SUMMER 2010

Comparative Survey of Efficacy and Complications between the Blunt Cannula and the Tri-Port Cannula for sub-Tenon’s Block

Khan J A (1), Glasgow D (1), Knox A (2), Orr D (1)
(1) Department of Anaesthetics and Intensive Care, (2) Consultant Ophthalmic Surgeon
Craigavon Area Hospital, Portadown, Northern Ireland, UK
Correspondence to drjameel@hotmail.co.uk

Introduction

Sub-Tenon’s block is a well established method of providing anaesthesia for cataract surgery. It is a safer and effective alternative to peribulbar block (1). It can be performed by using different cannulae (2, 3), the most commonly used is the 19 gauge blunt curved posterior sub-Tenon’s cannula (25mm long curved, with a flat profile and a single blunt end hole) (4). The “Tri-Port sub-Tenon cannula” is a 21 gauge Sub-Tenon cannula with a pencil point tip having three ports (1 central and 2 lateral) (5).

Methodology

A prospective survey of patients undergoing cataract surgery under sub-Tenon’s block was conducted between Feb – Apr 2010. The ethical committee was approached and a formal ethical committee approval was deemed unnecessary by the ‘Trust review board’. In our department, there is Consultant preference to the use of cannula for performing sub-Tenon’s block. This was a survey of the current practice at our department. Sub-Tenon’s blocks were performed mostly by Consultants and experienced trainees. The anaesthetist performing the block documented the details following the block. Surgical assessment was done by the Consultant surgeon prior to surgery and the results documented. Chemosis, ptosis and motor block were graded subjectively. Amount of local anaesthetic (LA) used and additional top-up were documented. Surgical assessment included chemosis and local bleeding, graded subjectively.

Statistical Methods

Descriptive statistical analysis has been carried out in the present study. Results on continuous measurements are presented on Mean ± SD (Min-Max) and results on categorical measurements are presented in Number (%). Significance is assessed at 5 % level of significance. Chi-square and Fisher Exact test has been used to find the significance of study parameters on categorical scale between two or more groups.

Results

A total of sixty patients were included in the survey with 35 in the Blunt cannula group and 25 in the Tri-Port cannula group. A total of 7 patients in the Blunt group and 3 patients in the Tri-Port group were not assessed surgically, hence excluded from surgical assessment.
In our survey, there was no statistically significant difference in the demographics (gender, age). There was a statistical significant difference in moderate chemosis, (P 0.002) with a higher incidence in the Tri-Port group. The amount of local anaesthetic (LA) used was statistically significant, with smaller amounts 4 and 4.5 mls used in Tri-Port group compared to 5 ml in the Blunt group. There was no statistically significant difference in the ptosis seen and additional top-up between the groups. There was a statistical significant difference in motor blockade being complete (P 0.03) in 36 % of patients in Tri-Port group and 11.4 % in the blunt group. There was no statistically significant difference in chemosis and sub-conjunctival bleed observed prior to the start of the surgery.

Table 1: Anaesthetic assessment

Table 2: Surgical assessment

+ Suggestive significance (P value: 0.05<P<0.10)
* Moderately significant (P value: 0.01<P £ 0.05)
** Highly significant (P value: P £0.01)
The Statistical software namely SAS 9.0, SPSS 15.0, Stata 8.0, MedCalc 9.0.1 and Systat 11.0 were used for the analysis of the data.

Discussion

The sub-Tenon’s block has a lesser incidence of complications like retrobulbar haemorrhage, optic nerve damage and perforation of the globe when compared to techniques using sharp needles (1, 6). The main difference between performing the sub-Tenon’s block with the blunt cannula and using the Tri–Port cannula is that in the former a prior incision of the Tenon’s capsule is required using the Westcott scissors. The use of Tri-Port needle appears to be less invasive, with a lower incidence of conjunctival trauma and good quality block (5). In our survey, we had a statistically significant incidence of moderate chemosis (anaesthetic assessment) in the Tri-Port group but no statistically significant difference when assessed surgically. This could have been due to the chemosis resolving over the period between performing the block and surgical assessment. Also, there is a likelihood of inter-individual difference in assessment. The Tri-Port cannula technique requires a smaller opening when performing the block, hence lesser volume of LA is lost and the amount of LA is required to produce the block is less. The Tri-Port group had a statistically significant complete motor blockade when compared to the Blunt group, most likely due to less loss of LA during the block from a smaller opening in the conjunctiva. None of the patients had major complications during performing the block.

References

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an efficient and safe technique. Br J Ophthalmol 1997; 81: 673–676

2. http://www.eyeanesthesia.org/newsletter/fall2006/Kumar.pdf

3. Kumar CM, Williamson S, Manickam B. A review of sub-Tenon‘s block: current practice
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4. Stevens JD. Curved, Sub-Tenon Cannula for Local Anesthesia. Ophthalmic Surg 1993;
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5. Allman KG, Theron AD, Byles DB. A new technique of incisionless minimally invasive
sub-Tenon's anaesthesia. Anaesthesia. 2008 Jul; 63(7): 782-3.

6. Ruschen H, Bremner FD, Carr C. Complications after sub-Tenon‘s eye block. Anaesth
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