Blood vessels: The main arterial supply to the globe and orbital contents is from the ophthalmic artery which is a branch of the internal carotid artery and passes into the orbit through the optic canal inferolateral to the optic nerve and within the meningeal sheath of that nerve. In the elderly and hypertensive patient it is tortuous and vulnerable to needle trauma when it may bleed profusely. Venous drainage is via the superior and inferior ophthalmic veins.

The lacrimal apparatus has orbital and palpebral components. The orbital part lies in the lacrimal fossa on the anterolateral aspect of the orbital roof, and the palpebral part is situated below the levator palpebrae superioris aponeurosis and extends into the upper eyelid secreting tear fluid into the superior conjunctival fornix.

Lacrimal drainage occurs through superior and inferior lacrimal puncta near the medial ends of both lid margins which form entrances to the 10-mm long lacrimal canaliculi medially passing through the lacrimal fascia to enter the lacrimal sac. The nasolacrimal duct connects the inferior end of the lacrimal sac to the inferior meatus of the nose.

 
Types of regional anaesthesia for ophthalmic surgery:

• Peribulbar block (Pericone block)
• Retrobulbar block (Intracone block)

The most popular technique for regional anaesthesia in eye surgery is now a peribulbar block. This has largely replaced retrobulbar blocks and general anaesthesia for many types of eye surgery.

Preparations

1. An intravenous cannula is inserted to allow immediate venous access in case of emergency.

2. The conjunctival sac is anaesthetised with amethocaine 1%. Three drops are instilled and this is repeated 3 times at 1 minute intervals.

3. A 10ml syringe is prepared containing 5 ml bupivacaine 0.75% plain plus 5ml lignocaine 2% with 1:200,000 adrenaline. Hyaluronidase 75 units is added to improve diffusion of the anaesthetic mixture within the orbit, giving faster onset and prolonged duration of anaesthesia.

4. A 25 gauge, 2.5 cm disposable needle is attached to the syringe.

5. The patient lies supine and is asked to look directly ahead focussing on a fixed point on the ceiling, so that the eyes are in the neutral position.

Performance of the block

Two transconjunctival peribulbar injections are usually required.

Inferotemporal injection (figs 3 - 5). The lower lid is retracted manually and the the needle is placed half way between the lateral canthus and the lateral limbus.

The injection is not painful as it is passing through an already anaesthetised conjunctiva. If there is not enough room for the needle to be positioned correctly then the injection may be made directly through the skin. The needle is advanced in the sagital plane, parallel to the orbital floor passing under the globe. There is no need to apply pressure to the syringe as it will easily advance without resistance.

When the needle tip is judged to be past the equator of the globe the direction is changed to point slightly medial (20°) and cephalad (10° upwards) to avoid the bony orbital margin. Advance the needle until the hub (which is at 2.5 cm) is at the same depth as the iris. Following negative aspiration 5 ml of the solution is slowly injected. There should not be any resistance while injecting. If resistance is encountered, the tip of the needle may be in one of the extraocular muscles and should be repositioned. During the injection the lower lid may fill with the anaesthetic mixture and there may be some conjunctival oedema.

Within 5 minutes of this injection, some patients will develop adequate anaesthesia and akinesia (lack of movement), but the majority will require another injection.

(Continued...)

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