Anesthesia in Strabismus Surgery

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Article initiated by:

A. Paula Grigorian, MD

All contributors:

Carson Ercanbrack, A. Paula Grigorian, MD

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Description

Strabismus surgery is usually performed under general anesthesia due to the discomfort caused by traction on extraocular muscles and the length of the procedure. If the patient cannot tolerate the general surgery or wants to avoid the risks of intubation, other options can be used, like retrobulbar, sub-Tenon’s or topical.

General Anesthesia

Indications

General anesthesia is the most common and preferred form of anesthesia for strabismus surgery. It is definitively indicated in patients who are uncooperative, unable to communicate, have severe anxiety, have claustrophobia, are unable to lie completely still, or cannot lie flat comfortably. Therefore, many children require general anesthesia as they are unable to reliably be still for the entirety of the procedure.

Contraindications/ Warning and Precautions

General anesthesia can be performed in most cases where other forms of anesthesia are not possible, however caution should be taken in particular subsets of patients considered high risk. This is why patients have to undergo a preoperative evaluation by an anesthesia specialist. Patients with chronic kidney, heart or pulmonary insufficiency should have their condition optimized prior to anesthesia. High blood pressure or high blood sugar are relative contraindications. Substance use or abuse as well as smoking adds to the risk of anesthesia. Obesity with BMI>30 puts patients at higher risk and makes intubation difficult. As with most medication, general anesthesia should be avoided in pregnancy and lactation due to transfer to the fetus or infant.

Malignant hyperthermia and pseudocholinesterase deficiency was previously a contraindication to general anesthesia in strabismus surgery but is no longer considered to be valid, however it requires advanced planning to minimize the adverse reactions.

Of note, induction agents (e.g. propofol, thiopental, etomidate) and volatile anesthetics (e.g. halothane, desflurane, sevoflurane) can cause a decrease in intraocular pressure (IOP). Thus, if a patient has chronically low IOP, a different choice in anesthesia may allow for better intraoperative and postoperative results.

Nitrous oxide, an inhaled anesthetic that may be used for induction, should be avoided for 4-6 weeks after vitreoretinal surgery that employs a gas bubbleas it causes expanssion of the gas bubble with increased IOP and possible blindness. Nitrous oxide is contraindicated in the setting of venous or arterial emboli, pneumothorax, acute intestinal obstruction with bowel distention, pneumocephalus after dural closure, pulmonary air cysts, and prior tympanic membrane grafts.

Halothane is contraindicated if there is unexplained liver dysfunction following previous exposure to the gas, reduced ejection fraction heart failure, or if a patient has a pheochromocytoma.

Isoflurane and desflurane are relatively contraindicated in patients with severe asthma or if the patient has active bronchospasms.

Sevoflurane has a relative contraindication in patients with renal dysfunction.

Long-term effects of general anesthesia are not common. Immediately after surgery under general anesthesia, a patient may experience a sore throat or inability to eat due to traumatic intubation. However, this typically resolves on postoperative days 3-7 and there is no long-lasting sequela. Patients who receive inhaled anesthesia may also experience postoperative nausea and vomiting, occasionally requiring an anti-emetic.

Clinical Pharmacology

Inhaled anesthetics can induce neuronal depression by augmenting signals to GABA receptors and potassium channels. However, the exact mechanism of the inhaled anesthetics is not entirely known and remains as an area of study.

Blocks

Retrobulbar blocks consist of injection of anesthetic in the retrobulbar intraconal space while peribulbar blocks target the extraconal space.

Indications

Blocks are indicated in a wide variety of ocular surgeries, most commonly intraocular surgeries. Retrobulbar blocks are also suitable for patients that are unable to undergo general anesthesia. When compared to general anesthesia, retrobulbar blocks and other forms of local anesthesia were associated with fast recovery time and time to hospital discharge.

Retrobulbar (intraconal), peribulbar (extraconal) rarely used for strabismus due to myotoxicity to EOM when inadvertently injected in the muscle. It can cause progressive segmental fibrosis and/or hypertrophy of muscle. Vertical strabismus with fibrosis of the inferior rectus may complicate retrobulbar anesthesia used for intraocular procedures (cataract most commonly).

Contraindications

An absolute contraindication to retrobulbar blocks is prior allergic reactions. Local infections also preclude retrobulbar anesthesia. Relative contraindications include open ocular trauma, intraorbital vascular tumor, prior scleral buckling, myopic eye with axial length greater than 26 mm, thyroid-associated orbitopathy, bleeding diathesis, strong orthopnea, poor cooperation with block insertion, enophthalmos. It should be avoided in monocular patients due to the risks. Other forms of anesthesia also may be more beneficial with lengthy procedures.

Clinical Pharmacology

Retrobulbar blocks are most often a mix of 2% lidocaine with 0.75% bupivacaine. Bupivacaine inhibits NMDA receptors and sodium channels, reducing transmission in the dorsal horn of the spinal cord. Similarly, lidocaine inhibits sodium channels, preventing nerve depolarization. Occasionally, hyaluronidase is added to the mix of lidocaine and bupivacaine. Hyaluronidase is an enzyme that reversibly depolymerizes hyaluronic acid and was found to increase success with the initial block and akinesia while also reducing the need for additional blocks.

Warnings/Precautions

Like most periocular injections, complications are very uncommon but not impossible. Complications of the procedure include retrobulbar hemorrhage, ocular globe injury, optic nerve damage, EOM injury, and spreading of the anesthesia in the central nervous system. As previously discussed, retrobulbar anesthesia can also result in myotoxicity, leading to postoperative diplopia.

Subtenon

With subtenon anesthesia the medication is administered in the subtenon space. The anesthetic can be delivered through intact conjunctiva, or through a small conjunctival incision created in the quadrant, usually the same incision used to approach the EOM. A blunt intraocular canula, 19 G or 21 G, is used. A flexible intravenous catheter may also be utilized. A small amount of anesthetic (usually 1 ml) is injected in the sub-Tenon space surrounding the muscle insertion. Lidocaine 1% or 2% or bupivacaine 0.5% are commonly used and can be mixed with epinephrine.  

Indications

Sub-Tenon anesthesia can be used in a variety of ophthalmologic cases, including strabismus surgeries. It is used for patients that cannot tolerate or want to avoid the risks associated with the above methods. The advantages consist of lower risks of globe perforation, or retrobulbar hemorrhage as this method uses a blunt cannula.

Sub-tenon bupivacaine was also found to decrease incidence of the oculocardiac reflex and associated severe bradycardia compared to normal saline injections in strabismus surgery. It is also safer in patients taking anticoagulation and in eyes with longer axial lengths. Compared to retrobulbar anesthesia, sub-Tenon anesthesia has typically less associated pain. Sub-tenon anesthesia is commonly associated with varying degrees of intravenous sedation, depending on the patient’s anxiety.

Contraindications

Relatively few absolute contraindications exist for sub-Tenon anesthesia, aside from patient refusal, prior adverse reaction, inability to cooperate during administration, and infection at the injection site.

Clinical Pharmacology

Bupivacaine inhibits NMDA receptors and sodium channels, reducing transmission in the dorsal horn of the spinal cord. Lidocaine inhibits sodium channels, preventing nerve depolarization. Epinephrine can be added to lidocaine or bupivacaine to increase duration of anesthesia while simultaneously reducing bleeding in the operative field and risk of systemic absorption of the local anesthetic. Epinephrine does this by acting as a vasoconstrictor, counteracting the vasodilatory effects of local anesthesia in subcutaneous and submucosal tissues.

Warnings/Precautions

Complications are rare and consist of orbital hemorrhage, retrobulbar hemorrhage, retinal ischemia, optic nerve damage, orbital swelling, and rectus muscle dysfunction.

More commonly, chemosis and subconjunctival hemorrhages may occur following the administration of sub-Tenon anesthesia. However, these typically resolve and do not impair surgery.

Topical

Topical anesthesia can be obtained with drops of 0.5% proparacaine or tetracaine. A drop is administered and followed by 5% betadine to sterilize the area. A cotton swab soaked in tetracaine is placed in the conjunctival sac on the insertion of the muscle. An additional 0.5% proparacaine eye drops are instilled during the incision of the conjunctiva and Tenon's capsule. Drops can also be given as needed for pain during surgery. Gel forms of topical anesthesia are used by some surgeons; however, caution is advised as the betadine does not penetrate gels, increasing the risk of infection.

Indications

An advantage of topical anesthesia use in strabismus surgery is that it does not interfere with motility and cover tests. Therefore, these tests can be performed and can guide further muscle adjustments if necessary. Additionally, topical anesthetic may be used if the patient wishes to avoid injections or the pain associated with injectable anesthesia. Topical anesthesia also prevents risks associated with retrobulbar blocks and sub-Tenon’s anesthesia, such as retrobulbar hemorrhage or ocular perforation.

Contraindications

Prior patient allergic reaction and patient refusal are contraindications to topical anesthesia. Ester topical anesthetics (proparacaine, tetracaine, benzocaine, chloroprocaine) are contraindicated in patients with known allergies to para-aminobenzoic acid (PABA), sulfonamides, and hair dyes.

Topical anesthesia, most importantly, does not provide EOM akinesia. Therefore, a patient that who is unable to keep their eye still for the entirety of the procedure may be a better candidate for another form of anesthesia.

Clinical Pharmacology

Local anesthetics act by binding to voltage-gated sodium channels and preventing action potential initiation and propagation, ultimately inhibiting nerve transmission.

Warnings/Precautions

Topical anesthetic drops are typically well-tolerated and do not carry large risks. Topical anesthetic drops are capable of causing ocular surface damage but are usually only seen with topical ophthalmic anesthetic abuse. Therefore, these drops should not be prescribed.

Pregnancy

Strabismus surgery is an elective procedure therefore should be avoided in pregnancy.

Nursing Mothers

Most anesthetic agents, including volatile anesthesia, are considered safe in breastfeeding patients. However, codeine and meperidine analgesics should be avoided as their metabolites are capable of being passed to the infant via breast milk, leading to sedation of the infant. Some cases of hydromorphone being passed with breast milk have been reported and therefore should be used with caution. Bupivacaine and lidocaine are both considered safe in breastfeeding patients.

Pediatric Use

The majority of anesthetic agents, both regional and general, are safe in pediatric populations with weight-adjusted dosing and careful monitoring in the pre-, intra-, and postoperative period.