Accurate intraocular pressure (IOP) measurement is important in the diagnosis of glaucoma and uveitis and is a critical data point for monitoring response to treatment. The investigators had observed that many dogs referred for glaucoma were brachycephalic or had prominent eyes, and in many cases the diagnosis was erroneous. They hypothesized that this might be due to the method of patient restraint during the measurement of IOP. In this study, 30 dogs with normal eyes (n = 57) were used. Dogs received a complete ophthalmic examination and were excluded if any abnormalities were noted or if baseline IOP was > 25 mm Hg. IOP was then measured in one or both eyes during 6 randomized manipulations of the eyelid or upper body. These manipulations included: 1) baseline with minimal eyelid restraint and no pressure on the jugular vein, 2) maximal extension of the eyelids ventrally and dorsally with no pressure on the jugular veins, 3) extension of the eyelids laterally with no pressure on the jugular veins, 4) minimal eyelid restraint with manual pressure on the jugular vein ipsilateral to the eye being measured, 5) minimal eyelid restraint with manual pressure on both jugular veins, and 6) extension of the eyelids laterally with manual pressure on both jugular veins. Pressure was applied on the jugular veins for > 1 minute prior to measurement. Proparacaine hydrochloride 0.5% ophthalmic solution was applied to eyes at least 1 minute prior to measurement. Measurements were obtained until 3 with < 5% variance were recorded. The same tonometer was used throughout the study and calibrated each day. To minimize stress, no attempt was made to use a consistent body position; positions included sternal recumbency, sitting, and standing. Two manipulations caused the greatest significant increase in mean IOP: lateral eyelid extension with compression on both jugular veins and lateral eyelid extension alone. Dorsoventral eyelid extension and jugular compression alone also significantly increased IOP as compared with baseline.

Commentary: The Tono-Pen causes the most frustration of any ophthalmic instrument, and this article highlights some of the reasons why. Subtle differences in technique can make a huge difference in pressure readings. My advice to general practitioners is to make sure you are holding the eyelids open gently and that there is bone, not eyeball, under your fingers. Pressure from the way the eyelids are held open (ie, too close to lid margin, causing pressure on globe or overextension of eyelids) is probably the most common cause of falsely elevated pressure readings, so this simple tip can help tremendously.—Ellison Bentley, DVM, Diplomate ACVO

Effect of eyelid manipulation and manual jugular compression on intraocular pressure measurement in dogs. Klein HE, Krohne SG, Moore GE, et al. JAVMA 238:1292-1295, 2011.