A modified sub-Tenon's cannula

More than a hundred years ago, sub-Tenon's anesthesia was used by Turnbull to perform enucleation. Nevertheless, retrobulbar injection has become the preferred technique of local anesthesia for ocular surgery.

Though it is easy to learn and easy to perform, it has numerous unavoidable complications. The report of the Joint Working Party on Anesthesia in Ophthalmic Surgery by the Royal College of Ophthalmologist (March 1993) has shown that life threatening administered and serious complications occurred in 1:360 cases.

Julian D Stevens introduced a curved sub Tenon Cannula in 1993 for placement of local anesthesia in the sub-Tenon's. This has popularized the technique of sub-Tenon's anaesthesthesia in recent years. I have designed a modified sub-Tenon's cannula for the placement of anesthetic solution into sub-Tenon's space.

A modified, sub-Tenon's cannula for delivery of sub-Tenon's local anesthetic solution, was manufactured from a standard disposable a 24G needle.

1. It has a straight portion and a curved portion.
2. The straight portion is 5 mm long and the curved portion is 27 mm long.
3. The straight portion is tangential to the curved portion.

The advantages of the modification are:

1. The straight portion of the cannula is tangentially angled to the curved portion. The junction of the two portions will help the surgeon to know when to stop the entry of the cannula into the sub-Tenon's space and where the tip of the cannula will be.
2. The curved portion is designed to be congruous with the curvature of the eyeball so that this portion will always remain in the sub-Tenon's space and will precisely deliver the anesthetic solution in the sub-Tenon's space.
3. The length of the curved portion is 27 mm long so that it will be about 5 mm away from the optic nerve, and will not damage the optic nerve.
4. By virtue of it curvature, it will penetrate the sub-Tenon’s space very smoothly.

Bore

24G

Straight portion

5 mm

Curved portion

27 mm

Radius of curvature

14 mm

Eyeball radius (avg)

12 mm

1. The bore of the cannula is 24G. The 24G is sufficiently wide to easily deliver the anesthetic solution.
2. The tip is blunt and slightly flattened. The tip is blunt so that it will not penetrate the eye ball. The flat tip facilitates the smooth entry of the cannulaa into the sub-Tenon's space.
3. The straight portion is 5 mm long. The straight portion is 5 mm long so that it will be stiff and long enough to visualize this portion when the curved portion of the cannula is completely in the sub-Tenon’s space.
4. The radius of curvature of the curved portion is 14 mm. As the average radius of the eye ball is 12 mm, it helps the cannular to almost hug the eye ball and remains precisely in the sub-Tenon's space.
5. The curved portion is 27 mm long. When it enters at a point 5 mm from the limbus, in an average eye, the tip will be 5 mm away from the optic eye.
6. The junction between the straight and the curved portion indicates the exact point at which the penetration of the cannula should be stopped.
7. When the straight portion of the cannula (and the syringe) is radial to the eye ball and is 5 mm away from the limbus, it indicates that the curved portion of the cannula is in the right tissue plane (sub-Tenon’ space about 5 mm away from the optic nerve).

A 3 ml syringe was loaded with lignocaine 2% 1.5ml, Marcaine 0.5%ml and 0.25ml of Garamycin (10mg). The cannula was fitted to the syringe and kept aside.

1. The eye was prepared in the routine manner.
2. The lid speculum was applied.
3. Amethocaine 1% drop was applied every minute for 5 min into the conjunctival sac.
4. The patient was instructed to look upwards and outwards. In the infra-nasal quadrant, 5 mm away from the limbus, between four and five o’clock position, the conjunctiva was cauterized over an area of about 2 mm in diameter (Fig. 1).
5. The cauterized conjunctiva was held with a corneal forceps, and a nick was made in the conjunctiva with a Wescott-syle scissors. This exposed the pearly white Tenon’s facia.
6. The Tenon’s facia was held with a toothed corneal forceps. A nick was made in the Tenon’s facia. The gap in the Tenon’s facia exposed the bare sclera. About 5 mm of the tip of the scissors was introduced into the opening of the Tenon’s space. This facilitated introduction of the cannula into the sub-Tenon’s space.
7. The cannula was positioned in such a way that the curvature of the cannula conformed with curvature of the eye ball. The tip of the cannula was placed in the opening (Fig. 2). If the eye ball moved, the eye ball was stabilized by holding firmly with a toothed forceps, close to the limbus at either four or five o’clock position.
8. The cannula was gently pushed making sure that the tip of the cannula was in close proximity to eye ball as it was pushed behind the eye ball.
9. When the cannula was pushed in, there was resistance in some eyes, owing to scleral-Tenon bridging fibres near the equator of the globe. When there was any resistance, a small amount of the anesthetic solution was pushed in. This hydrodisected the resisting tissue. Then the whole curved part of the cannula was pushed into the sub-Tenon’s space. At this point, the remaining 0.5mm straight portion of the cannula and the syringe were radial to the eye ball. The anesthetic solution was gently emptied into the sub-Tenon’s space. In some eye, the initial resistance was high. Once the solution entered, the resistance became less. The solution was pushed slowly and gently till the syringe was empty. The cannula was pulled out gently in the curved path it entered. (Fig. 3)
10. The surgery was performed after 10 minutes.

Fifty patients were administered sub-Tenon’s anesthesia for the following surgeries: extra capsular cataract extraction with IOL implant (45), trabeculectomy (four), evisceration (one).

The administration of the anesthesia was painless. The anesthesia was excellent. At the beginning of surgery akinesia was incomplete in all the patients, especially the superior oblique muscle.

In 22 cases, slight action of the superior oblique was noted even after the surgery was over. The residual ocular movement did not interfere with surgery. There were no complications due to anesthesia.

There are already a few sub-Tenon’s cannula available for use. The features and benefits of this cannula over other cannulae are:

1. It is curved (radius of curvature 14 mm) in such a way that, when it is pushed into the sub-Tenon’s space it will remain in the sub-Tenon’s space. This will avoid damage the optic nerve ocular muscles, and the blood vessels.
2. The curved portion is 27 mm long. When the cannula is inserted at 5 mm from the limbus, it will be about 5 mm away from the optic nerve. Even in relation to the smallest eye ball, the tip will be 5 mm away from the optic nerve and will not damage it. In longer eye balls, the tip will be behind the equator so that it will not produce messy chemosis.

The straight position which is tangential to the curved position indicates the point at which the penetration should stop. This enables the surgeons to know when to stop the penetration into the sub-Tenon’s space.

This cannula is easy to use and will avoid complications which can be anticipated from other sub-Tenon’s cannulae.

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5. Report of the Joint Working Party on Anesthesia in Ophthalmic Surgery; Royal College of Anaesthetist — College of Ophthalmologists, March 1993.
6. Steven JD. A New Local anesthesia technique for Cataract extraction by one quadrant sub-Tenon’s infiltration; British Journal of Ophthalmology 1992, 76:670–674.
7. Hideharu Fukasaku, MD., James A. Marron, MD. Pinpoint anesthesia: A New approach to local anesthesia. J. Cataract Refract Surg Vol 20, July 1994.