Anesthesia for Eye Surgery

Anatomy and Physiology

Understanding the anatomical considerations of the orbit is essential to regional anesthesia and achieving akinesis of the globe. The bony orbit contains primarily adipose tissue; the globe is positioned in the anterior portion of the bony orbit. The bony orbit contains many structures, including the optic nerve (cranial nerve II) and vascular bundles. The ophthalmic branch of the trigeminal nerve (division V1, cranial nerve V) provides sensory innervation to the globe.[4] The oculomotor (cranial nerve III), trochlear (cranial nerve IV), and abducens (cranial nerve VI) provide motor innervation to the extraocular muscles.[4] 

All of these nerves, except the trochlear nerve, pass through the muscular cone of the orbit. Injecting local anesthetic in this area can reliably provide sensory and motor blockade resulting in akinesia of the globe, extraocular muscle paralysis, and analgesia. When injecting a local anesthetic into the orbit, the needle should be kept extraconal to minimize the risk of puncturing important structures resulting in vision loss, nerve damage, and bleeding.

Intraocular pressure, the pressure exerted by intraocular contents on the surrounding wall, can substantially increase during ophthalmic procedures. Normal intraocular pressure is usually considered to range from 10 to 20 mm Hg. Increased intraocular pressure can directly injure or impair the perfusion of ocular structures such as the retina, choroid, and optic nerve.[5] The injection of as little as 0.5 mL of volume into the intravitreal space can increase intraocular pressure by over 150% compared to baseline levels. Pressure fluctuations can compromise retinal and optic nerve perfusion, resulting in postoperative visual impairment.

Indications

There are various approaches to anesthetic care for patients undergoing ophthalmic procedures. Anesthetic plans should be patient- and procedure-specific, accounting for underlying comorbidities while promoting cooperation, comfort, and safety. Anesthetic plans may comprise moderate sedation, monitored anesthesia care, or general anesthesia. Including topical and regional anesthesia in anesthetic plans have improved patient care and surgical outcomes.

If regional or topical anesthesia is pursued as an adjunct to monitored anesthesia care, the patient must be able to follow directions, respond appropriately, and tolerate the procedure and positioning. Most ophthalmic surgeries are performed with the patient supine. Comorbidities, such as heart failure, chronic obstructive pulmonary disease, or obstructive sleep apnea, may prohibit supine positioning. Sterile draping for ophthalmic procedures may limit access to secure an airway in the event of intraoperative respiratory compromise. Neurocognitive disorders may preclude patient cooperation and interactions. Pediatric patients may demonstrate difficulties being still and may benefit from general anesthesia.

The needs of specific procedures also influence anesthetic care. Most procedures for cataracts or glaucoma are completed under monitored anesthetic care combined with topical anesthesia and regional anesthetic techniques. In the United States, peribulbar and topical anesthetic techniques are the most commonly utilized approaches for cataract procedures.[6] However, patients undergoing vitreoretinal procedures in the United States will typically receive a combination of a regional anesthetic technique with general anesthesia. Topical anesthesia is not utilized due to the length of most vitreoretinal procedures. In contrast, vitreoretinal procedures are usually performed under a peribulbar block in India and other countries.

Contraindications

An absolute contraindication to the use of anesthesia in ophthalmic procedures would be patient refusal.

A history of anaphylaxis to local anesthetics may be a relative contraindication to a regional block. Local anesthetics employed in ophthalmic procedures include aminoesters and aminoamides; lidocaine is an aminoamide commonly used in local and regional anesthesia techniques. However, true anaphylaxis to lidocaine is exceedingly rare; most episodes of anaphylaxis are due to an allergy to the preservative methylparaben. Preservative-free lidocaine is available.[7] 

Malignant hyperthermia may be a relative contraindication to general anesthesia. However, proper preoperative planning surrounding the choice of anesthetic agents and readiness for complications will facilitate general or local anesthesia use in patients with malignant hyperthermia.[8]

Nitrous oxide is an absolute contraindication in some vitreoretinal surgeries. During procedures to correct retinal detachment, the ophthalmologist may inject a gas bubble of either sulfur hexafluoride or perfluoropropane to intentionally tamponade the retinal break. This gas bubble must be reabsorbed entirely before administering nitrous oxide, which can enter and expand this gas bubble, increasing intraocular pressure and the subsequent risk of permanent blindness.[8]

Localized eye infections and an increased axial length of the globe may be relative contraindications to specific anesthetic plans.[9]

Equipment

Anesthesia for ophthalmic surgeries utilizes standard anesthesia equipment, including oxygenation, ventilation, circulation, and temperature monitors. Oxygenation can be assessed via pulse oximetry, ventilation via capnography, and circulation via electrocardiogram and blood pressure readings. An anesthesia machine, which typically includes a gas analyzer, may be required to deliver positive pressure ventilation, oxygen, and volatile anesthetics.

Emergency drugs such as epinephrine and atropine, neuromuscular blocking agents, and airway equipment must be readily available. Lipid emulsification should be available for inadvertent intravascular injection and resulting local anesthetic systemic toxicity. Local and regional anesthetic agents are required for those anesthesia plans. Hyaluronidase may assist in the spread of local anesthetic to achieve akinesia of the globe while reducing the infiltrative volume of the local anesthetic.

Depending on the technique employed, local and regional blocks require 25- to 27-gauge needles.

Personnel

Anesthesia for ophthalmic procedures is administered by the anesthesia care team, which may include anesthesiology assistants and nurse anesthetists (CRNA) under the direction of an anesthesiologist. 

Preparation

Patients presenting for ophthalmic procedures requiring anesthesia should undergo a thorough preoperative assessment. A comprehensive medical history and physical examination are necessary to assess comorbidities and develop the safest anesthetic plan that facilitates the intended procedure and optimizes patient outcomes.

Comorbidities such as obesity, heart failure, pulmonary disorders, anxiety, claustrophobia, and chronic back pain may impair the patient's ability to tolerate the supine positioning, head immobilization, and sterile draping utilized for most ophthalmic procedures. The use of medications with anesthetic implications should be identified. Many patients undergoing eye surgery have been prescribed topical ophthalmic medications such as timolol, carteolol, pilocarpine, and phenylephrine, which are frequently absorbed via the nasal mucosa and can have intraoperative hemodynamic effects.[10] Additionally, anticoagulant medications may negatively affect or prohibit regional anesthetic techniques.

The preoperative physical examination should assess the patient's ability to cooperate and communicate effectively. Ophthalmic procedures are commonly performed in children and older adults who may not possess the requisite cognitive function to participate in conscious sedation or monitored anesthesia care with adjunctive local or regional anesthesia.

Technique or Treatment

Topical Anesthesia

Topical anesthesia is one of the most commonly utilized anesthetic techniques for ophthalmic surgery. Topical anesthesia is routinely used during surgical procedures for cataracts or glaucoma. Topical anesthesia can also mitigate injection-related bleeding risks for patients taking anticoagulants. Topical anesthesia is an excellent choice for procedures that do not require akinesia of the globe. Topical anesthesia is administered directly over the cornea and conjunctiva using local anesthetic drops or gels such as lidocaine, proparacaine, and tetracaine.

Regional Anesthesia

Regional anesthesia techniques can provide varying degrees of akinesia and analgesia through transconjunctival or percutaneous injections. Patient cooperation is paramount to successful regional anesthesia. Patients must be able to follow instructions and maintain a straight-ahead primary gaze position to protect the optic nerve and ophthalmic artery during intraorbital injection with a sharp needle. Negative aspiration should be confirmed before injecting the local anesthetic.

Regional anesthesia via nerve blockade is usually accomplished via the retrobulbar, peribulbar, or sub-Tenon (episcleral) block. The sub-Tenon block is rarely utilized in the United States but is commonly used in Great Britain.

The intraconal retrobulbar block was one of the earliest regional anesthetic techniques applied to ophthalmic procedures. This block may be performed via a transconjunctival or percutaneous route, the choice of which is dictated by operator comfort and patient anatomy. Using anatomical landmarks, identify the space slightly lateral to the junction of the middle and lateral thirds of the inferior orbital rim.[9] In a percutaneous approach, applying slight pressure above the rim of the eye creates a space between the globe and the infraorbital bone. This space is used to guide the trajectory of the needle, decreasing the risk of globe perforation. In a transconjunctival approach, slight downward pressure is applied near the inferior orbit to retract the eyelid before advancing the needle upward and inward into the space behind the globe. Confirm negative aspiration. Inject 2 to 5 mL of local anesthetic using a 23- to 25-gauge needle with a length of <1.5 in. This needle size will reduce the risk of globe perforation and damage to the nearby optic nerve and vasculature. The benefit of the retrobulbar block is the deposition of local anesthetic in the intraconal portion of the eye, providing maximum akinesia in approximately 85% of patients.[11]

The peribulbar block can be performed with a 25- to 27-gauge needle with a maximum length of 1.25 in. The peribulbar block is safer than the retrobulbar block because the needle employed to inject the local anesthetic remains extraconal, and the injection trajectory is less steep. The peribulbar block has a decreased incidence of adverse effects such as globe perforation, optic nerve injury, and central nervous depression secondary to local anesthetic injection into the intrathecal space.[12][13] 

The peribulbar block may be performed via an inferotemporal or medial approach. The inferotemporal approach utilizes the same anatomical landmarks as the retrobulbar block but does not require the needle to enter the myofascial orbital cone. The medial peribulbar block utilizes the approximate 10-mm space between the medial orbital wall and the orbital fat pad. A total volume of 6 to 12 mL of local anesthetic is injected into this space and allowed to spread in all directions within the extraconal orbital space.

The peribulbar block paralyzes the orbicularis oculi muscle improving operating conditions during ophthalmic procedures.[14] Although the peribulbar block is considered a safer procedure with less exposure to the posterior globe and intraconal anatomical structures, there is a higher incidence of conjunctival chemosis due to the larger volume of injectate, and more time is required for this block to take effect.[15]

The sub-Tenon block is performed by creating a small incision through the conjunctiva and Tenon capsule, which are secured via forceps. The sclera is carefully dissected with blunt scissors. A 19-gauge catheter is placed through the incision, and approximately 2 to 5 mL of local anesthetic is deposited into this space. This local anesthetic can spread to the posterior globe via this anterior approach. This block may take approximately 5 minutes to affect analgesia and akinesia.

Regardless of the regional anesthesia technique employed, assessing for any motion of the globe or extraocular muscles before the ophthalmic procedure begins is essential.

General Anesthesia

Eye surgery can be performed under general anesthesia. Although uncommon, the decision to utilize general anesthesia is based on patient factors and the specific details of the intended procedure. Vitreoretinal surgery is one of the most common ophthalmic procedures performed under general anesthesia. Under general anesthesia, it is necessary to maintain a deep and stable plane of anesthesia to create the best surgical outcomes. [16] 

Knowledge of intravenous and volatile anesthetic agents for induction and maintenance, pharmacotherapy concerns of opioids and neuromuscular blocking agents, airway management, and emergency procedures is imperative when utilizing general anesthesia. Additionally, the relationship between these anesthetic agents and intraocular pressure (IOP) must be continuously evaluated. Most induction agents, including propofol, thiopental, and etomidate, decrease IOP by approximately 30%.[17] Volatile anesthetics, commonly used to maintain anesthesia after induction, also cause a decrease in IOP. The combination of propofol and sevoflurane decreases IOP significantly and is a reasonable approach to counter the increase in IOP from the sympathetic response to laryngoscopy for airway management.[18]

Choosing an anesthetic plan to secure the airway via endotracheal intubation or a supraglottic airway device (SGA) is important; both have advantages and disadvantages. The benefit of an SGA is a smaller increase in IOP and smoother emergence upon removal compared to endotracheal intubation.[19] However, endotracheal intubation is a more secure airway and should be chosen if the patient is at increased risk of aspiration secondary to gastroesophageal reflux disease, obesity, a hiatal hernia, or decreased gastric motility as seen with gastroparesis from diabetes mellitus. The disadvantage of endotracheal intubation is the increase in IOP, which can be attenuated with pretreatment of fentanyl, remifentanil, dexmedetomidine, and clonidine before laryngoscopy.[20] 

If intubation is selected, it may be beneficial to avoid depolarizing neuromuscular blocking agents with the risk of a transient increase in IOP.[21] Ecothiophate is a medication commonly prescribed to patients with glaucoma that is an irreversible inhibitor of pseudocholinesterase, an enzyme responsible for the degradation of succinylcholine; ecothiophate may result in prolonged paralysis. If nondepolarizing neuromuscular blockade is pursued, it may be beneficial to reverse the paralytic using sugammadex instead of neostigmine and glycopyrrolate, which are anticholinergic agents known to increase IOP.[22] 

Regardless of the airway device, it is crucial to maintain a deep plane of anesthesia to avoid laryngospasm and even the slightest movements that can be detrimental to surgical outcomes. General anesthesia under volatile anesthetics can cause severe postoperative nausea and vomiting, drastically increasing IOP. Steps must be taken to reduce the likelihood of postoperative nausea and vomiting.

Complications

Regardless of the anesthetic approach, the inadvertent intravascular injection of local anesthetic can result in systemic toxicity.

The retrobulbar block has the highest incidence of adverse events due to injuries to surrounding anatomical structures.[9] The complications associated with retrobulbar blocks include globe perforation, the risk of which is increased when the axial length of the eye exceeds 26 mm, respiratory arrest secondary to intrathecal spread of anesthetic to the brainstem from inadvertent puncture through the optic nerve dura, and severe vision loss from increased IOP related to retrobulbar hemorrhage or optic nerve compression from local anesthetic infiltration.[23] A retrobulbar hemorrhage can be sight-threatening; clinical signs like proptosis and increased IOP suggest retrobulbar hemorrhage and warrant consideration of a lateral canthotomy.

Complications of peribulbar blocks are less severe and less common, increasing the popularity of this regional block. The most common complication of a peribulbar block is temporary swelling of the conjunctiva secondary to the accumulation of larger volumes of local anesthetic within the confined space of the orbit. Similarly, a sub-Tenon block has lower complication rates because anesthesia can be achieved using a blunt needle. A sub-Tenon block is a reasonable approach in patients taking anticoagulants or with an increased axial eye length; using a sharp needle in these circumstances would increase the risk of adverse events. 

If the oculocardiac reflex is stimulated intraoperatively, hemodynamic compromise may occur. Stimuli of the oculocardiac reflex include pressure on the globe, ocular pain, manipulation of the eye, and traction on the extraocular musculature, particularly the medial rectus muscle. The afferent nerve of this reflex is the trigeminal nerve (cranial nerve V), and the efferent nerve of the reflex arc is the vagus nerve (cranial nerve X). The reflex triggers severe bradycardia, bradyarrhythmias, and hypotension, which must be immediately recognized and treated. Treatment entails cessation of surgical stimulation and pharmacotherapy with atropine or glycopyrrolate to increase heart rate; atropine is preferred due to its faster onset.[24]

Clinical Significance

Understanding the expected anesthesia-induced changes in IOP is essential to a successful ophthalmic procedure. A decrease in IOP can be achieved with most inhaled and intravenous anesthetics. Other physiological changes such as hypocarbia, hypothermia, and decreased mean arterial pressure will also lower the IOP. Increases in IOP can be seen with drugs like succinylcholine and ketamine, as well as events such as the Valsalva maneuver or the application of external pressure to the eye. Vomiting may increase IOP by approximately 30 to 40 mm Hg; perioperative prophylaxis of nausea and vomiting should be administered. When considering the options for postoperative nausea and vomiting prophylaxis, it is important to remember that scopolamine can precipitate acute angle-closure glaucoma and should be avoided in susceptible patients.

Nitrous oxide should be avoided for 4 to 6 weeks after vitreoretinal surgery that uses an injected gas bubble. Nitrous oxide can enter and expand this gas bubble causing ischemia of the optic nerve.

Many patients undergoing eye surgery may use topical eye medications such as echothiophate, phenylephrine, beta-blockers, and acetazolamide. All of these medications can be absorbed and exhibit systemic effects. Echothiphate may result in prolonged paralysis if succinylcholine is used during intubation. Topical beta-blockers and phenylephrine can influence heart rate and blood pressure by acting on their corresponding receptors.

The oculocardiac reflex may produce severe bradycardia, hypotension, and asystole. A light plane of anesthesia and physiological conditions that may arise under general anesthesia, such as hypoxia and hypercarbia, increase the risk of intraoperatively stimulating this reflex.

Enhancing Healthcare Team Outcomes

Providing anesthesia for ophthalmic procedures requires a collaborative effort from an interprofessional team, aiming to enhance outcomes following eye surgery through comprehensive patient evaluation, personalized treatment planning, optimized perioperative management, multidisciplinary support, continuity of care, and patient education.

The clinical nursing staff plays a crucial role in the perioperative period by preparing patients for surgery, obtaining intravenous access, preparing and reviewing procedural and anesthesia consent documents, reconciling medications, monitoring patients throughout the perioperative period, and preparing patients for discharge.

Effective communication between team members during eye surgeries is imperative. Surgical technicians and operating room nurses maintain and monitor the sterile field to minimize infection and patient injury. The ophthalmologist and anesthesia team monitor intraocular pressure and must effectively communicate any changes in this critical vital sign to avoid adverse patient outcomes.

Patient-centered care delivered by an interprofessional team enhances team performance, resulting in better patient outcomes.