How Do Different Types of Tonometer Work?


Tonometry is a diagnostic procedure in which the pressure inside your eye is detected using a Tonometer, an equipment designed to assess intraocular pressure. Hence, tonometer is considered as intraocular pressure measuring device. This test can help your doctor 

determine whether you're at risk for glaucoma. Glaucoma is a serious eye disease that can cause vision loss if left untreated. In most cases of glaucoma, the fluid that normally bathes and nourishes the eye drains too slowly, causing pressure to build up. The normal 
range for ocular pressure is 10 to 21 millimeters of mercury. The thickness of your cornea may alter measurements. Normal eyes with thick corneas have greater readings, while normal eyes with thin corneas have lower readings. There are several different varieties of tonometers, which are listed below. 

Types of tonometry

A) Applanation tonometry- The cornea is flattened in applanation tonometry, and the Intraocular Pressure, commonly known as IOP is calculated by adjusting the applanating force or the flattened area.

(i) Goldmann & Perkins applanation tonometry- The Goldmann applanation tonometer measures the force required to flatten a 3.06mm corneal region. The capillary attraction of the tear film meniscus for the tonometer head balances the cornea's resistance to flattening at this diameter. The flattening force (in kg) multiplied by 10 equals the Intraocular pressure (in mm Hg). To highlight the tear film, fluorescein dye is applied to the patient's eye. The image of the tear meniscus is divided into a superior and inferior arc using a split-image prism. When these arcs are lined so that their inner borders just touch, the intraocular pressure is measured.

(ii) Non- contact tonometry 

(a) Air puff tonometer- The applanating force in air puff tonometry is a column of air that is emitted with steadily increasing intensity. The air column is turned off at the point of corneal flattening, and the force measured and translated into mmHg. When compared to the Goldmann applanation tonometer, readings from these machines may underestimate IOP at high ranges and overstate IOP at low levels. Because IOP changes over the cardiac cycle, a minimum of three observations should be averaged to establish the mean IOP. 

(b) Ocular response analyzer- A newer type of non-contact tonometer is the ocular response analyser. The applanating force is similarly provided by a column of air of increasing intensity in this device. The moment of applanation is recorded by the ocular response analyzer, but the air column continues to emit with increasing intensity until the cornea is indented. The air column's force gradually lessens until the cornea reaches a point of applanation once more. The pressure difference between the two applanation points is a measurement of corneal elasticity (e.g., hysteresis). For high or low elasticity, mathematical calculations can be employed to "adjust" the applanation point. Other forms of applanated pressures are regarded to be more dependent on corneal thickness than this "adjusted" IOP.

(B) Indentation tonometry- A force or a weight will indent or sink into a soft eye more than into a hard eye, according to the principle of indentation tonometry. It is divided into three sections. 

(i) Schiotz Tonometer- A curved footplate is placed on the cornea of a supine person in the Schiotz tonometer. The pressure in the eye is indirectly proportional to the amount a weighted plunger attached to the footplate sinks into the cornea. The plunger will sink deeper into a soft eye's cornea than it will into a tougher eye's cornea. A scale at the top of the plunger provides a reading based on how far the plunger dips into the cornea, and a conversion table transforms the scale value into Intraocular pressure in millimeters of mercury.

(ii) Pneumotonometer- The pneumotonometer is a combination of an applanation tonometer and indentation tonometry. On the end of a piston that travels on a stream of air is a 5mm diameter, slightly convex silicone tip. The silicone tip indentates the cornea. The pressure pushed forward on the tip is equal to the IOP when the cornea and tip are flat. At this point, the device measures the pressure within the system, which is displayed in millimeters of mercury (mm Hg). Within normal Intraocular pressure levels, the values correspond well with Goldmann applanation tonometry.

(iii) Tono- pen- Both applanation and indentation processes are used in the Tono-Pen. It's a battery-operated portable device that's compact and portable. A little plunger protrudes minimally from the center of the tonometer's applanating footplate. The plunger gets resistance from the cornea and the IOP as the tonometer makes contact with the eye, providing a record of rising force by a strain gauge. The force is shared between the foot plate and the plunger at the time of applanation, resulting in a tiny drop from the continuously growing force. This is the point of applanation that is electronically recorded. The average of multiple readings is calculated. The IOP may then be determined because the area of applanation is known. Within normal IOP levels, the values correspond well with Goldmann tonometry. 

(C) Rebound Tonometry- The iCare gadget is the most recent version of the rebound tonometer (Helsinki, Finland). An electromagnetic field in a handheld battery-powered gadget keeps a 1.8mm diameter plastic ball on a stainless steel wire in place. When a button is pressed, a spring accelerates the movement of the wire and ball. The ball and wire decelerate as they impact the cornea; the deceleration is faster if the IOP is high and slower if the IOP is low. The instrument measures the rate of deceleration and converts it into IOP. There is no need for anaesthesia. The central corneal thickness has also been demonstrated to influence IOP measurements obtained with this tonometer, with greater IOP values with thicker corneas. Other biomechanical features of the cornea, such as corneal hysteresis and corneal resistance factor, have been demonstrated to affect this tonometer.

(D) Pascal Dynamic Contour Tonometer- The Pascal dynamic tonometer measures dynamic pulsatile changes in IOP using a piezoelectric sensor implanted in the tonometer's tip. In comparison to the Goldmann tonometer, DCT measurements are said to be less influenced by corneal thickness, as well as corneal curvature and rigidity. In vitro and in vivo manometric research back up these assertions. The ocular pulse amplitude can also be measured using DCT. For each measurement, disposable covers are used, and the digital display displays a Q-value that indicates the measurement's quality.


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