Peripheral nerve blocks
The most commonly performed nerve block is the palpebral nerve (branch of the auriculopalpebral, a division of the facial
VII), which provides motor innervation to the powerful orbicularis oculi muscles in both eyelids (Figure 1). Complete blockade
results in a flaccid upper eyelid, which can be raised without resistance. The palpebral nerve traverses the zygomatic arch
at its peak 8-10 cm from the dorsal orbital rim, and again at 2-3 cm, and it may be palpated or strummed through the skin.
A 25-gauge needle is inserted so that it is subcutaneous beside the nerve, and 1-2 mL lidocaine 2 percent is deposited. Substantial
resistance to injection indicates that the needle is intradermal. The lidocaine may be fanned out with the needle, or the
area can be rubbed subsequently to disperse the drug. Both of these techniques irritate horses. Nerve branches arborize while
approaching the eyelid, so more peripherally fewer branches will be blocked, and greater residual muscle tone is encountered.
Figure 3: Temporary tarsorrhaphy placed laterally, either with or without a stent. Examination of the cornea is reduced, but
enhanced corneal protection is provided.
Other branches arise around the orbital rim from the facial nerve, and to achieve complete paralysis, a more proximal auriculopalpebral
nerve block is performed posterior to the dorsal ramus of the mandible using a 22-gauge 1.5-in. needle buried to the hub and
depositing 3-8 mL lidocaine. Longer-acting anesthetic agents are less desirable because of the longer interval until normal
blinking. Topical ointment is a logical supplement at the conclusion of the examination to prevent dessication of the epithelium.
Skin sensation around the orbit is provided via two divisions of the trigeminal nerve (V): through the lacrimal n., infratrochlear
n. and frontal n. (ophthalmic division) and the zygomatic n. (maxillary division). The frontal and supraorbital nerves are
the same. The sites for blockade are shown in Figure 1, p. 6E. A head jerk is often experienced if the needle impinges on
sensory nerves. If inserted swiftly without the syringe, skin resistance will prevent its inadvertent removal during head
motion. When the head is still, the syringe is attached and the lidocaine injected. Small vessels accompany the nerves, and
blood may be seen in the hub. Gray horses appear to bleed more, and their hair color makes it more evident. Occasionally,
a small hematoma may form, but rapidly reduces with light pressure. Desensitization is useful for surgical repair, and familiarity
with the topographical areas assists in diagnosing traumatic damage to sensory innervation.
The infraorbital nerve exits the infraorbital foramen just anterior and dorsal to the facial crest between the globe and the
nostril, underneath the levator nasolabialis muscle. This nerve is blocked occasionally to remove trigeminal (V) sensation
in the evaluation of head-shakers, but has a more-practical use in that its desensitization permits suturing wounds of the
nostril and lip. A 20-22 gauge needle is inserted directed caudally into the foramen, or its vicinity and 3-10 mL lidocaine
are deposited. The block may require 20-40 minutes to achieve full desensitization.
A more advanced technique is the retrobulbar block, which is most commonly performed under general anesthesia. It prevents
strabismus, extraocular muscle tension during ophthalmic surgery, and if coupled with 1-2% v/v epinephrine (1:1000) it limits
hemorrhage if enucleation will be performed. Specific descriptions are available in current texts. It may also be performed
under standing sedation, with caution and dexterity. The eye should be protected from inadvertent trauma. Temporary blindness
should be anticipated if the optic nerve is blocked.