[Tono-pen, a new position-independent tonometer. Comparison with the Goldmann tonometer by applanation measurement]

A new hand-held tonometer, Tono-Pen (TP), was compared with the Goldmann tonometer (GM). The TP utilizes the operating principle of the Mackay-Marg tonometer. However, the TP is battery-operated, only 18 cm long, weighs only 60 g, and features electronic signal analysis (single-chip microprocessor) and a digital pressure display (mm Hg). In 108 subjects (216 eyes) presenting no ocular abnormalities, comparative tonometric measurements were performed in randomized order by two investigators (A: TP; B: GM). In 33 of these subjects (66 eyes), the intraocular pressure (IOP) was changed by means of a scleral suction-cup system. A total of 457 pairs of values was obtained in an IOP range extending continuously from 0 to 50 mm Hg. The relationship between TP (y axis) and GM (x axis) pressure readings is described exactly by a slightly curvilinear graph (y = 0.96x - 0.003x2 + 1.48; mm Hg). This may be approximated by a regression line with a slope of 0.8, a y intercept of 2.76 mm Hg and a standard deviation of 2.5 mm Hg (y values) in relation to the regression line. The mean values were 17.6 (TP) and 18.6 mm Hg (GM), which were not significantly different (P less than 0.05). The correlation coefficient amounted to 0.97. Below an IOP (GM) of 16 mm Hg, the TP slightly overestimated the IOP by a maximum of 1.48 mm Hg at GM = 0). Above 16 mm Hg, the TP increasingly underestimated, the IOP by a maximum of 8 mm Hg at GM = 50). Summing up, the TP is unique among hand-held tonometers in its miniaturization. In the low and physiological pressure range, there is an acceptable relationship between TP and GM readings. In the high pressure range, however, its readings deviate considerably from those of the tonometer.