Effect of auriculopalpebral nerve block on equine intraocular pressure measured by rebound tonometry (TonoVet^®)

Abstract

Objective To assess rebound tonometry intraocular pressure (IOP) in unsedated horses without and with auriculopalpebral (AP) nerve blocks. Animals Studied Twenty‐two client‐ and twenty university‐owned horses (84 total eyes) with unremarkable ophthalmic examinations were evaluated. Procedure One eye of each horse was chosen randomly, an AP block performed for that eye, and IOP measured in both eyes with a TonoVet®. The process was repeated for the contralateral eye 72 hours later under the same conditions as the initial measurements. Horses were unsedated for nerve blocks and tonometry. Linear mixed‐effects models were used for comparisons with statistical significance threshold of 0.05. Results Overall, blocked eyes had an 0.8 mm Hg lower average IOP than unblocked eyes (P = .039). IOP for client‐owned horses was on average 3.2 mm Hg lower than in UGA‐owned horses (P = .025) and was more impacted by AP block (1.4 mm Hg lower average in client‐owned blocked versus unblocked eyes (P = .006)). Block effectiveness was ranked on a subjective scale (“good”, “poor”, no block/control), and IOP was on average lower in eyes with a good block (P = .008). Conclusion Although there were statistically significant differences in IOP between blocked and unblocked eyes, between client‐ and UGA‐owned horses, and between eyes with good and poor AP blocks, these differences were not clinically significant. Thus, AP blocks remain a useful tool for evaluating equine ophthalmic health with minimal impact on IOP assessment.

Comparison of two rebound tonometers in healthy horses

“…It has been initially suggested that AP block could result in more reliable IOP readings in horses by minimizing tension on the globe caused by the eyelids 7,32 . However, more recent studies have repeatedly found AP block to cause no significant effect on the results or have observed a negligible statistical difference assumed to have no clinical relevance 10,22,33 . Nevertheless, in an unsedated horse, holding the upper eyelid open to avoid tonometer probe contact with the eyelid cilia is often required.…”

Intraocular pressure following four different intravenous sedation protocols in normal horses

“…Our finding that intravenous sedation type significantly influences IOP readings is clinically relevant because horses often require sedation to successfully perform tonometry, especially when potentially painful conditions such as uveitis and glaucoma are present. Thus, sedation type should also be considered alongside other factors known to affect IOP in horses, such as time of day, 10 head position 8 and use of nerve blocks 9 …”

“…All animals received auriculopalpebral nerve block prior to IOP assessment. While the IOP altering effects of this technique are minimal, 9 the immobilisation of the upper eyelid greatly facilitated IOP measurements, especially prior to the use of intravenous sedation. Topical ophthalmic anaesthetic was not administered to the study animals as this is generally considered unnecessary when using a rebound tonometer such as the TonoVet 17 .…”

“…Intraocular pressure (IOP) is an important parameter to assess for the diagnosis and management of equine glaucoma and uveitis 1,2 . Numerous exogenous and endogenous factors can affect IOP in horses: certain sedative drugs, 3‐7 head position, 8 the use of auriculopalpebral nerve block 9 and time of day 10 . These factors can make equine IOP assessment challenging but must all be considered when performing ocular examinations in this species.…”

Clinical insights: Ophthalmology in the eye of the beholder: Are we properly caring for horses with ocular disease?

“…Most sedation protocols have been shown to reduce IOP readings in horses. Additionally, many ophthalmologists recommend an auriculopalpebral nerve block is routinely undertaken prior to IOP measurement (to avoid the effect of blepharospasm artefactually increasing IOP readings); however, a recent study suggested the effect on rebound tonometry (Tonovet ™ ) readings was not clinically significant 12 . It should also be remembered that if a horse is sedated for IOP measurement, the head should be supported at a normal height during measurement as measurement when the head is below heart height will significantly increase IOP measurements 13 …”