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. While there is no method of ocular anesthesia that is completely efficacious and risk free, sub-tenon block is a newer technique that may be relatively safer.
Tenon´s capsule is a thin layer of connective tissue which surrounds the globe. Anteriorly it lies in close apposition to the conjunctiva and fuses with it at the level of the limbus. It extends posteriorly surrounding the globe and fusing with the dura of the optic nerve. The sub-tenon´s (also known as episcleral) space is a virtual space between the capsule and the sclera. 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.
A patient's suitability for a local anaesthetic technique must be established. Many patients undergoing ophthalmic procedures are elderly and may have significant co-morbidities. As a result, a local anaesthetic technique may be inappropriate.
The patient should be placed in the supine position and standard monitoring applied. Intravenous access is desirable but not essential.
The anesthetized conjunctiva should then be cleaned by carefully placing a few drops of povidone iodine beneath the lower eyelid. The rest of the orbital margin can then be cleaned with the remaining solution. The anesthetic solution is drawn up in to a syringe using aseptic technique. Several types of blunt-ended sub-tenon cannulae are available commercially, made from either metal or plastic. An eyelid speculum is inserted to improve access and prevent blinking. Asking the patient to look up and out will assist in exposing the inferonasal quadrant. A well positioned mark or cross on the wall or ceiling will often help in maintaining this line of gaze. 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. A small incision is made in the tissue using a pair of ophthalmic scissors, exposing the sclera below. The blunt ended sub-Tenon's cannula can then be inserted, 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. Anesthetic is injected slowly: smaller volumes (e.g 2ml lidocaine) are typically adequate for analgesia, larger volumes (e.g. 3-5ml, sometimes more) if akinesia is also needed.
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). 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.
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. If resistance is encountered, the cannula can be withdrawn slightly and repositioned. The onset of analgesia is usually rapid, whereas maximal akinesia may take up to 10 minutes to develop. If conjunctival insufflation is noted, the cannula is likely simply in the subconjunctival space. 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.
A sub-tenon block can be given after an inadequate retrobulbar or peribulbar block for further anesthesia and/or akinesia.
Lidocaine 2%, alone or mixed with an equal volume of bupivacaine 0.5 or 0.75%, 2-5 ml, is used. A vasoconstrictor is rarely added. Warming and increasing the alkalinity of the solution have been tried without being proven as beneficial.
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. The risk of a dural puncture with subsequent brainstem anaesthesia, globe puncture and inadvertent intravascular injection is rare. The main complications seen with this technique are subconjuntival swelling and subconjunctival haemorrhage, both of which usually resolve with gentle orbital pressure. It is worth noting that the conjunctival haemorrhage can be quite robust if the patient is on anticoagulation or antiplatelet therapy.
Safety of sub-tenon's blocks was reviewed in 2011. In the United Kingdom, a national Guideline on Local Anaesthesia in Ophthalmic Surgery (including sub-tenon's anesthesia) was published by the Royal College of Ophthalmologists and the Royal College of Anaesthetists (Anesthesiologists), 2012. This Guideline is available as a free download, from either College's website.
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- Local Anaesthesia in Ophthalmic Surgery [Guideline], 2012. London, UK. Royal College of Ophthalmologists, Royal College of Anaesthetists. Available from http://www.rcophth.ac.uk/page.asp?section=451§ionTitle=Clinical+Guidelines
- Kumar CM, Eid H, Dodds C. Sub-Tenon's anaesthesia: complications and their prevention. Eye (Lond). 2011 Jun;25(6):694-703. doi: 10.1038/eye.2011.69. Available from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3178142/