Subtenon

• Subtenon is a potential space in the eye, beneath Tenon’s capsule, important for drug delivery and anesthesia.
• The subtenon space anatomy involves layers between the sclera and Tenon’s capsule, facilitating the spread of substances.
• Subtenon injections are a common method for administering medications directly to the posterior segment of the eye.
• Subtenon anesthesia provides effective regional pain control for various ophthalmic surgeries.
• Subtenon block procedures offer an alternative to retrobulbar or peribulbar blocks, often with a favorable safety profile.

What is Subtenon?

Subtenon is a term used in ophthalmology to describe the anatomical space situated between the sclera (the white outer layer of the eyeball) and Tenon’s capsule. Tenon’s capsule, also known as the fascia bulbi, is a thin, fibrous membrane that covers the posterior five-sixths of the globe, extending from the limbus (the border between the cornea and sclera) to the optic nerve. This potential space is clinically significant as it provides a direct pathway for the administration of medications and local anesthetics to the eye’s posterior structures.

The concept of the subtenon space is fundamental to understanding several ophthalmic procedures, particularly those requiring localized drug delivery or regional anesthesia. Its unique anatomical position allows for effective diffusion of substances to target tissues while minimizing systemic absorption, thereby reducing potential side effects. This approach has gained prominence due to its efficacy and generally favorable safety profile compared to other methods.

Anatomy and Function of the Subtenon Space

The subtenon space anatomy is defined by its boundaries: the sclera internally and Tenon’s capsule externally. This space is not a true open cavity but rather a potential space, meaning it can be expanded by the introduction of fluid or air. It is traversed by delicate connective tissue septa and the extraocular muscles, which pierce Tenon’s capsule to insert into the sclera. These anatomical features influence how substances spread within the space, often allowing for broad distribution around the globe.

The primary subtenon space function in a clinical context is to serve as a conduit for therapeutic agents. Its proximity to the sclera and choroid makes it an ideal site for delivering drugs that need to reach the retina, choroid, and optic nerve. For instance, anti-inflammatory drugs or antibiotics can be delivered directly to the site of inflammation or infection, bypassing systemic circulation and achieving higher local concentrations. This targeted delivery mechanism is a cornerstone of modern ophthalmic treatment strategies.

Subtenon Injections, Anesthesia, and Block Procedures

The subtenon space is frequently utilized for various medical interventions due to its accessibility and effectiveness. These procedures include the administration of medications and regional anesthesia for surgical purposes.

• Subtenon Injections: A subtenon injection involves introducing therapeutic agents, such as corticosteroids or anti-VEGF drugs, into the subtenon space. This method is particularly effective for treating conditions like macular edema, uveitis, or certain retinal vascular diseases. The medication diffuses through the sclera to reach the posterior segment of the eye, providing sustained drug release and localized action.
• Subtenon Anesthesia: Subtenon anesthesia is a widely adopted technique for providing regional anesthesia during ophthalmic surgeries, such as cataract extraction. It involves injecting a local anesthetic into the subtenon space, which then spreads to block the ciliary nerves, resulting in akinesia (paralysis of eye movement) and analgesia (pain relief). This method is favored for its rapid onset and high success rate, often with fewer complications than other regional block techniques.
• Subtenon Block Procedures: A subtenon block procedure refers to the overall technique of administering local anesthetic into the subtenon space to achieve ocular anesthesia. It typically involves creating a small incision in the conjunctiva and Tenon’s capsule, followed by the insertion of a blunt cannula into the subtenon space to deliver the anesthetic. This approach offers a safer alternative to retrobulbar or peribulbar blocks, as it generally avoids direct needle penetration into the muscle cone or orbit, reducing risks such as globe perforation or optic nerve damage. According to a review published in the British Journal of Anaesthesia, subtenon block has a lower incidence of serious complications compared to sharp needle techniques for regional ocular anesthesia.