Introduction:
Subtenon infiltration anesthesia is a technique developed to avoid complications that arise due to sharp needles. It is done by using a blunt cannula under the fascia of the tenon. Tenon’s capsule, which surrounds the eyeball, is a thin fascial sheath that separates the eyeball from orbital fat. It is now widely used in almost all healthcare centers to surpass the sight-threatening complications of using sharp needles. It was discovered by Turnbull in the year 1884.
What Is a Tenon Capsule?
It is a fibrous, dense layer of elastic tissue that surrounds the eye and the muscles present extraocularly. A muscle called the rectus divides this capsule into anterior and posterior segments. The posterior portion is thinner compared to the anterior portion, and it separates the contents of retrobulbar space (area behind the globe) from the globe (hollow eye structure excluding the eye appendages). This allows for the creation of a space between Tenon’s capsule and the sclera (white layer outside the eye). Local anesthesia is injected into this space to allow the spread of anesthesia into the retrobulbar space, extraocular muscle sheaths, fascia around the lids, and the nerve supplying the globe.
What Are the Advantages of This Technique?
The advantages of utilizing this technique are
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It is less painful compared to other orbital blocks.
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Minimize the risk of complications in eyesight.
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Good intraoperative analgesia is provided.
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It is considered a virtually painless technique.
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The facial block is not required.
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It helps provide loss of eye movements during the surgery.
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This is considered a safe technique in patients undergoing anticoagulant therapy.
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Complications of sharp needles are avoided.
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It can be used in patients with lengthier eye dimensions (axial length).
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Intraocular pressure is minimized and prevented from rising.
How to Prepare for This Procedure?
Prior to surgery, the patient should fill in the health questionnaire mentioning their medical, surgical, and pharmaceutical history. A thorough clinical examination is done, and the patient's vital signs are checked before the surgery. Routine examination reports like blood tests are studied, and the patient is prepared for surgery. Patients should be conscious, comfortable, pain-free, and without anxiety. Effective local anesthesia is necessary to achieve this. It has to be noted that the patient has sufficient oxygen flow under the surgical drapes. This is done to prevent hypercarbia (increased carbon dioxide concentration in the blood) and hypoxia (decreased oxygen concentration in the blood). In case of language difficulties or other speech abnormalities, a relative or interpreter should accompany the patient in the theater. Intraoperative monitoring is done through pulse oximetry and ECG (electrocardiogram).
The anesthetic procedure is priorly explained to the patients. All the anesthetic and monitoring equipment in the operation theater is checked and sterilized. Adjuncts to check blood pressure and oxygen saturation are connected. The ECG leads are connected, and baseline recordings are obtained. A patent intravenous cannula is kept handy if there is a need.
Since local anesthesia is a relatively safe procedure, particularly sub-tenon’s and topical techniques, therefore it is not necessary to starve the patients preoperatively. This is advantageous to diabetic patients particularly. They will be able to continue food intake and consume their usual medications on the day of surgery. Also, preoperative administration of anxiolytic medication is not necessary. The patients who are to be operated on under heavy sedation or general anesthesia should fast.
How Is It Done?
The patient is put in sedation, and their eyelids are retracted using a speculum (a retractable device used for holding the eyelids open during eye surgeries) for injecting the anesthesia. For this purpose, cannulas of varying lengths have been used. The patient is asked to look upwards throughout the procedure. Under topical anesthesia, two to three-millimeter spots of cautery are made five millimeters from the limbus, a narrow tissue band that encircles the cornea. A two-millimeter incision is made in the conjunctiva (outer surface of the eye) with blunt dissection through the fascia. A blunt cannula is directed posteriorly, with an injection containing one to three milliliters of local anesthetic. A small degree of conjunctival edema is often seen. Analgesia is usually excellent.
Little resistance to injection is often found. For this, cannula withdrawal and reinserting of the needle may be required. Slight proptosis (eye bulging) is seen after an appropriately sited injection. If there is a ballooning of the conjunctiva, it refers to an incorrect plane of insertion of the needle. For this, the removal of the cannula and gentle massaging of the globe is done. In myopic patients (nearsightedness), extra care should be taken since they have thinner and longer globes.
What Are the Benefits of This Technique?
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It is a versatile and effective technique.
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It is used primarily for cataract surgery. It is a procedure in which the lenses are replaced with artificial lenses in cases of cloudy vision, also called a cataract.
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It is used in vitreoretinal surgery, a type of surgery performed for retinal detachment and advanced diabetic eye disease.
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Used in strabismus surgery done to correct eye muscles in strabismus-affected patients.
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It is used in a procedure called pan-retinal photocoagulation. It is a treatment modality for retinal iscṣhemic disease, and the procedure involves creating thermal burns.
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It is used for optic nerve sheath fenestration. It is a surgery done to decompress the optic nerves to preserve vision in some patients.
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It is safe to use in patients who are taking anticoagulants, NSAIDs (non-steroidal anti-inflammatory drugs), and Aspirin.
What Are the Disadvantages?
There are potential complications like subconjunctival hemorrhage and chemosis. Subconjunctival hemorrhage refers to the bleeding of the white part, the conjunctiva. Chemosis refers to eye irritation. Rarely has this technique been associated with sight-threatening complications. Some patients experience temporary postoperative strabismus (malalignment in eyes or crossed eyes) lasting for a few days post-surgery. This is contraindicated in patients with a history of scleral disease accompanied by friable sclera and possible scarring. A swelling of the conjunctiva is seen in 30 percent of cases.
Conclusion:
The sub-tenon block is considered very effective by surgeons. The patient acceptability rate for this technique is reported to be 98.8 percent. Sub-tenon cannula insertion followed by anesthesia administration induces minimal discomfort. It is reported to be completely painless by 69 percent of the patients. Complications have been relatively low, and the safety of the sub-tenon technique is impeccable.
