Study Design and Methodologies for Evaluation of Anti-glaucoma Drugs - Ocular Pharmacology and Toxicology - page 209

pneumatonometer in cats, dogs, and rabbits [ 24 - 26 ]; it is more
cumbersome to use and has a much longer learning curve than the
other applanation or rebound tonometers. It still underestimates
true IOP in rabbits [ 25 , 26 ]. It also requires much greater cooper-
ation on the part of the animal to get accurate readings. In mano-
metric studies its IOP estimates are comparable to those obtained
with the rebound tonometer, although the latter is much easier to
use. In practice the mean IOPs obtained with the Perkins in con-
scious cats and dogs are not statistically significantly different from
those obtained with the Tono-Pen [ 23 ]. As the sensitive tip is
reused cross-contamination between groups is possible.
Rebound or induction/impact tonometry was introduced
2004 as an offshoot of efforts to develop accurate methods of
measuring IOP in rats and mice for glaucoma research [ 27 ]. The
fundamental premise of this instrument is that a precisely character-
ized disposable tonometer probe resembling a sewing pin is
electromagnetically propelled (induced) to come into contact
with (impact) and then rebound from the corneal surface. The
motion parameters of the probe have been determined to vary
with IOP and this experimentally derived calibration data has
been used to create species-specific internal algorithms. The inter-
nal calibration curve for the TonoLab has been optimized for mice
and rats [ 27 ] whereas the related TonoVet has three independent
internal calibration curves (cat/dog ¼

p) [ 13 ]. The probe is so light (0.027 g) that it can be used to
estimate IOP without the instillation of topical ocular anesthesia.
This allows one to avoid this potentially confounding variable in
topical anti-glaucoma drug studies [ 28 ]. Six individual measure-
ments are obtained, internally averaged, and the IOP estimate is
displayed accompanied by a letter indicating the species-specific
calibration curve that was used. The rebound tonometer can also
display a variety of error messages reflecting poor standard devia-
tion of the measurements, problems with the probe motion, and
misalignment contact with the central cornea. The tip is relatively
inexpensive ($1.66 each) and faster to change than the pneuma-
tonometer, but not as fast as the Tono-Pen. In practice the probe is
so small that in the author's experience less than 0.0002 g of tear
(and even less test article) may adhere to it after use. Of the
commercially available tonometers the TonoVet in the “d” setting
comes the closest to estimating true IOP in monkeys/dogs/cats
but, like the Tono-Pen and pneumatonometer, the “d” setting
underestimates IOP in rabbits [ 13 , 25 , 29 , 30 ]. Measurements
with the TonoVet also tend to exhibit less variability than Tono-
Pen which may be useful in detecting statistically significant differ-
ences between groups.

¼ d, horse ¼ h and other