Ultrasound-guided posterior extraconal block in the dog: anatomical study in cadavers

Possible Brainstem Anaesthesia in a Cat after Ultrasound-Guided Retrobulbar Block

A 13-year-old neutered, blue-eyed female Siamese cat with a bodyweight of 4.8 kg was admitted for enucleation of the right eye. An ultrasound guided retrobulbar block with 1 mL of ropivacaine was performed under general anaesthesia. When the tip of the needle was visualised inside the intraconal space, negative aspiration of the syringe before injection and no obvious resistance during injection were confirmed. Instantly, after ropivacaine was administered, the cat became apnoeic, and its heart rate and the blood pressure increased significantly for a short period of time. During surgery, the cat needed cardiovascular support to maintain blood pressure and was under continuous mechanical ventilation. Spontaneous breathing returned 20 min after the end of anaesthesia. Brainstem anaesthesia was suspected, and after recovery, the contralateral eye was examined. A reduced menace response, horizontal nystagmus, mydriasis, and absence of the pupillary light reflex were present. The following day, mydriasis was still present, but the cat was visual and was discharged. The inadvertent intra-arterial injection of ropivacaine was suspected to be the cause of the spread into the brainstem. To the current authors' knowledge, possible brainstem anaesthesia has only been reported in a cat 5 min after a retrobulbar block but never instantly.

An Observational Survey Study on the Use of Locoregional Anaesthesia in Non-Conventional Species: Current Practice and Potential Future Developments

The objectives of this study were to investigate the current attitudes of veterinarians towards the use of locoregional anaesthesia in non-conventional animal species and to identify areas for future useful research on this topic. A questionnaire was circulated online. A total of 417 veterinarians, including American and European specialists/specialists-in-training in both zoological medicine and anaesthesia/analgesia (ACZM/ECZM and ACVAA/ECVAA), participated in the study. Fifty-nine percent of respondents performed locoregional anaesthesia in rabbits, with intratesticular injections and local infiltration being the most commonly-reported techniques. ACZM/ECZM specialists reportedly performed dental blocks in rabbits more frequently than ACVAA/ECVAA specialists (p = 0.030). Forty percent of respondents performed locoregional anaesthesia in rodents, with intratesticular injections, topical/splash blocks and local infiltration being the most commonly reported techniques. The proportions of respondents who reportedly used locoregional anaesthesia in ferrets, birds and reptiles were 37.9%, 34.5% and 31.2%, respectively. The use of Tuohy (p < 0.001) and spinal needles (p < 0.001), as well as of ultrasonography (p = 0.009) and nerve-stimulators (p < 0.001), was more common among ACVAA/ECVAA compared to ACZM/ECZM specialists. Major topic areas for future research were identified as dental block techniques for rabbits and rodents and blocks for the wings of birds.

Description of a novel ultrasound guided peribulbar block in horses: a cadaveric study

BACKGROUND

Standing surgery in horses combining intravenous sedatives, analgesics and local anaesthesia is becoming more popular. Ultrasound guided (USG) peribulbar nerve block (PB) has been described in dogs and humans for facial and ocular surgery, reducing the risk of complications versus retrobulbar nerve block (RB).

OBJECTIVE

To describe a technique for USG PB in horse cadavers.

METHODS

Landmarks and PB technique were described in two equine cadaver heads (Phase 1), with computed tomography (CT) imaging confirming contrast location and spread. In Phase 2, ten equine cadaver heads were randomised to two operators naïve to the USG PB, with moderate experience with ultrasonography and conventional "blind" RB. Both techniques were demonstrated once. Subsequently, operators performed five USG PB and five RB each, unassisted. Contrast location and spread were evaluated by CT. Injection site success was defined for USG PB as extraconal contrast, and for RB intraconal contrast.

RESULTS

Success was 10/10 for USG PB and 0/10 for RB (p < 0.001). Of the RB injections, eight resulted in extraconal contrast and two in the masseter muscle (p = 0.47).

CONCLUSIONS

The USG PB had a high injection site success rate compared with the RB technique; however, we cannot comment on clinical effect. The USG technique was easily learnt, and no potential complications were seen. The USG PB nerve block could have a wide application for use in horses for ocular surgeries (enucleations, eyelid, corneal, cataract surgeries, and ocular analgesia) due to reduced risk of iatrogenic damage. Further clinical studies are needed.

Analgesic effects of a retrobulbar block with 0.75% ropivacaine in dogs undergoing enucleation

OBJECTIVE

To assess the analgesic effects of a retrobulbar block with ropivacaine in dogs undergoing enucleation.

STUDY DESIGN

Prospective, randomized, masked placebo-controlled trial.

ANIMALS

A total of 23 client-owned dogs.

METHODS

Dogs were randomized to be administered a preoperative inferior-temporal palpebral retrobulbar injection of either ropivacaine 0.75% (1 mL 10 kg^-1; group RG) or equivalent volume of 0.9% saline (control; group CG). Intraoperative variables recorded to detect a response to noxious stimuli included heart rate (HR) and mean arterial pressure (MAP). Three observers assessed and recorded pain using a numerical rating pain scale and visual analog scale (VAS) before anesthesia (baseline) and postoperatively at 0, 0.5, 1, 2, 3, 4, 5, 6 and 24 hours after extubation. Rescue analgesia was administered if intraoperative HR or MAP increased by ≥ 20% from the previously recorded surgical time point, average postoperative pain scores totaled ≥ 9/20, scored ≥ 3/4 in any one category with VAS ≥ 35/100, or if VAS was ≥ 35/100 with a palpation score > 0/4.

RESULTS

Intraoperatively, there was no significant difference in HR or MAP between groups. Rescue analgesia was administered intraoperatively to four and one dogs and postoperatively to five and seven dogs in groups CG and RG, respectively, with no significant difference between groups. VAS scores were significantly lower in ropivacaine dogs at extubation (p = 0.02), but not at other postoperative time points. Adverse events were not observed in either group.

CONCLUSIONS AND CLINICAL RELEVANCE

Preoperative retrobulbar 0.75% ropivacaine injection (1 mL 10 kg^-1) provided analgesia in dogs following enucleation at extubation; however, intraoperative and postoperative pain control did not differ from a placebo injection with saline. Lack of differences between groups may have been influenced by sample size limitations.

Comparison of four peribulbar anaesthetic techniques: a preliminary study in equine cadavers

OBJECTIVE

To compare the peribulbar injectate distribution and probability of regional anaesthesia of four peribulbar anaesthetic techniques in equine cadavers.

STUDY DESIGN

Prospective experimental cadaver study.

ANIMALS

A total of 12 isolated equine cadaver heads and 24 eyes.

METHODS

The 24 orbits underwent one of four injection techniques (six orbits each) with a mixture (1:4) of contrast medium and saline (CM): 20 mL ventrolateral peribulbar injection (V-20), 20 mL dorsolateral peribulbar injection (D-20), combined ventrolateral and dorsolateral peribulbar injections 10 mL each (VD-20) or 20 mL each (VD-40). To evaluate and score CM distribution at the base of, within the extraocular muscle cone (EOMC), and around the optic nerve (before and after pressure application to the periorbital area), computed tomography was performed. To assess the probability of achieving locoregional anaesthesia, two criteria were applied and both scored as 'likely', 'possible' or 'unlikely'. To compare CM distribution scores between injection techniques, Kruskal-Wallis analysis of variance was used. Mann-Whitney U test was used for post hoc comparisons between groups when needed. A p value < 0.05 was considered significant.

RESULTS

The CM distribution within the EOMC and around the optic nerve circumference was detected as 'possible' only after pressure application in seven out of 24 orbits (V-20, 3; D-20, 1; VD-40, 3). It was never considered 'likely' either before or after pressure application. The CM distribution at the EOMC base was considered 'likely' to provide regional anaesthesia in 50% (V-20), 0% (D-20), 33% (VD-20), 100% (VD-40) and in 66% (V-20), 16% (D-20), 50% (VD-20), 100% (VD-40) before and after applying pressure, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Complete regional anaesthesia seems more likely using the VD-40 technique, although the authors advise caution due to the risk of potential complications. Future studies are necessary to evaluate the efficacy of the technique in vivo.

In-plane ultrasound-guided peribulbar block in the dog: an anatomical cadaver study

OBJECTIVE

To describe a novel in-plane ultrasound (U/S)-guided temporal approach to peribulbar block in dogs.

STUDY DESIGN

Prospective experimental cadaver study.

ANIMALS

A group of 10 Beagle cadavers.

METHODS

After describing the U/S anatomy, peribulbar injection was performed bilaterally in 10 thawed Beagle cadavers by two randomly assigned operators. A 5-8 MHz microconvex U/S probe was positioned caudal to the orbital ligament in the longitudinal plane. Using an in-plane technique, methylene blue dye was injected in five dogs (10 eyes total), while methylene blue dye and iohexol contrast mixture (50:50) were injected in the remaining five dogs. Injection volume was 0.2 mL cm^-1 of cranial length. A computed tomography (CT) scan was performed on dogs injected with dye and contrast to identify spread of contrast. Dissection to visualize dye spread in the orbit was performed in all dogs. Injection success was defined as spread of contrast into the peribulbar space. The pattern of distribution of contrast-dye was also assessed. Comparisons between operator and bilateral injections were assessed using a Student t test (p < 0.05). All other data are reported as number (n/N) and percentage (%).

RESULTS

Peribulbar spread was noted in 19/20 injections (95%) on dissection. CT imaging (five dogs) illustrated peribulbar contrast spread in 9/10 injections (90%), with mixed peribulbar/retrobulbar spread for the remaining injection. Contrast was present at the rostral alar foramen in 4/10 (40%) injections, orbital fissure in 5/10 (50%), oval foramen in 1/10 (10%), maxillary nerve in 3/10 (30%) and intracranial in 5/10 (50%). Coverage of the maxillary nerve was noted on 3/20 (15%) injections on dissection. No further dye spread was noted.

CONCLUSIONS AND CLINICAL RELEVANCE

This technique demonstrated peribulbar spread of injectate in 100% of injections for the 10 canine cadavers studied. Further studies are required to evaluate this technique clinically.

Evaluation of injectate distribution after ultrasound-guided peribulbar injections in canine cadavers

OBJECTIVE

To describe a contrast medium distribution pattern after peribulbar injection in cadavers.

STUDY DESIGN

A cadaveric study.

ANIMALS

A group of eight male dogs and four canine cadaver heads.

METHODS

In phase 1, ultrasound-guided peribulbar injections (1.0 mL dye) were performed in four orbits using a subzygomatic approach. After dissection, the procedure was considered successful if 1) no dye was found in the retrobulbar space, 2) the peribulbar compartment was stained and 3) the dye reached the orbital fissure. In phase 2, using 16 orbits, the same technique was used to inject 0.1 mL kg^-1 contrast medium into (group A, n = 8) or outside (group B, n = 8) the peribulbar compartment. After freezing at -20°C, orbits were imaged using computed tomography, and the contrast medium distribution was analysed.

RESULTS

In phase 1, the procedure was successful in all four eyes. In phase 2, two orbits (one in each group) were removed from the study: one was not injected and the other was injected twice. The contrast medium was found inside the peribulbar compartment in six of seven (85.7%) group A orbits and two of seven (28.6%) group B orbits. In all of these cases, distribution to the orbital fissure, rostral alar foramen, mandibular foramen and inside the skull was observed. The oval foramen was reached in three of seven (42.9%) group A orbits and one of seven (14.3%) group B orbits. In one/seven (14.3%) group A orbits and five/seven (71.4%) group B orbits, the contrast medium was found inside the masseter and temporal muscles, and no distribution towards the aforementioned structures was observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Ultrasound-guided peribulbar injection of 0.1 mL kg^-1 contrast medium in canine cadavers can lead to an extensive distribution of the injectate that reaches the orbital fissure, the rostral alar, oval, and mandibular foramina and can be found intra-cranially.