Sub-Tenon's Anesthesia

Key points at a glance

Key Points at a Glance

• Sub-Tenon anesthesia is a relatively new local ophthalmic anesthesia method introduced in 1990, widely adopted as an alternative to retrobulbar anesthesia.
• Because a blunt cannula is used, the risk of serious complications such as globe perforation, orbital hemorrhage, and optic nerve injury is lower than with retrobulbar anesthesia.
• Analgesic effect is comparable to retrobulbar anesthesia, but akinesia may be inferior.
• The standard technique involves making an inferonasal conjunctival incision and advancing the cannula into the sub-Tenon space.
• For cataract surgery, 1–2 mL of 2% lidocaine is standard. For longer procedures like vitrectomy, a long-acting anesthetic is mixed.
• Main complications are subconjunctival hemorrhage and chemosis, both generally mild and self-limiting.

1. What is Sub-Tenon Anesthesia?

Sub-Tenon anesthesia (Sub-Tenon Block) is a local ophthalmic anesthesia technique in which local anesthetic is injected into the potential space between Tenon’s capsule and the sclera (sub-Tenon space). It was introduced in 1990 as a relatively new method and has become popular as a safer alternative to retrobulbar anesthesia.

Retrobulbar and peribulbar anesthesia use sharp needles, posing risks of serious complications such as globe perforation, orbital hemorrhage, optic nerve injury, and spread to the subarachnoid space. Sub-Tenon anesthesia significantly reduces these risks by using a blunt cannula.

Once mastered, sub-Tenon anesthesia can be used for many intraocular surgeries under local anesthesia, including cataract surgery, glaucoma surgery, vitrectomy, and scleral buckling.

Comparison of Local Ophthalmic Anesthesia Methods

There is a trade-off between the degree of akinesia and ease of technique among different anesthesia methods.

Anesthesia Method	Akinesia	Ease of Technique	Main Complications
Retrobulbar anesthesia	Strongest	Difficult	Globe perforation, orbital hemorrhage
Sub-Tenon’s anesthesia	Moderate	Medium	Subconjunctival hemorrhage, conjunctival edema
Topical anesthesia	Weak	Easy	Intraoperative pain, eye movement present

Q Which provides better pain control during surgery: sub-Tenon's anesthesia or topical anesthesia?

A

A meta-analysis of six randomized controlled trials (RCTs) showed that sub-Tenon’s anesthesia significantly reduced intraoperative pain compared to topical anesthesia (SMD=0.64, 95% CI 0.43 to 0.84, low certainty evidence). However, pain during anesthetic administration tended to be slightly less with sub-Tenon’s anesthesia (SMD=−0.20, 95% CI −0.43 to 0.04, not statistically significant).

2. Main symptoms and clinical findings

Subjective Symptoms

This section describes the anesthetic technique, but also notes patient subjective symptoms related to anesthetic administration.

• Discomfort during anesthetic administration: Usually milder than retrobulbar anesthesia because a blunt needle is used.
• Sensation of proptosis: Patients may feel mild protrusion of the eye during injection of the anesthetic solution.
• Postoperative conjunctival hyperemia and swelling: Changes in appearance due to subconjunctival hemorrhage or chemosis may be seen after surgery.

Clinical Findings (Findings Confirmed by the Physician)

• Onset of analgesia: Usually rapid.
• Akinesia: The anesthetic solution spreads into the retrobulbar space and blocks motor nerves. Complete akinesia may take up to 10 minutes to achieve.
• Subconjunctival edema: Occurs when the cannula is inadvertently placed under the conjunctiva rather than in the sub-Tenon’s space. Repositioning is required.

3. Causes and Risk Factors

Anatomical and patient-related factors that make sub-Tenon’s anesthesia difficult are described.

• Scarring or adhesion of Tenon’s capsule: Adhesions are strong after extraocular myositis or scleral buckling surgery, making blunt needle entry difficult.
• Scleral thinning: In high myopia or after scleritis, the sclera is thin, posing a risk of inadvertent scleral injury.
• Anticoagulant or antiplatelet therapy: May result in prominent subconjunctival hemorrhage.
• Young patients (abundant Tenon’s capsule): Tenon’s capsule is thick, often requiring blunt dissection.
• Elderly patients (thin Tenon’s capsule): Caution is required because the needle tip can easily enter the sub-Tenon’s space instead of the capsule.

Prevention and Daily Care

If you are scheduled for eye surgery and are taking anticoagulants or antiplatelet drugs (such as warfarin or aspirin), be sure to inform your doctor. In most cases, surgery such as cataract surgery can be performed while continuing these medications, but it is important to confirm with your physician.

4. Diagnosis and Examination Methods

The following points should be confirmed when evaluating patients for sub-Tenon’s anesthesia:

• Assessment of general condition and comorbidities: Many patients undergoing eye surgery are elderly and may have serious comorbidities. Determine whether local anesthesia or general anesthesia is appropriate.
• Confirmation of anticoagulant/antiplatelet therapy: To assess the risk of subconjunctival hemorrhage.
• Evaluation of anatomical abnormalities: Check for high myopia (axial elongation), scleral thinning, and previous surgical history.
• Patient cooperation: Confirm whether the patient can maintain a supine position and follow instructions for gaze fixation.

Q Can sub-Tenon's anesthesia be used as an alternative to general anesthesia?

A

It can be used as an alternative to general anesthesia for many intraocular surgeries (cataract, glaucoma, vitrectomy, etc.). However, general anesthesia may be appropriate for prolonged surgeries or when patient cooperation is not possible (e.g., dementia, children).

5. Standard Treatment

Medications Used

Short-duration surgery (e.g., cataract surgery):

• 2% lidocaine alone, 1–2 mL
• Administration of 1 mL of 2% lidocaine provides an action time of approximately 1 hour.

Long surgeries (vitrectomy, scleral buckling, etc.):

• Mix equal volumes of 2% lidocaine and 0.5% bupivacaine hydrochloride (Marcaine®) or 0.75% ropivacaine hydrochloride (Naropin®).
• Inject approximately 3–4 mL, and add more as needed depending on the situation.

Specialized supplement: Choice of anesthetic

Ropivacaine is less toxic than bupivacaine and does not contain preservatives, making allergic reactions less likely. Avoid adding vasoconstrictors (e.g., epinephrine). The benefits of warming or alkalinizing the solution have not been proven.

Procedure

Preparation

1. Place the patient in the supine position. Intravenous access is desirable but not essential.
2. Disinfect the conjunctival sac and orbital rim with povidone-iodine.
3. Insert a lid speculum to secure the surgical field.
4. Ask the patient to look upward and outward to expose the inferonasal quadrant.

Access to the sub-Tenon space

5. Lift the conjunctiva 5–10 mm from the inferonasal corneal limbus in a tent-like manner using non-toothed forceps.
6. Make a small incision in the conjunctiva and Tenon’s capsule with Westcott scissors to expose the underlying sclera.
7. Insert a blunt sub-Tenon cannula through the incision and advance it posteriorly along the curvature of the globe until the tip passes the equator.

Injection of anesthetic solution

8. Inject the anesthetic solution slowly. Correct placement is indicated by lack of resistance and disappearance of most of the solution posterior to the globe.
9. If conjunctival swelling (chemosis) is observed, the cannula is likely in the subconjunctival space and repositioning is required.

Professional supplement: Instruments used

Use of a dedicated cannula designed for sub-Tenon anesthesia (e.g., 25G or 27G) is recommended. Alternatives include a smoothly curved 27G blunt needle or a Nipro ophthalmic disposable needle. Metal cannulas carry a higher risk of trauma, while plastic ones risk kinking; therefore, dedicated products are preferred.

Expanded indications

Sub-Tenon block can also be used as supplemental anesthesia when retrobulbar or peribulbar anesthesia is insufficient. Additional injection through the same incision is possible during surgery.

Treatment precautions

• Although a blunt needle alone is unlikely to directly cause scleral perforation, cases of accidental scleral damage during Tenon’s capsule dissection with scissors have been reported. Blunt dissection of Tenon’s capsule with the tip of scissors is recommended.
• Extreme caution is required in cases of high myopia, after scleritis, after extraocular myositis, and after scleral buckling.
• Identify syringes by color or shape to avoid confusing anesthetic solution for injection with saline for hydrodissection.
• Use a locking syringe for safety, as large-volume syringes require more force to inject.

6. Pathophysiology and detailed mechanism

Anatomy of Tenon’s capsule

Tenon’s capsule (episcleral membrane) is a thin layer of connective tissue that envelops the eyeball. Anteriorly, it fuses with the conjunctiva at the corneal limbus, and posteriorly, it fuses with the dura mater of the optic nerve. The sub-Tenon’s space (episcleral space) between Tenon’s capsule and the sclera is a potential space.

Tenon’s capsule attaches to the sclera approximately 3 mm posterior to the limbus. By dissecting between Tenon’s capsule and the sclera posterior to this attachment, a pathway is opened to advance a blunt needle posteriorly.

Mechanism of analgesia and akinesia

Anesthetic solution injected into the sub-Tenon’s space diffuses posteriorly into the retro-orbital space, blocking the sensory nerves (trigeminal system) and motor nerves (oculomotor, trochlear, and abducens nerves) that pass through it. This provides analgesia and akinesia.

The analgesic effect is equivalent to retrobulbar anesthesia, but akinesia may be less adequate compared to retrobulbar anesthesia. This is because the anesthetic solution does not reliably reach the intraconal space (retrobulbar space). If motor nerve blockade is prioritized, trans-Tenon’s retrobulbar anesthesia (penetrating the fascia deep to Tenon’s capsule and injecting into the intraconal space) may be chosen.

Comparison with topical anesthesia (evidence)

According to a meta-analysis published in the ESCRS guidelines for cataract surgery, sub-Tenon’s anesthesia reduces intraoperative pain more effectively than topical anesthesia, but carries a higher risk of conjunctival chemosis (RR 2.11, 95% CI 1.46–3.05, 4 trials, n=1042, moderate certainty evidence). The risk of eyelid hematoma tends to be lower with sub-Tenon’s anesthesia (RR 0.36, 95% CI 0.15–0.88, low certainty evidence).

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7. Latest research and future perspectives (research-stage reports)

For patients: Please read

The following content is currently at the research stage or under clinical trial, and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Safety review of sub-Tenon’s anesthesia

A review of the safety of sub-Tenon’s block was conducted in 2011. In the UK, the Royal College of Ophthalmologists and the Royal College of Anaesthetists published national guidelines on local anesthesia for ophthalmic surgery in 2012.

Ultrasound-guided sub-Tenon’s anesthesia

Research is being conducted on improving the accuracy of anesthetic injection under real-time ultrasound guidance. It is expected to optimize the confirmation of cannula tip position and the diffusion pattern of the anesthetic solution.

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8. References

1. Guay J, Sales K. Sub-Tenon’s anaesthesia versus topical anaesthesia for cataract surgery. Cochrane Database Syst Rev. 2015 Aug 27;2015(8):CD006291.
2. Alhassan MB, Kyari F, Ejere HOD. Peribulbar versus retrobulbar anaesthesia for cataract surgery. Cochrane Database Syst Rev. 2015.
3. El-Hindy N, Johnston RL, Jaycock P, et al. The Cataract National Dataset Electronic Multi-centre Audit of 55,567 operations: Anaesthetic techniques and complications. Eye (Lond). 2009;23:50-55.