Reliability and Application of the Pneumatonograph-Reply

Abstract

In Reply. —Very simply stated, two things bothered us about the PTGs Digilab loaned us. One was the under-estimate of IOPs in the glaucoma range. For instance, at an IOP of 35 mm Hg, the horizontally applied PTG probe gave a reading of 30.9 and the vertically applied probe a reading of 29.5 (closed-stopcock measurements on intact eyes simulating clinical tonometry). The other was the gross underestimate of IOP during tonography. In these open-stopcock measurements, an IOP of 35 mm Hg was registered by the PTG (loaded with a 10-g weight) as 25.1, and a pressure of 30 mm Hg was recorded as 21.3. The methods we used have been satisfactory in studies on the Schiotz, Goldmann, and Mackay-Marg tonometers and should be equally satisfactory for the PTG. The PTG depends on very small movements of the probe membrane, of the order of 0.00025 mm per scale unit; this could