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EQUINE VETERINARY EDUCATION / AE / AUGUST 2019


patient with trigeminal neuralgia, in which the classical approach using anatomical landmarks was confounded because of anatomical variations (Okuda et al. 2000). A similar approach could be used in equine cases in which anatomical variations impede the use of anatomical landmarks. In cows, magnetic resonance imaging scans were used to compare two different techniques for retrobulbar blocks using contrast medium in cadaveric heads (Pearce et al. 2003).


Nerve stimulator guided local techniques for the equine head


Since the inferior alveolar nerve is a sensory nerve with certain motor components as well, it is a very suitable nerve for nerve stimulator guidance when it needs to be desensitised. In various studies, the use of nerve stimulator guidance of the inferior alveolar block in man has been described (Simon et al. 2010; Espitalier et al. 2012; Kumar et al. 2012). Stimulation of the nerve using a lateral extraoral approach results in a motor response of the temporal and masseter muscles, apparent as a jaw jerk. Cheetham et al. (2009) used a peripheral nerve locator


to perform a bilateral block of the common trunk of the hypoglossal nerve in 10 horses to determine the role of the hypoglossal nerve in equine nasopharyngeal stability. During this study, no complications were associated with the technique. In literature, use of a nerve stimulator to assist locoregional


techniques in the equine head has not been described, but the technique should be feasible and merits further investigation.


Neuromodulation of trigeminal nerve branches in the equine head


Desensitisation of peripheral nerves of the equine head is very important for surgical procedures; however, perineural techniques are used as well in diagnosing peripheral neuropathies in horses (such as headshaking) and neuromodulation of these peripheral nerves can also be of therapeutic value in these cases. Various studies on the technique and reliability of the maxillary nerve block for diagnosing headshakers have been published (Newton et al. 2000; Roberts et al. 2013; Wilmink et al. 2015). Where few horses with idiopathic headshaking showed improvement on anaesthesia of the infraorbital nerve, the majority (13 out of 16 = 81%) showed complete or partial improvement after anaesthesia of the posterior ethmoidal branch of the maxillary nerve (Newton et al. 2000). This block is called caudal anaesthesia of the infraorbital nerve by Roberts et al. (2013) and caudal nasal nerve block by Dyce et al. (2002). This latter nerve branches off the maxillary nerve just proximal to the maxillary foramen and enters the caudal nasal foramen before running towards the dorsal meatus of the nasal cavity to innervate the nasal mucosa. A recent study by Roberts et al. (2016) describes the first


results of neuromodulation of the maxillary nerve in equine headshakers, by use of a percutaneous electrical stimulation protocol featuring alternating high (100 Hz) and low (2 Hz) frequency stimulations. The infraorbital nerve was located by ultrasound guidance and the nerve was stimulated for 25 min. Preliminary results show partial or complete remission


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for a prolonged period in five out of seven cases after several treatments (up to 12–28 weeks after four treatment sessions). Neurostimulation for the treatment of humans with neuropathic pain has been well described (Pap


uc and


Rejdak 2013; Shaparin et al. 2015; Maniam et al. 2016). Peripheral stimulation is categorised into transcutaneous electrical nerve stimulation (TENS), peripheral nerve stimulation (PNS) and nerve root stimulation. With TENS, surface electrodes are used while electrodes are percutaneously implanted to directly contact the nerve in PNS. In the study by Roberts et al. (2016) in horses, stimulation is realised through needle electrodes that are inserted in close proximity to the nerve, placing this technique more or less between TENS and PNS. After stimulation, the needles are removed in contrast to the practice with PNS in man (in which the electrodes stay in contact). There are various hypotheses about the working


mechanism of peripheral nerve stimulation. Stimulation is thought to produce paraesthesia that spreads along the territory innervated by the stimulated nerve (Abej


on and


Krames 2009). Exogenous electrical stimulation might also lead to signal modification of intrinsic electrical impulses (Shaparin et al. 2015), or stimulation may inhibit central nociceptive transmission or lead to partial sympathetic blockade and local blood flow alterations (Shaparin et al. 2015). Pap


uc and


Rejdak (2013) proposed that high frequency stimulation would lead to inhibition exerted by large-size afferents on spinothalamic pathways. Low-frequency stimulation is thought to activate the antinociceptive systems, mediated in part by the opioid system. The gate-control theory by Melzack and Wall (1965), which states that competing nociceptive and innocuous signals influence second-order neurons to transmit pain signals for higher processing, may also be useful for understanding the effect of peripheral nerve stimulation.


Conclusions


Local anaesthetic techniques have been used in equine practice for a very long time, but the use of ultrasound (or nerve stimulator) guided techniques is new and still limited to date in equine cases. Nevertheless, these techniques are very promising and can possibly be of similar benefit as they have been shown to be in human medicine. Ultrasound-guided techniques can also become very prominent in training young professionals in performing various local blocks. As the immediate real-time feedback results in steeper learning curves, the quality and accuracy of the blocks is most likely to improve considerably, as in human medicine. So far, most studies have been performed on equine cadavers with the main aim of describing the technique. Efficacy and safety studies have not been performed in horses yet. For the further development of the techniques and their implementation in practice, clinical studies such as those that are described in the various systematic reviews in humans are needed. This review shows that guided techniques (both by


ultrasound and nerve stimulator) and modern imaging techniques are becoming more important in veterinary practice and have huge potential. Furthermore, the implementation of peripheral nerve stimulation protocols such as described for equine trigeminus-mediated headshaking is a promising development. It is not unlikely that these techniques will become as important in equine pain management as they are in human pain management.


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