Local Anesthetics for the Eye
Parenterally, local nerve blocks are an excellent aid for routine ocular evaluation and diagnostic procedures in horses. 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 horse’s 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 min and can last up to 2–3 hr.
The same principles are used in food animals, such as cattle, in which both a retrobulbar and ring block may be used. A correctly placed retrobulbar block will block cranial nerves II, III, IV, the ophthalmic branch of V, and VI. The ring block is needed to inhibit sensory input from the skin around the eye.