\documentclass[10pt]{article}

\usepackage{fullpage}
\usepackage{setspace}
\usepackage{parskip}
\usepackage{titlesec}
\usepackage[section]{placeins}
\usepackage{xcolor}
\usepackage{breakcites}
\usepackage{lineno}
\usepackage{hyphenat}





\PassOptionsToPackage{hyphens}{url}
\usepackage[colorlinks = true,
            linkcolor = blue,
            urlcolor  = blue,
            citecolor = blue,
            anchorcolor = blue]{hyperref}
\usepackage{etoolbox}
\makeatletter
\patchcmd\@combinedblfloats{\box\@outputbox}{\unvbox\@outputbox}{}{%
  \errmessage{\noexpand\@combinedblfloats could not be patched}%
}%
\makeatother


\usepackage{natbib}




\renewenvironment{abstract}
  {{\bfseries\noindent{\abstractname}\par\nobreak}\footnotesize}
  {\bigskip}

\titlespacing{\section}{0pt}{*3}{*1}
\titlespacing{\subsection}{0pt}{*2}{*0.5}
\titlespacing{\subsubsection}{0pt}{*1.5}{0pt}


\usepackage{authblk}


\usepackage{graphicx}
\usepackage[space]{grffile}
\usepackage{latexsym}
\usepackage{textcomp}
\usepackage{longtable}
\usepackage{tabulary}
\usepackage{booktabs,array,multirow}
\usepackage{amsfonts,amsmath,amssymb}
\providecommand\citet{\cite}
\providecommand\citep{\cite}
\providecommand\citealt{\cite}
% You can conditionalize code for latexml or normal latex using this.
\newif\iflatexml\latexmlfalse
\providecommand{\tightlist}{\setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}%

\AtBeginDocument{\DeclareGraphicsExtensions{.pdf,.PDF,.eps,.EPS,.png,.PNG,.tif,.TIF,.jpg,.JPG,.jpeg,.JPEG}}

\usepackage[utf8]{inputenc}
\usepackage[ngerman,greek,english]{babel}



\usepackage{float}








\begin{document}

\title{Objective methods of nerve localisation to facilitate performing
locoregional anaesthetic techniques for horses undergoing surgical
procedures}



\author[1]{Luís Filipe Louro}%
\author[2]{Kate Loomes}%
\author[3]{David Bardell}%
\affil[1]{Veterinary Anaesthesia Consultancy Services}%
\affil[2]{Affiliation not available}%
\affil[3]{University of Liverpool}%


\vspace{-1em}



  
  \date{May 26, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
Perineural anaesthesia is a useful technique in equine surgery,
providing pre-emptive targeted anaesthesia of the surgical site,
reducing volatile anaesthetic requirements, improving recovery quality
and providing post-operative pain relief. Surgery under standing
sedation in horses has increased in popularity, mandating the need for
effective locoregional anaesthesia and analgesic techniques. Nerve
location techniques offer greater accuracy than blind techniques when
placing injectate. These methods can help to avoid structures such as
blood vessels and minimise direct nerve damage during needle placement,
reducing the chance of procedure-related complications. This review will
discuss the most pertinent research in the veterinary literature where
objective methods of nerve location have been used to perform peripheral
nerve blocks in horses. The efficacy of using objective methods to
perform nerve blocks in equine anaesthesia is discussed by the authors,
providing useful information to equine anaesthetists, and potentially
improve the quality of anaesthesia and analgesia in horses.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Neuraxial and peripheral regional anaesthetic and analgesic techniques
have been widely utilised in equine anaesthesia. These techniques
provide pre-emptive analgesia, reduce adjunctive anaesthetic
requirements, improve recovery quality and provide post-operative
analgesia in horses subjected to surgical procedures under general
anaesthesia (Goodrich \emph{et al.} 2002, Morris \emph{et al.} 2020,
Louro \emph{et al.} 2020, Hector \emph{et al.} 2020). The growing
popularity of standing surgery and the increasing focus on pain
management in horses have created a demand for effective regional
anaesthesia and analgesia techniques. Although direct palpation of
nerves can be highly effective, its application is limited to
superficially located nerves, restricting its use to target nerves that
are located closer to the skin.

In recent years, hundreds of studies in human and small animal
anaesthesia have been conducted as a result of the widespread use of new
methods and technologies, including peripheral nerve stimulation and
ultrasound-guided injections. The application of these methods for
targeting peripheral nerves in small animal anaesthesia have been
previously reviewed and have been demonstrated to improve accuracy and
precision, as well as reduce complications associated with regional
anaesthesia and analgesia (Portela \emph{et al.} 2018a, Portela \emph{et
al.} 2018b).

The purpose of this review is to discuss the most pertinent research in
the veterinary literature where objective methods of nerve location have
been used to perform peripheral nerve blocks in horses.

\textbf{Peripheral nerve stimulation}

Anatomical landmarks and response to needle placement are typically
employed to ensure that the needle tip is placed near the target nerve
when performing peripheral nerve blocks in conscious horses. A
peripheral nerve stimulator, on the other hand, uses motor reactions to
electrical stimulation in order to determine the proximity of a
peripheral nerve (Dalrymple and Chelliah 2006). Although anatomical
landmarks are still used to guide needle approach, direct contact
between the needle and the target nerve is not required, avoiding
potential trauma to the nerve associated with blind techniques
(Dalrymple and Chelliah 2006).

In 1780, Luigi Galvani, discovered that the muscles of dead frogs' legs
contracted when an electrical stimulus was applied (Galvani 1791), but
it was not until 1912 that Georg Perthes described the safe and
atraumatic localisation of peripheral nerves using electrostimulation
for the purpose of performing regional anaesthesia (Perthes 1912).

\textbf{Clinical use of peripheral nerve stimulators}

The nerve stimulator applies a small amount of direct current (DC) to
the tip of the needle. If the needle is close enough to a motor neural
structure, the stimulus is sufficiently strong and applied for an
appropriate duration, depolarisation of the neurons will occur
(Dalrymple and Chelliah 2006). This will produce an action potential
which will propagate and generate a response from the effector organ
(e.g. skeletal muscle). Even when administered for a lengthy period of
time, a weak stimulus will not result in an action potential. Likewise,
if a powerful stimulus is administered for a very little period of time,
an action potential will not be generated (Dalrymple and Chelliah 2006).

A peripheral nerve stimulator (Figure 1.) is composed of a case
featuring an LCD display, an on-off switch, and a dial for regulating
the target stimulation current. Additionally, the stimulator is equipped
with several buttons to adjust the current range, stimulus duration, and
frequency, providing precise control over the stimulation parameters. To
establish the circuit, two leads are utilised, with one lead connected
to the skin electrode through a skin clamp or an ECG skin pad, while the
other lead is attached to the nerve stimulation needle. In both the
United Kingdom and the United States of America, the cathode (negative
terminal) and anode (positive terminal) are commonly color-coded as
black and red, respectively. However, it is essential to note that
colour coding may vary among different manufacturers worldwide (Krone
2012). The cathode cable is attached to the nerve stimulation needle,
whereas the anode is attached remotely to the patient in order to
complete an electrical circuit (Krone 2012). A constant current is
produced between the anode and cathode which is maintained irrespective
of the impedance of the tissue. The shaft of the nerve stimulation
needle is insulated (e.g., Teflon) so that the electrical current
carried through the needle is applied to the tissues only from the tip
of the needle (Krone 2012).

Peripheral nerve stimulators used in veterinary medicine allow an
adjustable constant current output from 0.1 to 2 mA which can be
controlled by a dial (Otero \emph{et al.} 2019). Initially, a high
current (\textgreater{}1.0 mA) is employed, which will stimulate neural
structures through tissue and fascial planes distant from the tip of the
needle (Dalrymple and Chelliah 2006). When a correct motor response is
first elicited, the current is gradually decreased, in decrements of 0.1
mA, to 0.5-0.3 mA; the `threshold current'. If a motor response is still
present at the threshold current but not if further reduced to 0.2 mA,
it indicates the tip of the needle is likely to be close to the neural
structure, but not penetrating the nerve (Otero \emph{et al.} 2019). If
a response is still elicited at 0.2 mA, it is likely the needle tip is
directly impinging on, or embedded in, the nerve and it should be
withdrawn slightly to avoid intrafasicular injection and iatrogenic
nerve damage. When correct needle placement has been achieved it is
appropriate to inject local anaesthetic, preceded by initial aspiration
to confirm inadvertent vascular needle placement has not occurred
(Otero\emph{et al.} 2019). Initially a small volume of local anaesthetic
is injected to confirm lack of excessive resistance and causing
displacement of the neural structure away from the needle tip resulting
in immediate cessation of the motor response, the so called Raj test.
Beware that this phenomenon may be noticed even if the nerve is located
further away from the nerve stimulator needle (Otero \emph{et al.}2019).
Aspiration of blood prior to injection, excessive resistance to
injection, or failure to abolish muscle twitch after the test dose has
been administered should alert the operator to intravascular or
intraneural needle placement and the needle should be withdrawn and
repositioned.

The frequency of feedback can be adjusted by the operator, determining
the rate at which a motor response is produced. Generally, this is set
at 1 to 2 Hz. Too high frequencies may cause discomfort to the patient,
despite giving the operator more regular feedback. When frequencies are
too low this could result in inadvertent penetration of the nerve
between stimulations (Otero \emph{et al.} 2019).

\textbf{Physics of peripheral nerve stimulation}

The relationship between the nerve stimulation needle and the nerve can
be described by Coulomb's law, \(E=k\frac{Q}{r^{2}}\), where E is the
required stimulating charge, k is a constant, Q is the minimal required
current to evoke stimulation of a neural structure and r is the radius
reflecting the electrode-to-nerve distance. Consequently, as the
constant current decreases, the nerve stimulation needle approaches the
nerve more closely, facilitating more precise and targeted stimulation.

Different peripheral nerve fibres have distinct electrophysiological
characteristics, depending on fibre diameter, function, conduction
velocity and degree of myelination (Dalrymple and Chelliah 2006). The
threshold stimulation of a neural structure will also depend on the
constant current applied and the duration of the stimulus. Rheobase is
the minimal current necessary to activate an action potential in the
nerve (Figure 2.). Even if the current is applied for an extended
period, it cannot initiate an action potential below this threshold.
Chronaxie is the length of time that the current must be delivered to
initiate an action potential, when the constant current applied is twice
the rheobase (Dalrymple and Chelliah 2006) (Figure 2.).

A\selectlanguage{greek}α \selectlanguage{english}motor nerve fibres, which are fast conducting nerves, have a short
chronaxie due to a shorter refractory period when compared to sensory
nerve fibres, such as A\selectlanguage{greek}δ \selectlanguage{english}or unmyelinated C fibres, which are slower
conducting nerves. Because of these characteristics it is possible to
stimulate a motor nerve (A\selectlanguage{greek}α \selectlanguage{english}fibres) but not the sensory nerve (A\selectlanguage{greek}δ \selectlanguage{english}or
unmyelinated C fibres) by using a constant current of shorter chronaxie
(shorter duration of stimulation). Therefore, a motor response can be
elicited without producing pain (Dalrymple and Chelliah 2006).
Peripheral nerve stimulators used in veterinary medicine will allow an
adjustable duration of stimuli from 0.1 msec to 0.3 msec which can be
controlled in the peripheral nerve stimulator (Otero \emph{et al.}2019).

\textbf{Use of peripheral nerve stimulation guided techniques in horses}

One of the first descriptions of the use of a peripheral nerve
stimulator to aid loco-regional anaesthetic techniques in horses was
reported in 2009, by Cheetham and colleagues who performed bilateral
hypoglossal nerve blocks at the level of the ceratohyoid bone, in two
separate occasions, in ten horses (total of 20 blocks) to study the role
of this nerve in upper airway stability (Cheetham \emph{et al.} 2009).
These authors used a constant current of 2 mA at a frequency of 2 Hz and
duration of 0.15 msec, with synchronous caudal retraction of the tongue
seen as effector organ response. Following a positive response, the
current was reduced in 0.2 mA increments until a threshold current of
0.2--0.4 mA was reached. When this was achieved, 0.5 mL of mepivacaine
(concentration not stated by the authors) was injected following
aspiration and if a positive Raj test was observed and no resistance to
injection was noted, the remainder (1.0 --2.5 mL) of the mepivacaine was
injected slowly (Cheetham \emph{et al.} 2009). Tongue tone was tested 30
minutes later to assess success of the technique, and was found to be
absent in only two out of 19 cases. Nevertheless, using this technique,
nasopharyngeal instability was reported in ten of 19 cases
(Cheetham\emph{et al.} 2009).

Peripheral nerve stimulation has also been described for performing
pudendal nerve block in horses (Gallacher \emph{et al.} 2016). These
authors performed an initial cadaveric study to evaluate dye
distribution after blocking the pudendal nerve using electrolocation,
followed by clinical evaluation of the block in horses undergoing
reproductive surgical procedures under standing sedation or general
anaesthesia.

In the cadaveric study the authors determined that a combination of anal
and perineal twitch, rather than only anal or perineal twitch alone,
provided better results in terms of dye distribution over the pudendal
and caudal rectal nerves (Gallacher \emph{et al.} 2016). The authors
also noted that there was no difference in dye distribution or length of
nerve staining if a high volume (20 mL) was injected compared to a low
volume (10 mL). During the cadaveric study, complications were reported
in two out of seven horses, including one case of rectal puncture and
one case of vaginal puncture as a result of the block (Gallacher\emph{et
al.} 2016).

In the clinical part of the study, a bilateral pudendal nerve block was
performed in 27 horses (both mares and geldings) undergoing various
reproductive surgical procedures (Gallacher \emph{et al.} 2016). In all
cases, the block was performed under standing sedation, using the
ischial tuberosity, semimembranosus muscle, the external anal sphincter
and either the dorsal vulvar lips or the retractor penis muscle as
visual or palpable landmarks. The ventral aspect of the external anal
sphincter served as the upper limit and the lateral limit is indicated
by the semimembranosus muscle. Following skin desensitisation, an
insulated 21G, 100mm needle was inserted at an angle of 45\selectlanguage{ngerman}° to the
sagittal plane adjacent to the ventrolateral aspect of the external anal
sphincter. Initial stimulation variables were reported as a current of 1
mA, frequency 1-2 Hz and stimulus duration of 0.1 msec. When
contractions of both the anal sphincter and perineal muscles or vulvar
lips were observed at a threshold current of 0.2--0.4 mA, an appropriate
anaesthetic solution was injected. For male patients, a 10 mL injection
of either lidocaine 2\% or mepivacaine 2\% solution was injected at each
site. For female patients, a 20 mL injection of lidocaine 2\%,
mepivacaine 2\%, or bupivacaine 0.5\% solution was injected, depending
on the required duration of anaesthesia for the specific surgical
procedure. The authors reported a needle insertion depth of 5-10 cm
depending on patient size and perineal conformation, with variable
duration of action between five to fifteen minutes. In males, time from
injection to penile extrusion was reported to range from one to ten
minutes and lasted for less than five hours (Gallacher \emph{et
al.}2016). The block was successful at first attempt in 25 cases, with
two cases requiring an additional unilateral injection of 10 mL of local
anaesthetic, after which the block was deemed effective.

A comparison of nerve stimulator and ultrasound-guided approach to the
inferior alveolar nerve with ventral or caudal blind approaches in
Shetland pony submitted to non-recovery anaesthesia demonstrated the
nerve stimulator guided technique performed poorly compared to the other
groups (Lloyd-Edwards \emph{et al.} 2022).

A single case report describes successful nerve-stimulator guided
thoracolumbar paravertebral nerve block in a thoroughbred mare
undergoing a flank laparoscopy for unilateral ovariectomy under standing
sedation (Santos and Gallacher 2017). Using the approach described by
Moon \& Suter (1993), these authors initially performed local
anaesthetic infiltration of the skin, subcutaneous tissue and
superficial musculature (5 mL of lidocaine 2\% per site). A 20 G, 150 mm
insulated needle was inserted perpendicular to the skin, towards the
relevant lumbar transverse processes, which in the adult horse are
located at a depth of approximately 9 cm. If bone was contacted, the
needle was walked off the cranial edge until penetration of the
intertransverse ligament occurs. Application of a current of 1 mA was
used to evoke contraction of the external and internal oblique and
transverse abdominal muscles. A volume of 5 mL of mepivacaine 2\% was
infiltrated at maximum positive stimulation and another 5 mL was
infiltrated when retracting the needle 2 cm above this point. This
procedure was repeated at three sites to desensitise nerves T18, L1 and
L2 at the transverse process of L1, L2 and L3 respectively. The adequacy
of the block was checked 15 minutes later by sensory loss of the
surgical field to pinprick sensation (Santos and Gallacher 2017).

\textbf{Ultrasound guided nerve blocks}

Ultrasound is a non-invasive imaging technique which allows for the
precise localisation of anatomical structures. Ultrasound provides a
real-time, dynamic image of the scanned anatomical structures, which has
revolutionised the field of locoregional anaesthesia (Krone 2012). The
introduction of ultrasound-guided technique for peripheral nerve blocks
has enabled more accurate needle placement in relation to the target
nerve, and the spread of the local anaesthetic can be observed in
real-time (Krone 2012). In recent years, the importance of this tool has
been recognised in small animal anaesthesia and has recently gained some
attention in equine anaesthesia, leading to the development of new
equine locoregional anaesthesia techniques. In humans, using ultrasound
to locate nerves for peripheral blockade has been shown to be more
successful than the use of peripheral nerve stimulation guided
techniques, and reduced the rate of vascular puncture (Munirama and
McLeod 2015). While studies have shown that this technique results in
faster onset times, longer block durations, better predictability of
block success and reduced requirement for supplemental analgesia, it is
yet unclear whether ultrasound truly provides better success and safety,
especially in terms of reducing nerve injury, than other techniques in
both humans and small animal patients (Marhofer and Fritsch 2017).

\textbf{a. Erector spinae plane block}

Erector spinae plane (ESP) block is a technique involving the
infiltration of local anaesthetic within the inter-fascial plane between
the transverse process of the thoracic vertebrae and the erector spinae
muscular complex (formed by iliocostalis, longissimus, and spinal
muscles), resulting in desensitisation of the structures innervated by
the dorsal rami of the thoracic spinal nerves. Delgado et al. (2021),
conducted an experimental study to assess the viability of utilising an
ultrasound guided approach to performing ESP block by assessing nerve
staining and yellow permanent tissue marking dye distribution in equine
cadavers. The procedure was conducted using a convex transducer (5 MHz)
at the level of the 16\textsuperscript{th} thoracic vertebra (T16). The
transducer was positioned parasagittally and orientated longitudinally
just lateral to the dorsal midline, allowing for the identification of
the transverse process of T16. An 18 gauge, 200 mm spinal needle was
then advanced in-plane craniocaudally towards the transverse process of
the T16. With the bevel orientated ventrally, the needle was advanced
until the bevel touched the bony surface of the transverse process. A
small volume of saline solution was injected in order to confirm needle
placement by observing hydrodissection between the erector spinae
muscles and the transverse process prior to injection of a lidocaine-dye
mixture (0.2 mL/kg). Dye distribution was subsequently assessed during
dissection of the cadavers.

Staining of the thoracolumbar fascia was observed in 85\% of cases, with
an average dye distribution length of 4.8 ± 1.3 vertebral bodies. At
least one dorsal rami of the thoracic nerves were stained in all
examples that underwent additional dissection to evaluate nerve staining
(total of 14 injections), whereas the ventral rami (intercostal nerves)
were stained in only three. Although no dye was found in the thoracic
and abdominal cavities or on the sympathetic trunk, staining of the
epidural space was observed in 20\% of the injections and involved one
or two intervertebral spaces.

This loco-regional technique may be useful for desensitising the
structures supplied by the dorsal rami of the thoracic and lumbar spinal
nerves, specifically to identify and treat conditions affecting the
spinous processes of the vertebrae. Recently, three case-reports have
been published demonstrating the use of ultrasound-guided ESP (UG-ESP)
block in horses undergoing ostectomy and interspinous ligament desmotomy
(Chiavaccini et al., 2022; Perez et al., 2023; Rodriguez et al., 2022).
Perez et al (2023) performed the procedure bilaterally under standing
sedation, targeting the transverse process of T13 and T18. Utilising 20
mL 2\% lidocaine hydrochloride at each of the four sites, the authors
reported an effective block for at least 140 minutes, after which a
top-up dosage of 5 \selectlanguage{greek}μ\selectlanguage{english}g/kg of detomidine and a splash block with 40 mL of
2\% lidocaine in the surgical area were required due to an increase of
27\% in the pulse rate and slight movement of the horse during surgical
manipulation (Perez et al., 2023). Rodriguez and co-workers performed
the UG-ESP block under general anaesthesia, bilaterally at the level of
T15 utilising a total volume of 53.5 mL (0.1 mL/kg) 0.5\% bupivacaine
per injection site (Rodriguez et al., 2022). Additionally, Chiavaccini
et al. (2022) utilized this loco-regional technique in a case of
surgical wound exploration and debridement in a horse with multiple
lumbar spinous process fractures (Chiavaccini et al., 2022). These
authors performed bilateral UG-ESP block using a combination of 0.25\%
bupivacaine (55.6 mL per site; 0.1 mL/kg) and dexmedetomidine (1 \selectlanguage{greek}μ\selectlanguage{english}g/mL)
(Chiavaccini \emph{et al.} 2022). These clinical case reports suggest
that UG-ESP block is effective in providing analgesia to horses
undergoing surgery of the dorsal spinal processes. Despite the potential
epidural spread of local anaesthetics reported in the cadaveric study
conducted by Delgado et al., 2021, no motor deficits have been reported
in clinical cases, utilising either lidocaine or bupivacaine at
different concentrations. A prospective randomised controlled trial is
needed to compare the effects of UG-ESP block to the blind loco-regional
techniques commonly used for these surgical procedures, as well as to
assess the impact of epidural spread on motor blockade of either the
front or hind limbs, which could lead to serious instability.

\textbf{b. Transversus abdominus plane block}

The transversus abdominus plane (TAP) block desensitises the sensory
branches of the thoracic and lumbar spinal nerves, which run within the
interfascial plane between the transversus abdominis and internal
oblique muscles. Local anaesthetic injection into this fascial plane
should anaesthetise the relevant areas of skin, abdominal wall muscles
and parietal peritoneum (Cevik \emph{et al.} 2022). Ultrasound is
commonly used to accurately locate the targeted fascial plane prior to
injection in humans, dogs, cats and horses (Cevik et al., 2022). When
performed successfully, the TAP block can be beneficial in desensitising
the ventral abdominal wall, providing analgesia and muscle relaxation
during abdominal surgical procedures. It is important to mention that
the TAP block is not expected to provide analgesia for visceral pain
associated with intra-abdominal pathology in horses.

So far, three different techniques have been described for administering
the TAP block in cadaver studies in ponies and horses: subcostal,
ventral intercostal, and flank approaches (Baldo \emph{et al.} 2018,
K\selectlanguage{ngerman}üls \emph{et al.} 2020, Freitag \emph{et al.} 2021) (Figure 3.). The
first report of the TAP block in pony cadavers was performed using a
flank approach in dorsal recumbency (Baldo \emph{et al.} 2018). A 6-13
MHz linear US transducer was positioned midway between the caudal aspect
of the last rib and the cranial aspect of the iliac crest at a point
perpendicular to the caudal aspect of the umbilicus. The ultrasound
transducer was positioned in a transverse plane and shifted
ventro-dorsally and caudo-cranially until the external oblique, internal
oblique and transversus abdominis muscles were identified as three
separate layers. Once the ultrasonography landmarks were identified, a
21G, 100 mm needle was inserted in-plane in a ventral-dorsal direction.
To confirm correct needle placement between the internal oblique and
transversus abdominis muscles, a small amount of dye and local
anaesthetic was injected to induce hydrodissection. If separation of the
two muscles did not occur, the needle was repositioned. Upon
confirmation of proper needle position, a total volume of 0.5 mL/kg of
bupivacaine 0.5\% and dye (methylene blue 1\%) was injected. The authors
reported the technique was simple to perform and resulted in
\textgreater{}75\% success rate in staining the ventral branches of
nerves T16-L2 (Baldo \emph{et al.} 2018). The authors speculated that a
TAP block performed using a flank approach will only desensitise the
middle to caudal and ventral abdominal wall and parietal peritoneum, not
the cranial abdominal wall (Baldo \emph{et al.} 2018), due to the
innervation of the equine abdominal wall originating from spinal nerves
ranging from T5 to L2 (Cevik et al., 2022).

A ventral intercostal approach was developed by Küls, et al. (2020) to
address the limited cranial spread of local anaesthetic observed with
the flank approach. In this cadaveric study each hemiabdomen received a
TAP block at three different points: caudal to the
9\textsuperscript{th}, 14\textsuperscript{th} and
18\textsuperscript{th}rib. Cadavers were positioned in lateral
recumbency and the rib corresponding to T18, T14 and T8 identified. A
5-10 MHz linear transducer was positioned perpendicular to the rib, 10
cm ventrally to the costochondral junction of the corresponding
intercostal space. Once the ultrasonography landmarks were identified, a
21G, 100 mm needle was inserted in-plane in a caudo-cranial direction. A
test injection to confirm hydrodissection of the plane between the
internal oblique and transversus abdominis muscles was performed, prior
to injection of 0.3 mL/kg bupivacaine 0.5\% and 1\% methylene blue dye,
equally divided between the three injection sites. With this technique
all spinal nerves originating from T8 to T18 were stained. These authors
subsequently investigated the efficacy of this block in a prospective,
blinded, controlled trial in six Shetland ponies under standing
sedation, using either 0.1 mL/kg bupivacaine 0.125\% or a saline
solution at each injection site (Küls \emph{et al.} 2020). Efficacy of
the block was assessed using cutaneous pinprick at the level of T8 to
L2. The authors reported a success rate of four out of six ponies,
although it failed in the remaining two cases. The results presented,
however, are pooled for all cases, making it challenging to precisely
determine the overall success rate. A noteworthy concern is that a truly
successful block should result in the absence of a pinprick response.
However, the authors reported a statistically significant reduction in
response for nerves T8--T18, suggesting some level of efficacy but not
complete success. An onset of effect of 30 minutes was reported and a
duration up to 120 minutes for nerves T12--T17 and up to 180 minutes for
nerves T8--T11 (Küls \emph{et al.} 2020).

The subcostal approach to performing a bilateral TAP block is a
two-point technique described in horses in either lateral or dorsal
recumbency (Freitag \emph{et al.} 2021). For this procedure a line is
drawn between the xiphoid cartilage and the umbilical scar and the
ventral border of the cutaneous trunci muscle is identified. The first
point of injection is located midway between these two landmarks ventral
to the cutaneous trunci muscle limits. The second point of injection is
located at the first third of the line drawn from the umbilical scar and
the fist injection point, ventral to the cutaneous trunci muscle limits.
A 7.5-12 MHz linear transducer is positioned perpendicular to the
central axis at one of these points. The cutaneous trunci, rectus
abdominis and transversus abdominis muscle are the ultrasonographic
landmarks to be identified. An 18G, 100 mm needle was inserted in-plane
in a dorso-ventral direction. A test injection confirmed hydrodissection
of the plane between the rectus abdominis and transversus abdominis
muscles, followed by a total volume of 30 mL methylene blue 0.5\%
injected at the two sites, corresponding to a total volume of 60 mL
(0.12-0.16 mL/kg). The authors reported correct needle placement at
first attempt in 86.7\% of hemiabdomens injected, with all failures
occurring when the horse was positioned in dorsal recumbency
(Freitag\emph{et al.} 2021). Cadavers placed in lateral recumbency had a
higher percentage of nerve staining (\textgreater{}57.1\%) between
T9-T17 than those in dorsal recumbency (\textgreater{}50\%) between
T13-T18, and this difference was statistically significant (p = 0.0249).
This technique is believed to be a feasible solution for desensitising
the ventral cranial abdomen (Freitag \emph{et al.} 2021).

\textbf{Rectus abdominis sheath block}

In an anatomical description and prospective, crossover,
placebo-controlled, blinded study, Ishikawa et al., (2023) described a
technique to perform an internal rectus abdominis sheath (RAS) block for
providing antinociception to the abdominal midline in horses (Figure
3.). The abdominal wall is composed by four muscles: the external
abdominal oblique muscle, the internal abdominal oblique muscle and the
transversus abdominis muscle, which are located anterolaterally compared
to the rectus abdominis muscle which is located immediately lateral to
the linea alba in a more midline position than the transversus abdominis
muscle (Budras \emph{et al.} 2012). The first part of the study
conducted by Ishikawa et al., (2023) utilised two cadavers and compared
a one-point injection technique, performed on the right hemiabdomen with
a two-point injection technique performed on the left hemiabdomen. The
xiphoid and the umbilicus were used as main landmarks and a line between
this two structures was used as a reference for the injections. A linear
ultrasound transducer (6-13 MHz) was placed over the abdominal midline
in a transverse orientation to the reference line in order to locate the
linea alba at the midpoint of the reference line. The probe was moved 5
to 10 cm laterally toward the right side of the abdomen to view the
interior RAS between the lateral aspect of the right rectus abdominis
muscle and the medial aponeurosis of the right transverse abdominal
muscle. At this site, an 18G, 90 mm spinal needle was inserted in-plane
at a 30-45° angle in a dorsomedial direction. Correct placement was
confirmed when a test dose (2.0 mL) of the prepared 1\% methylene blue
and 0.5\% bupivacaine solution was administered and hydrodissection of
the rectus abdominis muscle from the transverse abdominal muscle was
identified. The two-point technique involved a similar technique, with
the first and second points located close to the abdominal midline
between the cranial and middle thirds and the middle and caudal thirds
of the reference line, respectively. A total volume of 0.25 mL/kg of 1\%
methylene blue and 0.5\% bupivacaine solution was injected at each
point. The authors reported successful staining of the ventral branches
of the ninth thoracic (T9) to second lumbar (L2) nerves when using the
two-point injection technique. Incomplete spread was observed with the
one-point injection technique that did not consistently stain T9 and T10
(Ishikawa \emph{et al.} 2023). For the second part of the study, the
authors conducted a prospective, crossover, placebo-controlled, blinded
study using a two-point injection technique with 0.2\% bupivacaine (0.25
mL/kg per injection site) in standing horses (Ishikawa \emph{et
al.}2023). Antinociception, evaluated using mechanical nociceptive
threshold with a 1 mm probe tip, demonstrated the technique provided
antinociception for at least five hours at the abdominal midline without
evidence of pelvic limb weakness, leading the authors to hypothesise
that this technique may be useful for reducing incisional pain
associated with exploratory laparotomy (Ishikawa \emph{et al.} 2023).

\textbf{d. Cervical plexus block}

Laryngoplasty is commonly performed under standing sedation. In order to
allow this procedure, surgeons perform extensive infiltration of the
skin and peri-laryngeal structures with local anaesthetic
(Rossignol\emph{et al.} 2015). An ultrasound-guided cervical plexus
block (targeting C2 and C3 nerve roots) has been described in horses,
providing an alternative to the more traditional local infiltration
approach (Campoy \emph{et al.} 2018). The wing of the atlas, the
vertebral body of C2, the linguo-facial vein, and the jugular vein are
used as anatomical landmarks. A linear transducer is placed at the
midlevel of the second cervical vertebral body, caudal to the parotid
gland and ventral to the omotransversarius muscle. The transducer is
then rotated to obtain a short axial view of the ventral spinal branch
of C2 located in the interfascial plane between the longus capitis and
the cleidomastoideus muscles. A 20G, 90mm Tuohy needle was then advanced
in-plane through this fascial plane, and a small amount of saline is
injected to confirm needle placement by observation of hydrodissection
of the two muscle planes and approximately 40mL 2\% mepivacaine
injected. Additionally, the authors recommended an injection of 10 mL
2\% mepivacaine subcutaneously in a vertical plane, 10 cm caudal to the
most caudal aspect of the incision site in order to anaesthetise the
cutaneous branches of C3 (Campoy \emph{et al.} 2018). Performing a
cervical plexus block improved surgical conditions in comparison to
conventional tissue infiltration. The local anaesthetic used during
conventional tissue infiltration distorts tissues sufficiently to
obscure essential surgical landmarks (e.g. linguofacial vein) which are
utilised to guide the first skin incision. Moreover, the majority of
local anaesthetics typically used induce local vasodilation, thereby
increasing the probability of surgical haemorrhage and tissue oedema.
This can further complicate recognition of important intraoperative
landmarks. Despite these encouraging results, it is important to note
that sensory innervation of the larynx is not provided by the cervical
nerves. In addition, no corresponding reduction in surgical time,
duration of procedural sedation, or amount of sedative or local
anaesthetic was observed by the authors (Campoy \emph{et al.} 2018). The
efficacy of this block in horses undergoing left-sided prosthetic
laryngoplasty under general anaesthesia was compared to horses
undergoing the same procedure without a locoregional block in a
retrospective study (Morris \emph{et al.} 2020). The authors noted that
the group receiving a cervical plexus block needed less additional
anaesthesia compared to the non-blocked group (Morris \emph{et
al.}2020). Due to the neuroanatomy of the area it is possible that
transient Horner's syndrome and/or transient laryngeal hemiplegia of the
right arytenoid may occur in some horses. The latter is suspected to
occur due to diffusion local anaesthetic resulting in blockade of the
right caudal laryngeal nerve, resulting in altered laryngeal dysfunction
(Morris\emph{et al.} 2020). However, no intra- or post-operative
complications were reported following a cervical plexus block in either
study (Campoy\emph{et al.} 2018, Morris \emph{et al.} 2020).

In a prospective experimental cadaveric study, researchers aimed to
assess the feasibility of ultrasonography-guided perineural injection of
the C7 and C8 ramus ventralis in four equine cadavers
(Touzot-Jourde\emph{et al.} 2020). Using the cervical vertebrae C6 and
C7 as anatomical landmarks, a micro-convex 5-8 MHz ultrasound probe was
positioned to identify the C7-T1 articular process joint and a
longitudinal section of the C8 ramus ventralis was identified. A 20G,
88mm spinal needle was inserted 2 cm caudo-ventrally to the transducer
and advanced in-plane to within 5mm of the nerve surface for injection
of either 7 or 14 mL of methylene blue (Touzot-Jourde \emph{et
al.}2020). The study demonstrated successful staining of a portion of
the nerve root in all injections, with eight rami showing uniform
transversal staining extending over 2 cm. However, incomplete staining
was observed in one C7 and one C8 nerve root. Epidural contamination was
reported by the authors, with injections closer to the articular
processes resulting in more epidural diffusion. Despite the variations
in injection points and volume, all injections were selective for the
targeted nerve. The study suggests that ultrasonography-guided
perineural injection of C7 and C8 ramus ventralis is a feasible
technique with potential applications in multimodal analgesia in horses
(Touzot-Jourde \emph{et al.} 2020).

A similar technique was described for injecting the caudal cervical
spinal nerve roots (C5 to C7) in 14 equine cadavers
(Cruz-Sanabria\emph{et al.} 2021). However, in this study, the
distribution of the injectate was assessed using magnetic resonance and
computed tomography imaging before cadaver dissection. The perineural
injection consistently delivered contrast agent to the targeted caudal
cervical spinal nerve root region in all cases. The authors suggested
that this technique holds potential for diagnosing and treating cervical
pain in horses, especially when intra-articular cervical articular
process joint injections have not yielded desired results (Cruz-Sanabria
\emph{et al.}2021).

\textbf{e. Pudendal nerve block}

An ultrasound-guided technique for performing a bilateral pudendal nerve
block has been described in male donkeys (El-Khamary \emph{et al.}2017).
In contrast to the blind technique and nerve-stimulator guided technique
discussed earlier in this review, this approach offers the advantage of
real-time visualisation of crucial anatomical structures, including the
rectum, blood vessels, and nerves, during needle placement. This
capability has the potential to lower the risk of inadvertent damage to
these structures, hypothetically making the technique safer and more
precise and accurate. After desensitising the skin at the dorsal aspect
of the ischiorectal fossa, a 5-10 MHz transrectal probe was positioned
ventrolaterally to locate the ischium, and dorsally to that, the
internal pudendal blood vessels were identified using colour Doppler.
The pudendal nerve, which is not always easily visible with ultrasound,
was located approximately 1 cm dorsal to the internal pudendal vessels.
To target this nerve, an 18G, 200 mm needle was inserted dorsally in the
ischiorectal fossa and directed cranioventrally. The needle was then
visualised on ultrasound and advanced dorsally, reaching a depth of
about 8 cm dorsally to the internal pudendal artery and vein. A mixture
of 10 mL lidocaine 2\% and 1 mL of methylene blue 1\% (0.05 mL/kg at
each site) was injected to the targeted area. The same procedure was
repeated on the opposite side (El-Khamary \emph{et al.} 2017).
Anaesthesia of the penis and perineal region was achieved within five
minutes and lasted for an average of 47 minutes. Penile protrusion was
obtained in eight minutes and maintained for 90 minutes. No persistent
priapism or paraphimosis were observed in the animals studied
(El-Khamary \emph{et al.} 2017).

\textbf{f. Head nerve blocks}

A number of advanced techniques for the equine head have recently been
developed, such as ultrasound-guided, advanced imaging-guided and nerve
stimulator-guided techniques. This topic has been recently reviewed in
two excellent narrative reviews, where the authors discussed the use of
perineural nerve blocks for the equine head, exploring the advantages
and disadvantages of blind approaches as well as approaches guided by
ultrasound, nerve stimulation, or advanced imaging techniques
(Hermans\emph{et al.} 2019, Johnson \emph{et al.} 2021). Furthermore,
the authors highlighted the wide range of clinical applications for
ultrasound-guided procedures of the equine head and compared human and
equine literature, examining the potential of perineural techniques for
both diagnostic and therapeutic purposes. As techniques in the field are
constantly evolving, ultrasound guidance is becoming an increasingly
popular and utilised modality worldwide for locoregional anaesthesia of
the head in equine patients (Hermans \emph{et al.} 2019, Johnson\emph{et
al.} 2021). Consequently, the authors of the current review have not
included locoregional techniques involving the head for dental or ocular
procedures.

\textbf{Further research applications}

There is an obvious growing interest in objective methods of nerve
location in equine anaesthesia, which follows the prolific research
performed in small animal and human anaesthesia. Nevertheless, much of
the published literature consists of cadaveric studies and case reports,
with only a few studies combining cadaveric anatomical description with
clinical efficacy studies using a limited sample size. The need for
further research on the impact of locoregional techniques in equine
anaesthesia is apparent, specifically in terms of evaluating the effects
on anaesthetic requirements, post-operative analgesia and pain scores,
the ability to performing standing surgery using such techniques, and
the incidence of complications associated with these objective methods
of nerve location. Moreover, larger sample sizes should be used to
better understand and assess the impact of these methods of nerve
location in comparison to the traditional blind locoregional anaesthetic
techniques which are already popular in equine anaesthesia and
analgesia.

\textbf{Conclusion}

The use of objective methods for locoregional anaesthesia in equine
patients has led to the development of various new techniques for
targeting different nerves and desensitising various anatomical areas,
allowing for standing surgery as well as reduced intraoperative pain
during surgery performed under general anaesthesia. To assess the
advantages and disadvantages of these objective methods in comparison to
traditional blind locoregional anaesthetic techniques, and to determine
their impact on equine anaesthesia, analgesia, and surgery, further
research is necessary. So far, objective methods such as nerve
stimulation and ultrasound-guided injections have been found to improve
accuracy and precision, as well as reduce complications associated with
locoregional anaesthesia and analgesia in small animal and human
anaesthesia. Thus, gaining a comprehensive understanding of the impact
of these objective methods could significantly enhance the integration
of such techniques into the routine practice of equine veterinary
surgeons. This, in turn, will enable clinicians to make well-informed
decisions when choosing the most suitable locoregional techniques for
their equine patients, leading to improved patient care and surgical
outcomes.

Figure Legends:

Figure 1. Peripheral nerve stimulator (Plexygon, Vygon (UK) Ltd,
Swindon, UK). The peripheral nerve stimulator is a small device that
enables the localisation of nerves through the emission of a
low-intensity electrical current.

Figure 2. A graphical representation illustrating the concepts of
chronaxie and rheobase in nerve fibers. Chronaxie is the minimum
duration of an electrical impulse required to elicit a response at twice
the rheobase current intensity. Rheobase is the minimum current
intensity required to elicit a response with a stimulus of infinite
duration. These parameters are crucial in understanding the excitability
and responsiveness of nerve fibers to electrical stimulation.

Figure 3: Techniques for transverse abdominis plane (TAP) and rectus
abdominis sheath (RAS) blocks. This schematic figure illustrates various
approaches to the transverse abdominis plane (TAP) block and the
technique for performing an internal rectus abdominis sheath (RAS)
block. Three distinct methods have been identified in cadaver studies
for administering the TAP block in ponies and horses: subcostal, ventral
intercostal, and flank approaches (Baldo et al. 2018, Küls et al. 2020,
Freitag et al. 2021). Additionally, the technique for performing an
internal RAS block to provide antinociception to the abdominal midline
in horses is showed (Ishikawa et al. 2023). Anatomical landmarks are
indicated by blue marker and white lines, while points of injection are
represented by red marker.

\textbf{REFERENCES}

Baldo, C.F., Almeida, D., Wendt-Hornickle, E. and Guedes, A. (2018)
Transversus abdominis plane block in ponies: a preliminary anatomical
study. \emph{Vet Anaesth Analg} \textbf{45} , 392--396.

Budras, K.D., Sack, W.O., Rock, S., Horowitz, A. and Berg, R. (2012) The
Abdominal Wall. In: \emph{Anatomy of the Horse} , Schluetersche. pp
70--77.

Campoy, L., Morris, T.B., Ducharme, N.G., Gleed, R.D. and Martin-Flores,
M. (2018) Unilateral cervical plexus block for prosthetic laryngoplasty
in the standing horse. \emph{Equine Vet J} \textbf{50} , 727--732.

Cevik, J., Hunter-Smith, D.J. and Rozen, W.M. (2022) Anatomical
differences in the abdominal wall between animal species with
implications for the transversus abdominis plane block: a systematic
review. \emph{Surgical and Radiologic Anatomy} \textbf{44} , 1171--1180.

Cheetham, J., Pigott, J.H., Hermanson, J.W., Campoy, L., Soderholm, L.
V., Thorson, L.M. and Ducharme, N.G. (2009) Role of the hypoglossal
nerve in equine nasopharyngeal stability. \emph{J Appl Physiol
(1985)}\textbf{107} , 471--477.

Chiavaccini, L., Cavalcanti, M., De Gasperi, D. and Portela, D.A. (2022)
Clinical efficacy of ultrasound-guided bilateral erector spinae plane
block for standing lumbar spinous osteotomy in a horse. \emph{Vet
Anaesth Analg} \textbf{49} , 517--519.

Cruz-Sanabria, J.A., Gaschen, L., Bragulla, H.H., Mitchell, M. and
Leise, B.S. (2021) A study of ultrasound-guided perineural injection of
the caudal cervical spinal nerve roots in equine cadavers. \emph{Vet
Anaesth Analg} \textbf{48} , 603--611.

Dalrymple, P. and Chelliah, S. (2006) Electrical nerve
locators.\emph{Continuing Education in Anaesthesia, Critical Care and
Pain}\textbf{6} , 32--36.

Delgado, O.B.D., Louro, L.F., Rocchigiani, G., Verin, R., Humphreys, W.,
Senior, M. and Campagna, I. (2021) Ultrasound-guided erector spinae
plane block in horses: a cadaver study. \emph{Vet Anaesth
Analg}\textbf{48} , 577--584.

El-Khamary, A., Nazih, M., El-Sherif, M. and Senosy, W. (2017)
Ultrasound-Guided Pudendal Nerve Block in Male Donkeys. \emph{J Vet Med
Res} \textbf{4} , 1--4.

Freitag, F.A.V., Amora, D. da S., Muehlbauer, E., Dornbusch, P.T.,
Machado, M., Montiani-Ferreira, F., Prisco Farias, E.L., Valverde, A.
and Duque Moreno, J.C. (2021) Ultrasound-guided modified subcostal
transversus abdominis plane block and influence of recumbency position
on dye spread in equine cadavers. \emph{Vet Anaesth Analg} \textbf{48} ,
596--602.

Gallacher, K., Santos, L.C., Campoy, L., Bezuidenhout, A.J. and Gilbert,
R.O. (2016) Development of a peripheral nerve stimulator-guided
technique for equine pudendal nerve blockade. \emph{Vet J} \textbf{217}
, 72--77.

Galvani, L. (1791) \emph{Aloysii Galvani De viribus electricitatis in
motu musculari commentarius} . Ex Typographia Instituti Scientiarium.
https://doi.org/10.5479/sil.324681.39088000932442

Goodrich, L.R., Nixon, A.J., Fubini, S.L., Ducharme, N.G., Fortier,
L.A., Warnick, L.D. and Ludders, J.W. (2002) Epidural Morphine and
Detomidine Decreases Postoperative Hindlimb Lameness in Horses after
Bilateral Stifle Arthroscopy. \emph{Veterinary Surgery} \textbf{31} ,
232--239.

Hector, R.C., Rezende, M.L., Mama, K.R. and Hess, A.M. (2020) Recovery
quality following a single post-anaesthetic dose of dexmedetomidine or
romifidine in sevoflurane anaesthetised horses. \emph{Equine Vet
J}\textbf{52} , 685--691.

Hermans, H., Veraa, S., Wolschrijn, C.F. and van Loon, J.P.A.M. (2019)
Local anaesthetic techniques for the equine head, towards guided
techniques and new applications. \emph{Equine Vet Educ} \textbf{31} ,
432--440.

Ishikawa, Y., Sakai, D.M., Im, J.S., Zhang, S., Reed, R.A., Quandt,
J.E., Baldo, C.F., Walters, B. and Barletta, M. (2023) Antinociceptive
effects of bupivacaine injected within the internal abdominis rectus
sheath in standing healthy horses. \emph{Vet Anaesth Analg} .
https://doi.org/10.1016/J.VAA.2023.02.013

Johnson, J.P., Vinardell, T. and David, F. (2021) Ultrasound-guided
injections of the equine head and neck: review and expert
opinion.\emph{J Equine Sci} \textbf{32} , 103--115.

Krone, S. (2012) Chapter 13: Equipment for regional anaesthesia.
In:\emph{Ward's Anaesthetic Equipment} , 6th Edition., Eds.: A.J. Davey
and A. Diba, Saunders, London. pp 311--322.

Küls, N., Trujanovic, R., Otero, P.E. and Larenza-Menzies, M.P. (2020)
Ultrasound-Guided Transversus Abdominis Plane Block in Shetland Ponies:
A Description of a Three-Point Injection Technique and Evaluation of
Potential Analgesic Effects. \emph{J Equine Vet Sci} \textbf{90} ,
102994.

Lloyd-Edwards, R.A., Ferrão - van Sommeren, A., Hermans, H., Tersmette,
A.A., Veraa, S. and van Loon, J.P.A.M. (2022) Comparison of blind,
ultrasound- and neurostimulator-guided methods of percutaneous inferior
alveolar nerve block. \emph{Equine Vet Educ} \textbf{00} , 1--7.

Louro, L.F., Milner, P.I. and Bardell, D. (2020) Epidural administration
of opioid analgesics improves quality of recovery in horses
anaesthetised for treatment of hindlimb synovial sepsis. \emph{Equine
Vet J} evj.13338.

Marhofer, P. and Fritsch, G. (2017) Safe performance of peripheral
regional anaesthesia: the significance of ultrasound
guidance.\emph{Anaesthesia} \textbf{72} , 431--434.

Moon, P.F. and Suter, C.M. (1993) Paravertebral thoracolumbar
anaesthesia in 10 horses. \emph{Equine Vet J} \textbf{25} , 304--308.

Morris, T.B., Lumsden, J.M., Dunlop, C.I., Locke, V., Sommerauer, S. and
Hurcombe, S.D.A. (2020) Clinical Assessment of an Ipsilateral Cervical
Spinal Nerve Block for Prosthetic Laryngoplasty in Anesthetized
Horses.\emph{Front Vet Sci} \textbf{7} .

Munirama, S. and McLeod, G. (2015) A systematic review and meta-analysis
of ultrasound versus electrical stimulation for peripheral nerve
location and blockade. \emph{Anaesthesia} \textbf{70} , 1084--1091.

Otero, P.E., Klaumann, P.R., Romano, M., Bartel, A. and Portela, D.A.
(2019) Regional Anesthesia. General Considerations. In: \selectlanguage{english}\emph{Manual of
small animal regional anesthesia : illustrated anatomy for nerve
stimulation and ultrasound-guided nerve blocks} , 2nd Edition., Eds.:
Pablo E. Otero, Diego A. Portela, S.E. Fuensalida and Marta Romano,
Inter-Medica. pp 1--46.

Perez, B.R., Hawkins, A., Fiske-Jackson, A. and Jimenez, C.P. (2023)
Opioid-free anaesthesia protocol for standing spinal surgery in a
horse.\emph{Vet Rec Case Rep} e590.

Perthes, G. (1912) Ueber Leitungsanlsthesie unter Zuhilfe- nahme der
elektrischen Reizung. \emph{Miinch Med Wochen-schr} \textbf{47} ,
2543--2548.

Portela, D.A., Verdier, N. and Otero, P.E. (2018a) Regional anesthetic
techniques for the pelvic limb and abdominal wall in small animals: A
review of the literature and technique description. \emph{The Veterinary
Journal} \textbf{238} , 27--40.

Portela, D.A., Verdier, N. and Otero, P.E. (2018b) Regional anesthetic
techniques for the thoracic limb and thorax in small animals: A review
of the literature and technique description. \emph{The Veterinary
Journal} \textbf{241} , 8--19.

Rodriguez, A., Medina-Serra, R., Lynch, N. and Veres-Nyeki, K. (2022)
Erector spinae plane block as part of a multimodal analgesic approach in
an anaesthetised horse undergoing dorsal spinous process ostectomy and
desmotomy. \emph{Vet Rec Case Rep} \textbf{10} , e345.

Rossignol, F., Vitte, A., Boening, J., Maher, M., Lechartier, A.,
Brandenberger, O., Martin-Flores, M., Lang, H., Walker, W. and Ducharme,
N.G. (2015) Laryngoplasty in standing horses. \emph{Vet Surg}\textbf{44}
, 341--347.

Santos, L.C. and Gallacher, K. (2017) Nerve stimulation-guided
thoracolumbar paravertebral block for flank laparotomy in a
horse.\emph{Vet Anaesth Analg} \textbf{44} , 187--188.

Touzot-Jourde, G., Geffroy, O., Tallaj, A., Gauthier, O. and Denoix,
J.M. (2020) Ultrasonography-Guided Perineural Injection of the Ramus
ventralis of the 7 and 8th Cervical Nerves in Horses: A Cadaveric
Descriptive Pilot Study. \emph{Front Vet Sci} \textbf{7} , 488815.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1/Figure-1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-2/Figure-2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-3/Figure-3}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

