A subtenon block is a form of regional anesthesia used for eye surgeries. It involves administering a local anesthetic solution into a space around the eyeball. This technique numbs the eye and surrounding structures, allowing for surgery without general anesthesia. It is a safe and effective alternative for ophthalmic procedures, providing pain relief and reduced eye movement. This method has become increasingly popular in many countries, including the United Kingdom and New Zealand, since its reintroduction in the 1990s.
What It Is and How It Works
The subtenon block targets the Tenon’s capsule, a thin layer of connective tissue that surrounds the eyeball. This capsule extends from the front of the eye to merge with the optic nerve at the back. The subtenon space is located between the Tenon’s capsule and the sclera.
When the local anesthetic is instilled into this subtenon space, it diffuses posteriorly into the retro-orbital area. This allows the anesthetic to block the sensory and motor nerves that supply the eye and its surrounding muscles. Blocking these nerves results in both analgesia (pain relief) and akinesia (reduced eye movement). The anesthetic solution spreads circumferentially around the posterior part of the eye, bathing the ciliary nerves, which accounts for the rapid onset of globe analgesia, typically within three minutes. Over the next five minutes, the solution continues to diffuse into the retrobulbar space, along the sheaths of the extraocular muscles, and into the eyelids, further enhancing the block.
When It Is Used
A subtenon block is commonly used for a wide range of ophthalmic surgical procedures. It is suitable for surgeries requiring the eye to be numb and still while the patient remains awake. This includes frequently performed operations like cataract surgery, which is the most common ophthalmic surgical procedure globally.
Beyond cataract removal, subtenon blocks are also suitable for vitreoretinal surgery, which involves the back part of the eye, and glaucoma surgery. Other procedures that can benefit from this type of regional anesthesia include adult strabismus surgery, which corrects eye alignment, and panretinal photocoagulation, a laser treatment for retinal conditions. The block can also be used for optic nerve sheath fenestration, a procedure to relieve pressure around the optic nerve, and for managing long-term postoperative pain.
Administering the Block
The administration of a subtenon block typically begins with the patient lying in a supine position. Topical anesthetic eye drops are applied to numb the conjunctiva. The anesthetized conjunctiva is then cleaned with an antiseptic solution.
An eyelid speculum is then inserted to hold the eyelids open, providing access and preventing blinking during the procedure. The patient is often asked to look upwards and outwards, exposing the inferonasal quadrant of the eye, the most common site for the injection. A small tent of the conjunctiva and Tenon’s capsule is then lifted with non-toothed forceps near the limbus. A small opening is made through these layers with blunt ophthalmic scissors to expose the sclera.
A blunt-ended subtenon cannula is then inserted through this opening, following the curvature of the eyeball. The cannula is advanced posteriorly until its tip has passed the equator of the globe, ensuring it is within the subtenon space. The local anesthetic solution is then slowly injected into this space. The injection process is usually quick, lasting less than a minute.
Patient Experience and Considerations
Patients generally experience minimal discomfort during a subtenon block. Some may feel a sensation of pressure as the anesthetic spreads. The onset of pain relief is typically rapid, often within three minutes, while maximal reduction in eye movement may take up to 10 minutes to develop.
After the block, common temporary effects include blurred vision in the treated eye. Patients may also experience subconjunctival hemorrhage, appearing as redness or a bruise on the white of the eye. The incidence of this can vary, potentially approaching 100% with certain cannula lengths or in patients taking blood thinners. Another frequent, though temporary, side effect is chemosis, a swelling of the conjunctiva. This occurs due to the forward spread of the anesthetic solution and often resolves spontaneously or with gentle pressure.
Patients remain conscious during the procedure, though mild sedation may be offered to help with anxiety. While the block provides excellent pain control, complete akinesia is not always achieved, though residual movement rarely causes issues during surgery. The subtenon block is generally well-tolerated and is associated with high rates of patient satisfaction. Significant complications are rare, and any cosmetic effects from chemosis or hemorrhage are usually transient.