Sub-Tenon’s Block

Introduction

Sub-Tenon’s block (also known as episcleral block or parabulbar block) is a regional ophthalmic anesthesia technique that provides excellent analgesia and akinesia for various eye surgeries. It involves injecting local anesthetic into the sub-Tenon’s space, allowing diffusion to anesthetize the ocular nerves.

This technique is considered safer than retrobulbar and peribulbar blocks because it avoids deep needle penetration, reducing the risk of complications such as globe perforation and retrobulbar hemorrhage.


Anatomy of the Sub-Tenon’s Space

1. Tenon’s Capsule

• Tenon’s capsule is a fibrous sheath that envelops the eyeball from the limbus to the optic nerve, forming a potential space between the sclera and the capsule.

• The sub-Tenon’s space lies between the sclera and Tenon’s capsule, allowing the anesthetic to diffuse and block the sensory and motor nerves of the eye.


2. Nerve Supply to the Eye

• Sensory supply: Ophthalmic division of the trigeminal nerve (CN V1)

• Motor supply:

• Oculomotor nerve (CN III) → Medial, superior, and inferior recti, inferior oblique

• Trochlear nerve (CN IV) → Superior oblique

• Abducens nerve (CN VI) → Lateral rectus

Sub-Tenon’s block effectively anesthetizes CN III, CN IV, CN VI, and CN V1, providing sensory and motor blockade.


Technique of Sub-Tenon’s Block

1. Patient Preparation

• The patient is positioned supine with the head neutral.

• Topical anesthesia (e.g., lidocaine 2% drops) is instilled in the eye before the procedure.

• The eye is cleaned with povidone-iodine 5%, and a sterile drape is applied.


2. Equipment Required

• Syringe: 5 mL syringe

• Cannula: Blunt-tipped sub-Tenon’s cannula (19G or 22G) (safer than a sharp needle)

• Local Anesthetic Agents: Lidocaine 2% (fast onset),Bupivacaine 0.5% (longer duration), Ropivacaine 0.75%,Hyaluronidase (5–15 IU/mL) to enhance spread


3. Injection Procedure

• A small conjunctival incision is made in the inferonasal quadrant (to avoid major blood vessels).

• The blunt-tipped cannula is inserted through the incision into the sub-Tenon’s space, advancing it posteriorly along the sclera.

• 3–5 mL of local anesthetic is injected into the space, allowing it to diffuse around the eye.


4. Onset & Duration

• Onset: 2–5 minutes

• Duration: 2–4 hours, depending on the anesthetic used


5. Confirmation of Block Success

• Analgesia (Sensory Block): No pain with corneal touch

• Akinesia (Motor Block): No eye movement

• Ptosis: Due to paralysis of the orbicularis oculi


Advantages of Sub-Tenon’s Block

✅ Safer than retrobulbar/peribulbar blocks (avoids sharp needles, reducing risk of hemorrhage and globe perforation)

✅ Provides excellent anesthesia and akinesia

✅ Lower complication rate

✅ Can be used in anticoagulated patients


Disadvantages of Sub-Tenon’s Block

❌ Requires conjunctival dissection (mildly invasive)

❌ Mild subconjunctival hemorrhage is common

❌ Less effective for deep posterior globe surgeries

Comparison: Sub-Tenon’s Block vs. Other Ophthalmic Blocks

Feature

Sub-Tenon’s Block

Retrobulbar Block

Peribulbar Block

Injection site

Sub-Tenon’s space

Inside muscle cone

Outside muscle cone

Needle type

Blunt cannula

Sharp needle

Sharp needle

Depth of injection

Superficial

Deep

Moderate

Onset time

Fast (2–5 min)

Fast (2–5 min)

Slower (5–10 min)

Akinesia

Good

Excellent

Variable

Risk of hemorrhage

Low

High

Moderate

Complication rate

Lowest

Highest

Moderate


Complications of Sub-Tenon’s Block

Complication

Cause

Management

Subconjunctival hemorrhage

Small vessel rupture during dissection

Usually self-limiting, apply cold compress

Oculocardiac reflex (OCR)

Stimulation of ciliary nerves

Atropine 0.6 mg IV if severe

Globe perforation

Very rare (due to blunt cannula use)

Urgent ophthalmic evaluation

Retrobulbar hemorrhage

Very rare

Supportive care, surgical drainage if severe

Incomplete block

Poor diffusion of anesthetic

Repeat with additional LA or supplement with topical anesthesia


MCQs on Sub-Tenon’s Block

1. What is the main advantage of sub-Tenon’s block over retrobulbar block?

a) Provides better akinesia

b) Faster onset

c) Lower risk of complications

d) Requires less local anesthetic

• Answer: (c) Lower risk of complications


2. What type of needle is used in sub-Tenon’s block?

a) 21G sharp needle

b) 23G beveled needle

c) 25G blunt-tipped cannula

d) 18G Tuohy needle

• Answer: (c) 25G blunt-tipped cannula


3. Where is the injection given in sub-Tenon’s block?

a) Inside the muscle cone

b) Outside the muscle cone

c) Between Tenon’s capsule and sclera

d) Directly into the anterior chamber

• Answer: (c) Between Tenon’s capsule and sclera


4. What is the commonest side effect of sub-Tenon’s block?

a) Retrobulbar hemorrhage

b) Globe perforation

c) Subconjunctival hemorrhage

d) Brainstem anesthesia

• Answer: (c) Subconjunctival hemorrhage


5. Which ophthalmic anesthesia technique is the safest?

a) Retrobulbar block

b) Peribulbar block

c) Sub-Tenon’s block

d) Topical anesthesia

• Answer: (c) Sub-Tenon’s block