Ex vivo Evaluation of Tono-Pen and Pneumotonometry in Cat Eyes

PURPOSE: To evaluate the validity and intraobserver reliability of intraocular pressure (IOP) measurements with both pneumotonometry and the Tono-Pen in a closed ex vivo system in cat eyes. METHODS: IOP was increased step by step in 5 enucleated cat eyes, while taking IOP measurements with the Tono-Pen and pneumotonometry. The outcomes were compared to readings of a digital manometer simultaneously measuring the actual pressure in the anterior chamber. RESULTS: Pneumotonometry overestimated IOP below 15 mm Hg and underestimated pressures above 20 mm Hg. Tono-Pen tonometry considerably underestimated IOP over the whole spectrum in all of the eyes tested. The pneumotonometer was identified as the more valid and reliable instrument for cat eyes. CONCLUSION: Both tonometers are clinically useful tools to assess IOP for glaucoma studies using a cat animal model. However, one has to consider underestimation of IOP in the upper ranges. A correction formula can be used to calculate the actual IOP.