Duration of corneal anaesthesia following multiple doses and two concentrations of tetracaine hydrochloride eyedrops on the normal equine cornea.
Authors: Monclin S J, Farnir F, Grauwels M
Journal: Equine veterinary journal
Summary
# Editorial Summary: Tetracaine Hydrochloride Duration in Equine Corneal Anaesthesia Whilst tetracaine hydrochloride (THCl) eyedrops are commonly used for topical ocular anaesthesia in equine practice, the specific duration of effect and optimal dosing protocols have lacked rigorous documentation. Monclin and colleagues measured corneal touch threshold (CTT) in ten healthy horses before and after three treatment regimens—a single 0.5% drop, two 0.5% drops administered one minute apart, and a single 1% drop—recording recovery to baseline sensitivity every five minutes throughout the anaesthetic period. A single 0.5% drop provided only 30 minutes of reduced corneal sensitivity with just 5.5 minutes of complete anaesthesia (CTT of 0 mm), whereas doubling the dose to two 0.5% drops extended this to 60 minutes overall and 16 minutes of maximal effect; the 1% concentration occupied middle ground at 50 minutes and 15.25 minutes respectively, achieving complete anaesthesia in 80% of eyes compared to 55% with single-dose 0.5%. For practitioners performing diagnostic procedures or therapeutic interventions, these findings provide evidence-based guidance: a single drop offers sufficient protection only for brief examinations, whilst either a second application or higher concentration substantially improves both duration and reliability of anaesthetic cover, though the choice between these strategies may depend on corneal sensitivity, procedure length and individual patient factors.
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Practical Takeaways
- •For brief diagnostic procedures (corneal evaluation, foreign body removal), a single 0.5% drop provides 30 minutes of anaesthesia—plan procedures accordingly
- •For longer therapeutic procedures, use either two 0.5% drops at 1-minute intervals (60 min coverage) or a single 1% drop (50 min coverage) to ensure adequate anaesthesia throughout
- •Maximal anaesthetic effect appears rapidly (within 5 minutes) with higher concentrations or multiple doses, reducing recovery time between instillation and procedure commencement
Key Findings
- •Single drop 0.5% tetracaine produced corneal anaesthesia for 30 minutes with maximal effect lasting 5.5 minutes
- •Two drops 0.5% tetracaine extended anaesthesia to 60 minutes with maximal effect lasting 16 minutes and present in 90% of eyes
- •Single drop 1% tetracaine produced 50 minutes of anaesthesia with maximal effect lasting 15.25 minutes in 80% of eyes
- •Multiple dosing or increased concentration significantly prolonged both duration and intensity of corneal anaesthesia