The Ophthalmic Examination as It Pertains to General Ocular Toxicology: Basic and Advanced Techniques and Species-Associated Findings - Ocular Pharmacology and Toxicology - page 189

Fig. 56 A Tonopen Vet ® being calibrated prior to use

stress, diurnal or circadian rhythm [ 92 , 98 ], eye position, sedation
or anesthesia, corneal thickness, and several other variables. If
possible, the IOP should be obtained for all animals at the same
general time of day throughout the study and by the same exam-
iner, using the same tonometer, same handling personnel, and same
technique each time. Determination of IOP should be performed
prior to pharmacologic dilation. When IOP is a critical aspect of a
study, it is also advisable for the animals to be acclimatized to both
the procedure and restraint techniques prior to study initiation.
Applanation tonometry is most commonly performed using
the Tonpen XL ® , Tonopen Vet ® , Tonopen Avia ® , or pneumotono-
graph. It requires topical anesthesia of which 0.5 % proparacaine is
the most common topical ophthalmic anesthetic of choice. This
technique measures the force required to applanate or flatten a
given area of cornea and then converts this into an IOP value in
mmHg. These tonometers have the advantage of being able to be
self-calibrated for a GLP study (Fig. 56 ) and the pneumotonograph
can also provide a hard paper copy for record keeping. The Tonpen
XL and Tonopen Vet ® obtain four independent readings, average
them, and indicate both the IOP and the % error indicating the
variability between the four readings obtained. The Tonopen Avia ®
obtains ten independent readings, averages them, and reports the
IOP and reports the variability in readings as a % confidence. For
the Tonopen XL ® the % error should be 5 % and for the Tonopen

<

Avia ® the % confidence should be 95 %.

>