Refined IOP measurement with rebound tonometry in the rabbit

Abstract

 

Purpose

 

An animal model for glaucoma research requires accurate and reproducible measurements of intraocular pressure (IOP).The induction-impact (rebound) Tonovet tonometer provides a non-invasive method of measuring IOP. Although rabbit models are often used in drug-related and surgical glaucoma research, IOP measurement by Tonovet has not been fully validated in this species. We aim to design a standardized method of IOP measurement with the rebound tonometer in rabbit.

 

Methods

 

All experimental procedures complied with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. IOP was set and measured manometrically in generally anesthetized rabbits (n=8) after anterior chamber cannulation through the peripheral cornea. The 26-gauge needle used for anterior chamber cannulation was connected to a pressure transducer-controlled syringe pump from Harvard Apparatus. IOP was raised in 5mmHg steps from 5 to 50 mmHg using an open stopcock method. The IOP was measured with the TonoVet in both “p” and “d” modes by taking four to six readings at each point by two independent observers. IOP was measured by orienting the tonovet in horizontal or vertical positions. TonoVet measurements were compared with the corresponding manometric measurements. The differences between IOP of left and right eye, ages of rabbits were also compared.

 

Results

 

The position of the tonovet handle makes no difference to IOP measurements.TonoVet readings correlated well with manometric IOP (“d” mode: r(2) coefficient = 0.9966; “p”mode: r(2) coefficient = 0.9941). No significant differences were observed when comparing right and left eyes (t-test, p>0.05). The TonoVet had a tendency to underestimate IOP compared to anterior chamber cannulation. TonoVet readings obtained with “d” mode were closer to manometric IOP than those obtained with the “p” mode, although conversion formulae were possible to calculate for both modes. IOP recover at day 3 after cannulation. Younger rabbits have lower IOP compared with older rabbits (P< 0.01).

 

Conclusions

 

The TonoVet is a reliable and precise tool for the measurement of IOP in rabbits. The measured IOP tended to underestimate at IOP values from 5 to 50 mmHg. To compensate for this systematic error and calculate the actual IOP, correction factors were needed. The “d” mode is more useful than “p” mode in measuring rabbit IOP.

 

 

Correlation of cannulation and d mode

 

 

correlation of cannulation and p mode

 

Keywords: 568 intraocular pressure