Sub-Tenon block


Contributions by Dr Philip Guise


Introduction: The sub-Tenon block is arguably the eye block of choice when ocular akinesia is desired. Sub-tenon's block is associated with sensory and motor block intensity as good as retrobulbar and peribulbar block, but without the same complication risk. 12 Because sub tenon's block is performed with a blunt cannula rather than a sharp needle, it does not carry the same risk of retrobulbar haemorrhage or globe perforation (and inadvertent intraocular injection). 3

For cataract surgery, sub tenon's block has fallen in popularity over the last decade as less invasive non-akinetic methods for ocular anesthesia (topical, sub-conjunctival) have become feasible alternatives for this surgery. That said, akinetic blocks are still generally required for vitreoretinal surgery and other more major intra-ocular procedures e.g. pan-retinal photocoagulation.

With sub-Tenon's block, LA is injected posterior to the globe deep to the Tenon fascia - the fascia immediately surrounding the globe. 12

Skill level: moderate/advanced: mainly due to the fact the block requires an element of surgical skill. However, this skill can be easily taught to non-surgical trainees. 4


Procedure time: 2-5 mins (2 mins for experienced practitioners)


Common indications

Common – cataract surgery, vitreoretinal surgery, pan-retinal photocoagulation, extraocular procedures such as scleral buckling.


Sedation: e.g. midazolam 1-2 mg + alfentanil 250-500 mcg depending on the level of consciousness required at the start of surgery.


Anatomy: The globe is enveloped by a thin membrane called Tenon's capsule, which separates the globe from the surrounding orbital fat (Fig. 1). The sub-Tenon's space is a potential space under Tenon's capsule, which separates the capsule from the sclera. This space provides a conduit for the passage of a blunt cannula around to the posterior aspect of the globe for deposition of LA. However, multiple delicate bands of connective tissue cross the space from the fascial sheath to the sclera, which can necessitate blunt dissection to enable successful advancement of the cannula. Anteriorly, Tenon's capsule is firmly attached to the sclera about 5 mm lateral to the limbus (corneoscleral junction). Posteriorly, the capsule fuses with the sclera and meninges around the exit of the optic nerve from the globe.

LA injection deep to Tenon's capsule blocks afferent sensation from the eye through blockade of the short ciliary nerves as they penetrate Tenon's capsule to the globe. Concomitant blockade of anterior motor nerve fibres as they enter the extraocular muscles results in globe akinesia. Vision may also be blocked by LA action on the optic nerve.

The inferonasal aspect of the globe is free of the overlying extraocular muscles (Fig. 2) and vortex veins, so is therefore the preferred path under Tenon's capsule to the posterior orbit.

 

Tenon's capsule immediately surrounding the globe

                                        

Fig. 1. Sub Tenon's block: tenon's capsule immediately surrounding the globe.



 

                     Extraocular muscles. The inferonasal aspect of the globe has no overlying muscle                                        

Fig. 2. Extraocular muscles relevant to sug Tenon block. The inferonasal aspect of the globe has no overlying muscle.

 


Surface landmarks: Mark the forehead to be blocked.


Needle: 19G blunt curved metal cannula (e.g. sub-Tenon's cannula)


Setup (Fig. 3)

1. Sub-Tenon's cannula (e.g. 19G 2.54 cm blunt ended sub-Tenon's metal cannula)

2. Blunt (Moorfield's) scissors (avoid using the sharp ended type, as these will convert the technique from a blunt to a sharp technique, and therefore carry a similar complication risk as retro/peribulbar block)

3. Lid retractor

4. 5 mL syringe

5. 5% povidone-iodine.

6. Topical LA drops (e.g. benoxinate)

  

Instruments required for the sub-Tenon block

Fig. 3. Instruments required for the sub-Tenon block.



Procedure (Video 1)

1. First instill LA onto the conjunctiva and cornea (e.g. using amethocaine/benoxinate drops). Wait > 2 mins.

2. Insert lid retractor (Fig. 4).


lidretractor

Fig. 4.Sub-Tenon block: lid retractor insertion.


3. Instill 5% povidone-iodine into the inferonasal cornea/conjunctiva (site of the subsequent incision).

4. While asking patient to look upwards and outwards, with non-toothed forceps, grip Tenon's capsule and the conjunctiva as far laterally as practicable (at least 5 mm from limbus). Laterally, Tenon's capsule is less adherent to the underlying sclera and therefore more easily lifted. The gripped tissue is then retracted a mm or so to lift from the underlying sclera (Fig. 5).


forceps

Fig. 5. Sub-Tenon block: gripping and lifting the conjunctiva and Tenon's capsule. Choose a point as far laterally as practicable. Scleral indentation with the forceps to achieve a good grip of Tenon's capsule is not apparent from the image.


5. With the Moorfield's scissors, make a small incision through the retracted conjuctiva/Tenon's capsule. Ideally, the dull looking sclera should then become visible (Fig. 6).


moorfieldscut

Fig. 5. Incission through the conjunctiva and Tenon's capsule for sub-Tenon block.


6. Through this small incision (approx. 0.5 cm width), advance the closed scissors under the fascial layers and around to the back of the globe (Fig. 6). If resistance is encountered, apply firm pressure with the scissors until the resistance breaks. If resistance still persists (representing probable bands of connective tissue), consider using blunt dissection to create a tract under Tenon's capsule. Blunt dissection involves partially opening the scissors without advancing, then advancing while closed followed again by partially opening without advancing.


moorfieldstract

Fig. 6. Sub-Tenon block: passage of the Moorfield's scissors under Tenon's capsule around the globe to the posterior orbit. 


7. Swap the scissors for the sub-Tenon's cannula, and with the two fascial layers still retracted with the forceps, advance the cannula (through the incision and the tract previously created) to the back of the globe.


syringe

Fig. 7. Sub-Tenon's block: advancement of the sub-Tenon cannula to the posterior orbit.


8. Deposit 3.5-5 mL of LA (Fig. 8).


inject

Fig. 8. LA injection during sub-Tenon's block.


9. To minimise conjunctival haemorrhage, with the lids closed, apply gentle digital pressure over the site of the incision for 2 mins (Fig. 9). 5


digitalpressure

Fig. 9. Digital pressure.


finish

Appearance of the eye after sub-Tenon's block. Note conjunctival vasodilation but minimal haemorrhage.


Local anesthetic regimen: 3.5-5 mL lidocaine 2% or bupivacaine 0.5% (or a mixture of both) + hyaluronidase 50 IU/mL..

 

Specific Complications

1. Retrobulbar haemorrhage.

2. Intraocular perforation/injection.

Both are exceedingly rare when using blunt forceps and cannula.

3. Extraocular muscle trauma. Very unlikely when keeping to the inferonasal quadrant.

 

Clinical PEARLS

1. To maximise the chances of gripping Tenon's capsule with the forceps, firmish pressure should be applied to the sclera with the open non-toothed forceps (until there is a visible "dent" in the sclera) before grabbing (closing the forceps and then retracting) the conjunctiva/Tenon's capsule.

2.  Slow LA injection will lessen injection discomfort.

3. While injecting LA, rotate the cannula 30 degrees laterally then superiorly to promote adequate spread.

4. If chemosis (subconjunctival oedema) occurs, the cannula was probably not advanced far enough posteriorly. Closing the lids and applying gentle circular digital massage for 1 min can be helpful in spreading the anterior LA posteriorly.


Videos


Video 1. Sub Tenon block.The initial attempt at cannula advancement revealed an unsatisfactory tissue plane (? superficial to Tenon's capsule). Blepharospasm on cannula insertion can occur but is not typical. This patient received midazolam 2 mg + alfentanil before block placement. Immediately after surgery, the patient had no recollection of pain during the block.

 


References


1. Stevens JD. A new local anesthesia technique for cataract extraction by one quadrant sub-Tenon's infiltration. Br J Ophthalmol. 1992;76:670-4.

2. Kumar CM, Williamson S, Manickam B. A review of sub-Tenon's block: current practice and recent development. Eur J Anaesthesiol. 2005;22:567-77.

3. Kumar CM, Dowd TC. Complications of ophthalmic regional blocks: their treatment and prevention. Ophthalmologica. 2006;220:73-82.

4. Guise PA. Sub-Tenon anesthesia: a prospective study of 6,000 blocks. Anesthesiology. 2003;98:964-8.

5. Fredrickson MJ, Mantell NM, Watson AS, Vincent AL. A simple technique to minimize conjunctival haemorrhage following sub-Tenon's block. Clin Experiment Ophthalmol. 2007;35:685.