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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2019) 31 (8) 432-440 doi: 10.1111/eve.12757
Review Article
Local anaesthetic techniques for the equine head, towards guided techniques and new applications H. Hermans†*
, S. Veraa‡, C. F. Wolschrijn§ and J. P. A. M. van Loon†
†Department of Equine Sciences, ‡Division of Diagnostic Imaging, §Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. *Corresponding author email:
h.hermans@uu.nl
Keywords: horse; anaesthetic; head; perineural nerve blocks; ultrasound
Summary Perineural nerve blocks are often used in equine practice, especially since the use of diagnostic and surgical procedures in the standing sedated horse have expanded over recent decades. The purpose of this review is to discuss the different perineural nerve blocks for the equine head. The review starts with the currently most used blind approaches as described in textbooks and scientific studies. In human medicine, the role of guided techniques, such as ultrasound guidance, advanced imaging guidance and nerve stimulator guided techniques, is very extensively described. These techniques are promising to use in equine medicine as well. The first studies that describe these techniques in equine cases are also discussed in this review, as well as the possibilities for neuromodulation in equine pain syndromes like equine trigeminus-mediated headshaking and the role of perineural nerve blocks in diagnosing this syndrome.
Introduction
In modern equine veterinary practice, surgical and diagnostic procedures in the standing sedated horse are expanding, especially with the growing interest in minimally invasive surgery (Dixon et al. 2005; Coomer et al. 2011; De Linde Henriksen and Brooks 2014; Menzies and Easley 2014). Reliable and stable sedative planes are very important and various studies have been performed to assess the effects of sedative and analgesic pharmaca (Ringer et al. 2013; Marly et al. 2014). Both in the sedated horse and in the horse under general anaesthesia, the beneficial effects of locoregional techniques (diminished levels of sedation or anaesthesia, prevention of harmful reflexes and pre-emptive analgesic effects) are clearly described (Ong et al. 2005; Oel et al. 2014). Furthermore, in the standing equine patient it is especially important to provide a reliable local anaesthetic block regarding the safety of the horse, the veterinary surgeon and animal handlers. Previous reviews have very extensively described the use
and anatomical landmarks for all relevant local anaesthetic techniques for the equine head (Tremaine 2007; Labelle and Clark-Price 2013). Experience of the performer is very important in the success rates of local anaesthetic techniques, as was shown by Wilmink et al. (2015) for the perineural block of the maxillary nerve. Although blind techniques used to be the gold standard, nowadays ultrasound-guided techniques are quickly being adopted. They are broadly used in human medicine where there has been a rapid development in different ultrasound-guided
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approaches in recent years (Helayel et al. 2007). Ultrasound-guided local anaesthetic techniques enhance both the quality and duration of peripheral-nerve blockade and reduce the incidence of complications. Meta-analyses comparing ultrasound-guided local anaesthetic techniques to blind techniques in man have shown that ultrasound guidance reduces complication rates and improves quality of the block (both for sensory and motor blocks); the technique further reduces performance time and results in quicker onset of the block in peripheral nerve blockade in adult humans (Walker et al. 2009; Lewis et al. 2015). Furthermore, ultrasound guidance reduces the incidence of local anaesthetic systemic toxicity (neuro- and cardiotoxicity consequent to unintended intravascular injection or delayed tissue uptake; El-Boghdadly and Chin 2016; Neal 2016) and unintended paresthesias (Soeding et al. 2005). For infants, a recent systematic review by Guay et al. (2016) showed that ultrasound guidance improved the success rate and duration of perioperative neuraxial and peripheral local blocks. Additional data are required to assess the potential effect of ultrasound guidance on reducing the rate of inadvertent puncture of blood vessels. Besides these advantages, ultrasound guidance was found to improve the learning curve of clinicians and it has been stated that the technique should have a role in future training (Marhofer et al. 2005). To date, the role of ultrasound-guided techniques and advanced imaging is still limited in the performance of local nerve blocks in the equine patient. This review discusses the perineural nerve blocks for the
equine head and describes blind approaches as well as those guided by ultrasound, nerve stimulation, or advanced imaging techniques and compares human and equine literature. As perineural techniques can also have diagnostic and therapeutic value, the possibilities for neuromodulation in equine pain syndromes such as equine trigeminus-mediated headshaking and the role of perineural nerve blocks in diagnosing this syndrome are also discussed.
Perineural nerve blocks for the equine head: classic approach
The most commonly used local anaesthetic techniques for the equine head are well described in various review articles (Tremaine 2007; Labelle and Clark-Price 2013) and veterinary textbooks (Easley et al. 2011; Gilger 2011). Perineural nerve blocks are often used in equine dentistry and equine ophthalmology. Figure 1 shows the four anatomical locations for the techniques most often used in equine dentistry: the
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