David A. Wilkie, DVM, MS, DACVO
The Ohio State University Veterinary Hospital Dept.
Columbus, OH, USA
Intraocular Pressure Measurement
Determination of intraocular pressure (IOP) is indicated in all eyes with diffuse corneal edema, anisocoria, fixed and dilated pupils, episcleral congestion, blindness, buphthalmos and anterior uveitis. In addition, animals with medically or surgically controlled glaucoma require sequential determination of IOP to ensure adequate control, and animals with unilateral primary glaucoma and breeds predisposed to primary glaucoma require monitoring of the IOP in the unaffected at risk eye. Determination of intraocular pressure requires an instrument that is fast, accurate, portable and user friendly. As clinicians we must feel we can rely on the accuracy of the test and make treatment decisions based on the results. In my opinion, most of us do not feel this way about the Schiotz tonometer and consequentially, this instrument remains in its case in a drawer.
There are 2 specific ways to determine intraocular pressure: indentation tonometry and applanation tonometry.
The indentation tonometer measures the amount of corneal indentation that occurs when a given weight is placed on the cornea. The result is inversely proportional to the intraocular pressure and the actual pressure must be obtained from a table of values. The Schiotz tonometer is an indentation tonometer. The Schiotz tonometer requires assembly, disassembly and cleaning in order to ensure its accuracy. The foot plate is large and the patient must be cooperative in order to place the foot plate on the cornea in a vertical position. If the animal is fractious or the eye painful then it is unlikely that accurate placement will be obtained and erroneous values will result. My clinical experience is that practices with a Schiotz tonometer either do not use it or do not believe the results obtained. In many instances, pressures obtained by this method are not confirmed on referral to a specialist. The result is that glaucoma is not diagnosed or monitored accurately and IOP determination is not performed at the frequency indicated by the breed or clinical signs.
Applanation tonometry determines IOP by evaluating the force require to applanate or flatten a given surface area of cornea. Typically in veterinary ophthalmology these are electronic or battery powered tonometers. They have been shown to be highly accurate across species and a wide range of IOPs. In recent years, the Tonopen has been evaluated and shown to be similar to other applanation tonometers in accuracy. It is light weight, portable, accurate, self calibrating and averages several readings and gives a % error to ensure accuracy. In addition, the small foot plate allows this tonometer to be used on painful eyes in less cooperative patients, as only a small area of cornea is required to obtain a reading and the position of the patient's head is not related to obtaining the reading. Finally, this is the only applanation tonometer that is portable enough to allow routine IOP determination in the equine patient. Equine glaucoma in recent years has received attention and has increased in prevalence solely due to the availability of the Tonopen and the increase in the number of equine eyes that are evaluated. The newest Tonopen is the Tonopen Avia which allows for faster and increased numbers of readings increasing accuracy.
An alternative to the Tonopen is the Tonovet which is considered a rebound tonometer. It has also been evaluated in several species and has clinical application in veterinary ophthalmology. It should be noted however that the two tonometers, the Tonopen and Tonovet, are not interchangeable as the readings may differ significantly.
The ease of use, accuracy and comfort level the applanation tonometer gives your practice will ensure its frequent use, increase your hospital's awareness of glaucoma, allow early and prompt referral of glaucoma patients to a specialist if indicated, and subsequent referral of your glaucoma patients back to your hospital for monitoring following laser or other glaucoma surgery by a specialist. In addition, the Tonopen will allow you to incorporate IOP determination as a routine part of the physical examination in those breeds predisposed to primary glaucoma. In an equine practice it allows evaluation of IOP in horses with Equine Recurrent Uveitis and in those suspected of having glaucoma.