Trabeculectomy postponed due to volume of sub‐Tenon local anaesthetic

Abstract

We report a case of glaucoma in a middle aged gentleman of South-East Asian origin. On examination he was short, with small, deep-set eyes. However he did not appear to have the shallow orbit, and marked supra-orbital fat pad both classically found in South-East Asia. He had no external eye abnormalities and no other significant history. His only risk factor for normal pressure glaucoma was a nephew, blind because of glaucoma. His intraocular pressure pre-operatively was 20 mmHg. The eye was prepared for local anaesthesia with 0.5% proxymetacaine and 5% povidone iodine eye drops and the skin was cleaned. A Barraquer speculum was inserted and a sub-Tenon’s block was performed in the infero-nasal quadrant using 5 ml 0.5% bupivaciane and 75 iu hyalase via a blunt cannula. There was considerable filling of the upper eyelid during injection, but no signs of overfilling of the orbit (Fig. 3). Complications were not noted until after injection, when there was considerable difficulty removing the speculum and the upper eyelid could not be opened sufficiently to allow surgical access. After waiting 15 min for the local anaesthetic to disperse, the surgery was abandoned. Two hours after injection the intraocular pressure was still 20 mmHg. Significantly, the surgeons found access difficult even under general anaesthesia, noting a relatively narrow palpebral fissure 2 weeks later. Showing filling of the upper eyelid during injection, but no signs of overfilling of the orbit. The ocular orbit is conical and has a volume of approx 30 ml in Caucasians and 28 ml in Asians [1]. It contains the globe, optic nerve, external ocular muscles, ophthalmic artery and its branches, orbital veins and nerves and the lacrimal gland. Fibrous septa and orbital fat fill the remaining space, acting as a supportive cushion. Sub-tenon's local anaesthetic injection is increasingly used as the anaesthetic technique of choice for many ophthalmic procedures as ‘a safe and effective alternative to retrobulbar and peribulbar anaesthesia’ [2]. Other than chemosis, superficial haemorrhage and conjunctival haematoma, more serious complications are very uncommon [2, 3]. Literature search revealed no cases where trabeculectomy was abandoned due to the volume effects of 5 ml of local anaesthetic. Standard texts suggest between 3 and 5 ml for sub-Tenon's injection but do not enlarge on when to use this range of volumes [2, 4]. Ripart [5] states that 4 ml is sufficient to surround the globe and provide akinesia with a sub-Tenon's medial canthus injection. Finally, in Guise’s prospective study of 6000 blocks [3], the average injectate for all sub-Tenon's injections is 3.8 ml (range 3–8 ml). The following factors suggest that the volume of local anaesthetic could have been reduced in our case: oriental background, glaucoma, small stature and small apparent volume of globe that was deeply set in the orbit. The patient's background is an important consideration as the Asian orbit is shallower than the Caucasian orbit, (on average 48.3 mm vs 50.0 mm deep) which produces the smaller orbital volume [1]. We know from Ripart’s study, on eight cadaveric eyes, that 4 ml of radio opaque dye completely fills the sub-Tenon's space on CT scanning, and that any volume above 4 ml overflows the sub-Tenon's space [5]. In terms of the pressure effects, Riemann et al. [6] found that 5 ml of retrobulbar local anaesthetic increases tissue tension in the orbit from 4.4 (SD 2.2) mmHg pre-injection to 12.0 (SD 3.6) mmHg, declining to 6.7 (2.2) mmHg after 5 min. Orbital volume and ethnic origin are not recorded in either of these studies and no comments are made on the effect of orbital volume either on filling the sub-Tenon's space or the increased tissue tension. It is possible, that our patient has a shallow orbit and small deep set eyes that contributed to the overfilling of the orbit.