Duration of corneal anaesthesia following multiple doses and two concentrations of tetracaine hydrochloride eyedrops on the normal equine cornea.

This study aims to determine the duration of corneal anaesthesia in horses owing to the instillation of varying dosages and concentrations of tetracaine hydrochloride (THCl). The research discovers that applying more than one dosage or using more concentrated eyedrops significantly extends the duration of both anaesthesia and its maximum effect.

Objective and Methods

• The objective of this study was to ascertain the period of corneal numbness following the application of multiple doses and two concentrations (0.5% and 1%) of THCl in ten healthy horses.
• To achieve this, the corneal touch threshold (CTT), a measure of corneal sensitivity, was determined in both eyes before and after the application of the eyedrops.
• CTT was measured at intervals of 5 minutes until a full recovery of the basal CTT was observed.
• The entire processes were repeated after an interval of at least one week for each horse.

Results

• The study found that after the application of one drop of 0.5% THCl, corneal sensitivity was significantly reduced for approximately 30 minutes. This duration increased to 60 minutes when two drops of 0.5% THCl were used and reduced to 50 minutes when a drop of 1% THCl was used.
• The maximal anaesthetic effects, defined as a CTT of 0 mm, lasted for 5.5, 16 and 15.25 minutes respectively after the application of one drop of 0.5% THCl, two drops of 0.5% THCl, and one drop of 1% THCl.
• Around 55, 90, and 80% of the horses underwent maximal anaesthetic effects 5 minutes after the corresponding treatment methods.

Conclusions

• The researchers concluded that the application of a second drop or the use of a more concentrated THCl solution significantly lengthens the duration of both anaesthesia and its maximal effect.
• By understanding the duration of corneal anaesthesia following the instillation of THCl, better use can be achieved when performing diagnostic and therapeutic procedures on horses.
• The authors also noted that comparison between tetracaine and other ocular anaesthetics should be made in future publications, to further establish potential variations and implications in anaesthetic administration.