1 Department of Anaesthesia, Southern General Hospital, 1345 Govan Road, Glasgow G51 4TF, UK. 2 Department of Anaesthesia, Wycombe General Hospital, Queen Alexandra Road, High Wycombe, Buckinghamshire HP11 2TT, UK
Corresponding author. E-mail: cholliday@doctors.org.uk
Accepted for publication: August 9, 2002
Abstract
Br J Anaesth 2003; 90: 78793
Keywords: anaesthetic techniques, regional, Sub-Tenons block; surgery, ophthalmological
Local anaesthesia is the anaesthetic technique of choice for surgery on the eye in many instances. The expansion of day-case facilities has encouraged its use, and the development of less invasive surgical techniques has rendered general anaesthesia largely unnecessary. Retrobulbar and peribulbar blocks are commonly practiced.10 27 While providing excellent conditions for operating on the eye, these techniques are associated with serious but uncommon side-effects. The most dangerous is respiratory arrest as a result of brain stem anaesthesia.19 43 Less invasive techniques for providing local anaesthesia, such as subconjunctival or topical application of local anaesthetic, are devoid of such risks but fail to provide adequate immobility of the eye and postoperative analgesia.
Recent reports suggest that the use of Sub-Tenons block is becoming more widespread amongst anaesthetists and ophthalmological surgeons.7 10 It has been suggested that it has a more acceptable risk profile than traditional ophthalmic anaesthesia techniques.7 This article reviews the technique with special focus on possible complications.
Anatomy
Tenons capsule is a fascial layer of connective tissue surrounding the globe and extra-ocular muscles. It is attached anteriorly to the limbus of the eye and extends posteriorly over the surface of the globe until it fuses with the dura surrounding the optic nerve (Fig. 1.)
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Sensory innervation to the globe and its internal structures is derived from the ophthalmic branch of the trigeminal nerve via the long nasociliary and short ciliary nerves. The ciliary ganglion lies near the apex of the retrobulbar cone and is associated with the optic nerve and artery. It is composed of sympathetic fibres, sensory fibres, and parasympathetic fibres. Whereas the sympathetic and sensory fibres transit through the ganglion, the parasympathetic fibres synapse within it. The efferent nerve, the short ciliary nerve, progresses anteriorly to provide sensation to the globe and autonomic motor function to the iris. Its path to the anterior segment is diffuse, but traverses both the retrobulbar cone and Sub-Tenons space where it is susceptible to block by local anaesthetic agents.
Performance of the block
The patient should be assessed for anaesthesia in the standard manner. Non-consent, inability to comply with the anaesthetists instructions as, for example, a result of deafness or mental incapacity, inability to lie flat, allergy to local anaesthetics, lack of i.v. access, or excessive uncontrollable anxiety are absolute contraindications to Sub-Tenons anaesthesia. The procedure should be explained to the patient to allay anxiety and assist the anaesthetist in carrying out the block.
The injection is made on a supine patient with an i.v. cannula in situ. The conjunctiva is anaesthetized with topical anaesthetic solution. An eyelid speculum should be inserted at this point to improve access. Throughout the procedure, the patient is asked to look up and outwards to expose the infero-nasal quadrant. A small tent of conjunctiva is raised with a pair of fine non-toothed forceps approximately mid-way between the limbus of the eye and the visible edge of the inferonasal portion of the conjunctiva. A small incision is made in the tented conjunctiva with a pair of ophthalmic scissors (Fig. 2). The closed scissors are introduced through the aperture created and a tunnel is fashioned to the bare sclera by blunt dissection through Tenons capsule. A curved, blunt irrigating cannula is then inserted with the syringe of anaesthetic solution attached (Fig. 3). Stevens describes a metal cannula designed for this purpose48 (VisitecTM Sub-Tenons Anaesthesia Cannula 19 g x 25 mm) (Fig. 4). The cannula is introduced along the contour of the globe and gentle contact of its tip is maintained with the sclera until it lies in the posterior segment (Fig. 1). Occasionally, resistance to the needle is felt around the equator where a fibrous band can form as the ocular muscles broach the capsule. This is usually easily overcome by gentle pressure.
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Normally, little resistance is found to injection. If resistance is felt, withdrawal and re-insertion of the needle using gentle pressure of the tip against the globe to ensure entry into the correct tissue plane will help. Slight proptosis of the eyeball is normal after a correctly sited injection. There may be slight leakage of solution from the tunnelled point of entry of the cannula into the conjunctiva. Should ballooning of the conjunctiva occur, the solution might be in the incorrect tissue plane. After removal of the cannula, pressure is applied by massaging the globe. To ensure flaccidity of the extra-ocular muscles, an additional facial nerve block was originally described,47 but is rarely necessary in our experience.
Common pitfalls relate to the positioning of the conjunctival incision and the confirmation of dissection down to the sclera. The incision must be made at a sufficient distance from the limbus to avoid the oblique insertion of Tenons capsule into the sclera. A distance of 710 mm from the limbus is acceptable. Detecting the end-point of dissection comes with experience. The sclera has a different, more fibrous, and whiter appearance than either the capsule, or the continuation of the fascial layer of the extra-ocular muscles. Good illumination is very important for this difference to be apparent. While some ophthalmologists may prefer to use magnification, the procedure is normally performed satisfactorily with the naked eye.
Extra care should be taken with myopic patients. They have longer and thinner globes, and there is an increased risk of scleral perforation. The technique is relatively contraindicated where there is a history of scleral disease with possible scarring and friability of the sclera. Previous retinal detachment surgery can be associated with scleral buckles and adhesions, which may hinder dissection or spread of anaesthetic solution, and increase the risk of globe perforation in the quadrant dissected. While the extensive experience of Guise and colleagues17 and Friedberg and colleagues12 supports the safe use of Sub-Tenons block in patients with scleral pathologies, a recent case report of scleral perforation should be viewed with concern.14 This is discussed more fully below.
Mode of action
Spread of the local anaesthetic solution into the retro-orbital space through the thin posterior section of Tenons capsule may occur.50 This accounts for the further development of the sensory block, supplementing the direct effect on sensory nerves within the sclera.37 53 The development of akinesia is related to direct spread into the Tenons sheaths surrounding the insertion of the extra-ocular muscles, and to effects within the retrobulbar cone on both these muscles and their motor nerves.38 Diffusion of solution into the periorbital tissues has been demonstrated, which explains the akinesia of the eyelids.36
Composition of local anaesthetic mixture
Many studies use an anaesthetic mixture compromising of lidocaine and bupivacaine in equal volumes to a total of 4 ml. Hyaluronidase (75150 iu) is added to promote the rate and extent of diffusion to the peri- and retro-orbital tissues.9 18 This practice has been used to reduce the need for a facial nerve block.3 21 41 In our hospitals, lidocaine 2% and bupivacaine 0.75% are mixed in equal volumes then a small volume (0.20.4 ml) of hyaluronidase is added.
There is a relationship between the volume of local anaesthetic injected into the orbital cavity and the rise in intra-ocular pressure. When 3 ml of anaesthetic solution is slowly administered, a similar volume to a retrobulbar block, there is a similar rise in intra-ocular pressure. Peribulbar injection requires a larger volume to be administered, and the degree of intra-ocular hypertension is correspondingly greater.49
Efficacy
On administration of anaesthetic solution to the Sub-Tenons space, analgesia develops rapidly and effectively. Roman and colleagues40 reported that 99.1% of 109 patients scored the anaesthetic procedure as painless, and 97% reported the various operative procedures as devoid of pain. Stevens assessed analgesia by visual analogue pain scoring (scale 010) in 50 patients for cataract surgery.47 Insertion of the block produced a mean and median score of 1 (range 05). Forty-one patients scored 1 or less. When pain associated with the anaesthetic injection is compared, Sub-Tenons is comparable with topical,6 26 32 or subconjunctival52 anaesthesia and appears less painful than both peribulbar2 5 22 32 52 or retrobulbar20 21 52 blocks (Fig. 5). Sub-Tenons provides a similar degree of intra-operative analgesia to retrobulbar,20 21 52 and peribulbar blocks,2 5 22 32 52 and more adequate analgesia than topical methods6 26 32 52 (Fig. 5) particularly if scleral cautery is used.5 6
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Akinesia is less consistent than the production of adequate analgesia. Maximal akinesia will develop over 515 min. The site of injection, its timing relative to surgery, and the composition and volume of solution administered are each important in producing immobility of the globe. With standard volumes of 23 ml of solution and an inferonasal approach, a degree of lateral rectus function may prevail. The muscles most affected are normally adjacent to the site of injection. This can be an advantage for strabismus surgery.15 39 46 Volume appears critical, with Li and colleagues25 reporting reliable akinesia with 11 ml of solution injected for posterior segment surgery.
Inadequate blocks can be successfully managed by supplementing the anaesthetic with a 11.5 ml extra of solution via the tract already prepared through the conjunctiva and Sub-Tenons space. Supplemental injections are no more commonly needed than with peribulbar blocks,5 and with a Sub-Tenons block the ability to place solution through the pathway already prepared has the advantage of producing less pain and fewer complications.
A polyethylene catheter fixed into the Sub-Tenons space, permits extension of anaesthesia perioperatively for a theoretically unlimited period.4 Behndig also suggests that infusion of anaesthetic solution through such a catheter, rather than bolus administration by injection, could minimize chemosis and provide better akinesia.
An almost painless block, along with adequate analgesia for the surgical procedure, reduces patient anxiety and sedation requirements; this is particularly advantageous for day case patients. Improved patient cooperation minimizes both residual eye movement and blepharospasm during the surgical procedure.
The quality of the data comparing different regional anaesthesia techniques for ophthalmic surgery is limited by a scarcity of randomized controlled trials. Use of Sub-Tenons block is currently based on cohort studies and this limits its undisputed recommendation.13 42 44 Guidelines concerning the technique are therefore based on Level B+ evidence with inadequate grounds for Grade 1 recommendations. A large, correctly conducted randomized trial would add hugely to the validity of the technique.
Complications and safety
Sub-Tenons block is perceived as having an acceptable safety profile because a sharp needle is not placed within the orbital cavity. The design of the catheter and the nature of the blunt dissection suggest that intradural puncture is unlikely if not impossible. As a consequence, intradural, subarachnoid, and central nervous system injection is improbable. Intravascular or intraneural injection is also unlikely and the risk of scleral perforation is reduced.
Swelling of the conjunctiva has an incidence of 39.4% with Sub-Tenons blocks.40 Some chemosis is attributable to anterior leak of injectate but its extent can be much greater if the solution is administered incorrectly into either the anterior compartment of the Sub-Tenons space, or the subconjunctival space. It does not usually impede the surgical field. Chemosis might be reduced by fashioning a radial conjunctival incision.38
Subconjunctival haemorrhage occurs in 3256% of cases.40 47 Bleeding is usually localized and infrequently associated with significant consequences. To reduce haemorrhage, cauterization of the conjunctival incision has been advocated.51 Using a standard i.v. cannula (22 G Venflon) to puncture the anaesthetized and tented conjunctiva may reduce the occurrence of chemosis and subconjunctival haemorrhage.1 While the tip of the needle is kept in position and visible under the conjunctiva, the cannula is advanced posteriorly in the Sub-Tenons space, before administration of anaesthetic solution.
Global perforation
Use of a sharp needle to access the Sub-Tenons space is associated with the risk of globe perforation. This complication is reduced by keeping the needle tip in constant view.
Retrobulbar haemorrhage and anticoagulation
A vortex vein in the sclera may be ruptured during the blunt dissection. This is less likely in the inferonasal quadrant. Should retrobulbar bleeding occur, it might decompress anteriorly through the path dissected by the catheter. If patients are receiving anticoagulants, which cannot be reversed safely, and akinesia is desirable, Sub-Tenons anaesthesia is often considered to be the method of choice.22 34 The International Normalized Ratio should be appropriately controlled and conjunctival cautery is recommended.
Operator experience
The serious complications associated with sharp needle techniques are rare (Table 1). User-dependent factors, such as inexperience of the operator and the frequency of performing particular blocks, contribute to the incidence of complications. Such extensive practical and published experience has yet to be accrued for Sub-Tenons blocks, and it is conceivable that significant but rare complications may yet be uncovered.
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Case reports
We have identified four case reports of complications attributed to the administration of a Sub-Tenons block. Frieman and Friedberg14 reported an episode of globe perforation associated with a Sub-Tenons procedure. A patient with scleral scarring as a result of previous retinal detachment surgery presented with symptoms attributable to recurrence. Initially, the procedure was uneventful and the sclera was accessed by blunt dissection with Westcott scissors. These were then passed posteriorly and resistance was met because of scleral adhesions, which were cut with the scissors. The surgeon continued to advance the scissors posteriorly and encountered a sudden loss of resistance due to scleral perforation. While no long-term visual deficit was reported, prolonged management, including laser treatment to the associated retinal tear, was required. Once the sclera has been exposed, we believe it is unnecessary and unwise to use the scissors to dissect posteriorly in Sub-Tenons space, particularly when scleral scarring is present. The use of Sub-Tenons block in patients with previous retinal detachment surgery is not fully established and randomized trials in these patients would be of particular value.
There has been one case report of significant retrobulbar haemorrhage associated with Sub-Tenons block.33 Acute pain was the presenting symptom which occurred immediately on injection of 3 ml of local anaesthetic. There was no bleeding at the site of incision and lateral canthotomy and cantholysis were performed successfully to reduce intra-orbital pressure.
Mein and Woodcock29 reported one patient in their series of 58 who developed raised intra-orbital and intra-ocular pressures on injection of 1.5 ml of perilimbal anaesthetic solution. Lateral canthotomy and anterior paracentesis were performed, but there was no evidence of intra-ocular haemorrhage, and the scleral buckle procedure was completed successfully.
A self-limiting strabismus associated with clinical palsy of the ipsilateral superior oblique muscle occurred in a 68-yr-old man who underwent phacoemulsification and intra-ocular lens implantation.45 The Sub-Tenons injection was administered through the superior temporal quadrant and direct trauma to the muscletendon complex or the trochlear nerve by the needle were the most likely causes. His symptoms resolved within 4 weeks.
The National Survey of Local Anaesthesia for Ocular Surgery by the Clinical Sub-Committee of the Royal College of Ophthalmologists10 11 provides substantial evidence of the relative safety of the Sub-Tenons block. In an estimated population of 4384 patients receiving Sub-Tenons block, Eke and colleagues11 report one major expulsive haemorrhage. Because of the confidential nature of reporting to this audit, no further information is available. No life-threatening complications of the block itself were reported. Severe systemic effects such as hypertension, claustrophobia, and confusion were reported, Sub-Tenons producing a similar incidence to other blocks. There was no evidence of inadequate block of the oculocardiac reflex.
There is one further suggestion of a significant complication attributable to Sub-Tenons block.28 A verbal report of globe perforation was made at a meeting of the European Society of Anaesthetists (ESA) but, unfortunately, does not seem to have been reported further.
The perceived safety of Sub-Tenons block has led to its endorsement by The Royal Colleges of Ophthalmologists and Anaesthetists as a procedure where an anaesthetist need not be present,42 when appropriate patient selection has occurred. An anaesthetist must be present if i.v. sedation, performance of a sharp needle block, or general anaesthesia is required. The anaesthetist therefore continues to have an important role in the multidisciplinary team, and the appropriate administration of local anaesthetic is an important part of this role.
Conclusions
Anaesthetists frequently perform Sub-Tenons block for a variety of ophthalmological procedures. It is a relatively pain-free block to perform and is effective in producing sensory block sufficient for surgery without the use of sedation. Total akinesia may not be obtained. The patient should be warned of chemosis and conjunctival haemorrhage occurring although these do not normally impede the procedure. The technique of blunt dissection and the type of cannula used to perform the block avoid the potentially serious side effects of retro- or peribulbar blocks. The relatively few complications reported in the 11 yr since the technique was first described suggest that it is safe.
Acknowledgement
We would like thank Professor G. N. Dutton of the Tennent Institute of Ophthalmology, Western Infirmary Glasgow for his helpful comments.
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