Evaluation of the basic validity and clinical usefulness of the Mackay-Marg tonometer.

The present form of the Mackay-Marg tonometer consists of a 1.5 mm. diameter fat-based plunger mounted essentially flush with a flat footplate. The rubber mounting of the plunger acts as a stiff spring. Pressure measurements are derived from intraocular pressure acting through the flattened cornea against the plunger and causing microplunger movements. It has been shown that the response of this instrument is directly related in a linear fashion to hydrostatic pressure on the rubber membrane covering the plunger and to open-manometer pressure in enucleated eyes of various species. The slope of the linear relationship appears to vary slightly with the species. Clinical comparison was made between estimates of intraocular pressure obtained with the Mackay-Marg tonometer and the Goldmann applanation tonometer. Anesthesia was used for all examinations. There was a highly significant correlation between the results with the two instruments, the Mackay-Marg results being systematically greater and the Goldmann remits systematically lower than their mean remits. Comparison was also made between Mackay-Marg tonometer readings on the same eyes loithout and with corneal anesthesia. When anesthesia was not used the mean pressure was about 2 mm. Hg higher than when anesthesia was used. Potential errors and advantages were discussed.