Comparison of intraocular pressure measurements between the Tono-Pen XL® and Perkins® applanation tonometers in dogs and cats

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To compare the accuracy between two applanation tonometers, Tono-Pen XL® and Perkins®, in ophthalmoscopically normal dogs and cats.


Both eyes of 25 conscious and healthy dogs and cats were evaluated. Both eyes of five dogs and cats immediately after sacrifice were used as controls for the postmortem study.


In conscious animals, the tonometry was performed with topical anesthesia using 0.5% proxymetacaine eye drops for both tonometers and 1% fluorescein eye drops for the Perkins tonometer. Readings of intraocular pressure (IOP) in the postmortem study were taken using manometry and tonometry by Tono-Pen XL® and Perkins®.


The correlation coefficient (r2) in dogs between manometry and applanation tonometers Tono-Pen XL® and Perkins® were, respectively, 0.896 and 0.981 and in cats were 0.905 and 0.988. The mean IOP values in conscious dogs with Tono-Pen XL® and Perkins® were, respectively, 17.5 ± 3.7 mmHg (10.0–25.0 mmHg) and 15.3 ± 2.1 mmHg (12.0–19.8 mmHg) and in conscious cats were 16.8 ± 3.6 mmHg (10.5–24.5 mmHg) and 15.5 ± 1.3 mmHg (13.0–18.5 mmHg).


There was a strong correlation between the IOP values obtained by direct ocular manometry and the Tono-Pen XL® and Perkins® tonometers in dogs and cats. There was no statistically significant difference between the mean IOP obtained with both tonometers in conscious animals, there was, however, a difference between the minimum and mainly in the maximum values that were on average 5–6 mmHg higher with Tono-Pen XL® than those measured with Perkins®, which justifies a table of normal values differentiated for each tonometer.

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