Practical Matters: The disadvantages of digital tonometry in estimating intraocular pressure

May 01, 2004

Rhea V. Morgan
Although historically digital tonometry has been recommended as a means of estimating intraocular pressure (IOP) in dogs and cats, I find the method extremely unreliable. With digital tonometry, I have overestimated IOP in cats, brachycephalic dogs, and dogs with rigid sclera, such as Boston terriers and Chihuahuas. I have also underestimated IOP in dolichocephalic animals and animals with enophthalmos.

The accuracy of estimating IOP with digital palpation is affected by the anatomy of the surrounding orbit, the position of the globe within the orbit, and the status of the periocular soft tissues. I have found little correlation between the estimation of IOP by using digital palpation and the actual measurement of IOP by using either an indentation (Schiøtz) or applanation (Tono-Pen Vet—Medtronic Solan) tonometer. I highly recommend that small-animal veterinarians use either an indentation or applanation tonometer to measure IOP and that equine practitioners consider adding applanation tonometers to their practices. Although the indentation tonometer is being replaced by the applanation tonometer in many practices, it is still a reliable tool, especially when used on a consistent basis.

Rhea V. Morgan, DVM, DACVIM, DACVO
Smoky Mountain Veterinary Services
1212 Sawtooth Ridge Way
Walland, TN 37886