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The Schiotz tonometer consists of a curved footplate which is placed on the cornea of a supine patient. A weighted plunger attached to the footplate sinks into the cornea. A scale then gives a reading depending on how much the plunger sinks into the cornea, and a conversion table converts the scale reading into IOP measured in mmHg.
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]
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. [1] Most tonometers are calibrated to measure pressure in millimeters of mercury .
Goldmann applanation tonometer is based on the Imbert–Fick principle, which states that for a dry thin-walled sphere, the pressure (P) inside the sphere equals the force (F) necessary to flatten its surface divided by the area (A) of flattening (i.e. P = F/A).
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.
For eye pressures, a value of 28 hPa (21 mmHg) above atmospheric pressure 1,010 hPa (760 mmHg) is often used, with higher pressures leading to a greater risk. [2] [26] However, some may have high eye pressure for years and never develop damage. [2] Conversely, optic nerve damage may occur with normal pressure, known as normal-tension glaucoma. [27]
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It has only recently [when?] become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism. The appearance of visual complaints such as halos , glare and monocular diplopia after corneal refractive surgery has long been correlated with the ...