A variety of ophthalmic surgeries can be performed with retrobulbar anesthesia. The list of procedures includes cataract surgery, many types of corneal surgeries, pterygium surgery, and eye muscle procedures. A retrobulbar injection of an anesthetic agent into the orbital tissue behind the eye is often used as an anesthetic block so that patients have decreased sensation and limited mobility of the eye. The anesthetic effect can last after surgery to produce extended post-surgical pain relief.
Although many anesthesiologists are trained to perform retrobulbar anesthetic blocks, in clinical practice the task is usually delegated to the ophthalmologist. The ophthalmologist performs this type of block just before surgery. Immediately prior to the retrobulbar block, the anesthesiologist often provides sedation to make the block tolerable and comfortable for the patient.
The goal of a retrobulbar block is to block the ciliary ganglion, ciliary nerves, and cranial nerves II, III, and VI. Cranial nerve IV is not affected since it located outside of the muscle cone. Cranial nerve IV, also known as the Trochlear nerve, as responsible for rotational movement of the eye.
The anesthetic is delivered within the muscle cone or it may infiltrate the muscle cone when placed within the orbit. The ciliary ganglion, a parasympathetic ganglion, is located adjacent to the lateral border of the optic nerve and between the back of the eye and the posterior wall of the orbit. Parasympathetic nerve fibers originating in the oculomotor nerve and postganglionic fibers innervate the ciliary body and pupillary sphincter muscles.
The nasociliary nerve, a branch of the ophthalmic nerve, supplies sensory innervation of the cornea, iris, and ciliary body through the short ciliary nerves. These nerves are small filaments that travel with the ciliary arteries.
METHOD OF RETROBULBAR BLOCK
In adults, a 35 mm length needle with a 25 gauge diameter is used to perform a retrobulbar injection. A needle that is longer risks injury to the blood vessels in the back of the orbit.
Performance of a retrobular injection may include the following steps:
1) Palpation of the orbital rim.
2) Insertion of the needle through the skin and through the inferior orbital space. The needle is placed adjacent to the orbital rim.
3) The needle is advanced slowly, being careful not to touch or injure the eyeball. The surgeon usually observes movement of the eyeball to monitor for inadvertent touch by the needle.
4) The needle should penetrate peribulbar fat and the intermuscular septum. Resistance of the needle may indicate placement in the muscle, optic nerve or wall of the eye. If this happens, the needle should be withdrawn and repositioned.
5) The syringe is aspirated before injection to confirm that the needle is not in the subarachnoid space. Aspiration of turbid fluid is indicative of cerebrospinal fluid. Aspiration of blood indicates entry of the needle into a blood vessel.
6) The anesthetic is injected when the needle is at its deepest level. Attention is directed to the eye and adjacent tissue to monitor for excessive anesthetic and swelling of tissue.
7) Some surgeons gently massage the eye for a few minutes with the eyelids closed. This helps facilitate the spread of anesthetic.
The most commonly used local anesthetic agents are 2% lidocaine or bupivicaine. Many surgeons prefer the addition of epinephrine or hyaluronidase (Vitrase) to the anesthetic. Hyaluronidase assists in the spread of anesthetic to maximize its effect.
COMPLICATIONS OF RETROBULBAR BLOCKS
1) Oculocardiac Reflex: Bradycardia, junctional heart rhythm, or cardiac asystole can occur when there is pressure on the eye and ocular muscles. This is known as the oculocardiac reflex. This happens infrequently, occurring in less than 1 in 100,000 cases. Hypoxia, hypercarbia, light anesthesia, and eye massage can influence this reflex. Gentle eye massage is aborted when the oculocardiac reflex is detected, to avoid exacerbation of bradycardia. The anesthesiologist may administer intravenous atropine to treat bradycardia.
2) Retrobulbar Hemorrhage. This is one of the more common complications associated with retrobulbar injections and results from inadvertent puncture of vessels within the retrobulbar space. It is commonly recognized when, despite an excellent block, progressive swelling of the eyelids results in closure of the eyelids. Bleeding behind the eye can cause the eye to bulge forwards (proptosis) and an increase in intraocular pressure. Subconjunctival blood and eyelid ecchymosis may be seen as the hemorrhage extends anteriorly. Retrobulbar hemorrhage can lead to other complications including central retinal artery occlusion, and stimulation of the oculocardiac reflex. Surgery is usually discontinued when this complication is recognized. A lateral canthotomy may be considered by the surgeon to reduce pressure on the eye from excessive bleeding.
3) Central Retinal Artery Occlusion. Retrobulbar hemorrhage can result in compression of the the central retinal artery. A lateral canthotomy and/or an anterior chamber paracentesis may be considered to decompress the tissues around the eye to alleviate excessive pressure. Central retinal artery occlusion can also occur if the dura surrounding the optic nerve is penetrated and local anesthetic injected into the subarachnoid space.
4) Puncture of the Eyeball (Globe). This complication occurs when the needle used to administer anesthetic inadvertently enters the eyeball. This complications is more likely to occur in patients with severely myopic eyes because of their additional length and thin outer shell. Patients usually experience significant pain when the eyeball is penetrated with an anesthetic needle. Intraocular hemorrhage and retinal detachment may develop with needle puncture of the eyeball. To help avoid this complication, a surgeon may elect to use an anesthetic needle with a blunt tip. The most common variety of this needle is known as an Atkinson needle.
5) Penetration of the Optic Nerve. The optic nerve may be inadvertently penetrated by the retrobular needle. Direct injury to the optic nerve can result in permanent visual loss. Needle injury to the sheath that encapsulates the optic nerve can result in bleeding with nerve compression. Prolonged and elevated nerve compression can result in ischemia of the optic nerve. When this unfortunate complication happens, patients develop recognizable progressive optic atrophy and loss of vision.
6) Accidental Brain Stem Anesthesia. Inadvertent penetration of anesthetic from a retrobulbar injection into the cerebrospinal space can result in brain stem anesthesia. The patient may exhibit disorientation, altered breathing, decreased heart rate, loss of consciousness, and hemiplegia.
Fortunately, this complication is rare. However, it is important for this complication to be recognized as soon as possible so the proper treatment and monitoring may be administered. The anesthesiologist usually provides cardiac and respiratory support until the anesthetic agent has worn off.
7) Epinephrine reaction. Epinephrine in retrobulbar injection constricts surrounding blood vessels to minimize inadvertent bleeding. However, epinephrine that is inadvertently injected into a blood vessel may cause tachycardia, an irregular heart rate, or angina. This is of particular concern in patients with systemic hypertension or heart disease.
8] Allergic complications. Allergic reactions to local anesthetics or any of their accompanying additives are rare. Allergic reactions are typically recognized by acute swelling of orbital tissues.
9) Conjunctival edema (chemosis). A portion of anethetic may dissect along tissue planes and into the subconjunctival space. The resulting edema along the surface of the eye usually resolves quickly and without adverse effects on vision.
10) External bruising. Superficial vessels in the skin may be affected by the retrobulbar needle. Generally, this does not pose a significant problem. Depending on the amount of bleeding, post operative bruising usually clears within a few days to a few weeks.
CONTRAINDICATIONS TO RETROBULBAR ANESTHETIC BLOCKS
1) Lengthy procedure lasting more than one hour. Retrobular injections wear off gradually. Depending on circumstances, patients who undergo prolonged procedures may require additional local anesthetic or general anesthesia.
2) Patients who have an uncontrollable cough. Uncontrollable cough can result in excessive movement that makes performing eye surgery difficult or impossible.
3) Patients with uncontrollable movement. Patients who have chronic tremors or a movement disorder may also pose a problem for the surgeon who relies on steadiness of position to carry out surgery. Taping of the head to the headrest may be considered to minimize undesirable movement.
4) Emotional distraction. Patients who have claustrophobia or high anxiety can have unpredictable behavior while under the closed environment of surgical drapes.
5) Inability to assume a proper position.
Retrobulbar anesthetic blocks are usually very safe and effective. However, there are risks and potential complications from virtually any type of anesthetic block. Complications from retrobulbar blocks need to be properly recognized and treated appropriately to help prevent permanent injury and vision loss.