In equine first-opinion and referral practice, diagnostic and treatment procedures involving the head and dentition are often performed with the animal conscious and sedated in a standing position. To enable such tasks to be performed safely, humanely and effectively, regional analgesia can be helpful, if not essential, in many cases. With the risk of injury to equine clinicians being of concern, a more scientific and rational approach than has been practised historically is now required. Over the past 20 years there have been a number of scientific articles in the literature describing and validating the use of different sedative–analgesic combinations that enable surgical and dental procedures of an advanced nature to be performed safely and effectively in conscious horses in the standing position. Allied to these, there have been descriptions of effective regional analgesia in the head and the development of some new techniques for its application. This article describes these approaches for those intending to perform painful procedures, such as dental, ophthalmic and paranasal sinus treatments, on a conscious horse.
Statistics from Altmetric.com
Henry Tremaine qualified from the Royal Veterinary College, London in 1989 and entered mixed and equine practice for four years before undertaking a surgery residency at the University of Edinburgh, gaining an MPhil and then going to Ohio State University, USA. He rejoined the faculty in Edinburgh where he became a diplomate of the European College of Veterinary Surgeons and subsequently gained specialisation in equine surgery, before moving to the University of Bristol in 2002 and becoming a specialist in equine dentistry. He currently works at B&W Equine Group, Gloucestershire.
It is beyond the scope of this article to describe in detail all the pharmacological sedative–analgesia options available for use in a dental or surgical procedure to the head of a conscious horse. The combination of sedation, muscle relaxation and analgesia required varies between procedures and a balanced approach is appropriate, as is the case with modern management of general analgesia. The author’s preference is to select the combination based on the procedure planned, its duration and the character of the horse. With so many variables involved, to suggest an empirical ‘one size fits all’ combination is neither appropriate or helpful.
When undertaking any potentially painful procedure on the head, a combination of phenothiazine tranquilliser (eg, acepromazine), α2 sedatives (xylazine, romifidine or detomidine), opiate analgesia (butorphanol, buprenorphine or morphine), benzodiazepine muscle relaxation (diazepam or midazolam) and occasionally dissociative agents (eg, ketamine) can be used. These drugs are selected depending on case-specific requirements and can be administered as boli or by using a constant-rate infusion system (particularly for α2 sedation). In addition to the sedation regimen, selected regional local analgesia is routinely practised by the author for all painful procedures in the head.
Infusion vs bolus
The use of infusion or a bolus depends on the duration of the procedure. In most field situations, the administration of a weight-calculated bolus at the correct dose of sedative is effective, with incremental sedative doses given after a predetermined time interval or administered to effect. For longer procedures, intravenous infusions are appropriate with a variable, calculated infusion rate. A drip set can be used for simple infusions but an infusion pump is much more accurate and preferable for combinations of drugs.
A quiet environment is important for effective sedation. Stocks are useful to control movement, for the suspension of infusions, for improved instrument set-up and, most importantly, to reduce any risk to the operator (although not the horse necessarily). Headstands or suspension using a sling are necessary to reduce head movement and help balance ataxia when undertaking dental procedures or other head surgeries. A well-managed, clean stable can be used, provided due care is paid to operator and horse safety. Closed doors to limit human and canine traffic, and extraneous movement and noise in the horse’s plane of vision and hearing, are advisable; blinkers and earmuffs (for the horse) are also helpful.
Local anaesthetic agents
Local anaesthetics are weak bases and the characteristics of each drug are governed by the pharmacological properties including lipid solubility (determining potency), dissociation constant (onset of action) and free ionisation (duration). Lignocaine, prilocaine and mepivacaine are licensed for use in horses, but other agents can be used according the cascade. The agent selected depends on the duration of analgesia required: in general, shorter-acting agents have a quicker time of onset. Increasing lipid solubility and plasma binding result in an increase in duration of action but, potentially, a slower onset.
Table 1 lists agents that are commonly used in horses, with some of their properties and examples of their use. Products without a veterinary licence can be used according to the cascade system with owner consent.
In general, the patient should be sedated before attempting to apply any nerve block as the local anaesthetic can be accompanied by a stinging sensation. Clipping and aseptic preparation of the skin at the injection site is desirable and a full aseptic preparation is recommended for the deeper blocks and any intra-articular blocks. Some nerves are visible superficially (Fig 1) but many are in deeper tissues. A more extensive clip is desirable if ultrasound-guided needle placement is undertaken.
For the larger diameter nerves, regardless of which drug is used, sufficient time should be allowed for diffusion into the nerve bundles and a minimum of 10 to 15 minutes is recommended before the peak effect is anticipated, although lignocaine will elicit desensitisation more quickly. It has been estimated that three nodes of Ranvier need to be contacted by the drug for depolarisation of the nerve, which translates to an estimated contact distance with the local anaesthetic of 0.6 cm.
Nerve blocks can be applied by local infiltration, topically, perineurally or intra-articularly to desensitise and paralyse a particular area.
Mental nerve block
A mental nerve block desensitises the mental branch of the mandibular nerve (the mandibular branch of the trigeminal nerve [CN V]). It is used for:
Surgery to the lower lip;
Lower incisor endodontic and restorative procedures;
Lower wolf tooth removal;
Lower canine treatments.
The mental foramen is located on the lateral aspect of the mandible in the interdental space (Fig 2) and can be palpated externally. A vein emerges from the foramen alongside the nerve. A 21G x 40 mm needle with its hub bent at a slight angle is introduced into the skin approximately 1 cm rostral to the foramen and directed caudally using a digit to guide the point into the foramen (Fig 3). A small amount of local anaesthetic (1 to 2 ml) can be injected subcutaneously and the needle is then introduced further into the foramen. By occluding the foramen, the injected local anaesthetic will advance up the opening and desensitise the branch of the nerve that continues in the mandible and supplies the incisors and rostral part of the bone (although recent research has suggested this to be unnecessary if the needle is within the canal [Bardell and others 2010]).
An alternative intraoral approach is to introduce a 25G x 25 mm needle on a dental syringe subgingivally from within the everted lower lip, along the mandibular cortex and direct into the mental foramen, using a digit on the outside (Fig 4).
Inferior alveolar nerve block
The inferior alveolar (IA) branch of the mandibular branch of the CN V is desensitised by an IA nerve block. This technique is used for:
Dental procedures, including the extraction of mandibular dentition;
Rostral mandibular surgery;
Periodontal treatments involving mandibular cheek teeth;
Repair of mandibular fractures.
The large IA nerve innervates the mandible and mandibular dental alveoli, and its mental branch emerges to innervate the lower lip and gingiva. The mandibular nerve gives off a branch of the lingual nerve just dorsal to the mandibular foramen. This lingual branch passes medially before fusing with fibres of the glossopharyngeal nerve to provide sensory and motor innervation to the caudal aspect of the tongue and gingiva (sublingual branch of the mandibular branch of the CN V). If local anaesthetic is injected proximally (ie, dorsally) to this branch it will also desensitise the lingual branch of the mandibular nerve and desensitise the caudal third of the tongue and its mucosa on that side (Fig 5).
The mandibular foramen is on the medial side of the mandible, 1 cm caudal to its rostral border. It lies at a level along an imaginary line joining the buccal occlusal edges of the maxillary cheek teeth (Fig 6).
A long-handled syringe in combination with a curved 25G x 40 mm needle is advanced into the mouth over the tongue from the contralateral side and the needle is directed medially to the vertical ramus of the mandible lingually and caudally to the third molar (Fig 7). Once the mucosa has been penetrated, the needle is directed laterally along the medial mandibular cortex to place 2 to 5 ml of local anaesthetic adjacent to where the IA nerve enters the mandibular foramen. A tongue depressor is often required to insert the needle accurately.
The horse’s head should be supported on a stand in an extended position if using a transcutaneous approach.
The skin ventral and medial to the mandible is clipped and prepared aseptically. A skin bleb of 1 ml can be injected with local anaesthetic medial to the mandible caudal to where the facial vessels and salivary duct traverse the ventral mandibular ramus. An 18G x 30 cm spinal needle is inserted caudally to the facial vessels and medially to the mandible in a direction parallel to and 1 cm caudal to the rostral margin of the vertical mandibular ramus (Fig 8). The needle is advanced dorsally in the direction of the lateral ocular canthus until the tip is level with an imaginary line along the horizontal ramus of the mandible extending caudally. If the bevel of the needle faces outwards (laterally), the tip of the needle will avoid catching in periosteum and often will contact the convex surface of the cortex just dorsal to where the nerve enters the mandibular foramen. Injection of 5 to 7 ml of local anaesthetic is sufficient to effect the block.
The entry point for the needle using a caudoventral approach is at the junction of the horizontal ramus with the vertical ramus of the mandible on the medial side. The needle should perforate the skin medial to the mandible, with the bevel facing away from the cortex. To facilitate lateral needle advancement (to stay close to the medial cortex), the patient’s head can be turned slightly away from the side to be injected. The distance from the skin to the foramen is less using this technique than via the ventral approach, and in one small cadaver study it was found to be more accurate. The needle is directed medially to the mandible in a rostrodorsal direction towards a marker preplaced at the estimated site of the mandibular foramen, then towards the dorsal point of the foramen, the site of which is estimated as described above.
Using the extraoral approach, the branch of the lingual nerve that diverges from the mandibular nerve just proximal to the foramen can also be desensitised. As this results in desensitisation of the caudal third of the tongue, a simultaneous bilateral IA blockade is contraindicated because lingual self-trauma has been reported (Caldwell and Easley 2012). If bilateral treatment is necessary, the procedures should be staged and a short-acting local anaesthetic agent should be used.
Infraorbital nerve block
An infraorbital nerve block desensitises the infraorbital (IO) branch of the maxillary branch of the CN V. It is used for:
Desensitisation of the upper lip and nostrils;
Dental procedures to the upper incisors, canines, wolf teeth, incisive bone and rostral premolars.
The horse should be sedated before performing this nerve block as patients can respond violently to it. A 21G x 40 mm needle that has been bent to facilitate syringe attachment should be used.
The IO foramen, from where the IO nerve emerges from the maxillary bone (Fig 2), is obscured by the musculus levator nasolabialis but can be located by displacing the muscle dorsally. It lies 1 cm dorsal to an imaginary line joining the rostral facial crest to the nasoincisive notch. The direction of the canal is worth noting, as the needle should ideally be introduced parallel to it.
The foramen is approached from 1 cm rostrad. Ideally, the needle is placed at the foramen and 1 cm of local anaesthetic is introduced subcutaneously before advancing the needle further (Fig 9). If the needle is advanced into the foramen at its ventral aspect, the risk of trauma to the nerve is minimised and, when in position, 4 to 5 ml of local anaesthetic can be injected slowly. This then flows along the canal, desensitising the branches to the maxillary premolars. The IO nerve is extremely sensitive and even in heavily sedated horses a sudden vertical movement of the head can be evoked when the nerve is stimulated or contacted.
The location of the foramen and nerve can also be determined accurately using ultrasound guidance (Fig 10).
Maxillary nerve block
The maxillary branch of the CN V is desensitised when using a maxillary nerve block. This blockade is used for:
Maxillary sinus surgery;
Maxillary dental procedures;
Nasal septum surgery;
Incisive bone fracture repair;
Desensitisation of the maxillary nerve in the diagnosis of trigeminal neuropathic pain (headshaking syndrome).
The maxillary nerve is a very large sensory branch of the CN V that emerges from the foramen rotundum, courses along the pterygopalatine fossa and enters the maxillary foramen in the palatine bone just dorsal to the palatine foramen (Fig 11). It passes just dorsal to the maxillary artery and deep facial vein, and diverges to form the caudal nasal nerve just after entering the foramen. The IO branch (where it enters the foramen) is bounded by the IO artery laterally and the descending palatine artery ventrally. The maxillary nerve innervates structures in the maxillary sinuses, nasal septum and ventral concha, and gives branches within the IO canal to the maxillary cheek teeth. A rostral branch continues within the bone to innervate the incisors. The main nerve emerges from the IO foramen and branches to innervate the nostrils and skin and muscles of the upper lip.
It is worth noting that the maxillary nerve in the pterygopalatine fossa has been confusingly referred to as the caudal ethmoidal nerve in some articles.
There are four approaches in common use for performing a maxillary nerve block: caudal, lateral, lateral periorbital and dorsal (Fig 12) .
The caudolateral approach (used by the author) (Tremaine 2007) aims to deposit 4 to 6 ml local anaesthetic at the entrance to the pterygopalatine fossa adjacent to the maxillary nerve (Fig 13). The site is clipped and prepared aseptically just below the zygomatic arch and a skin bleb is injected just ventrally to this structure. A 22G x 9 cm spinal needle is directed at 45° to the skin caudorostrally and parallel to the facial crest, and advanced until it contacts the palatine bone immediately dorsal to the nerve entering the palatine foramen (the direction is approximately towards a point imagined on the rostral contralateral facial crest).
The lateral approach aims to deposit local anaesthetic almost at the same site as for the caudolateral approach – adjacent to the palatine bone near where the nerve enters the palatine foramen – and is usually performed using a 21G x 40 mm needle. The insertion point is almost perpendicular to the skin just ventral to the facial crest and the needle is directed approximately 6 cm through the masseter muscle and retrobulbar fat pad, but not as far as the bone. This reduces the risk of arteriopuncture allowing more widespread diffusion from the fat pad (Lowder 2012, Henry and others 2014).
Ultrasound-guided lateral approach
To assist with accurate needle placement, a microarray curvilinear ultrasound probe can be placed dorsally or ventrally to the needle insertion site, caudal to the facial crest (Fig 14), with the beam in a vertical plane to visualise the pterygopalatine fossa and maxillary vessels (Lowder 2012). Ultrasound machines with a Doppler function can reveal the artery (Fig 15), and the needle – if parallel to the ultrasound beam – can be guided to deposit local anaesthetic adjacent to the nerve.
Lateral periorbital approach
The lateral periorbital approach is preferred by some veterinarians to avoid inadvertent puncture of the deeper maxillary vessels. A 19G x 40 mm needle is inserted perpendicular to the skin, ventral to the facial crest at the level of the lateral ocular canthus, and directed through the masseter muscle into the retrobulbar fat pad. Local anaesthetic diffuses through the fat pad and contacts the maxillary nerve to achieve the blockade.
Larger volumes (10 to 20 ml) of local anaesthetic are typically used for this technique but this can reduce precision and excessive volumes are likely to increase the risk of side effects such as ocular prolapse due to extraocular muscle paralysis. However, the technique is simple and demonstrates a high level of accuracy and safety if smaller volumes of anaesthetic are used (Edwards 1930, Lowder 2012).
The dorsal approach (Stephenson 2004) involves advancing an 18G x 210 mm spinal needle from the temporal fossa, lateral and close to the temporal and pterygoid bones, in a ventral direction to place local anaesthetic adjacent to the maxillary nerve in the pterygopalatine fossa. It is hard to judge the length of advancement for this technique so it is less frequently used than the other approaches, as an excessively long needle could penetrate the pharynx.
The side effects of maxillary nerve blocks include haematoma formation after inadvertent articular puncture (the haematoma is usually self-limiting and resolves within 24 hours), globe prolapse due to inadvertent extraocculomotor muscle paralysis, Horner’s syndrome (Fig 16) and a failure to sensitise the nerve, nostril self-trauma (Fig 17) if the horse experiences an unpleasant sensation as the block recedes and a painful cellulitis in the pterygopalatine fossa if infection is introduced. These side effects can be avoided by:
Accurate needle placement;
Use of narrow 19 to 22G spinal needles;
Avoiding excessive volumes of local anaesthetic (5 to 10 ml is sufficient if accurately placed);
Appropriate restraint of the horse.
Ethmoidal nerve block
An ethmoidal nerve block affects the ethmoidal branch of the nasociliary branch of the CN V (ophthalmic branch). It is used to desensitise the frontal sinus mucosa or ethmoid or dorsal conchal sinus. The nasociliary nerve branches from the ophthalmic branch of the CN V after it emerges through the foramen ovale on the medial aspect of the temporal bone. A branch of the nasociliary branch then retroverts to re-enter the skull through the ethmoidal foramen to become the ethmoidal nerve.
This nerve block (described by Caruso and others ) enhances desensitisation of the frontal sinus and associated compartments, thereby reducing the morbidity of surgery of these compartments and of the ethmoidal area. The author performs it routinely for frontal sinus surgery, as there is improved patient tolerance.
When performing an ethmoidal nerve block, the aim is to place local anaesthetic perineurally as it enters the foramen ovale (Fig 18). A 19 to 22G x 60 to 90 mm spinal needle should be inserted laterally to the temporal bone and caudally to the zygomatic arch, and directed at 110° to the surface of the frontal skin in both transverse and sagittal planes (Fig 19). The needle is advanced approximately 50 to 60 mm and, when in contact with the temporal bone, 3 to 5 ml of local anaesthetic (mepivacaine) can be injected.
Temporomandibular joint nerve block
A temporomandibular joint (TMJ) nerve block desensitises the capsule of the TMJ and surrounding structures. There are two synovial compartments of this joint – discotemporal and discomandibular – and, usually, only the ventral discomandibular compartment is infiltrated as it is larger and easier to inject into. This blockade is used for:
Diagnostic confirmation of pain attributed to the TMJ;
Analgesia for surgical procedures.
The mandibular condyle, the articular margin of the temporal bone and the joint space are palpable caudal to the lateral ocular canthus ventral to the zygomatic arch. A 23 to 21G x 25 mm needle is inserted perpendicular to the skin and 3 to 5 ml mepivacaine injected into the caudal compartment. Ultrasound guidance can help needle placement (Fig 20).
Intraligamental dental nerve block
An intraligamental dental nerve block is highly effective for analgesia of the gingiva and coronal periodontium, and is useful for:
Prilocaine gel can be injected intraligamentally into the periodontal ligament of a tooth using a 21G intraligamental needle to desensitise this structure effectively (Fig 21).
Submucosal infiltration of lignocaine using a butterfly catheter for desensitisation of the gingiva and mucosa in the caudal oral cavity is useful for dental extraction and standing soft palate treatments (Fig 22). A dental syringe and fine 25G or 27G needle can be useful for this during removal of wolf teeth (vestigial first premolar) in a field situation when the local anaesthetic is deposited palatally and buccally to the tooth to be removed (Fig 23). The grove in which the greater palatine artery lies should be appreciated and intra-articular injection should be avoided.
Ocular nerve blocks
Fig 24 shows the approximate areas of skin that are sensitised by nerves around the eye.
Auriculopalpebral nerve block
The palpebral branch of the auriculopalpebral branch of the facial nerve can be palpated as it traverses the zygomatic arch after emerging through the stylomastoid foramen, underneath the parotid salivary gland. The nerve innervates the powerful orbicularis oculi that closes the eye (Fig 25). Deposition of 1 to 2 ml of lignocaine at this site will paralyse the upper eyelid. The nerve can be blocked at other sites and the auricular branch to the pinna will be included if it is blocked more proximally.
Supraorbital nerve block
A supraorbital nerve block enables complete desensitisation and akinesis of the eyelids, as well as blockade of the lacrimal nerve on the lateral aspect of the lower eyelid (ophthalmic branch of the CN V), blockade of the zygomatic nerve on the ventral aspect of the lower eyelid (maxillary branch of the CN V) and blockade of the infratrochlear nerve (maxillary branch of the CN V), which innervates the skin around the medial canthus.
The supraorbital branch of the ophthalmic branch of the CN V emerges from the supraorbital foramen that is palpable on the dorsal aspect of the orbit. Injection of 2 ml of local anaesthetic using a 23 to 25G x 18 mm needle subcutaneously will desensitise the upper eyelid and conjunctiva (Fig 25).
Retrobulbar nerve blocks
Complete retrobulbar blockade results in exophthalmos, mydriasis, globe paralysis and corneal desensitisation.
There are many ways to desensitise structures deep in the orbit for diagnostic and surgical procedures. The method chosen depends on the effect desired (and side-effects that are not desired) and the preference of the individual.
A lateral and medial subconjunctival approach involves introducing a 22G x 90 mm spinal needle subconjunctivally at the medial and lateral canthi (Fig 26). The needle is advanced following the contours of the orbit while flexing in a curve to enable injection of local anaesthetic deep in the orbit and close to the optic nerve cone.
Alternatively, a modified Peterson block, which is similar in approach to the fat pad technique for the maxillary nerve block, can be used, and by using larger volumes (20 ml) of local anaesthetic the retrobulbar area and extraocular muscles can be desensitised/paralysed.
Corneal desensitisation is best achieved using ophthalmic amethocaine or tetracaine drops (Ophthaine, Minims) that are pharmacologically sparing to the cornea and have a rapid onset of action.
Techniques for local analgesia in the head have enabled a wide range of surgical and dental procedures to be performed in conscious, standing horses in recent years. Careful selection of the products used, a multimodal approach to the sedation, mastering of the precise techniques and a sensible awareness of the limitations are of considerable benefit to both the patient and veterinarian to enable more ambitious treatments to be performed with greater safety, accuracy and precision.
In Practice has partnered with BMJ OnExamination to host the self-assessment quizzes provided with each clinical article. These can be completed online and found at the end of the online version of each article at http://inpractice.bmj.com
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.