Retrobulbar Block

Background

Retrobulbar block is type of regional anesthetic nerve block used in intraocular surgery. In this technique, local anesthetic is injected into the retrobulbar space, the area located behind the globe of the eye. This injection provides akinesia of the extraocular muscles by blocking cranial nerves II, III, and VI, which prevents movement of the globe. Retrobulbar block also provides sensory anesthesia of the cornea, uvea, and conjunctiva by blocking the ciliary nerves. ^[1]

Indications

Retrobulbar block is used for any ophthalmic surgery or procedure requiring globe anesthesia and akinesia.

Contraindications

Absolute contraindications include the following:

• Hypersensitivity or allergy to local anesthetic

• Orbital infection or significant inflammation

Relative contraindications include the following:

• Increased axial length of the globe

• Bleeding diathesis

• Thyroid-associated orbitopathy

• Space-occupying lesion within the orbit

• Previous scleral buckling

For increased axial length of the globe, many physicians will use 25 mm as a cutoff, although this exceeds the point at which a retrobulbar block should not be performed.

Many physicians will still perform a retrobulbar block even in the setting of aspirin or warfarin use.

Relevant Anatomy

Muscle Cone

The extraocular muscles form a cone about the globe. The apex is at the optic foramen and the base is formed by the insertions of the muscles to the globe.

A matrix of connective tissue forms the intermuscular septum. In addition to further separating extraconal and intraconal space, this controls the spread of anesthetic injected into the orbit.

Structures located within the cone (after passing through the annulus of Zinn) include the motor innervations to the rectus muscles (cranial nerves III and VI) and the afferent sensory fibers from the globe, which are carried by the short and long posterior ciliary nerves before joining the nasociliary nerve (a branch of cranial nerve V1).

Motor Innervation

Cranial nerve VI (abducens) innervates the lateral rectus muscle. Cranial nerve IV (trochlear) innervates the superior oblique muscle. Cranial nerve III (oculomotor) innervates all other extraocular muscles.

While cranial nerves III and VI pass within the cone, cranial nerve IV travels outside of the muscle cone to innervate the superior oblique muscle. It is likely affected by the retrobulbar block through diffusion of the anesthetic.

The motor nerves enter the rectus muscles on the intraconal surface.

Sensory Innervation

Afferent fibers from the globe travel via the long and short posterior ciliary nerves. These fibers pass through the intraconal ciliary ganglion.

The fibers then join the nasociliary nerve, which is a branch of the superior division of the trigeminal nerve (cranial nerve V1).

Equipment

A retrobulbar needle, also called an Atkinson needle, is commonly 23- or 25-gauge and 1.5 inches (38 mm) in length.

The needle is commonly placed on a 10-mL syringe.

Patient Preparation

Anesthesia

Intravenous sedation is useful to provide analgesia and amnesia, as well as to improve patient cooperation.

Short-acting agents such as propofol or midazolam are often employed.

The ocular surface is anesthetized with topical anesthesia in the form of tetracaine 1% or other similar medications.

Positioning

The patient is placed supine. The globe should be positioned in primary gaze (staring straight up at the ceiling).

Monitoring & Follow-up

Cardiac and respiratory systems should be monitored. Supplemental oxygen is usually given.

Complications

There is 1-3% risk of complications with retrobulbar block, ranging from mild to severe. Possible complications include the following:

• Retrobulbar hemorrhage

• Ocular perforation (< 0.1% incidence, but risk is increased with myopic eyes)

• Subarachnoid or intradural injection (very rare but can cause unconsciousness and/or cardiorespiratory collapse)

• Respiratory depression or arrest (0.29% incidence)

• Optic nerve injury ^[2]

• Retinal vascular occlusion

• Seizure

• Bradycardia

• Eyelid ptosis

• Diplopia due to extraocular muscle injury

• Chemosis

• Corneal abrasion

Choice of Anesthetic and Additives

The choice of local anesthetic depends on the desired duration of anesthesia and akinesia and physician preference. Lidocaine 2% is usually adequate for short cases, but is frequently used in a 1:1 mixture with bupivacaine 0.75% to increase duration of action. ^{[3, 4]}

Some physicians advocate the use of a painless local. This entails diluting 1.5 mL of local anesthetic with 15 mL of balanced salt solution. Then 1 mL of this mixture is injected subconjunctivally in the inferotemporal fornix via a 30-gauge needle. This initial injection dampens the pain associated with the retrobulbar block, although it is infrequently used.

Several additives can be combined with the anesthetic to modify the effect.

Epinephrine 1:200,000 can be added. It may increase duration of action by causing local vasoconstriction. The effect seems to be limited when bupivacaine is used, and the risk of retinal artery constriction in elevated.

Sodium bicarbonate has been used to neutralize the pH of the anesthetics and has been shown to speed the onset of bupivacaine.

Hyaluronidase (7.5 U/mL) can be added to the injectate to help facilitate the spread of anesthetic through the muscle cone.

Injection

If sedation is used, the physician waits until adequate sedation is achieved. The level of sedation can be confirmed by ensuring that the patient’s response to tactile stimulus of the eyelashes is absent or significantly blunted.

The lower eyelid is then prepped with topical isopropyl alcohol.

The globe should be in primary gaze, looking straight up towards the ceiling. Prior teaching required the patient to look up and in. However, this method has since been discontinued because it actually brings the optic nerve closer to the path of the needle.

The inferior orbital rim is palpated through the lower eyelid. While palpating the orbital rim, the globe can be elevated with the same finger using mild pressure on the inferior aspect of the globe. This maneuver furthers clears the path of the needle because it will pass the equator of the globe.

Prior to injection, the syringe is oriented so the volume scale is clearly visible to the injecting physician. The needle should be oriented with the bevel facing up towards the globe. This further protects the globe from penetration during injection.

The needle is then inserted through the lower eyelid, just superior to the lateral third of the inferior orbital rim. The temporal limbus is used as a guide, as shown in the image below.

The needle is advanced posteriorly parallel to the orbital floor, which has an approximate incline of 15 degrees.

When the needle is approximately 50% passed (at this point the tip of the needle will have passed the equator of the globe), the angle of injection is shifted medially and further superiorly to 45 degrees allowing the needle to enter the intraconal space. ^[5]

The path of the needle is shown in the image below.

During this process, one must be mindful of any resistance to the needle or any significant rotation of the globe that could indicate engagement of the sclera by the needle tip.

When the needle is felt to be within the muscle cone, the syringe is aspirated to ensure that no blood returns. At this point, up to 3 to 4 mL of anesthetic is slowly injected.

One can observe slowly developing proptosis of the globe as the anesthetic is injected, correlating to the volume of anesthetic injected.

Once the anesthetic is delivered, the needle is removed.

Ocular compression is then performed for approximately 15 seconds on, then 5 seconds off for a total of 1 to 2 minutes. This technique serves to quickly tamponade mild hemorrhage within the orbit as well as returning intraocular pressure to normal levels.

The degree of anesthesia and akinesia can be assessed after 5 minutes.

The retrobulbar block procedure is demonstrated in the video below.

Alternatives

There are several other anesthetic techniques that can be used to provide adequate anesthesia for modern intraocular surgery. The choice of technique should include anesthetic goals as well as the individual characteristics of each patient.

General anesthesia is rarely used for intraocular surgery. However, for certain patients such as children or adults with limitations to their mental status, general anesthesia may be the only option.

Peribulbar block is very similar to the retrobulbar block. Anesthetic is injected into the orbit; however, it is administered outside of the muscle cone. Because of this fact, this technique is lower risk than the retrobulbar block, but achieves a lesser degree of anesthesia and especially akinesia. ^[6]

Sub-Tenon block involves the use of topical anesthesia followed by incision of the conjunctiva and Tenon capsule. Anesthetic is then introduced in the sub-Tenon space using a blunt-tipped cannula. This technique provides the advantages of fast onset and improved anesthesia and akinesia when compared to topical anesthesia alone. ^[7]

Topical anesthesia is used in 60% of all cataract surgery in the United States. Topical anesthetic drops are used to anesthetize the cornea. Intraocular lidocaine can be used in conjunction to improve anesthesia. This method does not provide any akinesia. ^[8]