Nerve blocks or opioids for rhinoscopy in dogs?

Our latest Pain Update reviews the literature around controlling the response to rhinoscopy in dogs. The main paper we review (Kim et al 2022) aimed to evaluate the clinical effects of ethmoidal and maxillary nerve blocks during rhinoscopy in dogs. In our clinical patients when the rhinoscope is introduced we are monitoring for increases in physiological parameters with the aim of ensuring our plane of anaesthesia is adequate to prevent gross purposeful movement, which is clearly undesirable. It is these outcomes that were examined in the papers we discuss here.

In an earlier rhinoscopy study, Cremer and colleagues compared the subzygomatic approach to the maxillary nerve with the infraorbital nerve block. A control group received saline instead of lidocaine. They documented that gross head movement occurred in 75% of dogs in the infraorbital block group, reducing to 25% in the maxillary block group. In this study, cardiovascular changes did not accurately indicate success or failure of the technique – although CV parameters were more stable with the maxillary block group. Regarding sensory innervation, it is clear that other sensory nerves must be blocked in addition to the maxillary nerve.

In the Cremer et al study the anaesthetic protocol consisted of acepromazine, propofol and isoflurane so the only analgesic was lidocaine. In clinical practice we would use our local blocks as part of a multimodal analgesic protocol with opioids and alpha 2 agonists – which are likely to further decrease response to rhinoscopy. My choice for an ASA I or II patient would be medetomidine/dexmedetomidine at 5ug/kg combined with methadone 0.2 mg/kg. The analgesic benefits of the alpha 2 agonists is reviewed here and the versatility of methadone discussed here.

Subsequently, Kim et al (2021) examined the benefit of adding an ethmoidal block to the maxillary block having reviewed the anatomy. Considering the sensory innervation of the nasal meatus, the caudal branch of the maxillary nerve supplies the lateral mucosa. The ethmoidal nerve is a branch of the nasociliary nerve and supplies the medial mucosa of the nasal meatus. This paper is the first to describe the ethmoidal nerve block.

Image A ethmoidal nerve block:

 For the ethmoidal (E) block the needle is introduced at the most medial point of the groove of the angularis oculi vein, maintaining contact with the medial wall of the orbit, directing the needle towards the ethmoidal foramen. The volume of local anaesthetic used in this study was 0.1 ml/kg. In the cadaver stage of the study, with injection of CT contrast, this volume was deemed to provide adequate coverage of the ethmoidal nerve. In image A the needle tip is just above the ethmoidal foramen.

Image B maxillary nerve block via infraorbital canal:

 For the maxillary (M) nerve block the needle was introduced via the infraorbital foramen and advanced into the infraorbital canal (image B). The needle was inserted to a depth pre-measured to the lateral canthus. The volume of local anaesthetic used was 0.1 ml/kg. We have our reservations with this technique because it involves inserting a hypodermic needle with a cutting bevel into the canal which risks nerve damage – of course another option is an intravenous cannula with should be less traumatic to the nerve. An alternative technique is shown in this video.

In the stage of the study using live patients, the dogs were split into four groups;

Control (C) – saline injected bilaterally at each site – M&E

Maxillary (M) – lidocaine injected bilaterally for the M nerves and saline injected for the E nerves

Ethmoidal (E) – lidocaine injected bilaterally for the E nerves and saline injected for the M nerves

Combination (ME) – lidocaine injected bilaterally for M & E nerves.

Dogs were anaesthetised with acepromazine premedication, alfaxalone IV induction and isoflurane maintenance. The testing was conducted as follows;

• Cotton swabs were inserted and withdrawn five times within 10 seconds, and the insertion depth was to the level of the medial canthus

• A bilateral pinching test on the nose tip was performed using mosquito hemostatic forceps to verify whether the maxillary block was successful. The jaw of the forceps was inserted 2mm into the nostril and the forceps were locked at the first ratchet for 5 seconds

• Stimulation-induced changes (D) of the cardiopulmonary variables (DHR, DSAP, DMAP, DDAP and DfR) were defined as the difference between the baseline and the peak value after stimulation

The results of the study demonstrated that for cotton bud stimulation all variables increased significantly in all groups. These increases were smaller in the ME group – however total absence of stimulation was not achieved. For the nasal pinch outcome measure, the increase in cardiorespiratory parameters was significant in all groups, although these increases were lower in the M group versus C group.

These authors then published a further paper comparing ethmoidal & maxillary (EM) blocks to opioid alone. Dogs in the EM group received ethmoidal and maxillary nerve blocks with lidocaine, prior to rhinoscopy. Dogs in the opioid group received hydromorphone 0.05 mg/kg. As in previous studies, heart rate, arterial blood pressure and end tidal isoflurane concentrations were recorded.

In the opioid only group, the heart rate increase was higher, the increases in blood pressure were higher and end tidal isoflurane concentrations were higher. One of the outcome measures was response to endoscope insertion – head movement was observed in 83% of dogs in the opioid group and 12% of dogs in the nerve block group. As in the previous study, there were still increases in the measured parameters in the EM group in response to endoscope insertion.

It is therefore still clear that the EM block does not provide total anaesthesia to the nasal meatus, and a multimodal approach is required. It would appear that the addition of the ethmoidal block does confer an advantage to a maxillary block alone. The ethmoidal block involves insertion of the needle very close to the globe and I can imagine that would be of concern to many readers.

My conclusion to these studies is that there is certainly a benefit to adding the maxillary block as part of your multimodal anaesthetic protocol. It’s always easy to say that further research is required! The next step in this story is to examine the benefit of a multimodal approach -  with the addition of alpha 2 agonists and opioids to the premed, as suggested above. Another really simple option to also consider is the benefit of spraying lidocaine as a mist into the nasal cavity to determine if this could make a difference – or whether it would be a useful addition to those other local anaesthetic techniques.

References

Cremer J, Sum SO, Braun C, Figueiredo J, Rodriguez-Guarin C. Assessment of maxillary and infraorbital nerve blockade for rhinoscopy in sevoflurane anesthetized dogs. Vet Anaesth Analg. 2013 Jul;40(4):432-9. doi: 10.1111/vaa.12032.

Kim H, Jang M, Shin CW, Han H, Cha J, Lee I, Son WG. The effects of maxillary nerve block, ethmoidal nerve block and their combination on cardiopulmonary responses to nasal stimulation in anesthetized Beagle dogs. Vet Anaesth Analg. 2021 Jan;48(1):92-100. doi: 10.1016/j.vaa.2020.08.005. Epub 2020 Sep 8. PMID: 33012661.

Kim H, Kim D, Shin D, Kim J, Sung T, Rhee S, Lee I, Son WG. Ethmoidal and maxillary nerve block versus systemic opioid administration during rhinoscopy in dogs: a non-randomised clinical trial. J Small Anim Pract. 2022 Nov;63(11):816-820. doi: 10.1111/jsap.13537. Epub 2022 Jul 20. PMID: 35859528.

This post was written by Matt Gurney.

Matt & Carl established Zero Pain Philosophy to provide educational resources & telemedicine to veterinary professionals enabling optimal management of pain.

Matt Gurney is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and works at Anderson Moores Veterinary Specialists. Matt is Past President of the European College of Veterinary Anaesthesia & Analgesia and works at Eastcott Referrals in the UK.

Carl Bradbrook is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and is Past President of the Association of Veterinary Anaesthetists. Carl works at Anderson Moores Veterinary Specialists in the UK.

The intended audience for this pain update is veterinary professionals. This pain update is based on clinical experience and independent opinion.