Evaluation of intraocular pressure in conscious ferrets (Mustela putorius furo) by means of rebound tonometry and comparison with applanation tonometry.

To determine normal intraocular pressure (IOP) values by means of rebound tonometry in unanaesthetised ferrets, and to compare rebound and applanation tonometry, 55 clinically healthy ferrets were included in an observational, prospective, blinded study. On 52 ferrets, IOP was measured by means of rebound and applanation tonometry. On 3 ferrets, rebound tonometry was performed every two hours over a 24-hour period. Mean IOPs of the 104 eyes obtained with the rebound tonometer were 14.07±0.35 (95% CI 13.37 to 14.77) tonometer-units (TU). The IOP was significantly higher in males than in females, controlling for age and weight. A study-ordered decrease in coefficient of variation (CV) was observed (measurement 1-52=21.2±1.4 per cent v measurement 53-104=14.4±1.1 per cent) and high CVs (>30 per cent) were significantly less frequent in measurement 53-104. A significant difference in IOP during the 24- hour measurements was found, with the lowest IOP recorded at 22:00. The tonometers presented poor agreement, and IOP values were not correlated. The difference in IOP estimation increased with the magnitude of the measurements. Applanation tonometry presented a significant higher frequency of per-eye IOP values exceeding 25 and 30 TU, and a significant lower repeatability (CV=37.1±2.6 per cent v 17.8±1.2 per cent) compared with rebound tonometry. In conclusion, several factors need to be considered when measuring IOP in ferrets.