Sub-Tenon's block is a popular local anaesthetic technique for ophthalmic surgery . It's main advantage is perceived to be the avoidance of sharp needles and their associated complications . Although the sub-Tenon's space may be accessed in any quadrant , a commonly used approach is that described by Stevens, which involves an infero-nasal (IN) injection . This has the advantage of being distant from the operation site and enables the operator to steady their hands by gently resting on the patient's forehead and bridge of nose. However, the IN quadrant may not always be available due to the presence of conjunctival lesions (pterygium, etc.), previous retinal detachment surgery, medial rectus surgery or bony orbital abnormalities . In these circumstances an alternative is to use the infero-temporal (IT) approach. Passing mention of the IT approach  is made in the published literature but there are no data on the onset, distribution of block or complication rates with this route. The aim of this prospective, randomized study was to compare IN and IT approaches of sub-Tenon's block.
Local Ethics Committee approval was given. Informed consent was obtained from 100 American Society of Anesthesiologists (ASA) I–III patients undergoing routine cataract extraction surgery. Patients were excluded if they were unwilling to take part, if there were communication or language problems, if there was any history of allergy to study medication or they had a pre-existing extra-ocular muscle palsy. Patients were randomly allocated to one of the two groups using sealed envelopes. Group IT received an IT sub-Tenon's injection, whereas Group IN received an IN injection.
Patients were not fasted, nor did they receive sedatives. On arrival in the induction room standard monitoring (electrocardiogram, pulse-oximetry and non-invasive blood pressure) was commenced, and intravenous access secured. Baseline globe movements were assessed, then a sub-Tenon's injection performed by two experienced ophthalmic anaesthetists. This involved topical anaesthesia of conjunctiva and cornea by administering two to three drops of proxymetacaine 0.5%. The eye was then cleaned with several drops of specially formulated 5% aqueous povidone iodine solution. An eyelid speculum or an assistant's hand was used to keep the eyelids apart. The patient was then asked to look upwards and outwards, to expose the IN quadrant, or upwards and inwards to expose the IT quadrant. The conjunctiva and Tenon's capsule were gripped with non-toothed Moorfield forceps 3–5 mm away from the limbus in the IN, or the IT quadrant. A small incision was made through these layers with Westcott scissors to expose the white sclera. The tip of the posterior sub-Tenon's cannula (metal, 1 inch long) was then passed carefully backwards into the orbit so that the tip lay beyond the equator of the globe. Following test aspiration, lidocaine 2% 3 mL with 15 IU mL−1 hyaluronidase was injected slowly. The volume of injected local anaesthetic was increased to 4 mL if loss of local anaesthetic agent through the conjunctival wound during the injection was considered excessive. Manual compression and gentle massage of the eyeball was performed. At 2, 4, 6 and 8 min after the injection a trained anaesthetic nurse assessed the patient's ocular movements. Ocular movements were scored in the secondary directions of gaze (abduction, adduction, elevation and depression) using the Brahma scoring system (full movement, 3; partial movement, 2; a flicker of movement 1 and no movement, 0) . An eye with full movement in the secondary directions of gaze would score 12, whereas an eye with no ocular movement would score zero. If after 8 min the block score was greater than four, then the block was deemed inadequate for surgery and a supplementary injection of 3 mL of the test solution would be performed by the same technique.
Patient characteristics data (age, gender and axial length), akinesia scores, requirement for supplementary anaesthesia, time to start of surgery, duration of surgery and complications (chemosis and sub-conjunctival haemorrhage) were recorded. The anaesthetist administering the block asked patients to score the pain of injection and discomfort during surgery using a verbal analogue scale from 0 (no pain) to 10 (worst imaginable). The surgeon was also asked to score the block in terms of good, adequate or poor.
Differences of akinesia scores between groups were compared using a Wilcoxon's signed rank sum test. Patient characteristics data were compared using parametric (t-test) or non-parametric (Fisher's exact test) where applicable. A P < 0.05 was taken as significant.
There were no significant differences in patient characteristics data between the groups (Table 1). The mean (SD) volume of administered local anaesthetic was 3.3 mL (0.4) in both groups. Two minutes after the injection the mean (SD) akinesia score was 2.7 (2.18) in Group IN and 2.2 (2.23) in Group IT (P = 0.26). At 4 min the mean (SD) akinesia score was 1.1 (1.66) in Group IN, compared to 0.9 (1.52) in Group IT (P = 0.54). At 6 min the mean (SD) akinesia score was 0.4 (1.35) in Group IN, compared to 0.8 (1.68) in Group IT (P = 0.19). At 8 min the mean (SD) akinesia score was 0.2 (1.03) in Group IN, compared to 0.3 (1.10) in Group IT (P = 0.65). No patients required supplementary injections. Mean pain scores for the injection, per operatively and postoperatively were 0.9, 0 and 0 for Group IN, compared to 1.1, 0 and 0 for Group IT. Chemosis occurred in 14 patients in Group IN, compared to 22 in Group IT (P = 0.21). A sub-conjunctival haemorrhage was noted in 14 patients in Group IN and 19 patients in Group IT (P = 0.52). The surgeons scored all the blocks as ‘good’ except for one patient in each group.
Sub-Tenon's block involves gaining access to the sub-Tenon's space, inserting a sub-Tenon's cannula and administering local anaesthetic agent into the sub-Tenon's space. Tenon's capsule, or the fascial sheath, is a dense fibrous layer of elastic connective tissue surrounding the globe and extraocular muscles [7–9]. It originates at the limbus, and extends posteriorly to the optic nerve, and has sleeves along the extraocular muscles. The penetration by the rectus muscles divides Tenon's capsule into anterior and posterior portions. Anterior Tenon's capsule is adherent to episcleral tissue from the limbus posteriorly for about 10 mm and is fused with the intermuscular septum of the extraocular muscles and overlying bulbar conjunctiva. As the conjunctiva fuses with Tenon's capsule in this area, the space under the capsule can be easily accessed through an incision 2–3 mm behind the limbus. The posterior sub-Tenon's capsule is thinner and passes round to the optic nerve, separating the globe from the contents of the retrobulbar space. Posteriorly, the sheath fuses with the openings around the optic nerve and with the sclera around the exit of the optic nerve. Close to the optic nerve the fascial sheath of the eyeball is pierced by ciliary nerves and vessels, and by the vortex veins . As the Tenon's capsule encircles the globe, it is possible to gain access to the sub-Tenon's space from all quadrants of the globe except at the point of insertion of muscles into the globe.
Access to the sub-Tenon's space by the IN 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 [3,4]. Access to the sub-Tenon's space in other quadrants such as IT , superotemporal , superonasal  and the episcleral block (sub-Tenon's) by medial canthal approach  has been described. The superotemporal approach utilizes a very small amount of local anaesthetic agent (1 mL) to achieve anaesthesia and no akinesia is possible. There are no data on the superonasal approach either. Access to this quadrant may be difficult due to technical difficulty in dissecting the conjunctiva and Tenon's capsule which in this area has relatively rich blood supply . The medial canthus access during episcleral block is achieved by using a needle. Although the block has been successfully evaluated and used extensively [12–16], there are reservations expressed in using this approach as a needle rather a blunt cannula is used .
Most anaesthesia providers are familiar with the IT quadrant as many will have been trained in the use of sharp needles techniques, where the relatively avascular IT quadrant is favoured. It is likely that most would feel comfortable performing a sub-Tenon's block on this side as long as the resulting block has been shown to be as good as the traditional IN approach. Until now there has been a paucity of data to provide this re-assurance. We have shown that the IT approach of sub-Tenon's block provides an equally rapid and dense block compared to the IN approach without a significant increase in minor complications. The IT approach offers a good alternative in patients where IN access is a limiting factor.
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