Lidocaine is an amide local anesthetic used in strengths of 0.5%, 1%, and 2% (with or without epinephrine) for injection; 2% as a gel, and 4% as a solution for topical mucosal anesthesia. It yields a rapid (4–6-minute) retrobulbar or eyelid block that lasts about an hour (2 hours with epinephrine). The topical solution, applied to the conjunctiva with a cotton swab for 1–2 minutes, reduces the discomfort of subconjunctival injections. Topical lidocaine is preferable to cocaine or proparacaine for conjunctival biopsy because it has less effect on epithelial morphology. Lidocaine is also extremely useful for suppressing a cough during ocular surgery. For local injection in adults, the maximum safe dose of the 2% solution is 15 mL. A common adverse effect is drowsiness.
Mepivacaine is an amide drug used in strengths of 1%–3% (with or without a vasoconstrictor). It has a rapid onset and lasts approximately 2 hours; 2% is the most commonly used strength and has a maximum safe dose of 25 mL.
Bupivacaine is an amide anesthetic with a slower onset of action than lidocaine. It may yield relatively poor akinesia but has the advantage of a long duration of action, up to 8 hours without epinephrine. It is available in 0.25%–0.75% solutions (with or without epinephrine) and is frequently administered in a mixture with lidocaine or mepivacaine to achieve a rapid, complete, and long-lasting effect. The maximum safe dose of a 0.75% solution is 25 mL.
Hyaluronidase can be combined with local injection of anesthetics to increase the dispersion of the anesthetic drug(s) for intraocular, adnexal, or orbital surgery. Hyaluronidase catalyzes the hydrolysis of hyaluronic acid, a constituent of the extracellular matrix; it temporarily lowers the viscosity of the extracellular matrix and increases tissue permeability. Increased dispersion of the anesthetic drug may reduce the IOP rise in the limited orbital space, minimize distortion of the surgical site, decrease the risks of postoperative strabismus and myotoxicity, and increase akinesia of the globe and eyelid; lower volumes of anesthetic may be used.
Hyaluronidase products approved by the FDA include those derived from bovine and ovine sources, as well as a recombinant human product. Because of a lack of reliable animal sources and a shortage of supply from manufacturers, compounded formulations of hyaluronidase from animal-derived active pharmaceutical ingredients are only occasionally used. FDA regulations for compounding pharmacies are not as stringent as are regulations for pharmaceutical products, and concerns have been raised about the potency and purity of compounded hyaluronidase products from animal sources. There have been reports of hypersensitivity reactions to retrobulbar or peribulbar blocks associated with use of animal-derived hyaluronidase. For retrobulbar or peribulbar injection, 1 mL of hyaluronidase (150 USP U/mL; single-dose vial of recombinant human product) can be added to a syringe of the anesthetic to be administered.
Several other drugs are commonly used for topical anesthesia of the ocular surface. Because of their higher lipid solubilities, these medications have a more rapid onset than other topical anesthetics; thus, the initial discomfort caused by the drops is reduced. Proparacaine is an ester anesthetic available as a 0.5% solution. The least irritating of the topical anesthetics, it has a rapid onset of approximately 15 seconds and lasts approximately 20 minutes. Its structure is different enough from that of other local anesthetics that cross-sensitization apparently does not occur.
Benoxinate (also known as oxybuprocaine) is an ester anesthetic available in a 0.4% solution with fluorescein for use in tonometry. Its onset and duration are similar to those of proparacaine. Benoxinate is also available alone as a topical anesthetic in Europe.
Tetracaine is an ester anesthetic available in 0.5% solution and approved for short-duration ocular surface procedures. Its onset of action and duration of action are longer than those of proparacaine, and it causes more extensive corneal epithelial toxicity.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.