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EQUINE VETERINARY EDUCATION / AE / AUGUST 2019
a)
b) B C B A D A
Fig 3: Localisation of the lingual and inferior alveolar nerves. a) In-situ anatomical orientation of inferior alveolar nerve (A) and lingual nerve (B). The latter branches off the mandibular nerve (C) before it enters the mandibular foramen (D) as the inferior alveolar nerve. b) Schematic orientation of both nerves. Figure 3b reproduced with permission by Caldwell and Easley (2012).
and an additional block of the nerve in addition to the maxillary nerve block can be beneficial in these cases. Caruso III et al. (2016) described a technique to desensitise the ethmoidal nerve. The ethmoidal nerve branches off the nasociliary nerve, which proceeds as the infratrochlear nerve. The ethmoidal nerve is blocked at the rostromedial aspect of the supraorbital fossa and the technique has proven to be reliable and simple.
Inferior alveolar nerve block The mandibular or inferior alveolar nerve block has been described in various studies. Harding et al. (2012) described two extraoral approaches (vertical vs. angled technique), while Henry et al. (2014) described an intraoral technique. In the latter, a custom-made device inserted into the mouth is used to anaesthetise the inferior alveolar nerve with a relatively small volume of local anaesthetic (5 mL). This alternative technique could decrease the risks of side-effects of this block such as self-inflicted trauma of the tongue due to accidental desensitisation of the lingual nerve that branches off the mandibular nerve very close to the mandibular foramen (Fig 3). Several cases of this self-inflicted tongue trauma after bi- and unilateral inferior nerve blocks have been described by Caldwell and Easley (2012). Localisation of the mandibular foramen on the medial side of the mandible can be realised using the perpendicular lines passing through the occlusal surface of the maxillary and mandibular cheek teeth and the lateral canthus of the eye (Tremaine 2007; Fig 2b). Harding et al. (2012) determined the accuracy of this technique of localising the mandibular foramen using radiography, revealing that the mandibular foramen was consistently located in close proximity to the intersection of these perpendicular lines. The described topographical landmarks were found to be accurate in locating the mandibular foramen (Harding et al. 2012). The relative position of the lingual and the inferior alveolar nerves is shown in Figure 3.
Ophthalmic nerve blocks For ophthalmic procedures, various blocks can be used to enable minor and major surgical procedures in the standing horse or in the horse under general anaesthesia. The retrobulbar block (Tremaine 2007; Labelle and Clark- Price 2013) can effectively be used in surgical procedures such as enucleation and for minor procedures such as
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intraocular injection of tissue plasminogen activator in cases with recurrent uveitis with fibrin formation in the anterior chamber or for episcleral placement of cyclosporin implants. With the retrobulbar block, the oculomotor, trochlear and abducens nerves are desensitised, resulting in paralysis of all the straight and oblique ocular muscles, and the retractor bulbi muscles, which leads to a stable forward-positioned eye. Additionally, the ophthalmic and maxillary branches of the trigeminal nerve and the optic nerve are blocked, providing desensitisation of the eye and ocular adnexa. Figure 4 shows the ‘diamond block’, which comprises the supraorbital, lacrimal, infratrochlear and zygomaticofacial nerves. This block results in desensitisation of respectively the medial two-thirds of the superior eyelid, the temporal canthus of the eye, nasal eye canthus and the temporal 75% of the inferior eyelid. The block is useful for surgical repair of eyelid lacerations or several diagnostic procedures of the eye.
Ultrasound-guided local anaesthetic techniques for the equine head
In modern human and veterinary clinical anaesthesiological practice, ultrasonography is becoming a more important technique that improves accuracy and safety of local anaesthetic techniques. Ultrasound can help to determine the exact location of the needle placement relative to the anatomical landmarks and the peripheral nerve that is aimed for. By means of ultrasound guidance, the amount of local anaesthetic needed to desensitise a nerve can be minimised due to the close proximity of the needle in relation to the peripheral nerve. The resultant influence on volume and concentration of the local anaesthetic at the level of the nerve improves the anaesthetic block quality in terms of time of onset and duration of effect. Ultrasound can also help to determine anatomical structures such as adjacent blood vessels, hence reducing the risk of side-effects such as haemorrhage from puncturing a vessel or inadvertent injection of the nerve. On ultrasound, nerves appear as single or multiple round or oval hypoechoic areas surrounded by a relatively hyperechoic area in the transverse scanning orientation (Alexander and Dobson 2003). In the longitudinal view, the nerve presents as a hyperechoic band characterised by multiple discontinuous hypoechoic stripes separated by hyperechoic lines, creating a fascicle pattern
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