Intraconal (retrobulbar) block involves injection of local anaesthetic agent into the part of the orbital cavity behind the globe.

Extraconal (peribulbar) block refers to the placement of needle tip outside the muscle cone.

A combination of intraconal and extraraconal blocks is described as combined retroperibulbar block.

Sub-Tenon’s block refers to the injection of local anaesthetic agent beneath the Tenon capsule. This block is also known as parabulbar block, pinpoint anaesthesia and medial episcleral block.

anatomy

The orbit is an irregular four-sided pyramid with its apex placed posteromedially and its base facing anteriorly bounded by the orbital margins.

Four rectus muscles arise from the annulus of Zinn at the back of the orbit and inserted into the globe just anterior to its equator forming an incomplete cone.

Optic nerve, trunk of ophthalmic artery, ciliary ganglion and nerves to the muscles are in the cone.

Superior rectus, levator palpebrae, medial rectus, inferior rectus and inferior oblique muscles are supplied by the oculomotor nerve.

Lateral rectus is supplied by abducent nerve.

Superior oblique is supplied trochlear nerve which runs outside the cone.

Sensory innervation is very complex.

Corneal and perilimbal conjunctival and superonasal quadrant of the peripheral conjunctival sensation are mediated through the nasociliary nerve. The remainder of the peripheral conjunctival sensation is supplied through the lacrimal, frontal, and infraorbital nerves coursing outside the muscle cone, hence intra-operative pain may be experienced if these nerves are not blocked.

Tenon capsule is a thin membrane that forms a socket for the eyeball.

The inner surface is smooth and shiny and is separated from the outer surface of the sclera by a potential space called the episcleral space.

Crossing the space and attaching the fascial sheath to the sclera are numerous delicate bands of connective tissue.

Anteriorly the fascial sheath is firmly attached to the sclera about 1.5cm posterior to the corneoscleral junction.

Posteriorly, the sheath fuses with the meninges around the optic nerve and with the sclera around the exit of the optic nerve.

The tendons of all six extrinsic muscles of the eye pierce the sheath as they pass to their insertion on the eyeball. At the site of perforation the sheath is reflected along the tendons of these muscles to form on each a tubular sleeve.

Localization of anaesthetic agents after injection

Local anaesthetic agent diffuses from one compartment to other during needle blocks.

Sub-Tenon’s space opens during injection giving a characterstics T-sign and local abnaesthetic diffuses into the retrobulbar space.

Indications

Local anaesthetic technique is choice for eye surgery except when contraindicated.

Limiting factors are the ability of the patient to lie comfortably and still for the requisite time.

Specific contraindications

• Children
• Patient’s refusal
• Infected orbit
• Uncontrolled body movements (Severe Parkinson’s, severe chronic obstructive airway disease)
• Uncontrolled sneezing or coughing
• Serious psychiatry problems

Preparation of patients

Preoperative preparation and assessment vary worldwide.

Routine investigations are not essential as they do not improve health or outcome of surgery.

Tests can be done to improve general health of the patient if required.

There is an increased risk of haemorrhage in p atients receiving anticoagulants. Patients are advised to continue medications. Clotting results should be within the recommended therapeutic range. Currently there is no recommendation for patient receiving antiplatelet agents.

Knowledge of axial length is essential before needle block. Patients with long eyes have thinner sclera and outpouching of sclera (staphyloma) and have increased incidence of globe damage following needle block.

Patients are usually not starved and are encouraged to take their usual medications.

Patients should receive adequate explannations regarding anaesthesia and complications.

Monitoring during the block

A pulse oximeter, electrocardiograph and blood pressure recorder should be used. Intravenous access should be secured and full resuscitation equipment and drugs should be available .

Sedation

Sedation is common during topical anaesthesia.

Routine use of sedation during akinetic block is discouraged because of increased intra-operative events.

When sedation is administered, a means of providing supplementation oxygen, equipment and skills to manage any life-threatening events must be immediately accessible.

Needle block

Retrobulbar and peribulbar techniques are considered as separate techniques but the injection is made into the same adipose tissue compartment and the difference is merely a matter of needle direction and depth of insertion.

Equipments

• Intravenous cannula
• Proxymetacaine 0.5% or benoxinate 0.4% or tetracaine 1% and 5% povidone eye drop
• 2% lidocaine
• Hyaluronidase
• 27 G , 1.2 cm needle for dilute local injection
• 27 G , < 3.1 cm long needle for main injection
• Balanced Salt Solution (15 ml vial)
• Gauge swab

Retrobulbar technique

While the patient supine and looking straight ahead, the conjunctiva is anaesthetised with topical local anaesthetic drops.

Dilute painless local solution is prepared by adding 2ml of concentrated local anaesthetic agent to 13 ml of Balanced Salt Solution (BSS). 1.5-2 ml of this dilute solution is injected via 27G, 1.2cm needle through the conjunctiva under the inferior tarsal plate in the inferotemporal quadrant.

A sharp 27G, needle <3.1cm is inserted through the conjunctiva or skin in the inferotemporal quadrant as far lateral as possible below the lateral rectus muscle while the patient is looking in the primary gaze position.

Initial direction of the needle is tangential to the globe, then passes below the globe and once past the equator as gauged by axial length of the globe, is allowed to go upwards and inwards to enter the central space just behind the globe.

The globe is continuously observed during the needle placement. 4-5 ml of local anaesthetic agent is injected.

A sharp 25G, 3.1cm long needle is inserted through the conjunctiva as far lateral as possible in the inferotemporal quadrant while the patient is looking in the primary gaze position. Once the needle is under the globe, it is not directed upward and inward, but is directed along the orbital floor.

A volume of 5ml of local anaesthetic agent is injected. Many patients require supplementary injection.

This is usually performed to supplement inferotemporal retrobulbar or peribulbar injection particularly when akinesia is not adequate. A 25G or 27G needle is inserted in the blind pit between the caruncle and the medial canthus to a depth of 15-20mm. 3-5ml of local anaesthetic agent is usually injected.

Some use medial peribulbar as a primary injection technique for anaesthesia particularly in patients with longer axial lengths.

Author’s preferred technique

Movie of akinetic block using a short needle

Specific complications

Complications of needle blocks range from simple to serious and could be limited to the orbit or systemic.

Orbital complications include

• failure of the block,
• corneal abrasion,
• chemosis,
• conjunctival haemorrhage,
• vessel damage leading to retrobulbar haemorrhage,
• globe perforation,
• globe penetration,
• optic nerve damage
• and extraocular muscle damage.

The systemic complications such as local anaesthetic agent toxicity, brainstem anaesthesia, cardiorespiratory arrest may be due to injection or spread or misplacement of drug in the orbit during or immediately after injection.

Sub-Tenon’s block

This block was re-introduced into the clinical practice as a simple, safe and effective technique because of continuing concerns over rare but serious complications of sharp needle blocks.

The technique involves gaining access to the sub-Tenon's space by cannula and administration of local anaesthetic agent into the sub-Tenon’s space.

Injection of local anaesthetic agent under the Tenon capsule blocks sensation from the eye by action on the short ciliary nerves as they pass through the Tenon capsule to the globe and akinesia by direct blockade of motor nerves.

The sub-Tenon’s technique involves obtaining surface anaesthesia, instillation of antiseptic, access to the sub-Tenon’s space, insertion of cannula and subsequent administration of local anaesthetic agent into the sub-Tenon’s space.

Equipments

• Intravenous cannula
• Proxymetacaine 0.5% or benoxinate 0.4% or tetracaine 1%, 5% povidone eye drop
• 2% lidocaine
• Hyaluronidase
• Eye speculum
• Westcott Scissors
• Moorfields Forceps
• 5 ml syringe
• Gauge swab
• Sub-Tenon’s cannula

Technique

Surface anaesthesia is achieved by instilling topical local anaesthetic.

5% povidone iodine eye drop is instilled before dissection.

An eye speculum is applied or an assistant retracts the lower eyelid.

Patient is asked to look upwards and outwards to expose the inferonasal quadrant.

The conjunctiva is grasped with non-toothed forceps.

A small button hole incision is made with blunt spring scissors about 5-10mm from the limbus in the inferonasal portion of the conjunctiva and Tenon’s capsule.

Bare sclera is seen.

Conjunctival is held up with the forceps.

Blunt curved cannula is passed into the sub-Tenon’s space following the curve of the globe.

If a resitance is felt, small amount of local anaesthetic is injected. If injection is not possible, it is better to reinsert the cannula.

3-5ml of local anaesthetic is injected.

While the eyelids are closed, a gentle pressure is applied on the globe.

The block is asessed for akinesia after 5 minutes.

Author’s preferred technique of sub-Tenon’s block

Movie of the author's preferred technique of sub-Tenon's block

Uses of sub-Tenon’s block

• Cataract surgery
• Viteroretinal surgery
• Panretinal photocoagulation
• Strabismus surgery
• Trabeculectomy
• Optic nerve sheath fenestration
• Delivery of drugs

Specific complications of sub-Tenon’s block

Minor complications such as pain during injection , chemosis, conjunctival haemorrhage and over spilling of local anaesthetic are common. Akinesia is variable and volume dependent.

Major complications include

• rectus muscle paresis and trauma,
• orbital and retrobulbar haemorrhage globe perforation,
• central spread of local anaesthetic,
• orbital cellulitis and others.

Most complications have occurred following the use of 2.54cm metal cannula. Smaller and flexible cannulae appear to be safer but the incidence of minor complications increase.

Choice of local anaesthetic agent

All the modern, full potency local anaesthetic agents are suitable for ophthalmic block.

Studies have shown little difference in quality of anaesthesia, analgesia and akinesia. Concentration up to but not exceeding 2% lidocaine is commonly used.

Vasoconstrictor

Vasoconstrictor (epinephrine and felypressin) are commonly mixed with local anaesthetic solution to increase the intensity and duration of block, and minimise bleeding from small vessels.

Epinephrine may cause vasoconstriction of the ophthalmic artery compromising the retinal circulation.

The use of epinephrine containing solution should be avoided in elderly patients suffering from cerebrovascular and cardiovascular diseases.

Hyaluronidase

Hyaluronidase is known to improve the effectiveness and the quality of needle block.

Its role in sub-Tenon’s block is controversial.

The amount of hyaluronidase should not be used more than 15 IU.ml -1.

pH alteration

Alkalinisation has shown to decrease the onset and prolong the duration after needle blocks.

Its role in sub-Tenon’s block has not been fully investigated.

Others

Addition of clonidine, muscle relaxant and other chemicals are known to increase the onset and potency of orbital block but their use is not routine.

Catheter insertion technique

Prolonged anaesthesia and analgesia are obtained by inserting a catheter in the retrobulbar or sub-Tenon’s space.

Block assessment

There is no objective method of assessing anaesthesia following eye block.

Dense motor block (akinesia) is a good predictor of anaesthesia.

Intraoperative care

The patient should be comfortable and soft pads are placed under the pressure areas.

Patients should be monitored with pulse oximetry, ECG, non-invasive blood pressure measurement and maintenance of verbal contact. Patients should receive an oxygen-enriched breathing atmosphere to prevent hypoxia.

Rescue anaesthesia

Rescue analgesia is provided by supplemental topical local drops but repeat injection may be necessary to achieve good anaesthesia and akinesia Caution is required during repeat injection as the anatomy of the orbit is altered by previous injection.

Selection of anaesthesia

A naesthetic requirements for ophthalmic surgery are dictated by the nature of the proposed surgery, the surgeon’s preference and the patient’s wishes.

Needle block is the most popular technique practised around the world but sub-Tenon’s block is preferred in many countries.

Needle block is not suitable for inexperienced anaesthetists, long eye, previous extensive eye surgery and altered anatomy of the eye.

Sub-Tenon’s block may be difficult in patients who had previous eye surgery in the same eye, orbital trauma and infection of the globe.

Conclusion

Eye blocks provide excellent anaesthesia for ophthalmic surgery and success rates are high. Satisfactory anaesthesia and akinesia can be obtained with both needle and cannula. Although rare, orbital injections may cause severe local and systemic complications. Knowledge of orbital anatomy and training are essential for the practice of safe orbital regional anaesthesia.