Local anesthetics act by inhibiting sodium channels in nerve membranes, slowing the rate of membrane depolarization, and preventing action potentials. They are used topically and with local infiltration. Topical anesthetics include proparacaine, tetracaine, oxybuprocaine (also known as benoxinate), and lidocaine. Lidocaine and oxybuprocaine are often combined with fluorescein drops. Onset of action is 30–60 seconds, and duration ranges from 10–20 minutes. Topical anesthetics are used to facilitate tissue manipulation (third eyelid, conjunctiva, and cornea), observe the degree of spastic versus anatomic entropion, and provide conjunctival anesthesia before subconjunctival injections and diagnostic procedures (tonometry, corneal scrapings, conjunctival swabs and biopsies). Although the preservatives in topical anesthetics can interfere with culture results, this is unlikely to be clinically significant in most cases.
Before sampling for corneal/conjunctival cytology, two or three applications are required to ensure adequate anesthesia. Splash blocks may be used during extraocular surgery on the lids and cornea and during enucleation after globe removal. They should not be used therapeutically for pain relief because they are toxic to corneal epithelium.
Parenterally, local nerve blocks are an excellent aid for routine ocular evaluation and diagnostic procedures in large animals. The auriculopalpebral block is the most helpful block to limit blepharospasm during examination. This procedure blocks some of the motor nerves of the upper eyelid and enables the examiner to control the upper eyelid. The auriculopalpebral nerve is a branch of the facial nerve and can be palpated as it runs across the superior margin of the zygomatic arch. To block sensory input, a supraorbital nerve block or a ring block is used. The supraorbital nerve is a branch of the frontal nerve that traverses the supraorbital foramen of the upper orbit. If placed correctly, a dose of 1–2 mL of lidocaine is usually sufficient to block either the auriculopalpebral or supraorbital nerve. The block is usually effective within 3–5 minutes and can last up to 2–3 hours.
Retrobulbar blocks are used before enucleation and in lieu of neuromuscular paralysis for corneal and intraocular surgery. A correctly placed retrobulbar block will block cranial nerves II, III, IV, the ophthalmic branch of V, and VI. A combination of lidocaine and bupivicaine can be used. In small animals, 0.5–2 mL is usually sufficient for an intra- or extraconal block. In cattle and horses under heavy sedation or general anesthesia, the retrobulbar tissues can be anesthetized for enucleation using either a Peterson block (10–20 mL) or a 4-point block of the orbit (5–10 mL/site). The ring block is needed to inhibit sensory input from the skin around the eye. In all animals, toxicity can be systemic or local. Systemic toxicity includes CNS depression, bradycardia, and decreased myocardial contractility. Retrobulbar injection can cause death if inadvertently injected into the orbital foramina. Cats are quite sensitive to CNS effects from bupivacaine. Local toxicity can be ischemic necrosis due to large volumes and tissue damage from the injection itself.
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Also see pet health content regarding drugs used to treat eye disorders Drugs Used to Treat Eye Disorders The 3 primary methods of administering medications to the eye are topical, local ocular (such as under the conjunctival tissue or into the vitreous portion of the eye), and systemic (given by... read more .