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HERBERT E. KAUFMAN; Pressure Measurement: Which Tonometer?. Invest. Ophthalmol. Vis. Sci. 1972;11(2):80-85.
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In the search for more accurate ways to determine intraocular pressure, the Halberg applanation tonometer may represent an alternative to the Schiøtz tonometer for routine screening pressure measurements. Although between 100 to 150 measurements are generally required to gain proficiency with the Halberg, the instrument is accurate in patients with normal corneas even with very abnormal scleral rigidity, which would make Schiøtz readings inaccurate. In patients with corneal disease and irregular corneal surfaces due to edema or scarring, however, the optical applanation tonometer cannot be accurately used. In these patients, also, the Schiøtz tonometer is not only inaccurate but also may consistently read low and be grossly misleading. As validated by direct measurements with a cannula inside the eyes of patients with irregular corneal surfaces, the electronic applanation tonometer (MacKay-Marg tonometer) provided the first measure of the true intraocidar pressure in patients with corneal edema and corneal scarring. The Bausch and Lomb air tonometer could provide readings of generally comparable accuracy but is considerably less convenient to use in patients with irritated eyes or nystagmus or after keratoplasty. In patients with corneal disease and corneal edema, even modest elevations of intraocular pressure can contribute to edema and disease. Conversely, through neglect of this factor, the group with previous ocular damage has an increased likelihood of having elevated intraocular pressures and suffering irreversible damage. An adequate standard of ophthalmologic care now seems to require some provision for the measurement of pressure in patients with corneal disease.
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