by Noelle La Croix, DVM, Dip. ACVO
Some Basic Principles of Tonometry
The diagnostic measurement of intraocular pressure (IOP) is important in accessing ocular disease. Most practices today use an applanation tonometer (Tono-Pen Vet™ – Reichert, Inc.) that infers IOP by flattening a small section of the cornea. This article will describe some of the basic principles that underlie this instrument’s use.
First some basic physics… pressure is defined as force per unit area. For example, the pressure exerted by a gas is due to the force of its moving atoms which collide with a surrounding container. Fluids are similar but have some unique properties. The pressure of a static fluid (at rest) depends upon its depth, density, and gravity. When a static fluid completely fills a container then its pressure is exerted equally across the entire container (the atoms of the liquid act like one giant enclosed object).
When a container is surrounded by equivalent pressures it will not distort and thus appears “flat.” A distorted, or curved container, must have unequal pressures exerted on its sides. One can therefore flatten a curved container by applying an opposing force to equalize the pressure of both sides. In the case of a container filled with a static fluid, the pressure at this flattened point is also equivalent to the pressure exerted by the entire fluid.
We can imagine the eye as a fluid-filled container with an internal static pressure surrounded by a curved container. An IOP can therefore be measured at any point along the container (the cornea for instance) that has been perfectly flattened. The flattening or bending force needed to measure IOP can be applied and precisiely measured with a tonometer.
The Tono-Pen Vet™ applanation tonometer can flatten a miniscule section of the cornea within 40 milliseconds. During this time period a footplate is advanced from the probe that strikes and momentarily flattens the cornea. When the footplate is completely covered with membrane the surface of the cornea must be flat against it. At this point the pressure exerted at the footplate must equal and oppose the IOP of the eye. The tonometer’s internal computer records multiple readings and displays the average as IOP in mmHg.
To properly use the Tono-Pen Vet™ it must first be properly calibrated. Calibration will adjust the readings for the effects of gravity and atmospheric pressure exerted upon the sliding footplate. The instrument is first held tip down to maximally extend the footplate, and then tip up to maximally retract the footplate. The tonometer’s computer will then adjust its measurements accordingly in use. During use the tonometer is held approximately perpendicular to the cornea to further minimize gravitational effects. The particular shape and molecular structure of the corneal surface is also inconsistent so that readings near the corneal rim are particularly inaccurate. The most accurate IOP readings are taken when the footplate flattens the axial cornea.
There are additional factors, besides corneal decentralization, which can increase inaccuracy when using this device. Excessive eyelid tension, air trapped between the footplate’s protective tip cover and the cornea, and pressure on jugular veins can alter IOP readings. The operator performing the tonometry should also hold the eyelids open without pressing upon the eyeball. I typically press the opened eyelids against the frontal and maxilla bones to avoid this (Figure 1).
Note that the tonometer’s footplate is performing the actual fattening or indentation of the cornea. A very light tap is all that is required to make contact with the cornea. Any further force can compress the globe itself and therefore raise IOP. A struggling patient can be sedated prior to tonometry but most sedatives will decrease IOP. The IOP of dogs are generally highest in the morning, and the IOP of cats are highest in the evening. The time at which tonometry was performed, and any prior sedation, should therefore be recorded with any IOP measurement.
For most dogs a “normal” IOP will range from 15 mmHg to 25 mmHg. In cats 17 mmHg to 20 mmHg is also considered normal. The increased IOP at which a referral to a veterinary ophthalmoligst is necessitated is not a set value. However, the makers of the Tono-Pen Vet™ are guided by the principle that the dangers of under-referral outweigh the disadvantages of over-referral. Therefore this tonometer’s computer tends to error towards slightly elevated readings. A good rule of thumb is to accept the most accurate IOP as the lowest IOP recorded by the Tono-Pen Vet™ after three or more successful readings.
Most practices today use a Tono-Pen Vet™ applanation tonometer as part of their diagnostic arsenal. Numerous other devices (Schiötz tonometer, pneumotonometer, rebound tonometer, etc.) have also been used to measure IOP based upon other physical principles. The Tono-Pen Vet™ is easy to use and can be highly accurate when used properly. If you have any questions or concerns about tonometry, please feel free to ask your veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: Applanation tonometry performed on a Toy Poodle.