Tonometry

As a patient with severe conjunctivitis or keratitis, can exhibit congestion of the conjunctival vessels, an important differential diagnosis to Conjunctivitis is glaucoma. Glaucoma manifests as a pathologic increase in the intra-ocular pressure (Magrane 1957). Also in the paradigm of "Red Eye" belongs Uveitis. In uveitis, aqueous flare (graded 1 to 4) (Nasisse 1999) and cells are often seen in the anterior chamber (Gionfriddo 1995). Flare and cells are most easily diagnosed and graded using a little ray of light, holding this light close to the cornea and examining the anterior chamber temporally (perpendicular to the light source). Uveitis can cause inflammation of the corpus ciliare, resulting in lesser production of aqueous humour and hypotony (Miller et al 1991, Blogg 1981) . Measurement of the Intra-ocular Pressure (IOP) can therefore aid the ophthalmologist in diagnosing Glaucoma or Uveitis.

While performing tonometry it is important not to apply force to the throat of the patient as compression of the vena jugularis can result in a rise in the IOP (Peiffer et al 1977a). The IOP in canines has most commonly been measured with the McKay-Marg tonometer and the Schiötz tonometer. In resent years the handheld digital tonometer (eg. TONOPEN® XL) has been developed. Although most Veterinary Ophthalmologists agree that the McKay-Marg tonometer is the most accurate tonometer (Miller and Pickett 1992, Priehs et al 1990), the Mackay-Marg is no longer produced.

Measurement of the IOP with Schiötz Tonometer

An anaesthetic is instilled topically onto the cornea.The nose of the dog is held in an upright position so the cornea is horizontal. Then the Schiötz tonometer is applied to the cornea and the reading is noted. The Schiötz tonometer is an indentation tonometer, where hardness of the eye in inversely proportional with the indentation. Therefore a "hard" eye (high IOP) will give a low reading. The reading on the scale is correlated to the IOP by using a table. Fiedenwald's empiric research in the 50'ies resulted in a human table (Friedenwald 1957). Since then papers have been published proposing use of different tables for Schiötz tonometry on the canine eye (Peiffer et al 1977a, Peiffer et al 1977b), but the latest conclusions are, that the original Friedenwald table (Friedenwald 1957) is the most accurate and applicable to the dog (Miller and Pickett 1992). With the Schiötz tonometer several measurements with different weights will make it possible to make a more accurate estimation of the IOP.

The digital tonometer(eg TONOPEN® XL ) is simply tuned on pushing a button. Several measurements are made with the tip pressed gently against the cornea, with the dog in any position, and a "beep" signals to the examiner that the readings are over. A percentage (standard deviation) shown on the display shows the accuracy of the readings, with the mean displayed in large numbers. The Tonopen works by applanation tonometry. The tip is gently applied to the cornea as force transmitted through the tip will result in a faulty reading of the IOP.

This page was authored by T. F. Evans October 2000.