Rachael E. Carpenter1 and Christopher R. Byron2 1 Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Virginia Tech University, Blacksburg, Virginia, USA 2 Department of Large Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Virginia Tech University, Blacksburg, Virginia, USA In horses, many diagnostic and surgical procedures can be performed safely and humanely by combining local anesthetic techniques with sedation and/or physical restraint. In conjunction with a good physical examination, and thorough palpation of the limbs including tendons and joints, one of the most important techniques for the equine practitioner to master is local and regional anesthesia and analgesia of the limbs. Use of these techniques can aid in localizing lameness, provide analgesia and anesthesia for standing procedures or diagnostics, and provide intra‐ or postoperative analgesia. Mastery of other techniques (e.g., epidural and craniofacial nerve blocks) will facilitate procedures on standing patients, avoiding the relatively high morbidity and mortality associated with general anesthesia of the equine patient. The choice of which local anesthetic(s) to be used is usually made based on the onset and duration of action of the individual agent(s) and the desired result. Improper injection technique or misidentification of the landmarks can result in incomplete block and other complications. Aseptic preparation of the skin is recommended for local anesthetic injection sites, especially when injecting into a joint or the epidural or subarachnoid space. Desired anesthetic effects without complications are obtained by using proper techniques, including aspiration before injection to avoid intravenous or intra‐arterial administration, and avoidance of injections through or into inflamed tissues. The blocks described in this chapter are most commonly accomplished with lidocaine (with the exception of the diagnostic blocks for lameness examination) unless specified otherwise. Lidocaine has a rapid onset of action and a relatively short duration of effect (approximately 1 h). Where a longer duration of action is desired (for analgesia), mepivacaine, bupivacaine, or ropivacaine may be substituted. Local anesthesia of the limbs for diagnostic nerve blocks is usually accomplished with mepivacaine because it has a slightly longer duration of effect than lidocaine (up to 2 h), but lidocaine may be substituted at the same volume if needed. Dexmedetomidine added to mepivacaine has been shown to double the duration of action of mepivacaine alone in a peripheral nerve block if a longer block is needed [1]. For more in‐depth information on the pharmacology (metabolism, elimination, toxicity, and individual medication profiles) of local anesthetics, see Chapter 29. Local anesthesia of the head is most commonly used clinically for dental and sinus surgery, ocular examinations, and laceration repair. A recent study assessing complications associated with regional nerve blocks in dental patients identified eight complications out of 270 regional nerve blocks performed resulting in a 2.96% incidence of complication. Complications included supraorbital hematoma, local hematoma, lingual self‐trauma, and nerve block failure [2]. The infraorbital nerve block facilitates surgery of the nasal area or incisors. Desensitization of the upper lip and nose is achieved by injecting 5 mL of local anesthetic with a 1 inch (2.5 cm) 20 gauge needle over the infraorbital nerve as it emerges from the infraorbital canal (Fig. 63.1g). The infraorbital canal can be palpated after displacing the flat levator labii superioris muscle dorsally and then palpating with the index finger approximately half the distance and 2.5 cm dorsal to a line connecting the nasomaxillary notch and the rostral end of the facial crest. To desensitize the teeth as far as the first molar, the maxillary sinus, the roof of the nasal cavity, and the skin almost to the medial canthus of the eye, 5 mL of local anesthetic is deposited within the infraorbital foramen using a 2 inch (5.0 cm) 20 gauge needle (Fig. 63.1h) [3]. The local anesthesia produced by this technique is generally not sufficient to allow standing extraction of the premolars or trephination of the maxillary sinus but may be a useful adjunct to general anesthesia in those cases. A study using computed tomography (CT) and contrast injections examined two alternative techniques for infiltration of the infraorbital nerve within the pterygopalatine fossa in the hopes of refining the technique for better analgesia of the cheek teeth [4]. Both techniques were successful in achieving distribution of contrast medium around the infraorbital nerve. There is evidence to suggest that idiopathic head shaking may be a facial pain syndrome resulting from trigeminal neuropathy. Idiopathic head shaking is a diagnosis without a conclusive diagnostic test, but one study has suggested utilizing desensitization of the infraorbital nerve as a diagnostic tool for this disorder [5]. Figure 63.1 Sites for needle placement for local anesthesia of the head. a, supraorbital nerve; e and f, auriculopalpebral nerve; g and h, infraorbital nerve; i, mental nerve; k, alveolar mandibular nerve. Desensitization of the maxilla, premaxilla, paranasal sinuses, and sinus cavity can be achieved by blocking the maxillary nerve at the pterygopalatine fossa (where the nerve enters the infraorbital canal). Two methods of infiltration have been described. In the first, a 3.5 inch (8.9 cm) 20–22 gauge spinal needle is inserted on the ventral border of the zygomatic process of the temporal bone at the narrowest point of the zygomatic arch and directed rostromedially and ventrally in the direction of the sixth cheek tooth on the contralateral maxillary arcade [6]. The second approach is accomplished by inserting the needle at a 90° angle to the head so that it enters the pterygopalatine fossa just caudal to the maxillary tuberosity [7]. A cadaver study evaluated the accuracy of methods used to infiltrate the maxillary nerve and determined that using an angulated needle placement or a perpendicular needle placement were equally accurate [8]. To further improve the accuracy of maxillary nerve blocks, recent investigations are exploring the possibility that ultrasound‐guided blocks may be more accurate than blocks done using surface landmarks [9]. Blocking the mandibular nerve will desensitize the ipsilateral side of the mandible and the associated dental structures. The mandibular nerve can be blocked as it enters the mandibular canal at the mandibular foramen where it becomes the inferior alveolar nerve. The location of the mandibular foramen may be approximated using the intersection of a line passing vertically downward from the lateral canthus of the eye and a line extending backward from the table of the mandibular molar teeth. A 6 inch (15.24 cm) 20–22 gauge needle is inserted at the ventral border of the ramus, just rostral to the angle of the mandible and then advanced to the location of the mandibular foramen. In that location, 15–20 mL of local anesthetic may be injected. The second approach involves inserting the needle at the caudal border of the ventral ramus of the mandible about 3 cm ventral to the temporomandibular joint and then advancing the needle to the approximate location of the mandibular foramen, taking care to stay as close to the medial aspect of the mandible as possible. To perform the block, 15–20 mL of local anesthetic is injected into the area. One study examined the accuracy of the two approaches to the inferior alveolar nerve block and found that both the traditional approaches were accurate, but that currently recommended doses of local anesthetics may be excessive [10]. An intraoral approach to the inferior alveolar nerve has also been described. The authors of one study found that this approach could potentially reduce the dose of local anesthetic needed by allowing the anesthetist to deposit the local anesthetic closer to the inferior alveolar nerve, which could reduce the risk of desensitizing the lingual nerve and causing self‐trauma to the tongue [11]. To desensitize the lower lip, the mental nerve is blocked with 5 mL of local anesthetic rostral to the mental foramen (Figs. 63.1i and 63.2i). After the tendon of the depressor labii inferioris is displaced, the lateral border of the mental foramen is palpated at the horizontal ramus of the mandible in the middle of the interdental space [12]. The lower incisors and premolars can be desensitized by inserting a 3 inch (7.5 cm) 20 gauge spinal needle into the mental foramen as far as possible in a ventromedial direction and depositing 10 mL of local anesthetic to desensitize the mandibular alveolar nerve. This technique is difficult and is probably best used as an adjunct to general anesthesia for extraction of teeth. Figure 63.2 Area of skin desensitization after blocking various nerves of the head. a, supraorbital nerve; b, lacrimal nerve; c, infratrochlear nerve; d, zygomatic nerve; g, infraorbital nerve; i, mental nerve. As many sedated patients do not tolerate placement of local anesthetic into the mental foramen, one study investigated an intraoral technique and found that local anesthetic could be deposited into the mental foramen via this approach [13]. Because of the strength with which the horse can close its eyelids and keep them closed, some form of sedation and local anesthesia is generally required for a complete ophthalmic examination. These same techniques will often allow satisfactory completion of minor diagnostic and surgical procedures. In addition to the specific blocks listed below, a line block (local anesthetic deposited along the superior and/or inferior orbital rims) and infiltration anesthesia may be used to facilitate surgical procedures and placement of subpalpebral lavage catheters. Topical anesthesia is usually required for examination of an eye that is a source of pain. In addition, topical anesthesia will facilitate minor diagnostic and surgical procedures of the cornea and conjunctiva such as collection of samples for cytologic examination, removal of superficial corneal foreign bodies, and subconjunctival injections. Proparacaine and tetracaine are the most commonly used topical ophthalmic anesthetics. Preservative‐free ophthalmic formulations are preferred for topical ocular use because preservatives can damage the corneal epithelium. Although proparacaine and tetracaine are generally interchangeable, tetracaine is more irritating to the cornea than proparacaine. Other local anesthetics that have been used topically include lidocaine, mepivacaine, and bupivacaine. In humans, the onset of action of proparacaine is approximately 15 s, and the duration of action is approximately 15–30 min [14]. In horses, using two drops of 0.5% tetracaine increases the duration of maximal anesthetic effects from 5.5 to 16 min, and increasing the concentration to 1% tetracaine increases the duration of maximal anesthetic effects from 5.5 to 15.25 min [15]. A study in horses compared the efficacy and duration of topically applied proparacaine ophthalmic solution, lidocaine injectable solution, mepivacaine injectable solution, and bupivacaine injectable solution and concluded that bupivacaine may be most appropriate for procedures requiring longer periods of corneal anesthesia. In that study, corneal sensitivity was decreased for 35 min following topical application of 0.5% proparacaine and 2% mepivacaine, for 45 min following 2% lidocaine, and for 60 min following 0.5% bupivacaine [16]. Another study compared 0.5% bupivacaine, 2% lidocaine, and 0.4% oxybuprocaine and showed that corneal sensitivity was decreased for 45 min with bupivacaine, 55 min for lidocaine, and 65 min for oxybuprocaine [17]. Topical lidocaine gel has been studied as a topical ophthalmic anesthetic in horses. One study found that 0.2 mL of 2% lidocaine gel applied topically produced a marked reduction in corneal sensitivity for up to 75 min. Some corneal irregularities were seen in treated eyes, but all resolved without treatment within 24 h. The authors concluded that this may be a useful topical anesthetic for minor ophthalmic procedures performed in the standing sedated horse [18]. Repeated use of topical anesthesia can reduce Schirmer tear test values, cause mild corneal epithelial damage, and suppress wound healing with prolonged use. Because of these potential adverse effects, topical anesthetics should not be prescribed as analgesic agents for painful ocular conditions. The most important nerve block used in ocular examinations to prevent closure of the eyelid is the auriculopalpebral nerve block. This block primarily affects the motor innervation to the orbicularis oculi muscle, without affecting the sensory innervation to the eyelids. The auriculopalpebral nerve is usually blocked where the nerve is easily palpable along the most dorsal aspect of the zygomatic arch (Fig. 63.3e) or the depression caudal to the mandible at the ventral edge of the temporal position of the zygomatic arch (Fig. 63.3f). In each location, the needle is placed subcutaneously in the area where the nerve is palpated and 1–2 mL of local anesthetic is injected over the nerve [12,19]. Auriculopalpebral nerve blocks will diminish the blink reflex, so artificial tears should be applied to the cornea to prevent drying, and care should be exercised to protect the eye from accidental trauma or debris until recovery is complete. While auriculopalpebral nerve block does decrease intraocular pressure (IOP), the difference in IOP between blocked versus unblocked eyes is not clinically significant [20]. The supraorbital nerve block will desensitize the majority of the upper eyelid and can be used to facilitate minor surgical procedures of the nasal portion of the upper lid (Fig. 63.2a). The nerve emerges through the supraorbital foramen, which can be easily palpated with the index finger about 5–7 cm dorsal to the medial canthus and in the center of an imaginary triangle formed by grasping the supraorbital process of the frontal bone with the thumb and middle finger and sliding medially (Fig. 63.4A). Approximately 2 mL of local anesthetic is injected subcutaneously over the foramen, 1 mL as the needle is inserted into the foramen, and 2 mL as the needle is inserted to its full depth (2.5 cm) into the foramen (Fig. 63.4B). Successful completion of this block will desensitize the forehead, including the middle two‐thirds of the upper eyelid, and since this block will also desensitize some of the terminal branches of the auriculopalpebral nerve, motor function of the orbicularis oculi muscle may be affected [21]. Figure 63.3 Needle placement for periocular nerve blocks. a, supraorbital nerve; b, lacrimal nerve; c, infratrochlear nerve; d, zygomatic nerve; e and f, auriculopalpebral nerve. Figure 63.4 A. Palpation of the supraorbital nerve. B. Insertion of a 2.5 cm, 25 gauge needle into the supraorbital foramen. For more complete desensitization of the eyelids, the supraorbital block may be combined with techniques that block the lacrimal, infratrochlear, and zygomatic nerves. The lacrimal nerve is desensitized by inserting a needle percutaneously at the lateral canthus of the eye and directing it medially along the dorsal rim of the orbit (Fig. 63.3b). Deep injection of 2–3 mL of anesthetic at this site desensitizes the lateral canthus, lateral aspect of the upper eyelid, lacrimal gland, local connective tissue, and temporal angle of the orbit (Fig. 63.2b) [19,21]. Anesthesia of the medial canthus, lacrimal glands, nictitans, and connective tissues (Fig. 63.2c) is achieved by injecting 2–3 mL of local anesthetic around the infratrochlear nerve. The needle is inserted through the bony notch or the palpable irregularity on the dorsal rim of the orbit near the medial canthus (Fig. 63.3c) [19,21]. To infiltrate the zygomatic nerve and desensitize the lower two‐thirds of the lower eyelid, skin, and connective tissue (Fig. 63.2d), the needle is placed subcutaneously on the lateral aspect of the bony orbit and supraorbital portion of the zygomatic arch and 3–5 mL of local anesthetic is injected (Fig. 63.3d) [19,21]. If general anesthesia is contraindicated or financially undesirable, standing enucleation may be an option. Standing enucleation may be accomplished with sedation in addition to blockade of the auriculopalpebral, infratrochlear, lacrimal, and zygomatic nerves [22], or with blockade of the supraorbital, infratrochlear, lacrimal, and zygomatic nerves [23]. One study reviewed the use of auriculopalpebral, supraorbital, lacrimal, zygomatic, and infratrochlear nerve blocks in addition to a retrobulbar block and sedation for enucleation and orbital implant placement in 20 horses. The authors concluded that the standardized perioperative protocol described provided a safe effective alternative to general anesthesia [24]. Various techniques may be used to desensitize specific structures of the equine limb for diagnostic or therapeutic purposes. These include perineural, intra‐articular, and intrabursal injection techniques and local infiltration (e.g., ring blocks and line blocks) of local anesthetic. These techniques are frequently used therapeutically to provide temporary relief from pain or to facilitate procedures for treatment or diagnosis of diseases. In addition, local anesthesia techniques are an integral part of the procedures used to diagnose lameness in horses. The blocks used for therapeutic and diagnostic purposes are similar. However, clinicians should keep in mind that blocks performed for diagnosis of lameness should be carried out in a manner that desensitizes the most specific region possible (to allow identification of the affected structures with confidence), whereas blocks performed for therapeutic relief from pain or to allow performance of a procedure (e.g., surgery) may be applied so that a larger (i.e., less specific) area is desensitized. Therefore, the goal of the local anesthesia procedure should be considered when performing these techniques. In general, perineural injections performed as distally as possible using the smallest effective amount of local anesthetic increase the specificity of such blocks. Skin at injection sites should be cleaned and prepared prior to needle insertion to reduce the likelihood of infection of deeper tissues. Such preparation may include the use of alcohol only or povidone‐iodine or chlorhexidine gluconate scrub followed by a sterile water, saline solution, or alcohol rinse. Skin should also be aseptically prepared prior to insertion of a needle in a synovial structure, and the use of sterile gloves, syringes, needles, and unopened bottles of local anesthetic is recommended owing to the potential for septic arthritis. Clipping of hair prior to aseptic preparation for injection of synovial structures is often performed; however, clients may object to this. Removal of hair prior to preparing the skin may not be necessary; the number of bacterial colony‐forming units on skin over midcarpal and distal interphalangeal joints after a povidone‐iodine and 70% alcohol scrub is not significantly different between areas that are clipped and those that are not clipped to remove hair [25]. Removal of hair in horses with thick coats (i.e., hirsute horses or during cold weather), for horses with soiled coats, and to aid identification of landmarks for select injections may be warranted. Clipping of hair with an electric clipper and a size 40 clipper blade decreases contamination of joints with tissue debris and hair after needle insertion [26]. In addition, the use of stylets for spinal needles, 22 gauge needles rather than 20 gauge needles, and angled needle insertion reduces joint contamination when hair is not clipped [26]. Aseptic preparation is typically not necessary before performing perineural blocks; however, the skin should be clean and the area should be briefly scrubbed or wiped with alcohol. As a precaution, the skin should be thoroughly prepared whenever there is a risk of inadvertent puncture of a synovial structure. In addition, needles should only be handled on the outside surface of the hub when sterile gloves are not worn. Depending on the block performed, injections may be performed with the limb in a weight‐bearing or flexed position. Changes in position may aid the identification of anatomic landmarks before needle insertion. Restraint of the horse by a capable handler should be applied in all instances; the use of a twitch is typically warranted. The ipsilateral or a contralateral limb may be held by an assistant if the horse is reluctant to stand still during a block. However, the handler must exercise extreme care in such instances so that neither the clinician nor the assistant is injured; this is particularly important when the contralateral limb is held and should not be attempted with fractious horses. Sedation may be necessary to perform a block; however, this should be avoided when the horse must be walked or trotted as part of a lameness examination. Sedation of horses with detomidine may not alter the severity of lameness but can alter the pattern of locomotion [27]. In another study evaluating the effects of xylazine, detomidine, and romifidine alone or in combination with butorphanol, xylazine alone had the least impact on objective lameness measures [28]. Use of low‐dose detomidine may minimize effects on coordination but does have an impact on other objective gait parameters [29]. Therefore, xylazine may have the least effect on lameness and be the best choice when light sedation is necessary during lameness examination. Intra‐articular blocks may be best performed with mepivacaine because the severity of toxic effects of that local anesthetic on equine articular chondrocytes is less than that with lidocaine and markedly less than that with bupivacaine [30]. Although not in common use for horses, there is evidence that ropivacaine may have lower chondrotoxicity than mepivacaine [31]. Choice of needle gauge is based primarily on personal preference. However, 20 gauge needles are useful for a wide variety of injections. Use of smaller (22–25 gauge) needles can be advantageous for perineural injections and local anesthesia of superficial synovial structures in the distal aspects of limbs because they may reduce the horse’s reaction and can be easier to place precisely. Use of larger (18–19 gauge) needles is typically warranted for injections performed with long (greater than 1.5 inch or 3.8 cm) needles, through thick fascia or muscle, or when movement of the horse may cause bending and breakage of the needle. Choice of needle length should be made with consideration of the location of the structure to be injected and the locations of other deeper structures that may sustain iatrogenic damage during needle insertion. For horses that are prone to move during an injection or in instances during which motion of the needle may cause deposition of local anesthetic outside the region of interest, extension set tubing may be used between the syringe and needle. For perineural injections, the needle may be directed in a proximal to distal direction or a distal to proximal direction. Use of a distal to proximal direction may decrease the risk of needle breakage if the horse moves, particularly for small gauge needles. Use of a proximal to distal direction may aid retention of the needle at the site of insertion if the horse moves. For nerves within a perineural sheath, deposition of local anesthetic inside the sheath may be required to ensure effective desensitization [32]. Confirmation of needle placement can be made via ultrasonography. The efficacy of a perineural block is typically confirmed by detection of loss of skin sensation distal to the site where the block was performed. However, this may not be a reliable indication of efficacy in all instances, and the use of other signs (e.g., improvement in lameness or loss of response to hoof tester pressure or limb flexion) should also be used to assess the block further. Loss of skin sensation is not a reliable indicator of the efficacy of perineural blocks performed in the proximal aspect of a limb. When checking skin sensation, a blunt object that will not cause skin trauma should be used and light to moderate pressure should be applied. Horses may move because of the presence and movements of the clinician; for such horses, it may be helpful to check skin sensation from the contralateral side or to have an assistant cover the horse’s eyes. The digital neurovascular bundles are located in the palmar or plantar aspect of the pastern region (between the coronary band and metacarpophalangeal joint) medial and lateral to the deep digital flexor tendon. The digital nerves are the most palmar or plantar structures in these neurovascular bundles (Figs. 63.5a and 63.6a). Local anesthesia of these nerves may be performed with the limb in a weight‐bearing or flexed position. Digital nerve blocks are typically performed with a 1 inch (2.5 cm) 22 gauge needle or a ⅝ inch (1.5 cm) 25 gauge needle. The needle is advanced through the skin, and approximately 1.5 mL of local anesthetic is injected perineurally. The palmar or plantar aspect of the foot (including the heel bulbs, sole, and navicular bursa, and associated structures) is desensitized starting within 5–10 min after performance of the block (Fig. 63.7A). Figure 63.5 Needle placement for left distal forelimb nerve blocks. a, lateral and medial palmar digital nerves; b, dorsal branches; c, lateral and medial palmar digital nerves (basisesamoid); d and g, lateral and medial palmar nerves; e, lateral and medial palmar metacarpal nerves; f, communicating branch; h, location of high suspensory block. Digital nerve blocks do not significantly reduce the response of non‐lame horses to flexion of the distal limbs [33]; therefore, clinicians should keep in mind that these blocks may not improve distal limb flexion responses in lame horses. Digital nerve blocks should be performed just proximal to the collateral cartilages of the foot to reduce the likelihood that proximal interphalangeal joint pain would be alleviated by the block [34]. Local anesthesia of palmar digital nerves may alleviate lameness attributable to pain of the sole of the foot [35] or the distal interphalangeal joint [36]. Local anesthesia of palmar digital nerves affects kinematic gait analysis variables in horses with navicular disease [37]. Palmar digital nerve blocks performed with mepivacaine are fully effective between 15 min and 1 h after the block, and effects may persist for more than 2 h [38]. Mepivacaine appears to be more reliable and has a longer duration than lidocaine for diagnostic local anesthesia [39]. Results of another study indicate that the onset of desensitization is similar among local anesthetics for palmar digital nerve blocks in horses; lidocaine has a significantly shorter duration than bupivacaine, which in turn has a shorter duration than prilocaine or mepivacaine in this application [40]. The combination of dexmedetomidine and mepivacaine greatly increases the duration of sensory blockade compared with mepivacaine alone; perineural dexmedetomidine alone does not significantly affect the mechanical nociceptive threshold [1]. The addition of epinephrine to lidocaine has been shown to increase the duration of palmar digital nerve block local anesthesia [41]. Liposomal bupivacaine is also an effective agent for local anesthesia of palmar digital nerves [42]. Structures of the dorsal aspect of the foot and pastern region can be desensitized by blocking the medial and lateral dorsal branches of the palmar or plantar digital nerves (Figs. 63.5b and 63.6b). This block is typically performed in addition to the digital nerve block; a 1–1.5 inch (2.5–3.8 cm) 20–22 gauge needle is used to inject 1.5–3 mL of local anesthetic subcutaneously as the needle is passed in a dorsal direction from the site of injection of the palmar or plantar digital nerves; the needle should be inserted up to the hub. Performing the block in the proximal aspect of the pastern provides analgesia for a larger area than is provided when the block is performed in the distal aspect of the pastern; therefore, use of a distal location yields a block that is more specific for structures of the dorsal region of the foot and dorsodistal aspect of the pastern. The medial and lateral palmar or plantar digital nerves (including dorsal branches) can be blocked at the level of the distal abaxial aspect of the proximal sesamoid bones (Figs. 63.5c and 63.6c); 3–5 mL of local anesthetic is injected perineurally with a 1 inch (2.5 cm) 22 gauge needle or a ⅝ inch (1.5 cm) 25 gauge needle. Local anesthesia at this level provides analgesia of the foot and the palmar or plantar aspect of the pastern region. As with digital nerve blocks, abaxial sesamoid nerve blocks may not have an effect on lameness that is exacerbated by flexion of the distal aspect of a limb [33]. In a substantial number of horses, local anesthetic may diffuse proximally after this block [43]. Although liposomal bupivacaine is an effective perineural local anesthetic in this location, the duration does not seem to be significantly longer than bupivacaine hydrochloride [44]. The use of ketamine hydrochloride as a sole agent in a basisesamoid block has been assessed; use in this peripheral location did not significantly improve lameness [45]. Figure 63.6 Needle placement for left distal rear limb nerve blocks. a, lateral and medial plantar digital nerves; b, dorsal branches; c, lateral and medial plantar digital nerves (basisesamoid); d and g, lateral and medial plantar nerves; e, lateral and medial plantar metatarsal nerves; f, communicating branch; h, location of high suspensory block The medial and lateral palmar or plantar nerves and palmar metacarpal or palmar metatarsal nerves are desensitized with this block (Figs. 63.5d,e and 63.6d,e). For palmar or plantar nerves, local anesthetic is injected between the suspensory ligament and the deep digital flexor tendon. Because of the proximity to the digital flexor tendon synovial sheath, the area should be thoroughly prepared, the hub of the needle should be checked to ensure that synovial fluid is not obtained, and the injection should be performed at a level approximately 1 cm proximal to the distal ends of the splint bones. For the palmar metacarpal or plantar metatarsal nerves, local anesthetic is injected between the third metacarpal or metatarsal bone and the suspensory ligament at a level just distal to the distal extent of the splint bones. Because of the proximity to the palmar or plantar pouch of the fetlock joint, the area should be thoroughly prepared and the hub of the needle checked to ensure that synovial fluid is not obtained. If synovial fluid is obtained, the needle should be withdrawn and redirected. Although this block may be performed with the limb in a weight‐bearing or elevated position, elevation of the limb may increase the risk of digital flexor tendon sheath puncture [46]. These blocks provide analgesia for most structures of the fetlock joint and structures distal to that joint (Fig. 63.7B). Subcutaneous injection of additional local anesthetic in a dorsal direction may be necessary to provide complete analgesia. Local anesthetic diffusion in a proximal direction is minimal after performance of a low palmar nerve block [47], and diffusion is unlikely to cause a decrease in the severity of lameness attributable to pain in the proximal aspect of the metacarpal region [48]. The high palmar or plantar (high four‐point) block desensitizes the medial and lateral palmar or plantar nerves and palmar metacarpal or plantar metatarsal nerves in the proximal aspect of the metacarpal region proximal to the communicating branch of the medial and lateral palmar or plantar nerves (Figs. 63.5f–h and 63.6f–h). The palmar or plantar nerves are desensitized between the suspensory ligament and the deep digital flexor tendon; this area is typically easiest to palpate with the limb in a weight‐bearing position. The palmar metacarpal or plantar metatarsal nerves are desensitized via injection of local anesthetic along the axial aspects of the splint bones; this is easiest to accomplish with the limb in a flexed position. For complete analgesia of the dorsal aspect of the limb in this region, local anesthetic should be injected subcutaneously in a ring block to the dorsal midline of the limb (Fig. 63.7C). The palmar aspect of the carpometacarpal joint may be entered when performing this block [49]; therefore, the area should be thoroughly prepared and suction applied after connection of the syringe to the needle to ensure that synovial fluid is not obtained. Inadvertent injection of the carpometacarpal joint would provide anesthesia to the middle carpal joint, which could lead to erroneous conclusions during a lameness examination. The origin of the suspensory ligament may not be completely desensitized with this block. Gas may be introduced into tissues while performing this block and can temporarily interfere with ultrasonographic examination of structures of the proximal palmar metacarpal region [50]. The origin of the suspensory ligament may be desensitized via direct injection of the structure with local anesthetic. The block is performed with the limb in a flexed position. The needle is advanced into the proximal aspect of the suspensory ligament along the axial aspect of the splint bone, and local anesthetic is injected in several sites in a fan‐shaped pattern. The injection may be performed from the medial and lateral aspects, or just the lateral aspect of the limb (particularly in hind limbs, where injection from the medial side may be difficult). Use of an 18–19 gauge needle may prevent inadvertent needle breakage if the horse moves while performing this block. Figure 63.7 Desensitized subcutaneous area after various lower limb nerve blocks. A. Palmar/plantar digital nerve block. B. Low palmar/plantar (low four‐point) nerve block. C. High palmar/plantar (high four‐point) nerve block plus ring block to desensitize the dorsal metacarpal/metatarsal region. D. Proximal metacarpal/metatarsal (high suspensory) nerve block. The lateral palmar nerve can be desensitized at the level of the accessory carpal bone to provide analgesia for the deep structures of the palmar aspect of the forelimb, including the proximal aspect of the suspensory ligament. The block can be performed from the lateral or medial aspect of the limb. For the lateral approach, local anesthetic is deposited perineurally between the distal aspect of the accessory carpal bone and the proximal aspect of the fourth metacarpal (lateral splint) bone; this can be performed with the limb in a weight‐bearing position or with the carpus flexed. Use of a medial approach to the nerve may prevent inadvertent injection of the carpal synovial sheath [51]. This technique is performed with the limb in a weight‐bearing position. A ⅝ inch (1.5 cm) 25 gauge needle is inserted in a groove in the flexor retinaculum fascia at the palmaromedial aspect of the accessory carpal bone. The needle is directed in a mediolateral direction until it contacts bone, and then, local anesthetic is injected. Local anesthesia of the deep branch of the lateral plantar nerve provides analgesia of the proximal aspect of the suspensory ligament in the hind limb. A 1.5 inch (3.8 cm) 18–19 gauge needle is inserted 1.5–2 cm distal to the head of the fourth metatarsal bone (lateral splint bone) and directed axial to that bone [52,53]. The needle may be directed either proximodorsally or dorsally; insertion of the needle in a proximodorsal direction may provide more specific local anesthesia of the deep branch of the lateral plantar nerve and lead to less diffusion of the local anesthetic versus insertion of the needle in a dorsal direction [53]. It is important to note that anesthesia of the lateral plantar nerve is also likely when blocking the deep branch of the lateral plantar nerve, so the procedure is not strictly specific to the suspensory ligament origin [54]. Because of a small risk of tarsometatarsal joint and tarsal sheath puncture, the skin should be thoroughly prepared before performing the block. The tarsal sheath and tarsometatarsal joint are inadvertently entered in 16% and 6% of horses, respectively, during performance of this block [55]. Performance of these blocks in combination provides analgesia for structures of the antebrachium, carpus, and distal aspect of the forelimb. These blocks may be performed before a procedure or for diagnostic purposes. Local anesthesia of the median nerve is performed just distal to the superficial pectoral muscle at the caudomedial aspect of the radius, cranial to the flexor carpi radialis muscle (Fig. 63.8a); approximately 10 mL of local anesthetic is injected. The ulnar nerve is desensitized approximately 10 cm proximal to the accessory carpal bone at the caudal aspect of the limb in a palpable groove between the flexor carpi ulnaris and ulnaris lateralis muscles (Fig. 63.8b); a 1 inch (2.5 cm) 20 gauge needle should be inserted to the hub, and approximately 10 mL of local anesthetic injected as the needle is withdrawn. The medial cutaneous antebrachial nerve is desensitized just cranial to the cephalic vein at the dorsomedial aspect of the middle of the radius (Fig. 63.8c); approximately 5 mL of local anesthetic is injected. The medial cutaneous antebrachial nerve provides analgesia only for skin, so the block is not required for diagnostic purposes. The median and ulnar nerve blocks are not commonly performed for diagnostic purposes; however, these blocks can be very useful for determining a diagnosis in horses with forelimb lameness that is not affected by intra‐articular blocks or perineural blocks of the distal aspect of a limb. As such, the median and ulnar nerve blocks can be useful for the diagnosis of problems causing pain in the carpal bones or the distal aspect of the radius and pain attributable to soft tissue problems between the distal aspect of the antebrachium and proximal aspect of the metacarpus. Although the median nerve block site is close to the cubital joint, performance of this block does not improve lameness originating in that joint and is unlikely to confound interpretation of results in horses with complex upper forelimb lameness [56]. Figure 63.8 Needle placement for proximal forelimb nerve blocks. a, median nerve; b, ulnar nerve; c, medial cutaneous antebrachial nerve. Inset: cross‐sections and desensitized subcutaneous areas of left forelimb. Use of ultrasound guidance may aid accurate perineural injection of local anesthetic for median and ulnar nerve blocks [57]. The authors have also found ultrasound guidance useful for medial cutaneous antebrachial nerve blocks. When performing these nerve blocks for therapeutic purposes, a solution of 2% lidocaine with epinephrine has similar duration of efficacy as 2% mepivacaine and seems to be a suitable alternative [58]. Buffering mepivacaine with sodium bicarbonate results in faster onset and greater reduction in lameness compared with non‐buffered mepivacaine [59]. Performance of median and ulnar nerve blocks with 3% chloroprocaine results in faster and more complete resolution of lameness versus 2% mepivacaine [60]. Local anesthesia of the tibial and peroneal nerves desensitizes bone and soft tissues of the distal aspect of the tibia, the hock, and structures distal to the hock. For local anesthesia of the tibial nerve, a 1.5 inch (3.8 cm) 20 gauge needle is inserted 10 cm proximal to the tuber calcis caudal to the deep digital flexor muscle (Fig. 63.9a); 15–20 mL of local anesthetic is injected. Although the nerve is located medially, the injection may be performed from the medial side or from the lateral side. When performing the block from the lateral side, the needle should be inserted to a depth sufficient to ensure deposition of local anesthetic perineurally without penetration of the skin on the medial side of the limb. Ultrasound guidance may improve accuracy of tibial nerve blocks [61,62]. Because it is difficult to maintain contact between the syringe and needle when the horse moves, it may be useful to connect extension set tubing between the needle and syringe. The superficial and deep peroneal nerves are desensitized by insertion of a 3.5 inch (8.9 cm) 18 gauge needle 10 cm proximal to the tuber calcis in a palpable groove between the long and lateral digital extensor muscles on the lateral aspect of the limb (Fig. 63.9c). The needle is inserted until the caudolateral edge of the tibia is contacted, and 20–30 mL of local anesthetic is injected as the needle is withdrawn to desensitize both the deep and superficial peroneal nerves. A subcutaneous ring block may be necessary to provide complete skin analgesia for surgery. After performing these blocks, horses may drag their toe; caution should be used during lameness examination of such horses. As for the median and ulnar nerve blocks in the forelimb, these blocks are not commonly performed but may help reach a diagnosis in horses with lameness that is not affected by intra‐articular blocks or distal perineural blocks. Figure 63.9 Needle placement for proximal rear limb nerve blocks. a, tibial nerve; b, saphenous nerve; c, peroneal nerve. Inset: cross sections and desensitized subcutaneous areas of left rear limb. The saphenous nerve is desensitized by injection of 5 mL of local anesthetic perineurally near the saphenous vein approximately 10 cm proximal to the tibiotarsal joint with a 1 inch (2.5 cm) 20 or 22 gauge needle (Fig. 63.9b). This block is useful for providing analgesia to the skin of the medial aspect of the limb. It is not performed for diagnostic analgesia during a lameness examination. Several techniques can be used for injection of the navicular bursa. However, an approach from the distal palmar aspect of the limb in which the position of the navicular bursa is determined on the basis of external anatomic landmarks may yield the highest success rate [63,64]. This technique is typically performed with the foot on an inclined wooden block to position the distal aspect of the limb in a flexed position. The position of the navicular bone can be predicted to be at a location 1 cm distal to the coronary band at the midpoint between the most dorsal and most palmar aspect of the coronary band. Injection of 1–2 mL of local anesthetic subcutaneously at the needle insertion site may decrease movement of the horse while performing this block. A 3.5 inch (8.9 cm) 18–20 gauge needle is inserted at the midpoint between the heel bulbs just proximal to the coronary band and advanced toward the predicted position of the navicular bone (Fig. 63.10a). After the navicular bone is encountered, the needle is withdrawn slightly. Correct needle placement can be confirmed by aspiration of a small amount of synovial fluid (which is rarely obtained) or via radiography. Then, 3–5 mL of local anesthetic is injected. An alternate technique introducing the needle from the lateral aspect of the limb just proximal to the collateral cartilages of the foot under ultrasonographic guidance avoids penetration and possible damage of the deep digital flexor tendon [65]. Improvement of lameness within 10 min after injection suggests that the lameness is caused by pain in the navicular bursa or associated structures [66,67]; however, distal interphalangeal joint pain may be improved by local anesthesia of the navicular bursa 20–30 min after injection [67]. Mepivacaine can diffuse between the navicular bursa and the distal interphalangeal joint [68]. Analgesia of the navicular bursa may improve lameness caused by pain in the dorsal aspect of the sole of the foot [69]. Figure 63.10 Needle placement for distal limb intra‐articular and intrabursal blocks. a, navicular (podotrochlear) bursa; b, distal interphalangeal (coffin) joint; c, proximal interphalangeal (pastern) joint; d, volar pouch of the fetlock joint capsule; e, digital flexor tendon sheath. For local anesthesia of the distal interphalangeal joint, a 1.5 inch (3.8 cm) 20 gauge needle is inserted 1 cm proximal to the coronary band and 2 cm lateral to the dorsal midline aspect of the limb (Fig. 63.10b). The needle is directed distomedially toward the palmar or plantar lateral aspect of the extensor process of the third phalanx (coffin bone). Alternatively, the needle may be inserted into the dorsoproximal pouch of the joint on the dorsal midline aspect of the limb, 1 cm proximal to the coronary band and directed in a palmar or plantar direction either parallel to the ground or distally toward the solar surface of the foot. The horse should be in a weight‐bearing position while performing this block. Injection of 6 mL of local anesthetic into the distal interphalangeal joint may desensitize the dorsal aspect (toe) of the sole of the foot, whereas injection of 10 mL may also desensitize the angles (heel) of the sole [70]. Therefore, injection of a maximum of 6 mL is recommended to increase the specificity of the block when it is performed for diagnostic purposes. Because mepivacaine can diffuse between the distal interphalangeal joint and the navicular bursa [68], intra‐articular distal interphalangeal joint blocks may not be specific for problems of that joint. The proximal interphalangeal joint is desensitized via insertion of a 1.5 inch (3.8 cm) 20 gauge needle into the palpable joint space under the extensor tendon on the dorsal aspect of the limb in a lateral to medial or medial to lateral direction (Fig. 63.10
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Equine Local Anesthetic and Analgesic Techniques
Introduction
Choice of local anesthetic agent
Anesthesia of the head
Infraorbital nerve block
Maxillary nerve block
Mandibular nerve block
Mental nerve block
Anesthesia for ocular procedures
Topical anesthesia
Auriculopalpebral nerve block
Sensory nerve blocks
Anesthesia of the limbs
Perineural anesthesia of the limbs
Palmar or plantar digital nerve block
Block of the dorsal branches of the digital nerves
Abaxial sesamoid (basisesamoid) block
Low palmar or plantar (low four‐point) nerve block
Proximal metacarpal or metatarsal region blocks
Median, ulnar, and medial cutaneous antebrachial nerve blocks
Tibial, peroneal, and saphenous nerve blocks
Intra‐articular and intrabursal anesthesia of the limbs
Navicular (podotrochlear) bursa
Distal interphalangeal (coffin) joint
Proximal interphalangeal (pastern) joint
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