The influence of residual methylcellulose solution on Tono-Pen readings

Calibration of the TonoVet and Tono-Pen Vet tonometers in the porcine eye

OBJECTIVE

The pig has an increasingly important role in ocular drug delivery models, but the most accurate tonometer in this species is unknown. The purpose of this study was to evaluate the accuracy of TonoVet and Tono-Pen Vet tonometers in the ex vivo porcine eye.

PROCEDURE

Four freshly enucleated normal porcine eyes were cannulated with two 25-gauge needles; one connected via tubing to a mercury manometer calibrated continuous physiologic recorder and the other connected to a reservoir of lactated Ringer's solution on an adjustable stand. Triplicate IOP readings were taken with the TonoVet and then the Tono-Pen Vet at 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 mmHg.

RESULTS

Linear regression showed strong linear trends for both the TonoVet (r² = 0.969) and Tono-Pen Vet (r² = 0.983). The TonoVet slightly underestimated IOP at lower pressures and slightly overestimated IOP at higher pressures (y = 1.092x - 4.0, where y = tonometer reading, x = manometer reading, and 4.0 = intercept). The Tono-Pen Vet consistently underestimated IOP (y = 0.773x - 2.1). These differences were statistically significant (P = <0.001, one-way repeated-measures ANOVA).

CONCLUSION

As in other species, both the TonoVet and Tono-Pen Vet tonometers do not measure true IOP in the porcine eye; however, the TonoVet more closely approximated true IOP in the normal porcine eye than the Tono-Pen Vet and may be the tonometer of choice for this species.

Tonometry in pediatric patients: a comparative study among Tono-pen, Perkins, and Schiötz tonometers

Measuring the intraocular pressure (IOP) in children can be difficult. We compared the accuracy of IOP measurements in pediatric patients using three tonometers: Perkins, Schiötz, and Tono-pen. Seventy-seven children (132 eyes; average age, 1 year, 7 months; range, 1 month to 60 months), mainly afflicted with retinopathy of prematurity (107 eyes), were evaluated. A good correlation (r = 0.867) was found between the Tono-pen and the Perkins tonometers. No statistical difference was found between the mean difference of IOP values obtained with the Tono-pen and the Perkins (P > .05). The Schiötz measurements were significantly higher than those obtained with the Perkins and the Tono-pen tonometers (P < .05). We concluded that the Tono-pen tonometer has a higher correlation coefficient, comparable to the Perkins tonometer, and can be used reliably to assess IOP in pediatric patients.