Feasibility Study of Sub-Tenons Anesthesia for Phacoemulsification in Dogs

Abstract

Cataract is one of the most common diseases that cause blindness in dogs and is conventionally treated with surgery such as phacoemulsification. During the surgery, exposure of surgical area (i.e. lens) by extraocular muscles akinesia and pupil dilation is essential. Additionally, because the eyes are densely innervated structures, surgical stimuli may result in ocular pain and pain management during the surgery and postoperative period is important. The purpose of the present study was to evaluate the effects of sub-Tenons anesthesia on extraocular muscles akinesia, pupil dilation and intraoperative and postoperative analgesia in dogs undergoing phacoemulsification. In chapter 1, the feasibility of sub-Tenons anesthesia was investigated in dogs with clinically normal eyes by injecting lidocaine into the sub-Tenons space. A cross-over design was used with both eyes from five Beagle dogs randomly injected, under general anesthesia, with 1 ml of 2% lidocaine (1 ml-lidocaine group), 2 ml of 2% lidocaine (2 ml-lidocaine group) or 2 ml of normal saline (control group). Each eye was assigned to all treatments with a minimum 14 day interval between injections. Changes in eye position, pupil diameter, and intraocular pressure (IOP) were evaluated during the procedure. All eyes in the 2 ml-lidocaine group exhibited akinesia and mydriasis (pupil diameter >10 mm). The onset times of akinesia and mydriasis were 6.5 ± 4.9 and 4.2 ± 4.3 min, respectively. In the 1 ml-lidocaine group, akinesia was induced in 9 eyes and mydriasis occurred in 7 eyes with an onset time of 10.7 ± 5.8 and 5.4 ± 2.4 min, respectively. No changes in eye position or pupil diameter were observed in the control group. Akinesia was maintained for 44.3 ± 26.7 min in the 1 ml-lidocaine group and for 88.5 ± 17.2 min in the 2 ml-lidocaine group. Duration of mydriasis was 51.7 ± 28.9 min in the 1 ml-lidocaine group and 82.9 ± 15.6 min in the 2 ml-lidocaine group. No significant change in IOP was observed between the mean pre- and post-injection values in all groups. These results suggest that a sub-Tenons injection of 2 ml of 2% lidocaine provided effective extraocular muscle akinesia and mydriasis in dogs. In chapter 2, the effect of sub-Tenons lidocaine injections on akinesia and mydriasis was compared to those of systemic atracurium and retrobulbar lidocaine injections in dogs. Three treatments were performed on 10 beagle dogs with a minimum 7-day washout period: intravenous injection of atracurium (0.2 mg/kg, AT group)

retrobulbar injection of 2% lidocaine (2.0 ml, RB group) in one eye

and sub-Tenons injection of 2% lidocaine (2.0 ml, ST group) in the opposite eye. When the akinesia was not obtained within 10 min, an additional 1 ml of lidocaine was administered in the RB and the ST groups. Onset of akinesia in the AT (1.5 ± 0.9 min) and the ST (3.8 ± 5.8 min) groups was significantly shorter than that in the RB group (9.0 ± 6.5 min). Duration of akinesia in the ST group (116.2 ± 32.8 min) was longer compared to the AT (60.6 ± 23.6 min) and the RB (89.0 ± 52.8 min) groups, even though there was only a significant difference between the AT and the ST groups. Mydriasis was achieved in five eyes in the RB group and nine eyes in the ST group. There was no significant difference in onset (3.6 ± 3.1 and 2.9 ± 2.3 min, respectively) or duration (91.4 ± 31.9 and 102.1 ± 35.8 min, respectively) of mydriasis between the groups. Sub-Tenons lidocaine injections provide excellent akinesia and mydriasis compared to systemic atracurium and retrobulbar lidocaine injections. Therefore, sub-Tenons anesthesia could be an alternative to the systemic administration of neuromuscular blockers and retrobulbar anesthesia for ophthalmic surgery in dogs. Based on the results of chapter 1 and 2, the effect of sub-Tenons anesthesia on akinesia, mydriasis, and intraoperative and postoperative analgesia were evaluated in dogs with normal eyes undergoing phacoemulsification in chapter 3. Dogs were anesthetized and assigned to 2 treatments: concurrent sub-Tenons injection of 2% lidocaine hydrochloride solution (2 ml) and intravenous injection of saline (0.9% NaCl) solution (0.02 ml/kg

lidocaine group [n = 7]) or concurrent sub-Tenons injection of saline solution (2 ml) and IV injection of 0.2 mg of atracurium/kg (0.02 ml/kg

control group [n=7]). Pupils were dilated by topical application of a combined tropicamide and phenylephrine ophthalmic solution. Ten min after the injections, pupil diameter was measured and phacoemulsification was performed. End-tidal isoflurane concentration was used to evaluate intraoperative pain. Subjective pain scores were recorded during the postoperative period. Akinesia was induced and maintained throughout the surgery in all eyes. Mean ± SD pupil diameter was significantly greater in the lidocaine group (13.7 ± 0.7 mm) than in the control group (12.2 ± 0.8 mm). Isoflurane requirements were significantly lower in the lidocaine group than the control group. However, postoperative pain scores were not significantly different between the groups. In the present studies, it was shown that sub-Tenon injection of 2 ml of 2% lidocaine was an effective method for inducing akinesia of extraocular muscles, mydriasis, and intraoperative analgesia for phacoemulsification in dogs. Therefore, this could be another option for surgical field exposure and pain management during phacoemulsification in dogs.