Facial Nerve Block in Veterinary Surgery

Overview

• Abolish motor function of the eyelids (akinesia) for ophthalmic examination/surgery
• Desensitize specific regions of the face for surgical procedures

Key Exam Point: Facial nerve blocks produce motor blockade only (akinesia). They do NOT provide sensory analgesia. For full anesthesia of the eye/eyelid, a separate sensory block (supraorbital, retrobulbar, or Peterson block) must be combined.

Branches Commonly Blocked in Veterinary Surgery

1. Auriculopalpebral Nerve Block (Most Important Exam Topic)

What is it?

Indications

• Ophthalmic examination (especially horses - their eyelid muscles are so powerful the eye cannot be opened without this block)
• Adjunct to ophthalmic surgery (reduces blepharospasm)
• Combined with retrobulbar/Peterson block for enucleation

Nerve course

Technique - CATTLE

• Injection site: Subcutaneously along the dorsal edge of the zygomatic arch (at the rostral end), just in front of the base of the ear
• Needle: 20-22 gauge
• Drug & Volume: 2% lidocaine, 5-10 ml subcutaneously
• Onset: 3-5 minutes
• Duration: 1-2 hours

Technique - HORSE

• Injection site: At the crest of the rostral zygomatic arch (insert 25 ga needle at the dorsal edge of the most dorsal point of the zygomatic arch) or adjacent to the nerve where it is palpable over the arch
• Needle: 25 gauge, 5/8-inch
• Drug & Volume: 2% lidocaine, 3-5 ml subcutaneously
• Onset: 3-5 minutes
• Duration: Up to 2-3 hours

Technique - DOG/CAT

• A needle is inserted dorsally to the zygomatic process and caudally to the tympanic bulla and the injection is made subcutaneously
• Drug & Volume: Lidocaine, bupivacaine, or ropivacaine; 0.5-1.5 ml depending on size
• Should be combined with other regional techniques for adequate analgesia during ocular surgery
• Adverse effects: Temporary inability to close eyelids - apply eye lubricant every 2-4 hours until blinking returns

What Auriculopalpebral Block Does NOT Do

• Does not provide sensory analgesia of the eye or eyelids
• Does not block sensation from cornea, conjunctiva, or periorbital skin
• Must be combined with topical anesthetic or retrobulbar block if sensory blockade is needed

2. Peterson Eye Block (Cattle - Exam Favourite)

Indication

• Enucleation of the eyeball and eyelid surgery in cattle
• Provides sensory blockade of the eye and orbit

Nerve Blocked

Anatomical Landmark (Key for Exam)

• Supraorbital process - cranially
• Zygomatic arch - ventrally
• Coronoid process of mandible - caudally

Technique

1. Skin desensitized with 5 ml of 2% lidocaine at the notch
2. A 14-gauge, 2.5-cm needle is placed into the notch as a guide cannula
3. A curved 18-gauge, 10-12 cm needle is passed through the cannula horizontally and posteriorly until it contacts the mandibular ramus (pterygoid crest)
4. The needle is then redirected medially and cranially toward the orbital apex
5. 10-20 ml of 2% lidocaine is deposited
6. Eye movement may be observed as needle approaches the nerves

Combined with:

• Auriculopalpebral block (to prevent eyelid closure during surgery)

Peterson vs. Retrobulbar Block

3. Retrobulbar Block

Indications

• Enucleation (all species)
• Intraocular surgery as alternative to neuromuscular blockade
• Oculocardiac reflex prevention - traction on optic nerve causes bradycardia/cardiac arrest; retrobulbar block prevents this

In Dogs/Cats

• 0.5-2 ml lidocaine + bupivacaine mix
• Single-point transpalpebral technique (ventrolateral approach described as easiest in small animals)

In Cattle

• 4-point block: Four injections around the orbital rim (easier due to bovine anatomy)
• Or Peterson block (more skill required but safer)
• Volume: 5-10 ml per site (4-point), 10-20 ml (Peterson)

In Horses

• Needle inserted caudal to the supraorbital process of the frontal bone near the supraorbital foramen

4. Summary Table for Exam Revision

High-Yield Exam Points

1. Auriculopalpebral block = motor only, NO sensation - this is the most commonly tested fact
2. Nerve is palpable over the zygomatic arch - used as a landmark in cattle and horses
3. Peterson block landmarks = supraorbital process (cranially) + zygomatic arch (ventrally) + coronoid process of mandible (caudally)
4. Always combine auriculopalpebral block with retrobulbar/Peterson block for enucleation
5. Oculocardiac reflex - reason retrobulbar block is mandatory before enucleation, even under general anesthesia
6. Eye lubricant must be applied when auriculopalpebral block is performed - patient cannot close eyelids
7. In horses, eyelid muscles are so strong that palpebral/auriculopalpebral block is required just to examine the eye
8. Local anesthetic of choice: 2% Lidocaine (onset ~3-5 min, duration 1-2 hrs); Bupivacaine for longer duration
9. Cornual nerve block (for dehorning) - uses cornual branch of zygomaticotemporal nerve (cattle) or both cornual branch of zygomaticotemporal + infratrochlear nerve (goat)
10. Peterson block is safer than retrobulbar (less risk of penetrating the globe) but requires more skill

Pudic (Internal Pudendal) Nerve Block - Veterinary Surgery

Alternative Names

• Pudic nerve block (older/classical term)
• Internal pudendal nerve block
• Larsen's technique (classical bovine approach - first described by Larsen, 1953)

Anatomy - The Pudendal (Pudic) Nerve

Origin

• Arises from the sacral plexus - ventral rami of S2, S3, S4 (in cattle and horse)
• Some texts cite S4 and S5 contributing the caudal rectal nerve (co-branch)

Course

1. Exits the pelvis through the greater sciatic foramen
2. Follows the medial surface of the sacrotuberous (sacrosciatic) ligament
3. Runs lateral-ventral to the pelvic wall, cranial and dorsal to the lesser sciatic foramen
4. Closely associated with the internal pudendal artery and vein (the neurovascular bundle is the key palpation landmark)
5. Travels through the ischiorectal fossa

Branches (Key Exam Point)

Full blockade requires all three branches to be blocked. Blockade of only the perineal and caudal rectal branches (as can occur with some techniques) omits the dorsal nerve of the penis/clitoris, leading to incomplete desensitization.

What the Block Achieves

• Anesthesia of: anus, perineum, vulva/vestibule (female); anus, perineum, glans penis, lamina interna of prepuce (male)
• Relaxation of the retractor penis muscle (crucial for penile prolapse/examination in bulls)
• Both sensory and motor effects (mixed nerve)

Indications

In CATTLE (Bull) - Most Common Use

• Operations on the penis - the primary classic indication
• Penile prolapse examination and reduction
• Penile hematoma treatment
• Preputial surgery
• Prevention of straining due to uterine or vaginal prolapse in cows
• Perineal/perianal surgery

In HORSES (Mares and Geldings)

• Perineoplasty (Caslick's procedure extensions)
• Urethroplasty
• Clitorectomy (mares)
• Penile examination (geldings)
• Phallectomy (penile amputation)
• Urethrostomy (perineal)
• Reproductive and perineal surgeries

In SMALL RUMINANTS

• Perineal urethrostomy (goat - electrolocation technique reported)
• Sheep - ultrasound-guided technique at ischial spine level described

Why Pudic Block is Preferred Over Epidural/GA in Bulls

Classical Technique in Cattle (Larsen's/Liverpool Technique - Through the Ischiorectal Fossa)

Animal Preparation

• Animal restrained in standing position (stocks/crush)
• Adequate sedation (xylazine commonly used in cattle)
• Rectal evacuation

Key Anatomical Landmarks (Palpated Per Rectum)

• The internal pudendal artery - felt as a pulsating vessel on the lateral-ventral pelvic wall
• The pudendal nerve lies cranial and dorsal to the lesser sciatic foramen
• The nerve is located on the lateral wall of the pelvis, approximately at the level of the cranial border of the sacrotuberous ligament

Needle Insertion

• A long needle (10-15 cm, 18-gauge) is inserted through the skin of the ischiorectal fossa
• Directed toward the nerve as guided by the operator's finger placed per rectum
• The per-rectal finger confirms needle position and guards against rectal wall penetration

Three Injection Points (A, B, C):

• Needle directed to the nerve adjacent to the internal pudendal vessels
• 8-10 ml of 2-3% lignocaine (lidocaine)

• Needle almost completely withdrawn and redirected more dorsally
• Per-rectal palpation confirms new position
• 5 ml of 2-3% lignocaine

• Needle again withdrawn and redirected ventrally toward the cranial part of the lesser ischiatic foramen
• Per-rectal finger guides placement
• Additional volume deposited

The procedure is performed bilaterally (both sides) for complete anesthesia.

Drug and Volume Summary (Cattle)

• Drug: 2% or 3% lidocaine (lignocaine hydrochloride)
• Volume per point: 5-10 ml
• Total per side: ~15-20 ml
• Bilateral block required = total ~30-40 ml

Technique in HORSES

Classical Blind Technique (Manual/Rectal Palpation-Guided)

• Similar principle - per-rectal palpation to locate the pudendal nerve along the medial surface of the sacrosciatic ligament
• Long spinal needle inserted through the perineal skin toward the nerve
• Limitation: More difficult due to larger anal diameter required for palpation; higher risk of rectal wall puncture

Peripheral Nerve Stimulator-Guided Technique (Gallacher et al., 2016)

• Peripheral nerve locator used to electro-locate the nerve
• Positive response: Simultaneous anal AND perineal twitch confirms pudendal nerve localization (anal twitch alone may only indicate caudal rectal nerve)
• More accurate, requires less local anesthetic volume
• Drug volume: 10 ml per site in males, 20 ml per site in mares
• Drugs: lidocaine, mepivacaine, or bupivacaine (choice based on required duration)

Ultrasound-Guided Technique (Emerging)

• Transcutaneous ultrasound to visualize pudendal neurovascular bundle
• Avoids rectal wall puncture
• Injection depth: ~75-90 mm
• Allows direct visualization to avoid sciatic nerve and other critical structures

Comparison of Techniques

Complications

1. Rectal wall puncture - most feared; can cause peritonitis/pelvic abscess
2. Vaginal puncture (in females)
3. Vascular puncture - internal pudendal artery/vein laceration
4. Intraneural injection - nerve damage
5. Intravascular injection - systemic local anesthetic toxicity (CNS depression, bradycardia)
6. Inadvertent sciatic nerve block - hind limb weakness/ataxia (the sciatic nerve lies only a few centimeters cranial to the pudendal nerve)
7. Incomplete block - if dorsal nerve of penis is not blocked, penile desensitization will be inadequate

Comparison: Pudic Block vs. Epidural vs. GA

High-Yield Exam Points

1. Pudic = Internal pudendal nerve - same nerve, older terminology used more in veterinary texts
2. Origin: S2-S4 sacral plexus - same as in humans
3. Primary indication in cattle = bull penis surgery (most classic exam question)
4. Also used in cows to prevent straining in uterine/vaginal prolapse
5. Three branches must be blocked: caudal rectal nerve + perineal nerve + dorsal nerve of penis/clitoris
6. Key landmark = internal pudendal artery palpated per rectum (the nerve runs with the vessel)
7. Larsen (1953) first described the ischiorectal fossa approach in bulls
8. Bilateral block required for complete perineal anesthesia
9. Advantage over epidural: preserves hind limb motor function - bull stays standing
10. Advantage over GA (cattle): eliminates regurgitation/inhalation risk
11. Nerve stimulator endpoint = anal + perineal twitch together (anal twitch alone = only caudal rectal nerve)
12. Sciatic nerve lies just cranial to pudendal nerve - inadvertent sciatic block causes ataxia
13. Retractor penis muscle relaxation is a key objective in bull penile surgery - only pudic block (and GA/epidural) achieves this, NOT local infiltration
14. In horses, used for phallectomy, urethrostomy, clitorectomy, perineoplasty - procedures not requiring recumbency
15. Drug of choice: 2% lidocaine (lignocaine); bupivacaine if longer duration needed