyzer (ORA, Reichert, Inc) utilizes non-contact tonometer (air puff) technology. However, in the ORA, the air pulse causes the cornea to go beyond applanation, into an indented state, before returning to normal curvature. As such, the cornea passes through applanation twice during the process, once on the way inward, and again on the way back out. Viscous damping (energy absorption) in the corneal tissue results in two different pressure values from the inward and outward applanation events. The difference between these two pressure values is termed "corneal hysteresis" and is a measurement of the cornea's biomechanical tissue properties. The ability of the ORA to measure these properties enables the calculation of an IOP measurement called IOPcc (Corneal Compensated IOP). This is an IOP measurement that is less influenced by corneal properties such as resistance or thickness. As such, it has been shown to be more accurate than other methods of tonometry. .
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