Which eyes need their pressure taken?

Although it sounds trite, the answer to the question “Which eyes need their pressure taken?” is easy. Every patient which may have altered intraocular pressure (IOP)! Let’s assess that answer a bit more closely.

This includes those which may have conjunctivitis (in which the IOP is always normal), uveitis (in which IOP tends to be low) or glaucoma (in which IOP is always elevated). Therefore, tonometry should be performed on any eye which is reddened, has discharge, is painful (blepharospasm in dogs or cats; lethargy in cats), has corneal opacity (“cloudiness”), vision loss, altered pupil size (anisocoria), or altered pupillary light reflexes. Once you have diagnosed uveitis or glaucoma, tonometry is one of the most important things to do at every recheck to guide adjustments in drugs administered and their dosages.

The Tono-Pen and the TONOVET

The availability of the easily-used and reasonably-priced tonometers such as the Tono-Pen Vet™ and the TONOVET™ make measurement of IOP easy in all species, particularly cats. For both techniques, the patient is minimally restrained so as not to artificially raise IOP via direct pressure on the jugular veins or the globe itself. For the Tono-Pen but not for the TONOVET, a drop of topical anesthetic is applied to the corneal surface. I like to rest the hand holding the Tono-Pen onto the hand holding the eyelids or onto the patient’s head itself and gently touch the central cornea with the Tono-Pen tip using minor “tremor-like” blotting or kissing movements. Particular attention should be paid to the “approach angle” of the Tono-Pen tip to the cornea so that the tip’s flat surface is parallel to the corneal surface. This is best achieved by viewing the interface between the cornea and the tip from the side. The “reliability” (coefficient of variance) of the result should be 5% or tonometry should be repeated.

The TONOVET is a rebound tonometer which ejects a small probe at a fixed distance from the cornea and assesses the motion of the probe as it returns to the instrument after rebounding from the cornea. The probe should be held about 6 mm from the cornea and held horizontal (parallel with the floor). Eyes with higher IOP cause a more rapid deceleration of the probe and a shorter return time to the instrument.

This technique is affected to some degree by corneal thickness and ocular surface tension, and therefore should be performed before application of any topical medications, including topical anesthetic. Tono-Pen applanation tonometers are probably less susceptible to erroneous readings attributable to these variables than are the rebound tonometers. However, unlike the Tono-Pen the TONOVET has been calibrated for normal dogs, cats, and horses, allowing the operator to select the correct calibration curve for each species. Because of this species-specific calibration in normal animals, the TONOVET tends to estimate IOP very close to true manometric pressure. In contrast, the internal calibration curve of the Tono-Pen has been optimized for humans, not animals, and although the IOP estimates with this instrument are highly correlated with manometric pressure, the Tono-Pen applanation tonometer tends to overestimate pressure at lower IOPs and underestimate IOP at higher pressures.

Interpreting IOP

Across large populations, normal canine and feline IOP is reported as approximately 10–20 mm Hg. However, significant variation is noted between individuals, technique, and time of day. Comparison of IOP between right and left eyes is therefore critical to interpretation of results. A good rule of thumb is that IOP should not vary between eyes of the same patient by more than 20%. The obvious application for tonometry is the diagnosis of glaucoma where IOP is generally elevated. However tonometry is also used to diagnose uveitis (in which IOP is lowered) and conjunctivitis (in which IOP is normal). Perhaps the most important role for tonometry is the monitoring and adjustment of therapy.