There is a substantial international variation in the care and provision of ophthalmic regional anaesthesia [1-5]. Both akinetic and non-akinetic methods are in use. Akinetic block using a needle, such as intraconal (retrobulbar), extraconal (peribulbar) and combined intraconal/extraconal (retro/peribulbar), are the commonest techniques practised around the world. The complications related to needle block, such as retrobulbar haemorrhage, globe perforation, retinal vascular obstruction, cardiorespiratory arrest and even death, although rare, have been reported [6,7]. Sub-Tenon's block is a simple, safe, effective and versatile alternative  to a sharp needle block. The exact frequency of the use of this technique is not known. It is commonly practised in certain parts of the world [9,10] but only 7% of ophthalmic departments in the UK practised this block in 1997 [4,5]. Its use now appears to have increased .
The technique was first described by Turnbull in 1884  and later by Swan in 1956 . More recently Mein and colleagues , Hansen and colleagues , Stevens , Greenbaum , Fukasaku and Marron  and Ripart and colleagues  have popularized this block. This block is also known as parabulbar block , pinpoint anaesthesia  and episcleral block .
The sub-Tenon's block involves obtaining surface anaesthesia, gaining access to the sub-Tenon's space, insertion of a cannula and subsequent administration of local anaesthetic agent into the sub-Tenon's space . Gaining access to sub-Tenon's space by dissection may initially be daunting to anaesthetists who are not used to holding eye surgical instruments. Therefore, knowledge of globe anatomy, especially Tenon capsule and the surrounding structures is particularly important before embarking on this technique.
The knowledge of anatomy of the orbit and its contents is essential to the safe practice of ophthalmic regional anaesthesia and many excellent textbooks of anatomy are available [20,21]. The orbit is an irregular four-sided pyramid, with its apex pointing posteromedially, and its base facing anteriorly. The annulus of Zinn, a fibrous ring which arises from the superior orbital fissure, forms the apex. The base is formed by the hind surface of the globe. Globe movements are controlled by the rectus muscles (inferior, lateral, medial and superior) and the oblique muscles (superior and inferior). The rectus muscles arise from the annulus of Zinn near the apex of the orbit and insert anterior to the equator of the globe forming an incomplete cone. The distance from the inferior temporal orbital rim to the annulus measures 42-54 mm. Within the annulus and, therefore, within the muscle cone lie the optic nerve (II), oculomotor nerves (III containing both superior and inferior branches), abducent nerve (VI), nasociliary nerve (a branch of V), ciliary ganglion and vessels. The superior branch of oculomotor nerve supplies the superior rectus and the levator palpebrae muscles. The inferior branch of oculomotor nerve supplies the medial rectus, the inferior rectus and inferior oblique muscles. The abducent nerve supplies the lateral rectus. The trochlear nerve (IV) runs outside and above the annulus and supplies the superior oblique muscle. The retained activity of this muscle is frequently observed as anaesthetic agent may fail to block this nerve. Corneal and perilimbal conjunctival sensation and superonasal quadrant of the peripheral conjunctival sensation are mediated through the nasociliary nerve. The remainder of the peripheral conjunctival sensation is supplied through the lacrimal, frontal and infraorbital nerves coursing outside the muscle cone. Intra-operative pain may be experienced if these nerves are not blocked.
The fascial sheath (Tenon capsule) is a thin membrane that envelops the eyeball and separates it from the orbital fat . It thus forms a socket for the eyeball. The inner surface is smooth and shiny and is separated from the outer surface of the sclera by a potential space called the episcleral space (sub-Tenon's space). Crossing the space and attaching the fascial sheath to the sclera are numerous delicate bands of connective tissue (Fig. 1). Anteriorly the fascial sheath is firmly attached to the sclera about 1.5 cm posterior to the corneoscleral junction (Fig. 2). Posteriorly, the sheath fuses  with the meninges around the optic nerve and with the sclera around the exit of the optic nerve (Fig. 3). However, the description varies and a major textbook of anatomy  suggests that the space under the Tenon capsule is a lymph space, and this follows the optic nerve and continues with the subarachnoid space. The tendons of all six extrinsic muscles of the eye pierce the sheath as they pass to their insertion on the eyeball. At the site of perforation the sheath is reflected along the tendons of these muscles to form a tubular sleeve. The superior oblique muscle sleeve extends as far as the trochlea and the inferior oblique muscle sleeve extends to the origin of these muscles. The tubular sleeves for the four recti muscles have expansions. Those for the medial and lateral recti are strong and are attached to the lacrimal and zygomatic bones and are called medial and lateral check ligaments respectively. The superior rectus expansion is thinner and less distinct and extends from the superior rectus tendon to that of levator palpebrae superioris. Similarly, the expansion from the inferior rectus extends to the inferior tarsal plate. The inferior part of the fascial sheath is thickened and is continuous medially and laterally with the medial and lateral check ligaments.
Preparation and assessment of patients
Preoperative preparation and assessment of patients undergoing ophthalmic surgery under local anaesthesia varies worldwide. There are specific evidence-based guidelines  and reports  available on this subject. The Joint Colleges Working Party Report  has recommended that patients are not starved but starvation policies vary considerably . Complication rates as a result of starvation or aspiration in ophthalmic regional anaesthesia are unknown and dangers remain if a patient vomits whilst undergoing anaesthesia and surgery. According to published guidelines and evidence reports [22,23], routine investigation of patients undergoing cataract surgery is not essential and does not improve health or outcome of surgery but tests can be done to improve general health of the patient if required.
The preoperative assessment should always include specific enquiry about bleeding disorders and drugs. There is an increased risk of haemorrhage in patients receiving anticoagulants and a clotting profile assessment is required prior to injection. Patients receiving anticoagulants are advised to continue medication . Clotting results should be within the recommended therapeutic range [25,26]. Currently there is no recommendation for patient receiving antiplatelet agents . Procedures under topical, subconjunctival, sub-Tenon or shallow peribulbar blocks are recommended .
Once the decision is made to operate, anaesthetic and surgical procedures are explained to the patients. All monitoring and anaesthetic equipment in the operating environments should be fully functional . Blood pressure (BP), oxygen saturation and electrocardiogram (ECG) leads are connected and baseline recordings are obtained . Although the insertion of an intravenous (i.v.) line has been questioned , a secure i.v. line is good clinical practice .
Local anaesthetic eye drops (proxymetacaine 0.5% or tetracaine 1%) are instilled onto the conjunctiva to obtain surface anaesthesia. The eye is cleaned with specially formulated 5% aqueous povidone iodine solution. An eyelid speculum or an assistant hand is used to keep the eyelids apart. The patient is asked to look upwards and outwards, to expose the inferonasal quadrant . Access to the space by the inferonasal quadrant is the commonest approach described because the placement of the cannula in this quadrant allows good fluid distribution superiorly while avoiding the area of surgery and reducing the risk of damage to the vortex veins .
Under sterile conditions, the conjunctiva and Tenon capsule are gripped with non-toothed forceps (Moorfield forceps) 3-5 mm away from the limbus . A small incision is made through these layers with scissors (Westcott scissors) to expose the white sclera .
A blunt curved posterior sub-Tenon's cannula (Fig. 4, 19-G, 25 mm long curved, with a flat profile and a blunt end hole) mounted on to a 5 mL syringe with local anaesthetic is inserted through the hole along the curvature of the sclera. The direction of the cannula is equidistant between the horizontal and vertical meridian of the globe. If resistance is encountered, a gentle pressure is applied and hydro-dissection usually helps in advancing the cannula. The resistance felt during insertion of the cannula is due to the intermuscular septum passing circumferentially between the extraocular muscles. Once this is negotiated, the cannula easily passes into the posterior sub-Tenon's space. If the hydro-dissection does not help or the resistance encountered is too great, it is advisable to reposition or reintroduce the cannula.
The local anaesthetic agent of choice is injected slowly. The patient is asked to return the eye to the primary gaze position. After removal of the cannula, gentle pressure is applied over the globe to favour the spread of the local anaesthetic agent. However, there are variations of technique relating to access, cannulae, agent, volume and adjuvant used.
Variations of access to sub-Tenon's space
Access to all other quadrants have been reported and are essentially similar in principle, such as superotemporal by Fukasaku and Marron , superonasal and inferotemporal by Roman and colleagues  and the medial canthal side by Ripart and colleagues . It is not known how frequently these quadrants are used for access. In addition, at present, there is no comparative data to support the ease of access to any particular quadrant.
Alternative sub-Tenon's cannulae
Alternative cannulae are available for this block. Some are specifically designed for this purpose while others have different primary purpose.
The specifically designed cannulae are made of metal or plastic. The metal cannulae vary in gauge, length, curvature and position of the end holes. A plastic cannula (Fig. 5) advocated by Greenbaum  is known as an anterior sub-Tenon's cannula. This is 15-G, 1.2 cm long, blunt, ‘D’ shaped and flat bottomed. The opening on the flat bottom is designed especially so it faces the sclera after insertion.
Non-specific sub-Tenon's cannulae include metal Southampton cannula , metal ophthalmic irrigation cannula [29,30], plastic i.v. cannula  and plastic mid-sub-Tenon's (Fig. 6) cannula . Recently an ultrashort metal cannula (Fig. 7, 16-G, 6 mm with blunt end hole) has been described .
The placement of a polyethylene catheter into sub-Tenon's space has been described for surgery of long duration .
Additionally, access to the sub-Tenon space through the medial canthal approach has been described using needles (Fig. 8) without dissection [19,35].
The selection of a cannula or needle depends on the availability, cost and the preference of the clinician. However, commercial posterior metal sub-Tenon's cannulae mostly feature in the published studies.
Choice of local anaesthetic agent
The ideal local anaesthetic agent for ophthalmic block should be safe, painless to inject and produce a rapid onset of dense motor and sensory block. The duration of the block must be sufficient for surgery yet not excessively prolonged . The speed of onset is partially determined by the properties of the anaesthetic, but more directly by the proximity to the nerves. Two percent lidocaine is the most commonly used agent and is considered the gold standard . All other agents appear to have a place in sub-Tenon's block. The choice depends on the duration of anaesthesia, availability and anaesthetist preference. Agents, such as mixtures of lidocaine and bupivacaine [37,38], mepivacaine , articaine , etidocaine  and prilocaine , have been used in sub-Tenon's block. However, there is no comparative data available on their relative effectiveness.
Volume of local anaesthetic agent
There is a wide variation in the volume of local anaesthetic used in sub-Tenon's block and this has been a subject of debate. The volumes vary from 1 to 11 mL [17,42] but 3 to 5 mL are generally used [8,16,43,44]. Smaller volumes usually provide globe anaesthesia  but larger volumes are required if akinesia is desirable .
Adjuvant and sub-Tenon's block
Vasoconstrictors (e.g. epinephrine) are commonly mixed with local anaesthetic solution to increase the intensity and duration of block, and minimize bleeding from small vessels . Absorption of local anaesthetic is reduced thus avoiding a surge in plasma levels. Epinephrine may cause vasoconstriction of the ophthalmic artery compromising the retinal circulation . The use of epinephrine containing solutions are avoided in elderly patients suffering from cerebrovascular and cardiovascular diseases . The role of epinephrine in sub-Tenon's block has been questioned . This is because ophthalmic surgery is usually of short duration and the duration of block achieved by lidocaine without epinephrine suffices for modern minimal invasive cataract surgery.
Hyaluronidase is an enzyme, which reversibly liquefies the interstitial barrier between cells by depolymerization of hyaluronic acid to a tetrasaccharide, thereby enhancing the diffusion of molecules through tissue planes . It is available as a powder readily soluble in local anaesthetic solution. The amount of hyaluronidase mixed with the local anaesthetic varies from 5 to 150 IU mL−1. There is conflicting evidence that hyaluronidase (30 IU mL−1) improves the effectiveness and the quality of sub-Tenon's block [46,47]. If hyaluronidase is to be used, 15 IU mL−1 is the recommended amount . It is an expensive drug  and although side-effects are rare, allergic reactions , orbital cellulites  and the formation of pseudotumours  have been reported after its use.
Commercial preparations of lidocaine and bupivacaine are acidic solutions in which the basic local anaesthetic exists predominantly in the charged ionic form . It is only the non-ionized form of the agent that traverses the lipid membrane of the nerve to produce the conduction block . At higher pH values a greater proportion of local anaesthetic molecules exist in the non-ionized form, allowing more rapid influx into the neuronal cells. Alkalinization of the local anaesthetic agent has been shown to decrease the onset and prolong the duration of needle blocks [53,54] but no such benefit has been observed in sub-Tenon's block .
Addition of clonidine , muscle relaxant [57,58] and other chemicals are known to increase the onset and potency of orbital needle block but their use is not routine.
Passage of local anaesthetic agent during sub-Tenon's injection
The passage of the local anaesthetic during sub-Tenon's block has been studied using ultrasound [59-61] and magnetic resonance imaging techniques . These studies confirm that when the anaesthetic agent is injected into the sub-Tenon's space, it opens the sub-Tenon's space giving a characteristic ‘T-sign’ (Fig. 9). As the local anaesthetic agent spreads through the sub-Tenon's space, it diffuses into intraconal and extraconal areas resulting in anaesthesia and akinesia of the globe and eyelids. Intense analgesia is produced by blockade of the short ciliary nerves as they pass through the Tenon capsule . Akinesia is obtained due to blockade of the motor nerves present in the intraconal and extraconal compartments.
Complications and problems
Common complications of sub-Tenon's block are mainly minor, although rare major complications have also been reported.
Pain during injection
Pain experienced during ophthalmic blocks is multi-factorial. The incidence of pain during sub-Tenon's injection with posterior metal cannula is reported in up to 44% of patients [8,16,29,63]. Pain scores on a visual analogue scale (VAS) [0: no pain; 10: worst imaginable] have been reported as high as 5  and smaller cannulae appear to offer a marginal benefit . Premedication or sedation of patients during sub-Tenon's injection does not add any benefit . Preoperative explanation of the procedure, good surface anaesthesia, gentle technique, slow injection of warm local anaesthetic agent and reassurance are considered good practice and may reduce the discomfort and anxiety during the injection.
Chemosis signifies anterior injection of the anaesthetic agent. This usually occurs if a large volume of local anaesthetic is injected and if the Tenon's capsule is not dissected properly . The incidence of chemosis varies from 25 to 60% [8,29] with a posterior cannula and the incidence increases to 100% with shorter cannulae . Chemosis may not be confined to the site of injection and has been known to spread to other quadrants [16,44]. This usually resolves after the application of digital pressure, and no intra-operative problems have been reported. Surgeons performing glaucoma surgery may feel that significant chemosis compromises the surgical procedure.
Minor conjunctival haemorrhage
Fine vessels inevitably severed on making the conjunctival dissection cause conjunctival haemorrhage. The incidence of haemorrhage varies from 20 to 100% and depends on the cannula used [16,29,44]. This can be minimized by careful dissection avoiding damage to fine vessels plus the application of cautery and the use of topical epinephrine [16,17]. Patients should be warned of the possibility of this occurrence preoperatively.
Loss of local anaesthetic volume during injection
Overspill of local anaesthetic during its administration is commonly observed [16,29,44]. This is likely to occur if the dissection of the sub-Tenon's capsule is not complete or if there is resistance to injection. Enlargement of the initial dissection following traction during injection and large injection volume also cause overspill. Careful dissection and use of diathermy may minimize the loss.
Anaesthesia and akinesia
Anaesthesia accompanying sub-Tenon's block is good as evident from most published studies on the subject. However, akinesia is variable and may not be complete . Akinesia is volume dependent and if 4-5 mL local anaesthetic agent is injected, a large proportion of patients develop akinesia . Superior oblique muscle and lid movements may also remain active in a significant number of patients .
These include short-lived muscle paresis , orbital and retrobulbar haemorrhage [66,67]. Recently, scleral perforation during sub-Tenon's block has been reported in a patient who had previously undergone retinal surgery . Trauma to inferior and medial recti muscles leading to restrictive functions resulting in diplopia have been reported following damage to the muscles by metal cannula . Other complications relate to optic neuropathy , afferent pupillary and accommodation defects . Retinal and choroidal vascular occlusion  and a case of central spread of the local anaesthetic agent leading to cardiorespiratory collapse have been reported . The mechanism of central spread is not clear but perhaps spread of the injected local anaesthetic agent into the subarachnoid space (previously described in the paragraph relating to anatomy) through the optic nerve sheath or back tracking of the local anaesthetic agent through one of the orbital foramina are possible explanations. The latter can particularly happen if there is an unintentional perforation of the Tenon capsule leading to the deposition of the local anaesthetic agent into the intraconal compartment.
Retained visual sensations during sub-Tenon's block
Published studies have reported that patients having phacoemulsification cataract surgery under topical, retrobulbar and peribulbar blocks experience light and various visual sensations during surgery [74-77]. Recent studies have also shown that patients experience a wide range of visual sensations undergoing phacoemulsification cataract surgery under sub-Tenon's block [78,79]. Although the majority of patients felt comfortable with the visual sensations they experienced, a significant proportion of patients (16%) found the experience to be unpleasant or frightening . Preoperative counselling appears to offer benefits in these patients . Hence patients receiving sub-Tenon's block should be offered preoperative advice and this may alleviate this unpleasant experience.
Intraocular pressure and role of ocular compression
The rise in intraocular pressure after administration of sub-Tenon's block is small or even non-significant [81,82]. There was a significant reduction in intraocular pressure using Honan's intraocular pressure reducer, but this did not make a significant difference in the effectiveness of anaesthesia to the surgeon .
Pulsatile ocular blood flow after sub-Tenon's block
It is known that retrobulbar and peribulbar injections decrease pulsatile ocular blood flow, at least for a short time . In a recent study  the changes in intraocular pressure and ocular pulsatile amplitude were compared during peribulbar and sub-Tenon's blocks. The intraocular pressure remained stable throughout the study with both the blocks. One minute after injection of the anaesthetic agent, the ocular pulsatile amplitude was significantly decreased in the injected eyes in both the sub-Tenon's (24%) and peribulbar (25%) groups. The decrease in the ocular pulatile amplitude in the sub-Tenon's group (14%) was also detectable after 10 min in the control eyes. Therefore, caution is required in the management of patients whose ocular circulation may be compromised and an alternative anaesthesia, such as general anaesthesia, may be desirable.
Who should administer sub-Tenon's block?
It is recommended that only appropriately trained anaesthetists or ophthalmologists should perform orbital local anaesthetic injection . Non-medically qualified staff may administer topical or subconjunctival anaesthesia.
In some centres, nurses have been trained to perform sub-Tenon's block . In the authors opinion a basic requirement for non-medical staff performing orbital block is that they must have a basic understanding and knowledge of basic science and techniques behind regional blocks. Clear lines of responsibility and indemnity must be in place.
Presence of the anaesthetist
Presence of the anaesthetist during sub-Tenon's block is not essential but local staffing availability will dictate whether an anaesthetist may be provided for ophthalmic lists . It is also recommended that monitoring should be the role of a member of the staff who remains with the patient throughout the monitoring period and whose sole responsibility is to the patient. This person must be trained to detect and act on any adverse events, and may be an anaesthetist, nurse, operating department practitioner or anaesthetic nurse as long as they are trained in life support .
The patient should be comfortable and soft pads are placed under the pressure areas. All patients undergoing major eye surgery under local anaesthesia should be monitored with pulse oximetry, ECG, non-invasive BP measurement and maintenance of verbal contact . Patients should receive an oxygen-enriched breathing atmosphere to prevent hypoxia. ECG and pulse oximetry should be continuous. Once the patient is under the drapes, verbal and tactile contact must be maintained .
Sedation during sub-Tenon's block
Although sedation is used commonly with topical anaesthesia , its use with sub-Tenon's anaesthesia should be limited to those for whom explanation and reassurance have proved inadequate. Short acting benzodiazepines, opioids or small doses of i.v. anaesthetic agents are favoured but dosage must be minimal. Routine use of sedation is discouraged  because of increased intra-operative events [86,87]. It is essential that when sedation is administered, a means of providing supplemental oxygen is available. Equipment and skills to manage any life-threatening events must be immediately accessible.
Uses of sub-Tenon's block
Sub-Tenon's block has been used for a large number of ophthalmic surgical procedures, which include mainly cataract surgery but also vitreoretinal surgery [88-90], panretinal photocoagulation [91,92], strabismus surgery , trabeculectomy , optic nerve sheath fenestration , chronic pain management  and therapeutic delivery of drugs .
Sub-Tenon's block in patients receiving non-steroidal anti-inflammatory drugs and anticoagulants
It is recommended and generally accepted that needle blocks should be avoided in patients who are receiving anticoagulants and non-steroidal agents. Recent reviews suggest that sub-Tenon's block may be used safely in these patients provided the blood results are within normal therapeutic ranges [25,26].
Advantages of sub-Tenon's block
The technique initially appears to be intimidating but with practice it becomes easier to learn and perform. It eliminates the risks of sharp needle techniques, provides reliable anaesthesia and has the potential for further supplementation for prolonged anaesthesia and postoperative pain relief . It can be safely used in patients with a long globe. It does not need a large injectate volume and is found to achieve a better success rate when compared to retrobulbar, peribulbar and topical anaesthesia alone, as shown in many published studies [97-110] relating to this block.
Limitations of sub-Tenon's block
Subconjunctival haemorrhage and chemosis are common and occur relatively less as experience increases. Residual muscle movement or incomplete akinesia does not cause intra-opesrative difficulties and is generally acceptable to the surgeons. The block may be difficult to perform in patients who have had previous sub-Tenon's block in the same quadrant, previous retinal detachment and strabismus surgery, eye trauma and infection to the orbit. Some glaucoma surgeons may have a dislike for sub-Tenon's block although this block has been used successfully for glaucoma surgery.
At present there is no absolutely safe orbital regional block technique. Sub-Tenon's block is a simple, effective, relatively safe and versatile technique. Although rare complications do occur following this block. A thorough knowledge of anatomy and understanding of the underlying principles is essential before embarking on a sub-Tenon's block.
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