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.
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.
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 .
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.
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.
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.
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.
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.
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 .
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].
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.
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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