Applanation tonometry is based on the principle that the force required to flatten a certain area of a sphere is the same pressure as that inside the sphere. Tonometry provides an estimate of intraocular pressure (IOP). This paper compared 2 applanation tonometers. The Tono-Pen XL probe is covered by a disposable latex device and a signal is produced when it contacts the cornea. The signal is digitized and sent to the internal microprocessor. A mean of 3 measurements is calculated. The Perkins tonometer uses the Goldmann applanation method, which involves a double prism to measure corneal indentation and IOP with formation of fluorescein semicircles. The prism must be removed and disinfected between patients.

A postmortem study was first performed for calibration of applanation tonometry with direct manometry. An in vivo study then evaluated 50 eyes from 25 dogs and 50 eyes from 25 cats using both tonometers. No significant difference in mean IOP values was found between the 2 tonometers, but there was a difference between the minimum and maximum values. Advantages of the Tono-Pen XL included ease of taking and visualizing the IOP reading, ease of disinfecting, and long-term battery life. However, it is more expensive, requires frequent calibration, and tends to underestimate IOP. Advantages of the Perkins tonometer included accuracy, inexpensive disinfection of the prism, long-term battery life, and low cost. However, a longer training period is required to achieve accurate IOP measurements.

Commentary
The Tono-Pen XL has been widely adopted by veterinary ophthalmologists despite its tendency to underestimate IOP. The Perkins tonometer (marketed for humans) does not provide a digital reading of IOP, which must be calculated based on subjective visual assessment of semicircle intersections on a dial. Maximum IOP that can be measured on the calibrated dial is ~50 mm Hg.

A major flaw of the study is that direct comparisons between instruments were not possible. Veterinarians should also be aware of the significant impact of inappropriate restraint and tear film and corneal pathology on the ability to obtain accurate IOP readings with applanation tonometers. Together, these important sources of inaccuracy need to be emphasized.—Gillian J. McLellan, BVMS, PhD, DVOphthal, DACVO, DECVO, MRCVS

Source
Comparison of intraocular pressure measurements between the Tono-Pen XL® and Perkins® applanation tonometers in dogs and cats. Andrade SF, Palozzi RJ, Giuffrida R, et al. VET OPHTHALMOL 15:14-20, 2012.