Fine cannula technique for sub-Tenon’s injection for ophthalmic anaesthesia

Sir,

Although various methods of local anaesthesia are in use for ophthalmic surgery, sub-Tenon’s anaesthesia is the most popular due to its safety and efficacy. A variety of cannulae have been described for this technique, which vary in guage (G), material, and length. The most popular is a 25 mm, 19 G, curved, blunt, metal cannula first described by Stevens.¹

Minor complications such as conjunctival chemosis and haemorrhage are common with the standard sub-Tenon’s block. Although these rarely present a problem for routine cataract surgery, they may interfere with glaucoma surgery and cosmesis in the weeks following surgery. In our experience, long-term scarring and consequent discomfort may also be associated with the conjunctival incision.

Inserting the anaesthetic cannula directly into sub-Tenon’s space without a prior conjunctival incision can reduce conjunctival damage. This also leads to a better, more reproducible block, due to less reflux of anaesthetic from a small insertion site. It also requires fewer instruments. Although incisionless techniques have been reported, we describe a modified incisionless technique using a finer, cheaper, more readily available cannula (26 G, 28 mm lacrimal cannula (Surgistar Inc., Vista, CA, USA) (Figure 1).

Figure 1.

Modified incisionless sub-Tenon’s block using the 26 G, 28 mm lacrimal cannula (Surgistar Inc., Vista, CA, USA).

Between October 2015 and March 2016, local anaesthesia for consecutive routine cataract surgery was administered by inserting a lacrimal cannula into the inferonasal sub-Tenon’s space without a prior conjunctival incision. Pain at each stage of the procedure was graded on a visual analogue scale (0–4). Measures of anaesthetic safety and efficacy were graded from 0 to 3.

Of 32 patients (10 males, 22 females), 57% developed conjunctival chemosis, 53% developed subconjuctival haemorrhage, 16% described pain during anaesthetic administration, 6% described pain during the procedure, and 87% had complete akinesia. The surgeon assessed the block as being ‘excellent’ in 93% of cases. There were no major complications and the majority of subconjunctival haemorrhage and chemosis involved only 1 quadrant (63 and 65%, respectively).

Our results show that our modified incisionless sub-Tenon’s block produces excellent anaesthesia and akinesia with mild conjunctival haemorrhage and chemosis. The absence of a relatively wide conjunctival incision may also reduce post-operative scarring and discomfort.

We feel that, out of all sub-Tenon’s blocks described, this technique causes the least trauma to the conjunctiva and the least reflux of anaesthetic, due to the small insertion site. This, in theory, should lead to better anaesthesia/akinesia and fewer complications. It is also cost-effective and uses a cannula that is readily available in all ophthalmology departments.

Our technique compares favourably with published data on the safety and efficacy of the standard sub-Tenon’s block (Table 1).^{1, 2, 3, 4}

Table 1. A comparison of the safety and efficacy of the sub-Tenon's block in different studies.

	Stevens1	Roman et al2	Guise3	Kumar et al4 (3 cannulas)	El-Khayat et al (present study, 2017)
Pain during block (%)	18	1	32		16
Pain during surgery (%)	4	3	7	0/0/0	2
Complete akinesia after block (%)	54	0		46/50/46	87
Complete akinesia after surgery (%)		0			59
Chemosis (%)		85	56	76/20/32	57
Subconjunctival haemorrhage (%)	34% (>1 Quadrant)	56	7	56/20/20	53

Limitations of this technique include pre-operative conjunctival scarring that makes insertion of the cannula without a prior incision difficult.

To our knowledge, there are no studies that look at the long-term scarring or discomfort from the conjuctival incision in standard sub-Tenon’s anaesthesia. Future studies looking at this and comparing scarring with incisionless techniques may further make the case for transitioning to incisionless techniques.