A comparison of retrobulbar and two peribulbar regional anesthetic techniques in dog cadavers

Improving rat welfare through the development of a peribulbar anaesthesia technique for ophthalmic procedures: A preliminary study

Rats are a commonly used animal model for the study of the pathogenesis and novel treatments of glaucoma, which is induced experimentally using invasive, painful procedures. Peribulbar anaesthesia (PBA) is frequently used in people and domestic animals prior to ophthalmic surgeries to provide excellent perioperative analgesia. Our goal was to develop a PBA technique adapted to rat anatomy, improving the welfare of animals used as a model for glaucoma. Eighteen rat cadavers (n = 36 eyes) were used to establish the optimal needle insertion location. Five injection techniques using 0.1 mL/100 g lidocaine 2% and a contrast agent (1:1 volume ratio) were compared via computed tomography (CT). CT images were scored for injectate distribution at four locations: extraconal, intraconal, around the optic nerve and at the orbital fissure (scale 0-8, where 0 = none and 8 = excellent). Median scores using the dorso-medial-75° (5; range 2-6) and medial-canthus (4.5; range 2-8) injection techniques were not different from the dorso-medial-45° (4; range 3-6) technique and were higher (better distribution) compared with mid-ventral (3; range 2-5) and ventro-lateral (2; range 1-3) techniques. The two superior techniques were used in two experimental rats (n = 4 eyes) to determine the volume of bupivacaine 0.5% necessary to affect corneal touch threshold (CTT) and periocular skin sensitivity (PSS). A volume of 0.05 mL/100 g decreased CTT and PSS for several hours, while a larger volume produced excessively long effects. Dorso-medial-75° or medial-canthus PBA using 0.05 mL/100 g bupivacaine are likely to provide ocular and periocular analgesia in rats, with minor transient adverse effects.

The use of sub-Tenon's anesthesia versus a low-dose neuromuscular blockade for canine cataract surgery: A comparative study of 224 eyes

OBJECTIVE

Assess the utility of a Sub-Tenon's anesthesia (STA) protocol to provide suitable operating conditions for canine cataract surgery and compare it to an alternative low-dose neuromuscular blockade (LD-NMB) protocol used for canine cataract surgery.

PROCEDURES

Clinical study of dog eyes undergoing cataract surgery with either an STA or LD-NMB protocol. While intraoperative vitreal expansion scores and intraoperative complications were collected prospectively, globe position, intraocular pressure, return of vision, and postoperative complications were collected retrospectively. Statistical testing was used to compare results between the STA and the LD-NMB groups for the data available.

RESULTS

A total of 224 eyes from 126 dogs were assessed, with 133/224 (59.4%) eyes from 99/126 (78.6%) dogs receiving STA and 91/124 (40.6%) eyes from 72/126 (57.1%) dogs receiving LD-NMB. Forty-five of these dogs (45/126; 37.7%) received STA for one eye and LD-NMB for the other eye. There was no significant change in intraocular pressure measurements following STA administration. This was not measured for the LD-NMB group. The globe achieved a central position in 110/133 (82.7%) of eyes that received STA. This was not measured for the LD-NMB group. Intraoperative vitreal expansion scores were slightly higher in STA-treated eyes compared to LD-NMB-treated eyes. The intraoperative complication rate for STA-treated eyes was higher (73/133; 54.8%) compared to NMB-treated eyes (12/91; 13.2%). The most common intraoperative complication for STA was chemosis (64/133; 48.1%), the risk of which increased with an increase in the volume of local anesthetic injected. The post-operative complication rate was higher in STA-treated eyes (28/133; 21.1%) compared to NMB-treated eyes (16/91; 17.6%). Post-operative corneal ulceration was the most common postoperative complication in STA-treated eyes (6/133; 4.5%).

CONCLUSION

The STA protocol described resulted in suitable operating conditions, but more intraoperative and postoperative complications compared to the LD-NMB protocol. Despite these complications, the STA protocol did not cause a significant deleterious impact on post-operative outcomes as defined in the present study.

Comparison of a blind and an ultrasound-guided technique for Retrobulbar anesthesia in dogs undergoing unilateral subconjunctival enucleation

OBJECTIVE

This study compared the quality of retrobulbar anesthesia using a blind inferior-temporal palpebral approach (ITP) with an ultrasound-guided supratemporal (ST) technique in dogs undergoing unilateral enucleation.

ANIMAL STUDIED

Twenty-one client-owned dogs were undergoing enucleation.

PROCEDURES

Dogs were randomly assigned to receive ITP (n = 10) or ST (n = 11) with 0.5% ropivacaine at 0.1 mL/cm of neurocranial length. The anesthetist was blinded to the technique. Intraoperative data included cardiopulmonary variables, inhalant anesthetics requirement, and requirement for rescue analgesia (intravenous fentanyl 2.5 mcg/kg). Postoperative data included pain scores, sedation scores, and need for intravenous hydromorphone (0.05 mg/kg). Treatments were compared using Wilcoxon's rank sum test or Fisher's exact test as appropriate. Comparison of variables over time were tested using a mixed effect linear model on rank. Significance was set at p = 0.05.

RESULTS

Intraoperative cardiopulmonary variables and inhalant requirements were not different between groups. Dogs receiving ITP required median (interquartile range, IQR) 1.25 (0, 2.5) mcg/kg intraoperative fentanyl while those receiving ST required none (p < 0.01). Intraoperative fentanyl was required in 5/10 and 0/11 of dogs in the ITP and ST groups, respectively (p = 0.01). Postoperative analgesia requirements were not significantly different between groups; 2/10 and 1/10 dogs in the ITP and ST groups, respectively. Sedation score negatively affected pain score (p < 0.01).

CONCLUSIONS

The ultrasound-guided ST technique was more effective at decreasing intraoperative opioid requirements than the blind ITP approach in dogs undergoing unilateral enucleation.

Clinical and Antinociceptive Effects of Distal Inferior Alveolar Nerve Block in Ponies With Tramadol 5% or Lidocaine 2

This study aimed to compare the antinociceptive effects of tramadol 5% and lidocaine 2% on mental nerve block in horses of the Brazilian Pony breed. Eight adult non-pregnant mares were used in this study. The ponies were tranquilized with acepromazine (5 µg kg^-1, IV), and the infiltration of the mental foramen was performed in Treatment 1- tramadol 5% (T, 150 mg) or Treatment 2- lidocaine 2% (L, 60 mg), both at a total dose of 3 ml in each foramen. Heart rate (HR), respiratory rate (RR), systolic arterial pressure (SAP), diastolic arterial pressure (DAP), mean arterial pressure (MAP), rectal temperature (RT), and formation of skin lesions (SL) were evaluated. Evaluation of nociception of the outer lip (OL), inner lip (IL), and gingiva (GG) were performed using an electronic von Frey device with the evaluation of the ponies' reactions to each stimulus. From these reactions, we determined latency time (LT) and duration of antinociception (DAN). Analysis of variance with 16 observations was performed for HR, RR, SAP, DAP, MAP, LT and DAN. Data were expressed as mean ± standard deviation and the means were compared by the SNK and Student's t-test (P< 0.05). Changes in HR, RR, SAP, DAP, and MAP between evaluation times were associated with the effects of acepromazine (P>0.05). No formation of skin lesions was observed. Latency time did not differ between treatments (P>0.05). Both lidocaine 2% and tramadol 5% produced an antinociceptive effect. We conclude that the duration of the antinociceptive effect of tramadol 5% is longer than that of lidocaine 2%.

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.

Computed Tomography Analysis of the Feline Infraorbital Foramen and Canal

Feline skull anatomic variation is plain to see with casual observation. Obtaining an in-depth understanding of this anatomic variability is critical to performing safe and effective regional anesthesia for dental procedures and maxillofacial surgeries. Maxillofacial anatomic variability is proven to impact the placement and efficacy of nerve blocks in dogs and horses, but similar studies have not been performed in cats. This study's main objective was to evaluate the anatomy of the infraorbital foramen and canal in relation to regional anatomic landmarks in brachycephalic and mesaticephalic cats. Significant anatomic variability was identified, particularly among cats with brachycephalic skulls.

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.

Locoregional Anesthesia of the Head

Locoregional (local and regional) anesthesia is used routinely during surgical procedures of the head. The goal of this article is to provide a practical guide for the clinician to safely perform different techniques in dogs and cats and to minimize the risk of complications associated with locoregional anesthesia.