An Injection Primer, Part 3
BY WILLIAM L. MILLER, OD, PHD, FAAO
In the final part of this series on injectables, I'll cover the use of anesthetic injectables. The aim of anesthetic injections in and around the orbit is to achieve a level of sensory and motor blockage ((the former to alleviate pain and discomfort; the latter to achieve muscle immobility). Anesthetics work by reversibly blocking sodium channels in the nerve, thus decreasing the level of action potential rise for nerve transmission. Indications for injectable anesthetics vary, but may include cataract surgery or lesion removal, as in the case of lid lesions.
Typically the injection will serve as a tarsal or local infiltrative block; however, in the case of cataract surgery, it's likely to be a peribulbar or retrobulbar block, both of which result in a level of extraocular muscle akinesia.
Sifting Through the Options
As in the case of chalazion injections, a 27-gauge to 30-gauge, one-half-inch needle is the instrument of choice for anesthetic injections. Many practitioners opt for lidocaine (with or without epinephrine), but you may also choose bupivacaine, prilocaine or levobupivacaine, each of which has advantages and disadvantages related to speed of onset, duration and adverse reactions.
Using an anesthetic with epinephrine provides vascoconstrictive properties that decrease bleeding at the injection site and minimize systemic absorption of the anesthetic.
You can either administer anesthetics as an infiltrative procedure in and around the lid lesion or in the area. Just place the needle bevel down at a 45-degree angle during the procedure with subsequent aspiration before delivering the anesthetic to ensure that the needle hasn't entered a blood vessel.
A small amount of anesthetic (0.5cc to 1.0cc) will provide anesthesia for tissue in and around the lesion, although you may want to increase this amount for more invasive procedures. A regional anesthesia is often required for oculoplastic surgery, and you can achieve this by placing the anesthetic at the base of the supraorbital, infraorbital and zygomatico-temporal nerves, which serve to innervate the upper and lower eyelids.
On the Surgical Scene
Similar needle sizes and anesthetics are used in cases of peribulbar/epibulbar blocks for cataract surgery. However, techniques vary and many choose to form a subcutaneous bleb between the middle two-thirds and outer one-third of the eye just superior to the infraorbital rim and another injection bleb just medial to the supraorbital notch. A second needle is inserted at a right angle to the blebs into the potential space between the globe and floor of the orbit. Surgeons aspirate and inject, allowing for muscle akinesia in the extraconal space. Akinesia of extraocular muscles is also accomplished with retrobulbar or modified retrobulbar techniques. But because of the crowded orbital space filled with important blood vessels and nerves, a greater risk exists.
A Lesson on Epinephrine
Epinephrine injections are allowable in many states, even though optometrists in those states prohibit the universal use of injections. Epinephrine 0.3cc to 0.5cc is available in a variety of formats for anaphylactic indications. You can administer it subcutaneously or intramuscularly at the first sign of anaphylaxis.
However, the route of administration is tailored to the severity of the reaction, and cases of severe anaphylaxis may necessitate intravenous delivery. Any medication can cause an immediate anaphylaxis or delayed hypersensitivity in sensitized individuals. Therefore, in the case of ocular injections it's prudent to have anticomplication medications such as epinephrine and emergency equipment and numbers available to handle such emergencies.
Dr. Miller is on the faculty at the University of Houston College of Optometry. He is a member of the American Optometric Association and serves on its Journal Review Board. You can reach him at firstname.lastname@example.org.