Measurement of Intraocular Pressure

The gold-standard method of intraocular pressure measurement is Goldmann applanation tonometry. The Goldmann tonometer is supplied as an accessory to the slit-lamp microscope. The principle of applanation is as follows: when two balloons are pushed together so that the interface is a flat surface, the pressure within the two balloons must be equal. By the same argument, when a fixed flat surface is pressed against a spherical surface, such as the cornea, at the point at which the spherical surface is exactly flattened, the intraocular pressure is equal to the pressure being applied. The app-lanation head is a small Perspex rod with a flattened end, which is fitted to a moveable arm. The tension applied to the moveable arm can be measured directly from a dial on the side of the instrument. The observer looks through the rod using the microscope of the slit-lamp, and the point at which exact flattening occurs can thus be gauged. For applanation tonometry, the patient is seated at the slit-lamp and not lying down but it is still necessary to instill a drop of local anaesthetic beforehand. Because the measurement of the intraocular pressure is such a basic requirement in any eye clinic, attempts have been made to introduce even more rapid and efficient devices. Perhaps the most ingenious to date is the tonometer, which measures the indentation of the cornea in response to a puff of air by a photoelectric method. This airpuff tonometer is less accurate than applan-ation, but it is useful for screening, although abnormal results should be confirmed by Goldmann tonometry.

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