Sub-Tenon’s Block

A sub-Tenon’s (episcleral) block:

Indications

Anaesthesia and akinesia of the globe, in a patient with:

  • ↑ axial length
  • Presence of anticoagulant or antiplatelet agents
    Sub-tenons may be preferred by some providers, for a potentially lower risk of significant haematoma.

Contraindications

Surgical factors:

  • Trabeculectomy
    May reduce operative success.
  • Previous sub-Tenon block in the same quadrant
    Adhesions may reduce efficacy of hydrodissection.
  • Complete akinesia required
  • Where chemosis or haemorrhage may reduce outcome
    Glaucoma filtration surgery.

Relative contraindications:

  • Eye trauma
  • Orbital infection
  • Ocular pemphigoid

Anatomy

The orbit:

  • Takes a truncated pyramid shape
  • Contains mostly fat
  • Is divided into three parts, based on their contents:
    • Intraocular compartment contains the globe:
      • Lies anteriorly
      • Equator of the globe lies at the lateral border.
    • The intraconal compartment contains the six extraocular muscles
      The inferior and lateral rectus muscles sit at inferiorly and nasally respectively, so the greatest gap between them lies is inferonasally.
      • The four rectus muscles form the retrobulbar cone
        Contact the walls of the orbit.
        • The intraconal space lies within these muscular attachments
          Intraconal injection of local anaesthetic produces anaesthesia and akinesia of the globe and extraocular muscles.
        • The extraconal space is a potential space lying between these muscles and the orbital wall
    • The extraconal compartment
      Consists of fat sitting between the extraocular muscles and the bony walls of the orbit, and communicates freely with the intraconal component.

The globe is:

  • Spherical
  • Covered in three layers:
    • Cornea and sclera
      • Cornea is translucent and covers the iris
      • Sclera is opaque, and covers the rest
        The circumferential junction of the cornea and the sclera is called the limbus.
    • Tenon’s capsule covers the scleral portion
      Connective tissue layer acts as the fascial sheathe of the globe. The capsule:
      • Lies deep to the conjunctiva
      • Superficial to the sclera
      • Anteriorly merges ~1mm with the limbus
      • Posteriorly merges with the optic nerve sheathe
  • Attached to each of the extraocular muscles
  • Divided into the anterior and posterior segments:
    • Anterior segment
      Small, containing:
      • Aqueous humor
    • Posterior segment
      Large, containing:
      • Lens
      • Vitreous
      • Retina
      • Macula
      • Optic nerve
    • Segments are separated by the iris, and communicate via the pupil

Equipment

  • Gauze
  • Antiseptic
    Typically 50% saline with 50% aqueous betadine
  • Topical local anaesthetic drops
  • Mix LA solution
    Many different options exist.
    • Lignocaine/Ropivacaine
      • Take 40mls of 0.75% ropivacaine, 10ml 10% lignocaine, and 1500 units of hyaluronidase
      • Into a 50ml syringe, draw up the local anaesthetics and mix in the hylauronidase
      • This gives a combination of 0.8% ropivacaine, 2% lignocaine, and 30U/ml hyaluronidase
        Hyaluronidase concentrations of 30U/ml or greater are associated with periorbital cellulitis.
    • Ropivacaine alone
      0.75-1% ropivacaine with 75-150 U/ml hyaluronidase.
  • Eye speculum
  • Fine non-toothed (Moorfields) forceps
  • Westcott Scissors
    Preferably spring scissors.
  • Sub-Tenon cannula

Technique

Assumes monitoring, IV access, time-out, topical local anaesthetic drops, and sedation have been performed, as discussed here.

The surgical approach involves:

  • Topical LA drops
    Onto intranasal quadrant as this is more comfortable.
  • Apply antiseptic
    Ensure there is some antiseptic under the lower lid and in the conjunctival sac.
  • Ask the patient to look directly up and out
  • Perform block:
    • With small forceps, push in and then grab the inferonasal quadrant of the globe 5-10mm from the limbus
      The infranasal quadrant is preferred as there are no muscle insertions and it is not used for a surgical approach.
    • Keep pressure on the globe with the forceps throughout the procedure
    • Use the scissors to create a linear incision in the Tenon’s capsule
      Bring the scissors in vertically to create a straight incision. A curved incision creates a conjunctival flap, and may cause a cyst.
    • Perform blunt dissection with the scissors past the equator of the globe
    • Insert a blunt cannula into the episcleral space, tangential to the globe
    • Inject LA
      • 3-5ml typically produces good analgesia but only partial akinesia
      • 8-11ml produces akinesia via spread of LA to extraocular muscle sheaths
  • Remove speculum

Complications

Mechanism of complications are similar to peribulbar blocks, but are typically much less common.

Minor complications:

  • Chemosis
    Almost universal.
  • Pain on injection
  • Subconjunctival haemorrhage

Major complications:

  • Brainstem anaesthesia
  • Globe perforation
  • Retrobulbar haemorrhage
  • Retinal ischaemia
    May occur in absence of retrobulbar haemorrhage, and result in permanent visual loss.
  • Optic nerve damage
  • Rectus muscle dysfunctikon
    May be due to direct trauma and haemoatoma formation, leading to muscular contracture.

References

  1. Ripart J, Mehrige K, Rocca RD. Local & Regional Anesthesia for Eye Surgery. NYSORA.
  2. Olive, D. Sub-Tenon’s Eye Block Version 2. Anaestricks. 2013.
  3. Kumar CM, Eid H, Dodds C. Sub-Tenon’s anaesthesia: complications and their prevention. Eye. 2011;25(6):694-703.
  4. Chuan A, Scott DM. Regional Anaesthesia: A Pocket Guide. 1st Ed. Oxford University Press