Sub-Tenon Anaesthesia

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Introduction

Local anaesthesia is one of the most used techniques to many procedures, since the use of “sharp instruments” in the peri and retrobulbar anaesthesia has been quoted as causing possible serious and even life threatening complications like globe perforation, orbital haemorrhage, optic nerve damage and subarachnoid diffusion, a relative new and safer technique is the sub-Tenon block.

Anatomy

Tenon´s capsule is a thin layer of connective tissue which surrounds the globe. Anteriorly it lies in close apoosition to the conjunctiva and fuses with it at the level of the limbus. It extend posteriorly surrounding the globe and fusing with the dura of the optic nerve. The sub-Tenon´s space is a virtual space between the capsule and the sclera. The instillation of local local anaesthetic into this space produces analgesia and akinesia by diffusing posteriorly into the retro-orbital space to block the traversing sensory and motor nerves.


Technique

A patients suitability for a local anaesthetic technique will need to be established. Many patients undergoing ophthalmic procedures will be elderly and may have significant co-morbidities. As a result a local anaesthetic technique, could 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 any remaining solution. 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. A blunt cannula preferably curved 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. If a curved needle is not available, other options include using a traight blunt or the plastic portion of an intravenous cannula (20 or 22G). 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 needle can be withdrawn slightly and repositioned. The onset of analgesia is usually rapid, whereas maximal akinesia may take up to 10 minutes to develop.

Medication

Lignocaine 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 while warming and increasing the alkalinity of the solution have been tried without being proven as beneficial.


Complications

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 is subconjuntival swelling and subconjunctival haemorrhage, both of which will usually resolve with gentle orbital pressure.


References

1.- Amin S, Minihan M, Lesnik-Oberstein S, Carr C. A new technique for delivering sub-Tenon anaesthesia in ophthalmic surgery. Br J Ophthalmol; 2001 2.- Yanoff M, Duker JS Augsburger JJ, et al. Ophthalmology. 3rd ed. St. Louis, MO: Mosby; 2004: 441-446 3.- Atkinson, W. Local Anesthesia in Ophthalmology 4.- Cass GD. Choices of local anesthetics for ocular surgery. Ophthalmol Clin North Am. 2006 Jun; 19(2):203-7