Surgery involving the cornea, anterior chamber, and lens can be performed with a retrobulbar or peribulbar block.
Retrobulbar block involves depositing local anesthetic inside the muscle cone. It aims to block the ciliary nerves, the ciliary ganglion, and cranial nerves III, IV, and VI. The ciliary ganglion is a parasympathetic ganglion, which lies about 1 cm from the posterior boundary of the orbit between the lateral surface of the optic nerve and the ophthalmic artery.
Older references describe the retrobulbar block as only affecting nerves “within the cone”, however Barash states that “Cadaveric dissections, however, have shown the fallacy of the classic concept of the cone.” (Barash 5/e p985)
A retrobulbar block is deeper and uses less volume than a peribulbar block. A retrobulbar block does not anesthetize cranial nerve VII (facial nerve), which leaves the patient able to close the eye with the orbicularis oculi (CN VII) but not open it with the levator muscle (CN III).
Complications of retrobulbar blocks
1. Allergic reactions – usually occur with ester-type local anesthetics
2. Retrobulbar hemorrhage – This is the most common complication seen. It is characterized by a motor block, closing of the upper lid, and simultaneous sudden rise in intraocular pressure causing proptosis. Retrobulbar hemorrhage can lead to central retinal artery occlusion and stimulation of the oculocardiac reflex. Usually surgery is postponed. Hemorrhage is rare with peribulbar block.
3. Central retinal artery occlusion – Can result from retrobulbar hemorrhage or if the dura is penetrated and local anesthetic is injected into the subarachnoid space.
4. Subconjunctival edema (chemosis) – Can be minimized by slowing rate of injection, can interfere with suturing.
5. Penetration or perforation (through one side and out the other) of the globe – more likely to occur in a myopic eye which is longer, but also thinner than normal. Symptoms include pain at the time the block is performed, sudden loss of vision, hypotonic, or a poor red reflex.
6. Central spread of local anesthetic – due to either direct injection into the dural cuff near the optic nerve or retrograde arterial spread. Symptoms include drowsiness, vomiting, contralateral blindness, convulsions, respiratory depression, and cardiac arrest within 5 minutes of injection.
7. Oculocardiac reflex – bradycardia which can follow traction on the eye. Retrobulbar or peribulbar block ablates the oculocardiac reflex by blocking the afferent pathway (ciliary ganglion to ophthalmic division of trigeminal nerve), but this reflex can occur with block placement. It can also occur several hours later if it is secondary to an expanding hemorrhage. Therefore, patients with hemorrhage should be closely monitored.
8. Optic nerve atrophy – Caused by direct damage to the optic nerve secondary to injection into the optic nerve sheath or hemorrhage within the nerve sheath. Symptoms include partial or complete visual loss.
9. Postoperative strabismus – may be caused by intramuscular injection of anesthetic
Peribulbar block involves injections above and below the orbit, with local anesthetic deposited in the orbicularis oculi muscle. This technique blocks the ciliary nerves, as well as CN III and VI, but does not block the optic nerve (CN II). There is less potential for intraocular or intradural injection since the local anesthetic is deposited outside the muscle cone. This block is technically easier to place and the risk of hemorrhage within the muscle cone and direct injury to the optic nerve is decreased. It is more difficult to get a complete, dense block with peribulbar technique, but it is still widely used, given its lower complication rate.
Complications of peribulbar block
- Spread of local anesthetics to the contralateral eye
- Periorbital ecchymoses
- Transient blindness
Contraindications to Peribulbar and Retrobulbar Block
- Age less than 15
- Procedures lasting more than 90-120 minutes
- Uncontrolled cough or tremors
- Disorientation or mental impairment
- Excessive anxiety or claustrophobia
- Language barrier or deafness
- Bleeding or coagulopathies
- Perforated globe