Schiötz tonometers. An assessment of their usefulness.

THERE is no doubt that applanation tonometry gives more accurate information about the level of intra-ocular pressure than does indentation tonometry. This increased accuracy is due to the fact that applanation readings approach very closely to the levels of pressure in the undisturbed eye (Po), owing to the small volume of fluid displaced by the application of the tonometer (Goldmann, 1957). An indentation tonometer, on the other hand, which compresses the eye with a considerable weight (16-5 G in the case of a standard Schiotz tonometer), raises the intra-ocular pressure considerably and the level of Po can only be obtained by reference to conversion tables which assume that the eye responds in a standard way to the application of the tonometer (Grant, 1951). It is now recognized that the effect of the application of a Schiotz tonometer may vary, not only from eye to eye, but also in the same eye from time to time. This variability of effect is related to what we call "scleral rigidity", a complex of factors which includes not only the resistance of the ocular coats to deformation, but also the compressibility of the vascular contents of the eye, and the ease with which fluid is expressed through the drainage channels, both of aqueous and blood (Perkins and Gloster, 1957). The currently accepted conversion charts for Schiotz tonometers (the 1955 scale) are based on Friedenwald's work and assume an average scleral rigidity (Friedenwald, 1957). When the scleral rigidity is within the normal range, Schiotz tonometers (provided that they are of standard construction) may be expected to give readings of intra-ocular pressure which are of an acceptable standard of accuracy. The purpose of this present paper is to attempt to assess the relationship between the readings of applanation and Schiotz tonometers. Before doing so, however, it seems necessary to draw attention to certain established facts about Schiotz tonometers, and in particular, tonometers of the mechanical type. Some of these considerations do not apply to electronic tonometers.