OBJECTIVE: To compare the TonoVet rebound tonometer with the Tono-Pen Vet applanation tonometer in a larger number of glaucomatous eyes and to evaluate the effect of different corneal pathologies on both tonometers. Procedure In 26 eyes with clinical signs of glaucoma, intraocular pressure (IOP) was measured using the TonoVet followed by the Tono-Pen Vet. In 29 eyes with focal corneal pathology (e.g., corneal scarring, edema, pigmentation), both tonometers were used successively to measure IOP in one unaffected area of the cornea, as well as on the lesion itself. Impact on measurement results was assessed comparing the deviation in IOP readings of each tonometer between the two localizations. Statistical data analysis included paired t-tests and regression analysis using SAS software (version 9.2; SAS Institute, Cary, NC).
RESULTS: In glaucomatous eyes, the TonoVet consistently yielded higher values of IOP than the Tono-Pen Vet as can be quantified by the regression equation IOP (TonoVet) [mmHg] = 1.12 * IOP (Tono-Pen Vet) [mmHg] + 11.5 with R2 = 0.91 and P < 0.0001. Depending on the type and degree of corneal pathology, the deviation in IOP resulting from measurements on altered cornea ranged from 6 to 16 mmHg for the TonoVet and 7 to 20 mmHg for the Tono-Pen Vet, respectively. On average, the effect of corneal disease on IOP measurements was lower for the TonoVet by 1.14 mmHg.
CONCLUSIONS: Rebound tonometry appears to be a valuable alternative to established applanation tonometry in patients with ocular disease such as glaucoma and corneal disorders. In patients suffering from glaucoma, the same type of tonometer should be used for follow-up examinations, as measurement results of the TonoVet and the Tono-Pen Vet differ substantially with increasing IOP. Corneal pathology has considerable influence on both tonometers with the degree of over- or underestimation of IOP depending on the alteration of biomechanical properties of the cornea inflicted by various corneal pathologies.