"Minified" Goldmann applanating prism for tonometry in monkeys and humans

Abstract

The tip diameter of the standard Goldmann applanating prism was reduced from 7.0 mm to 4.0 or 4.5 mm, but the endpoint of intraocular pressure measurement-applanation of a circle of cornea having a diameter of 3.06 mm-was not changed. The "minified" tonometers exhibited standard mechanical calibration characteristics in three standard configurations. The 4.5-mm tonometer was calibrated for cynomolgus monkey eyes in vivo by open and closed stopcock manometry. By open stopcock manometry, between pressures of 4 and 70 mm Hg, IOP(tonometer) = 1.01 IOP(manometer)-0.72, with very little scatter or curvature. During closed stopcock manometry, between pressures of 5 and 55 mm Hg, IOP(tonometer)=1.07 IOP(manometer)-1.32, again with very little scatter or curvature. The tonometer elevated manometric IOP by an average of about 7.5%; the percentage pressure elevation decreased slightly as pressure increased. The 4.0-mm tonometer was calibrated against a standard tonometer in living human eyes. Over the pressure range from 0 to 75 mm Hg, IOP(minitonometer)=0.98 IOP(standard tonometer)+ 0.82, with very little scatter or curvature. Similar comparison of two standard tonometers showed a virtually identical relationship and scatter. We conclude that minified Goldmann tonometers provide accurate and reprodible measurements of IOP in cynomolgus monkey and human eyes.