Accuracy of intraocular pressure measurements with two different tonometers through bandage contact lenses

The intraocular pressure (IOP) of nine cadaver eyes was set by a manometer at 10 mm Hg increments from 10 to 50 mm Hg. Pressure measurements at each of these settings were taken using the Digilab Pneumatonometer and Tono-Pen to determine the accuracy of these instruments. In addition, four brands of therapeutic contact lenses were placed on the eyes, and IOPs were measured through them to determine whether or not comparable IOPs could be obtained through a range of bandage contact lenses. We found a significant difference (p < or = 0.0000) between the measurements obtained by the two instruments at a given manometric setting. Digilab Pneumatonometer's pressures correlated well with manometric pressures. Tono-Pen consistently underestimated manometric pressures. At 50 mm Hg, Digilab Pneumatonometer's mean measurement was 47.5 mm Hg, whereas Tono-Pen's mean was only 38.9 mm Hg. IOPs assessed through all contact lenses were comparable to the measurements without lenses. Multiple regression of score showed that the measured pressure was explained by manometric pressure (81%), the choice of instrument used (5.5%), and the lens thickness (0.09%). The Pierson correlation coefficient was 0.93. This study showed that Pneumatonometer could accurately measure IOP with and without a therapeutic contact lens, but Tono-Pen was equally inaccurate with and without lenses, giving false low measurements.