Equine Eye Nerve Blocks

• Eye nerve blocks are essential techniques in equine ophthalmology for diagnostic and therapeutic purposes
• They provide analgesia, facilitate examination, and aid in the treatment of various ocular conditions

Supraorbital Nerve Block

• The supraorbital nerve is a branch of the trigeminal nerve (CN V)
• It provides sensory innervation to the upper eyelid and forehead region in horses
• The supraorbital foramen is about one inch above the supraorbital rim, along a line from the medial canthus to the base of the ear
• Palpation of the supraorbital foramen is crucial for accurate needle placement
• A 22 to 25-gauge needle is inserted perpendicular to the bone into the foramen
• Common local anesthetics are lidocaine (2%) or bupivacaine (0.5%)
• 3-5 mL of local anesthetic solution is administered
• Care is taken to avoid injecting directly into the nerve to prevent nerve damage
• Desensitization of the upper eyelid results, facilitating procedures

Auriculopalpebral Nerve Block

• The auriculopalpebral nerve is a branch of the facial nerve (CN VII)
• It controls motor function of the eyelids, specifically the orbicularis oculi muscle responsible for eyelid closure
• The nerve runs superficially along the zygomatic arch
• Palpation helps locate the nerve as it crosses the zygomatic arch
• A 22 to 25-gauge needle is inserted subcutaneously over the zygomatic arch
• 3-5 mL of local anesthetic solution is administered
• This block induces temporary akinesia (paralysis) of the upper and lower eyelids
• It facilitates thorough ocular examination and treatment by preventing blepharospasm (eyelid spasms)
• It does not affect sensation to the eyelids or globe

Palpebral Nerve Block

• The palpebral branch is another branch of the facial nerve (CN VII)
• It provides motor innervation to the eyelids by innervating the orbicularis oculi muscle
• The lateral canthus is the target area for needle insertion
• Insert a 25-gauge needle directed towards the upper and lower eyelid
• Deposit 1-2 mL of local anesthetic solution over each eyelid
• This block induces temporary paralysis of both eyelids, reducing blepharospasm
• Useful for short procedures needing eyelid akinesia

Lacrimal Nerve Block

• The lacrimal nerve is a branch of the ophthalmic branch of the trigeminal nerve (CN V)
• It provides sensory innervation to the lacrimal gland and the lateral portion of the upper eyelid
• The injection site is at the dorsolateral orbital rim
• A 25-gauge needle is inserted near the lacrimal gland
• 1-2 mL of local anesthetic solution is administered
• This block desensitizes the lacrimal gland

Infratrochlear Nerve Block

• The infratrochlear nerve is a branch of the ophthalmic branch of the trigeminal nerve (CN V)
• It provides sensory innervation to the medial canthus and adjacent nasal area
• The injection site is at the medial orbital rim, near the trochlea
• A 25-gauge needle is inserted
• 1-2 mL of local anesthetic solution is administered
• This block desensitizes the medial canthus region

Zygomatic Nerve Block

• The zygomatic nerve is a branch of the maxillary branch of the trigeminal nerve (CN V)
• It provides sensory innervation to the lower eyelid and lateral canthus
• The injection site is ventral to the lateral canthus
• A 25-gauge needle is inserted
• 1-2 mL of local anesthetic solution is administered
• This block desensitizes the lower eyelid and lateral canthus

Intratarsal nerve block

• The palpebral nerves run within the tarsal plate of the eyelid
• The injection is performed by eversion of the eyelid
• A 25- or 27- gauge needle is introduced into the tarsal plate
• 0.5 mL of local anesthetic solution is administered
• Indicated for procedures on the eyelid margin such as removal of masses

Considerations and Precautions

• Always use sterile needles and syringes to minimize the risk of infection
• Aspirate before injecting to avoid intravascular injection
• Use the minimal effective volume of local anesthetic to reduce the risk of systemic toxicity
• Be aware of potential complications, such as hematoma formation, nerve damage, or allergic reactions to the local anesthetic
• Avoid injection directly into nerves
• Proper restraint of the horse is essential to ensure accurate and safe administration of nerve blocks
• Knowledge of equine anatomy is crucial for successful nerve blocks

Local Anesthetics

• Lidocaine provides rapid onset and moderate duration of action (approximately 1-2 hours)
• Bupivacaine has a slower onset but longer duration of action (approximately 4-6 hours)
• Selection of the appropriate local anesthetic depends on the duration and intensity of analgesia required
• Always check the expiration date and concentration of the local anesthetic
• Use caution when administering local anesthetics to horses with cardiac or neurological conditions

Potential Complications

• Hematoma formation at the injection site
• Nerve damage resulting in temporary or permanent nerve dysfunction
• Infection if sterile technique is not followed
• Allergic reactions to the local anesthetic
• Systemic toxicity due to excessive absorption of the local anesthetic
• Corneal ulceration secondary to nerve block causing reduced blinking

Clinical Applications

• Facilitating ocular examination in cases of blepharospasm or pain
• Performing diagnostic procedures such as corneal scrapings or conjunctival biopsies
• Providing analgesia during surgical procedures, such as enucleation or eyelid laceration repair
• Managing ocular pain associated with conditions
• Delivering medication to specific ocular tissues

Monitoring

• Observe the horse for any signs of adverse reactions
• Monitor the injection site for swelling, redness, or discharge
• Assess the effectiveness of the nerve block by evaluating the degree of analgesia or akinesia achieved
• Provide appropriate post-operative care, including pain management and wound care

Additional Notes

• Nerve blocks can be combined to achieve more comprehensive analgesia or akinesia
• The choice of nerve block(s) depends on the specific clinical situation and the intended procedure
• Experienced equine veterinarians or veterinary ophthalmologists should perform nerve blocks
• Proper training and practice are essential for mastering these techniques
• Knowledge of the indications, contraindications, and potential complications of each nerve block is crucial for safe and effective use