Horn blocks

There are similarities in desensitizing the nerve supply to the horn in cattle, goats, and sheep. In cattle, the main sensory nerve supply to the horn arises from the cornual branch as well as the supraorbital and infratrochlear nerves, all of which originate from the ophthalmic branch of the trigeminal nerve (Fig. 66.1A). The ophthalmic branch divides into three nerves while still within the foramen orbitorotundum: the lacrimal, nasociliary, and frontal nerves. The lacrimal nerve consists of two strands located along the lateral surface of the lateral rectus muscle that later join between them and with the communicating ramus of the zygomatic nerve to form the zygomaticotemporal branch, which is located at the level of the dorsal and caudal aspect of the orbit and exits along the ventral aspect of the zygomatic process (supraorbital) of the frontal bone. The zygomaticotemporal branch continues as the cornual branch as it travels caudally toward the base of the horn along the temporal ridge, between the supraorbital process and the lateral edge of the base of the horn [16].

In adult cattle, the site of injection for the cornual branch is 3–5 cm in front of the base of the horn, where the nerve courses from ventral to dorsal from the temporal fossa to the frontal bone and branches into endings along the base of the horn (Fig. 66.1B). A 2.5–3.8 cm, 20 gauge needle is inserted along the area of the temporal line and frontal bone and 10 mL of 2% lidocaine is injected after negative aspiration of blood has been verified to avoid injection into the cornual artery or vein in the surrounding area.

The nasociliary nerve branches into the infratrochlear nerve as it enters the orbit, which ascends to the dorsal margin of the orbit to the level of the lacrimal bone, dorsal to the medial canthus, where it curves around the frontal bony margin of the orbit to travel caudally and along the frontal bone and may reach the base of the horn [16]. The site of injection for the infratrochlear nerve is 2–3 cm medial from the mid‐dorsal aspect of the rim of the orbit; a 2.5 cm, 20 gauge needle and 5 mL of 2% lidocaine are used (Fig. 66.1B). If the needle is directed an additional 2–3 cm caudomedially, then the supraorbital nerve to the horn can also be blocked from this approach. Alternatively, the infratrochlear nerve branches that reach the horn can be blocked with the supraorbital nerve as described below.

The frontal nerve travels from the foramen orbitorotundum to the orbital opening of the supraorbital canal, on the caudal and dorsal aspect of the orbit, to emerge as the supraorbital nerve at the supraorbital foramen of the frontal bone. The supraorbital foramen can be located about 3–4 cm from the temporal ridge and halfway along the distance between the supraorbital process and the medial edge of the base of the horn [16]. The site of injection for the supraorbital nerve is at the level of the supraorbital foramen using a 2.5 cm, 20 gauge needle and 5 mL of 2% lidocaine and taking care to avoid the supraorbital vein (Fig. 66.1B). Because most of the cornual branches of the infratrochlear nerve travel to the horn lateral to the supraorbital nerve at this same location, they can be blocked with this approach by directing the needle lateral toward the orbit, 2–3 cm from the rim of the orbit.

It is important to note that the proximity of the rostral auricular and zygomatic branches of the auriculopalpebral nerve to the sites of injection of the cornual, nasociliary, and supraorbital nerves often results in their blockade, producing relaxation of the ear and inability to close the eyelids [16].

In addition to these three main nerves, the caudal aspect of the base of the horn may be supplied by cutaneous branches of the second cervical nerve and these can be blocked by injecting local anesthetic close to the dorsal midline of the neck at a point level with the base of the ear.

In calves, nerve block for disbudding or dehorning can be performed as in adult cattle or simplified to two injection sites in younger calves (Fig. 66.2). For the simplified form of the block, a 1.5–2.5 cm, 20 gauge needle is inserted at the same site as for the cornual branch in adult cattle (i.e., where the nerve courses from ventral to dorsal from the temporal fossa to the frontal bone) and 5 mL of 2% lidocaine is injected above this ridge and 2 mL below the ridge. The dorsal volume of 5 mL is further divided to block the infratrochlear and supraorbital nerves as they travel toward the horn by directing the needle 2–3 cm from the ridge toward midline, and then blocking the cornual nerve along the ridge itself. The second injection is completed by redirecting the needle ventral to the ridge toward the temporal fossa, where 2 mL are injected to further block the cornual branch.

The use of a ring block can also be used in calves of less than 2 months of age, using a 2.5 cm, 20 gauge needle and injecting 6 mL of 2% lidocaine through four to five injection sites under the skin around the base of the horn [17]. However, as this block acts mostly on distal nerve endings from the major nerves to the horn, it has a shorter duration of action than the blocks described above.

Following surgery on the horns, anesthesia and analgesia are provided for as long as the duration of action of the local anesthetic. Due to the invasive nature of dehorning surgery, a non‐steroidal anti‐inflammatory drug (e.g., flunixin meglumine 2 mg/kg, IV or IM) is suggested for postoperative analgesia to supplement the anesthetic effect as it diminishes.

The cornual nerves of sheep and goats are very similar to those of cattle, although the cutaneous branches of the second cervical nerve are less likely to innervate the horn. The cornual branch of the lacrimal nerve (zygomaticotemporal) is blocked behind the root of the supraorbital process (Fig. 66.3). A 2.5 cm, 22 gauge needle is inserted to a depth of 1–1.5 cm, and 2–3 mL of 2% lidocaine is injected, halfway between the lateral canthus and the lateral edge of the base of the horn. The cornual branches of the infratrochlear nerve are blocked close to the dorsal rim of the orbit, halfway between the medial canthus and the medial edge of the base of the horn, by inserting a 2.5 cm, 22 gauge needle to a depth of about 0.5 cm and injecting 1–2 mL of 2% lidocaine [18].

Eye and adnexa block

The orbit has a rich presence of nerves behind the globe that are not exclusive to the eye. These nerves emerge from the cranial cavity through various foramina (e.g., the foramen orbitorotundum and optic foramen) to supply the eye and adnexa in addition to other extraocular structures. Nerves present in this location include the optic nerve as it emerges from the optic foramen as well as the nerves emerging from the foramen orbitorotundum, including the ophthalmic branches of the trigeminal nerve or extensions of these branches that eventually give rise to nerves supplying the horn (lacrimal nerve, infratrochlear nerve, and frontal nerve), the oculomotor nerve, the trochlear nerve, the abducens nerve, and the zygomaticofacial branch of the maxillary nerve (part of the trigeminal nerve) [16]. Therefore, injection of local anesthetic in this area can result in sensory or motor blockade of the nerves and the structures they serve, and not just of the eye.

The structures potentially anesthetized or paralyzed by local anesthetic effects on the oculomotor nerve include the dorsal, ventral, and medial rectus muscles, the superior levator palpebrae muscle, the retractor bulbi muscle, and the ventral oblique muscle. Local anesthesia of the trochlear nerve affects the dorsal oblique muscle while local anesthesia of the ophthalmic branches of the trigeminal nerve affects the upper eyelid, lacrimal gland, conjunctiva, third eyelid, skin of medial and lateral angle of the eye, iris, cornea, horn, and sections of skin over the frontal bone. Local anesthesia of the zygomaticofacial branch of the maxillary nerve affects the lower eyelid, and local anesthesia of the abducens nerve affects the lateral rectus muscle and the lateral part of the retractor bulbi muscle [16].

Anesthesia of the eye is most commonly performed for enucleation surgery. It is possible to perform an enucleation in adult cattle with the animal standing, using either a four‐point injection (retrobulbar block) to block the deep orbital nerves or a Peterson block to exclusively anesthetize the nerves as they exit the skull through the foramen orbitorotundum and optic foramen. Ocular surgery in small ruminants or young cattle is generally performed under general anesthesia but the use of an eye block is also recommended to decrease volatile anesthetic requirements, decrease the likelihood of eliciting the oculocardiac reflex (trigeminovagal), and provide postoperative analgesia.

Peterson block

This is technically more difficult than the four‐point block and requires careful needle positioning, making it less reliable. Its efficacy depends on accurate placement of the injected anesthetic at the site of emergence of the nerves from the foramen orbitorotundum since there is minimal distribution along tissue planes [19]. Due to the ventral location of the foramen orbitorotundum with respect to the optic foramen (about 1 cm ventral), direct blockade of the optic nerve may not result; however, increasing the volume of injection may facilitate reaching and therefore blocking the optic nerve (Fig. 66.4). In addition to the nerves emerging from the foramen orbitorotundum described for the retrobulbar block, the maxillary nerve (branch of the trigeminal nerve) is also involved, resulting in block of the zygomatic nerve and its zygomaticofacial branch (lower eyelid innervation); blockade of the pterygopalatine and infraorbital nerve provides anesthesia of the nasal passages and nose as described below [16]. The mandibular nerve (a branch of the trigeminal) is less likely to be affected because it exits the skull through the oval foramen, which is more ventrally located and separated from the foramen orbitorotundum by the pterygoid crest (Fig. 66.4) [16].

To perform this block, the animal is restrained with its nasal bones parallel to the ground. A 10 or 12 cm, 20 gauge needle is passed just in front of the rostral border of the coronoid process of the mandible, caudal to the notch formed by the zygomatic arch and supraorbital process and directed slightly ventrally and posteriorly for the length of the needle or until it strikes bone. The local anesthetic (15–20 mL, 2% lidocaine) is injected once the needle strikes bone in the area where the nerves travel toward the orbit [18,20] (Figs. 66.4b and 66.5A). Alternatively, the needle can be introduced below the zygomatic arch at the same horizontal position caudal to the notch of the zygomatic arch and supraorbital process, but in this case the needle is directed slightly dorsally and posteriorly to reach the same position as the previous dorsal approach (Fig. 66.5B). The same technique can be performed in small ruminants and calves using a 6.3 cm, 20 gauge needle and injecting 3–4 mL of 2% lidocaine, with the approach dorsal or ventral to the zygomatic arch.

Nasal passages and nasal block

Anesthesia of the nasal passages and nostrils can be achieved by blockade of the maxillary nerve at the foramen orbitorotundum as described for the Peterson eye block. This block will include the divisions of the maxillary nerve that enter the maxillary foramen, the pterygopalatine nerve, and infraorbital nerve. The pterygopalatine nerve supplies the soft palate (minor palatine nerve), hard palate (major palatine nerve), and ventral aspects of the nasal cavity and palate (caudal nasal nerve). The infraorbital nerve travels within the infraorbital canal from the maxillary foramen to the infraorbital foramen, where it emerges and continues to the nose and surrounding tissue, to supply the skin of the dorsal nasal area, nares, and upper lip [16]. To avoid including the innervation to the eye and adnexa, it is more common to only block the infraorbital nerve as it emerges from the infraorbital foramen, resulting in incomplete block of the nasal passages.

The infraorbital foramen can be readily localized by palpation, by extending a line from the nasoincisive notch to the first palpable cheek tooth (second premolar since the first premolar is absent), approximately 5 cm above the tooth (Fig. 66.6). A volume of 5–10 mL in adult cattle or 2–4 mL in calves of 2% lidocaine can be injected into the infraorbital canal using a 3.8 cm, 20 gauge needle introduced through the infraorbital foramen.

Line infiltration block

In adult cattle, this is the simplest block to perform and involves multiple injections of small volumes (5–8 mL) of 2% lidocaine per site with a 3.8 cm, 18 gauge needle along the previously clipped and aseptically prepared predicted incision line; the anesthetic spreads in different directions and depths from the point of entry. Therefore, this block mostly affects the nerve endings immediately proximal to the incision site. It is best to start from the most dorsal aspect of the flank and insert the needle in a dorsal to ventral direction and work toward the ventral end of the incision, as each injection provides progressive desensitization before the subsequent injection. A longer needle (8.9 cm) may be used instead of the shorter needle to reach the parietal peritoneum and improve the quality of the block.

With lidocaine, this block is usually only effective for 60–90 min due to the rapid systemic uptake of anesthetic from the vascular abdominal wall. Based on toxic doses for sheep, the total dose should not exceed 5–6 mg/kg since absorption from the muscle layers is probably rapid. This dose represents a volume of 125–150 mL of 2% lidocaine for a 500 kg cow.

In small ruminants or calves, the technique is the same but a 2.5–3.8 cm, 20 gauge needle and volumes of 1–2 mL 2% lidocaine per site are used, based on weight and a 5–6 mg/kg dosage (e.g., a volume of 10–12 mL of 2% lidocaine may be used in a 40 kg patient).

Inverted “L” or “7” block

The injection of local anesthetic following an inverted “L” or “7” shape along the caudal aspect of the last rib and the ventral aspect of the lumbar vertebrae transverse processes blocks the transmission of pain from the periphery (flank area) to the spinal cord. For correct nomenclature, the left flank is blocked with spread of local anesthetic in an inverted “L” or reversed “7” shape, whereas the right flank is blocked with a “7” shape.

The block is similar to the infiltration technique in that it requires similar volumes (5–8 mL per injection in adult cattle) of 2% lidocaine and a 3.8 or 8.9 cm, 18 gauge needle for injection of anesthetic along the shape of the block. To allow progressive desensitization before the subsequent injection (similar to line infiltration techniques) it is best to start from the angle of the “L” or “7” and spread the anesthetic to the caudal aspect for the horizontal plane and to the ventral aspect for the vertical plane. It is also important to remember that nerves will lie in different planes between muscle and fascial layers so several different depths of injection may be required for complete block of deeper layers.

This block provides more spread of the local anesthetic than the line block; however, because it is done on two axes, it is important to distribute the amount of local anesthetic evenly to avoid an incomplete block. Similar to the line block, the duration of action is approximately 60–90 min when 2% lidocaine is used.

In small ruminants or calves the block follows the same technique using a 2.5–3.8 cm, 20 gauge needle and volumes of 1–2 mL 2% lidocaine per site.

Proximal paravertebral block

This block is also known as the “Farquharson,” “Hall,” or “Cambridge” technique, and the term “proximal” refers to the proximity to the spine. Generally, nerves T13, L1, L2, and L3 are blocked. The inclusion of nerve L3 provides better anesthesia of the caudal third of the abdominal flank [21]. Branches of L3 occasionally contribute to the femoral nerve and blockade may potentially result in hindlimb weakness. However, the femoral nerve is mostly derived from L4, L5, and L6 [22], which makes ataxia very unlikely; therefore, the inclusion of L3 is recommended.

Dorsal and ventral branches from nerves T13 and L1 travel superimposed routes dorsoventrally for approximately 10 cm as the respective nerve emerges from the intervertebral foramen [23]. For nerves L2 and L3, the superimposition is less exact and a branch from the ventral branch of L2 joins the ventral branch from L3 at approximately 9–12 cm from midline (Fig. 66.7). The ventral branch from L3 is the only one of the four nerves involved that travels rostral to the dorsal branch [23].

In adult cattle, the proximal paravertebral technique involves the perineural injection of local anesthetic in proximity to the spinal nerves as they emerge from the vertebral canal. The dorsal and ventral branches of each nerve must be blocked if complete anesthesia of the flank is desired.

To locate the site of injection, it is best to identify the lumbar transverse processes by counting back from a caudal location as lumber transverse process 1 is not always palpable, depending on the degree of obesity of the animal, and may be confused with the lumbar transverse process 2. While cattle have six lumbar vertebrae, transverse process 6 is significantly smaller than that of 5 and is hidden by the iliac wing. Consequently, counting should begin at lumbar transverse process 5, which is the most proximal and cranial to the tuber coxae, and then move cranially. Each spinal nerve divides into a dorsal and ventral branch as it emerges and travels between lumbar transverse processes. Because the lumbar transverse processes are curved cranially, it is important to note that once a perpendicular line is traced from the middle of the width of the transverse process toward the spine, the nerve located in this area corresponds to the preceding process (e.g., for lumbar transverse process 4, the nerve located using this method is L3, and so on). Therefore, to block nerves T13, L1, L2, and L3, it is necessary to locate lumbar transverse processes 1 through 4 (see Figs. 66.7 and 66.8).

The epaxial area should be clipped and aseptically prepared from the last rib to lumbar transverse process 4, and from the lateral tip of the transverse processes to 3–4 cm from the dorsal spinous processes on midline. In adult cattle, the distal end of lumbar transverse process 4 is identified by placing the thumb and index finger on the width of the process. From here, a perpendicular line is traced from the middle of this width toward the spine and a 3.8 cm, 16 gauge needle is inserted full length approximately 5–6 cm laterally from midline. This will act as a cannula for the subsequent insertion of an 8.9 cm, 20 gauge spinal needle and prevent it from bending. Then, the spinal needle is directed toward lumbar transverse process 4 and, once in contact with it, is walked‐off the cranial edge until it advances through the intertransverse ligament and then situated ventrally for blockade of the ventral branch of nerve L3.

Avoid walking the needle off the caudal edge of lumbar transverse process 3 (or any of the subsequent transverse processes included in this block) because blockade at this location is less effective due to the routing of the nerves from the vertebral foramen along the transverse process. Aspiration to confirm negative pressure is important to avoid placement of the needle in the abdominal cavity. Following aspiration, 20 mL of 2% lidocaine is injected. The needle is then retracted to the point of no friction, which indicates placement above the intertransverse ligament, and the dorsal branch is blocked with 5 mL of 2% lidocaine and the needles withdrawn. This method is repeated for nerves L2 and L1 by identifying lumbar transverse processes 3 and 2, respectively. If palpation of lumbar transverse process 1 is not feasible, the distances between the previous injection sites should be symmetric, which will allow the distance between thoracic transverse process 13 and lumbar transverse process 1 to be estimated. The method described above can then be repeated to complete the block. Blockade of the four nerves as described requires approximately 100 mL of 2% lidocaine in a 500 kg cow, similar to the volume used for the infiltration block; however, paravertebral block provides a more precise but extensive block of the flank and is usually recommended over infiltration techniques. The duration of action tends to exceed that of the line infiltration and inverted “L” or “7” blocks since the administration of anesthetic is more circumscribed to the main nerves; with 2% lidocaine, blocks of 90–120 min are produced.

In addition to sensory blockade, motor and sympathetic fibers are also affected, which results in relaxation of the epaxial lumbar muscles and vasodilation, respectively. Therefore, the spine curves (scoliosis) toward the blocked site and the skin temperature over the flank increases. From the surgeon’s perspective, this means that tissues become tense due to the convexity that results on the surgical site, which tends to spread the tissues when surgically approaching the abdominal cavity. This combined with vasodilation of blood vessels, can result in increased bleeding if surgical hemostasis is poor.

In small ruminants and calves, the block follows the same technique for adult cattle and should include the same nerves (T13–L3). Anatomical studies in sheep have identified nerve branches from T13 that run obliquely and overlap with segments from L1 and L2 on the flank area as they spread toward midline [24–26], which is similar to cattle but occurs less commonly in goats [27,28]. Sheep have six (more common) or seven lumbar vertebrae, whereas goats have six but sometimes only five lumbar vertebrae, which can complicate the identification of vertebrae if counting backward as described for cattle. However, it is simpler in these species to identify lumbar transverse process 1 and count from there to verify the proper sites for needle insertion. In these species, the block is completed using a 2.5–3.8 cm, 20 gauge needle with volumes of 0.5–1.5 mL for the dorsal branch and 2–3 mL for the ventral branch of 2% lidocaine. It is important to accurately note body weight and not exceed the 5–6 mg/kg toxic dosage threshold [24,29,30].

For a longer‐lasting block, ropivacaine, bupivacaine, or lidocaine with epinephrine have also been used for the proximal paravertebral block in cattle and small ruminants. In one study of mixed‐breed sheep (44–49 kg) injected with 4 mL of local anesthetic (1.5 mL for the dorsal branch and 2.5 mL for the ventral branch), the duration of anesthesia as assessed with needle pinpricks, pinching with a hemostat, and ruminal fistulation surgery was 649 min for 0.5% levobupivacaine, and 569 min for 0.5% ropivacaine [30]. In another study with fat‐tailed lambs (23 kg) injected with 3 mL of local anesthetic (1 mL for the dorsal branch and 2 mL for the ventral branch), the duration of anesthesia as assessed with needle pinpricks and pinching with a hemostat was 65 min for 2% lidocaine, 95 min for 2% lidocaine with epinephrine (5 μg/mL), and 303 min for 0.5% bupivacaine [29].

Distal paravertebral block

This block is also known as the “Magda,” “Cakala,” or “Cornell” technique and the term “distal” refers to the distance from the spine. This distal approach is used to block the dorsal and ventral branches of the same nerves as for the proximal paravertebral block (T13, L1, L2, and L3) as they cross over and under, respectively, the transverse processes (Fig. 66.7).

For this block, a lateral approach with regards to the location of the lumbar transverse process is used and the area around the tip of each process is clipped and aseptically prepared. The transverse processes are located as described for the proximal paravertebral block; the L2 and L3 nerves are blocked from the location of lumbar transverse process 4, the L1 nerve is blocked from the location of lumbar transverse process 2, and the T13 nerve is blocked from the location of lumbar transverse process 1 (Fig. 66.7).

In adult cattle, approximately 5 mL of 2% lidocaine is injected above and 10–20 mL below the transverse process using a 6.4–8.9 cm, 18 gauge needle. The injection is started at the tip of the distal end of the transverse process and the local anesthetic is deposited along the process, as the needle is advanced toward the spine. It is important to keep the needle close to the transverse process as anesthetic deposited in the surrounding soft tissue rather than around the nerve may result in block failure. The duration of action is similar to the proximal paravertebral block.

Segmental dorsolumbar epidural block

In cattle, the dermatomes innervated by nerves T13 and L1–L3 can be blocked bilaterally or unilaterally by performing an epidural injection at the thoracolumbar (T13–L1) or first lumbar intervertebral (L1–L2) space [31–34].

An epidural injection refers to depositing the anesthetic drug in the space between the dura mater and the vertebral column. Technically, it is an intradural injection between two dural laminae since the dura mater only closely adheres as a fused double layer within the skull. In the vertebral column, it is separated and only the internal lamina, made of fibrous tissue, surrounds the spinal cord, and provides rigidity to help support the blood vessels that supply the spinal cord [35].

The injection of a reduced volume of local anesthetic or xylazine into the epidural space at T13–L1 or L1–L2 allows these drugs to affect only those segments of the spinal cord that innervate the flank without interfering with motor function of the pelvic limbs; this allows the animal to remain standing and prevents ataxia. This technique can be used for surgery on or performed via the flank and needle tip placement in the epidural space can be directed toward one side of the spinal cord to emphasize the block on the corresponding ipsilateral flank or the needle kept on the median plane with respect to the spinal cord to block both sides. The block is more technically challenging than any of the other techniques previously described for flank anesthesia; however, it can be readily learned and effectively used by practitioners [34,36].

Adequate restraint and sedation of the animal are necessary to facilitate the placement of the needle in the epidural space. The skin caudal to the T13 or L1 spinous process and contralateral to the flank region to be desensitized is aseptically prepared and injected subcutaneously with 2–4 mL of 2% lidocaine adjacent to the interspinous ligaments between T13–L1 or L1–L2 to facilitate the insertion of a short 2.5 cm, 14 gauge needle that serves as a cannula for the subsequent insertion of an 11.4 cm, 18 gauge spinal needle [32]. Alternatively, a 12 cm, 16 gauge Tuohy needle can be used by itself [37]. The mean distance from skin to the epidural space at this level is 8.1 cm in cows between 337 and 742 kg [38], but it is recommended to use needles that are slightly longer because if the needle does not reach the deeper planes of the epidural space, the injected anesthetic will remain between periosteum and epidural fat, rather than distribute between epidural fat and dura mater [37]. Fat is considered a barrier because it is present in a semifluid state that impedes spread and potentially prevents the actions of the anesthetic drug; the amount of fat in the thoracolumbar epidural space is greater in the dorsal aspect compared to the ventral aspect of the space [37].

The L1–L2 intervertebral space is localized on the path of an imaginary line drawn from side to side, 1–2 cm caudal to the tips of the two cranial edges of the second lumbar transverse process. The operator can then decide to insert the needles at the depression between L1 and L2 or move to the next cranial depression between T13 and L1. The spinal needle is advanced gradually through the interspinous ligament until it reaches and then penetrates the ligamentum flavum to enter the epidural space. Correct placement can be verified by use of the hanging drop technique in which the hub of the needle is filled with saline or local anesthetic after the stylet is removed. If the needle is correctly placed, the fluid is aspirated into the needle shaft and epidural space due to the subatmospheric epidural pressure. The stylet can also be removed before the needle penetrates the ligamentum flavum and the operator can detect the aspiration of the fluid once the needle enters the epidural space. The initial pressure of the epidural space is on average –21 mmHg (range of –17 to –23 mmHg), but within 1 min of needle insertion, this stabilizes at –14 mmHg (range of –9 to –17 mmHg) [38]. For this reason, it has been recommended to allow air to enter freely into the epidural space for approximately 1 min to decrease the effects of varying pressures on the distribution of anesthetic drug [33]. Additional verification of correct placement should include ease of injection into the epidural space and the absence of CSF prior to injection, which indicates that the dura and arachnoid membrane have not been pierced. Avoiding subarachnoid injection is important since epidural doses are significantly higher than those recommended for subarachnoid administration. An alternative modified technique involves introducing the needle an additional 0.7–1.0 cm or until the cow shows signs of discomfort, such as sudden movement or dipping of the back, to bypass the epidural fat and enhance the spread of the anesthetic drug [33]. Caution is advised with this modified technique. The needle should be removed immediately after injection to avoid damage to the spinal cord.

Studies using new methylene blue have shown that 5 mL injected at the L1–L2 epidural space of adult Holstein cows spread to the T12–L3 spinal dermatomes, whereas 10 mL spread to the T11–L5 spinal dermatomes [37]. Volumes of 6–8 mL of 2% lidocaine (0.24–0.32 mg/kg) or 5% procaine (0.6–0.8 mg/kg) are recommended in a 500 kg cow to desensitize the dermatomes of T13–L3. Xylazine (2%, 0.05 mg/kg) is also effective but the combination of xylazine (0.025 mg/kg) and lidocaine (0.1 mg/kg) diluted to a volume of 5 mL with 0.9% saline resulted in more consistent anesthesia than either drug alone diluted to the same volume [33]. Onset of anesthesia is approximately 10–15 min with a duration of 45–120 min [33,34].

For prolonged regional anesthesia or analgesia, an epidural catheter can be placed in the T13–L1 space to allow repeated injections of anesthetic drugs and prolonged duration of action. A 10.2 cm, 17 gauge Tuohy needle is placed in the epidural space as described above and the tip of the epidural catheter is advanced through the needle to the L1–L2 space for injection of the anesthetic drugs [39]. Because of the possibility of the catheter tip pointing to one side of the spinal cord, a unilateral block may be more likely with this technique.

Segmental thoracolumbar subarachnoid block

The insertion of needles into the T13–L1 or L1–L2 intervertebral spaces for subarachnoid injection of anesthetic is discouraged due to the high risk of trauma to the spinal cord when piercing of the dura mater and arachnoid membranes is attempted at these locations. Therefore, a catheter should be advanced from the lumbosacral (L6–S1) subarachnoid space to the T13–L1 space for the injection of local anesthetic (lidocaine or procaine) [39]. The distance from L6–S1 to T13–L1 is approximately 45 cm in adult cattle and care must be taken while advancing the catheter rostrally within the subarachnoid space to avoid kinking or curling it, which results in patient discomfort and potential damage to the spinal cord. Due to greater complexity and the need for a more sterile environment, this technique is less likely to be performed under field conditions.

The L6–S1 intervertebral space is localized 1–4 cm caudal to an imaginary line traced between the cranial edges of the tuber coxae. Following aseptic skin preparation, the subcutaneous and deep interspinous ligaments are desensitized with 5 mL of 2% lidocaine using a 15 cm, 18 gauge needle. A 15 cm, 17 gauge Tuohy needle is inserted at the L6–S1 intervertebral space with the bevel pointing cranially and slowly advanced into the epidural space, continuing until the dura and arachnoid mater are pierced and the subarachnoid space is entered. Correct placement is verified by aspiration of 2 mL of CSF. An 80–100 cm epidural catheter with a spring guide is advanced into the subarachnoid space for the estimated distance required to reach the T13–L1 space, which is usually approximately 60 cm due to the length and angle of the needle. The Tuohy needle and spring guide are removed, and the catheter distance is adjusted to the correct location and secured in place. Doses of 1.5–2 mL of 2% lidocaine or 5% procaine have been injected at a rate of 0.5 mL/min to induce unilateral or bilateral anesthesia from T10–L3 in 5–10 min for a duration of 54 min [39].

One disadvantage of this technique is the uncertainty of whether the resulting block will be on the intended side of the animal. The variation in block has been suggested to result from trapping of the catheter ventral to the spinal cord, which may impede adequate distribution of the injected anesthetic around the circumference of the pia mater due to the presence of the dorsal and ventral longitudinal ligaments [39].

Line infiltration block

The abdomen, subcutaneous tissues, and skin of the abdomen can be desensitized by infiltration of local anesthetic to allow procedures such as correction of umbilical and abdominal wall hernias and right paramedian abomasopexy to be performed. For these blocks, the technique and volumes described for the flank line infiltration block are used along the anatomic area selected for the surgery. Often a “V” shape block can be used with the angle of the “V” located at the cranial aspect of the incision and the wings along either side of the incision to block more proximal sensory nerves.

Lumbosacral, sacrococcygeal, or intercoccygeal epidural (cranial epidural) block

This technique is often referred to as a “cranial (anterior) epidural” and can be performed at any of the three sites (Fig. 66.9). Injections at the sacrococcygeal or first intercoccygeal epidural space require that the injectate volume of anesthetic drug is sufficient to facilitate its rostral spread from this site to the thoracolumbar area so that it can affect structures cranial to the pelvis (periumbilical region and flank). For lumbosacral epidurals, less volume is required.

The same principles discussed for the segmental dorsolumbar epidural technique apply here. This technique is used commonly in small ruminants for abdominal or pelvic surgery, most often to supplement general anesthesia or sedation. Despite their popularity, epidural injections often fail in providing complete analgesia/anesthesia for multiple reasons, including those listed in Box 66.1.

In adult cattle, volumes of local anesthetic of up to 150 mL (0.2–0.3 mL/kg), injected in the sacrococcygeal or intercoccygeal epidural space, have been recommended to desensitize the flank and umbilical region but these volumes also result in motor block of the pelvic limbs and the animal unable to remain standing. For this reason, cranial epidural blocks are not commonly practiced in adult cattle as they may injure themselves when motor control is lost or when attempting to stand. Another concern is that a high (cranial) epidural may result in hypotension secondary to blockade of sympathetic nerves, which results in vasodilation. Hypotension is more likely to develop in animals with pre‐existing hypovolemia.

Calves and small ruminants may require larger volumes on a mL per kg basis than adult cattle to achieve the same degree of cranial spread from the sacrococcygeal or intercoccygeal epidural space. Volumes of 0.4–0.6 mL/kg of local anesthetic are recommended to achieve analgesia of the umbilical region [40].

A lumbosacral epidural requires a smaller volume. In goats given 0.1, 0.2, or 0.3 mL/kg of new methylene blue at the lumbosacral space immediately after euthanasia, the average rostral spread was to L3–L4, T13–L1, and T10–T11, respectively [41]. A similar study in sheep demonstrated rostral spread to the first lumbar segment with a volume of 0.2 mL/kg [42]. To obtain sufficient anesthesia of the cranial abdomen and abdominal wall, it is often necessary for the anesthetic to reach the T10 dermatomal segment but, due to the possible variation in further rostral spread, there is also an increased risk of impairing pulmonary function. Volumes of 0.2 mL/kg or 1 mL/5 kg of the local anesthetic of choice (2% lidocaine, 0.5% bupivacaine, and 0.5% ropivacaine) are commonly used.

In goats and young calves, the lumbosacral space is easily palpable but may be less obvious in large, well‐nourished sheep. A lumbosacral epidural injection can be made with the animal in sternal or lateral recumbency. Spread of anesthetic is more likely to be even, and to produce bilateral blockade, when the animal is placed in a sternal position. Otherwise, the side to be approached should be the dependent side when performed in a lateral position; this facilitates contact of the local anesthetic with the desired nerve roots. The animal should be maintained in that position for at least 5–10 min to allow the block to take effect before moving it to the required position for surgery.

Strict aseptic technique and use of sterile gloves are indicated for epidural injections at the lumbosacral space, due to the close proximity of the spinal cord within the spinal canal. For most small ruminants and calves, a 3.8 or 7.5 cm, 20 or 22 gauge spinal needle is suitable. The landmarks consist of the distinct dorsal spinous processes of the lumbar vertebrae, which are readily distinguished from sacral vertebrae. Calves and small ruminants have six lumbar vertebrae, goats may only have five, and sheep can have as many as seven lumbar vertebrae. Therefore, it is advisable to palpate the anterior surfaces of both tuber coxae and draw an imaginary line between them; this borders the spinous process of the last lumbar vertebra. At this site, the index finger can palpate the space between the last lumbar vertebra and first sacral vertebra. At this point, the spinal needle enters the skin perpendicularly and is advanced into the epidural space. Correct placement is confirmed by a popping sensation as the needle penetrates the ligamentum flavum and a loss of resistance upon injection (the “hanging drop” technique, as described previously, may also be used). This distance is relatively short, especially in goats. Inadvertent advancement into the subarachnoid space often occurs since the spinal cord is present at this location. If the animal is in lateral recumbency and the subarachnoid space is penetrated, CSF drips from the hub following removal of the stylet; however, this is less obvious if the animal is in sternal recumbency. If CSF is encountered, the spinal needle can be withdrawn slightly to reposition it in the epidural space and avoid a subarachnoid injection.