![]() The main complications seen with this technique are subconjuntival swelling and subconjunctival haemorrhage, both of which usually resolve with gentle orbital pressure. The risk of a dural puncture with subsequent brainstem anaesthesia, globe puncture and inadvertent intravascular injection is rare. The sub-tenon´s block is perceived as a safe alternative to both the retrobulbar and peribulbar blocks, both of which require the use of a sharp needle. Warming and increasing the alkalinity of the solution have been tried without being proven as beneficial. Lidocaine 2%, alone or mixed with an equal volume of bupivacaine 0.5 or 0.75%, 2-5 ml, is used. If this occurs, one may need to either reposition the cannula to a deeper plane, or possibly repeat the "tent and snip" described above to expose the true sub-tenon space.Ī sub-tenon block can be given after an inadequate retrobulbar or peribulbar block for further anesthesia and/or akinesia. If conjunctival insufflation is noted, the cannula is likely simply in the subconjunctival space. The onset of analgesia is usually rapid, whereas maximal akinesia may take up to 10 minutes to develop. If resistance is encountered, the cannula can be withdrawn slightly and repositioned. On injection of the local anaesthetic, little resistance is usually encountered and most of the solution should disappear behind the globe resulting in slight proptosis. These metal cannulae are probably more likely to cause trauma, and the plastic cannulae can fold and block, therefore it is preferable to use a cannula that has been designed for sub-Tenon's anesthesia. If a dedicated sub-Tenon cannula is not available, other options include using a 21 gauge Rycroft cannula, a typical large bore "BSS cannula", lacrimal cannula, or the plastic portion of an intravenous catheter (20 or 22G). 3-5ml, sometimes more) if akinesia is also needed. Anesthetic is injected slowly: smaller volumes (e.g 2ml lidocaine) are typically adequate for analgesia, larger volumes (e.g. The blunt ended sub-Tenon's cannula can then be inserted through the newly created defect, with the syringe of local anaesthetic attached, and passed posteriorly, following the curvature of the globe, until its tip is perceived to passed the equator - making sure to insert the cannula in the subtenons space (just exterior to bare sclera) and not in the subconjunctival space. A small incision is made in the tissue using a pair of ophthalmic scissors (often Westcott tenotomy scissors), exposing the sclera below. A small tent of the conjunctiva and Tenon capsule is raised with a pair of blunt, non toothed forceps approximately 5-10 mm from the inferio-nasal limbus and can be as far inferior as the fornix depending on patient anatomy. A well positioned mark or cross on the wall or ceiling will often help in maintaining this line of gaze. Asking the patient to look up and out will assist in exposing the inferonasal quadrant (this can also be done in the inferotemporal quadrant). An eyelid speculum is inserted to improve access and prevent blinking. Several types of blunt-ended sub-tenon cannulae are available commercially, made from either metal or plastic. The anesthetic solution is drawn up in to a syringe using aseptic technique. The rest of the orbital margin can then be cleaned with the remaining solution. The anesthetized conjunctiva should then be cleaned by carefully placing a few drops of povidone iodine beneath the lower eyelid. Intravenous access is desirable but not essential. The patient should be placed in the supine position and standard monitoring applied. As a result, a local anaesthetic technique may be inappropriate. Many patients undergoing ophthalmic procedures are elderly and may have significant co-morbidities. The instillation of local anesthetic into this space produces analgesia and akinesia by diffusing posteriorly into the retro-orbital space to block the traversing sensory and motor nerves.Ī patient's suitability for a local anaesthetic technique must be established. The sub-tenon´s (also known as episcleral) space is a potential space between the capsule and the sclera. It extends posteriorly surrounding the globe and fusing with the dura of the optic nerve. Anteriorly it lies in close apposition to the conjunctiva and fuses with it at the level of the limbus. Tenon´s capsule (also referred to as the episclera) is a thin layer of connective tissue which surrounds the globe. While there is no method of ocular anesthesia that is completely efficacious and risk free, sub-tenon block is a technique that may be relatively safer. Local anaesthesia is one of the most used techniques for many ocular procedures, since the use of “sharp instruments” in peribulbar and retrobulbar anaesthesia have a risk of causing serious and even life threatening complications like globe perforation, orbital haemorrhage, optic nerve damage and subarachnoid diffusion.
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