The influence of the tonometer position on canine intraocular pressure measurements using the Tonovet^® rebound tonometer

Abstract

The objective of this study was to assess the variability of readings made using the Tonovet® rebound tonometer for measurement of intraocular pressure (IOP) in the peripheral cornea and in angulated positions on the canine corneal surface. Forty-six client-owned dogs admitted for ophthalmic evaluation at the Queen’s Veterinary School Hospital, University of Cambridge were included in the study. IOP readings were taken at a variety of locations and using the tonometer at a number of different angles to the cornea: 1) Perpendicularly at center of the cornea (CC); 2) At the center of the cornea but with the tonometer positioned at four angles, and 3) At four different points on the peripheral cornea. All values were compared with the values recorded at the recommended CC position. IOP values were significantly underestimated in seven positions, with median and interquartile range from 12.1 ± 4 mmHg (nasal on periphery) to 15 ± 5 mmHg (laterally angled at center), varying between 0 mmHg to 2.9 mmHg from the CC value. While dorsally angled in the central cornea were not significantly different from those at CC (p = 0.09). Median values were lower for measurements in peripheral positions when compared to angled central positions. These results demonstrate that angling the tonometer or measuring in peripheral regions can result in small but statistically significant underestimation of IOP values.

Comparison of two rebound tonometers in healthy horses

“…A similar benefit of this property of rebound tonometry has been described in human patients 29 . Keeping in mind that misalignment of the rebound tonometer probe will affect the IOP readings, 23,30 we included two patients with one eye each having a minor peripheral scar in the study, as these lesions were not located in the central part targeted when performing the tonometry. The IOP readings between the two eyes in both of these horses were very symmetrical with both tonometers used, and thus, the minor peripheral corneal scars were considered unlikely to have impacted the readings.…”

“…In dogs, misalignment of the rebound tonometer during measurement has been found to result in underestimation of IOP 24,30 . The equine cornea is larger, and thus, central placement is easy to achieve, but in an unsedated horse, the height of the patient relative to the height of the examiner can occasionally pose challenges and lead to misalignment of the device.…”

Comparative intraocular pressure measurements using three different rebound tonometers through in an ex vivo analysis and clinical trials in canine eyes

“…Three measurement methods, including indentation, applanation, and rebound tonometers, have been used in veterinary medicine to determine IOP. Of these, the rebound tonometer TONOVET ® has been widely used among veterinarians 6,10,20‐25 . Validation of this device has been reported not only for dogs but also for mice, monkeys, pigeons, birds of preys, and rabbits 12,15,26‐28 .…”

Intraocular pressure measurements using the TONOVET® rebound tonometer: Influence of the probe‐cornea distance

“…Even though tonometry performed by contact methods is reasonably accurate and very repeatable including in conscious animals, rebound tonometry using TonoVet ® and more recently TonoVet Plus ® (Icare) has become increasingly popular during the last decade for measuring intraocular pressure (IOP) in animals, including domestic, laboratory, and exotic and wildlife species 1‐15 . Rebound tonometry also is used in humans, especially in children 16‐19 .…”

“…Both TonoVet ® and TonoVet Plus ® have a smaller probe tip compared to applanation tonometers, which facilitates the IOP estimation in smaller animal species 1,20,21 . A potential disadvantage of the TonoVet ® is that the instrument needs to be positioned with the probe oriented horizontally, perpendicular to the axial corneal surface 1,11 . In general, this is not the case for applanation tonometry, which allows small variations in positioning 1,20 …”