Ophthalmic Blocks at the Medial Canthus

In Reply:—

We thank Dr. Lopatka et al.  for their interest in our work and for their pertinent comments. The main concern is the difference between the technique described by Hustead et al.  1that Dr. Lopatka et al.  seem to use and our technique. 2–7They are very different. Hustead uses a peribulbar (extraconal) injection. The local anesthetic is injected into the extraconal space, a part of the corpus adiposum of the orbit. This is a different route of injecting into the same space than when performing classic inferolateral peribulbar anesthesia. Using the technique of Hustead, the needle is inserted medially to the caruncle, at the medial end of the lid aperture, near the junction between the lacrimal portion of the inferior and superior lids (fig. 1). Our technique is an episcleral (sub-Tenon) injection. The needle is inserted more laterally, tangentially to the globe, directly into the episcleral space (fig. 1). As we stated in our first article 2when we began using this technique, we were convinced that it was another peribulbar approach, similar to the technique of Hustead. However, after intensive anatomic works, 6,7we found that the two spaces of injections are different. Peribulbar injection results in a spread of the local anesthetic into the whole corpus adiposum of the orbit, including the intraconal space in which are located all the sensory and motor nerves that must be blocked to ensure good akinesia and analgesia of the eyeball 8(fig. 2B). This spread is sometimes uncertain or incomplete, explaining some partial failures of peribulbar anesthesia. Injecting into the episcleral space forces the spread of the local anesthetic circularly around the scleral portion of the globe, thus encountering the ciliary nerves just before they enter the sclera and accounting for a good sensory block (fig. 2C). There is a continuity between the fascial sheath of the eyeball (the Tenon capsule) and the sheaths of the recti muscles. Thus, injecting into the episcleral space also guides the spread of the local anesthetic into the sheaths of the recti muscles, where it encounters the terminal motor nerves, accounting for a good akinesia of the globe. Because the fascial sheath of the eyeball fuses anteriorly with the bulbar conjunctiva, episcleral injection results in subconjunctival infiltration (chemosis). Finally, the excess local anesthetic flows into the lid, thus preventing blinking. This difference between the techniques explains the greater efficacy of our technique as compared with classic peribulbar injection. 5However, there is a small difference in the position of the needle between the technique of Hustead and our technique. A small misplacement of the needle during an intended Hustead approach may result in episcleral injection, and conversely, Dr. Lopatka outlines that an intended peribulbar anesthesia occasionally produces chemosis, which is the indicator of episcleral injection.

The work of Vohra and Good 9has outlined the interest of the medial canthus for preventing inadvertent globe perforation. One of the main risk factors of perforation is myopic staphyloma, which is frequently located posteriorly to the globe (with an increased risk of retrobulbar injection) or inferiorly (with an increased risk of inferolateral peribulbar injection) but very infrequently on the medial part of the globe. Theoretically, this should be an argument in favor of the safety of medial canthus approaches. However, introducing a needle into the orbit has its own hazards. Safety of eye blocks depends mainly on skill, experience, and strong anatomic knowledge.