Introduction
The aims of anesthetic management during eye surgery are pain-free procedures, quick recovery, and reducing complications. Patients who undergo eye surgery are mostly at the extreme end of their age and require vigilant intraoperative and postoperative monitoring and care.
Anatomy of the Eye
The eye and orbit can be viewed as a pyramid-shaped structure, whose base faces anteriorly and apex posteriorly. The eye’s motor nerves consist of the 3rd, 4th, and 6th cranial nerves that are responsible for eye motility and the 7th cranial nerve for the frontalis and orbicularis oculi movement. The 3rd cranial nerve (oculomotor) supplies the medial, inferior, and superior recti and the inferior oblique muscles. The superior oblique is innervated by the 4th cranial nerve (trochlear), and the lateral rectus is innervated by the 6th cranial nerve (abducens). The facial nerve is split into the upper zygomatic branch, which supplies the upper lid’s frontalis muscle and orbicularis oculi. The lower zygomatic branch supplies the orbicularis oculi of the lower eyelid. The 5th cranial nerve covers the sensory nerve supply of the eye and its adnexa.
Preoperative Evaluation
The preoperative medical assessment involves an examination of comorbid conditions and perioperative decision-making about prescribed medications to continue or not. People with diabetes should check blood sugar levels the day before surgery. On the morning of surgery, all diabetic medicine should be omitted. Hypertensive patients must get their blood pressure (BP) checked preoperatively.
All patients receiving regional anesthesia fasted from midnight of the day before surgery. Regular airway inspection should always be done. Detection of specific irregularities, including those symptoms that intervene with comfortable supine lying (e.g., congestive heart failure, chronic obstructive pulmonary disease [COPD], back pain, or claustrophobia), should be undertaken cautiously during preanesthetic evaluation.
Besides observing the patient’s oral drugs, the anesthesiologist should be cautious of possible systemic consequences of these chronically administered ophthalmic solutions. Timolol eye drop, nonselective beta-adrenoreceptor antagonists, can decrease heart rate and increase systemic vascular resistance. Some ophthalmic solutions’ systemic effects include increased heart rate (HR) or BP with muscarinic agonists like pilocarpine, increased blood pressure by phenylephrine, etc.
Choice of Anesthetic Technique
The type of operation, the estimated length, age, and fitness of the patient can affect the anesthetic technique. It can be done under topical, local, or regional and general anesthesia (Fig. 29.1).
Topical anesthesia is a safe option in situations where the surgeon may not need full akinesia. It is the most common anesthesia technique for cataract and intraocular glaucoma surgery. Other applications include:
The anesthesiologist applies a local anesthetic eye drop to the cornea and conjunctiva (e.g., 2% lidocaine, 0.5% proparacaine). It is rapid-acting (within 20 s) and lasts between 15 and 20 minutes. It is contraindicated in the presence of penetrating eye injury. It is the simplest method for anesthesia for the anterior eye chamber and can be employed as the sole anesthetic technique. Complication risk is minimal relative to other anesthetic techniques as no needle is used. It cannot provide eye akinesia. As the patient must remain motionless for the whole process, the proper patient selection is critical.
Topical anesthesia can be combined with 1% lignocaine solution injection into the anterior chamber through paracentesis or side-port incision. This gives iris and ciliary body anesthesia.
Regional anesthesia is more widely employed for cataract and glaucoma surgeries and less often for vitreoretinal surgery. It can be helpful in patients unable to tolerate general anesthesia. The objective of regional anesthesia is twofold. First, it achieves lid and eye muscle akinesia. Second, it provides analgesia to the eye. Lignocaine hydrochloride or bupivacaine with or without 1:1000 adrenaline and hyaluronidase are usually used in regional anesthesia. The incorporation of hyaluronidase enhances tissue permeability of the local drug and helps diffuse local anesthetic. Epinephrine is sometimes used to increase the anesthesia duration.
In regional anesthesia, facial nerve blocks are performed to obtain lids akinesia. Retrobulbar and peribulbar blocks are administered for both akinesia and globe analgesia.
It prevents squinting of the eyelids during operation and enables the speculation of the lid to be put. There are four techniques of facial block: Van Lint’s block, Atkinson block, O’ Brien block, and Nadbath block.
Van Lint’s block: The peripheral divisions of the facial nerve are blocked in van Lint’s block. This procedure induces orbicularis oculi muscle akinesia without facial paralysis. A total of 2.5 mL of anesthetic agent is administered at the proximity of the orbital rim; the injection site is 1 cm lateral from the lower outer edge of the orbit, at the crossing of lines drawn parallel to the lower and temporal rim of the orbit.
O’ Brien’s block: It is also known as the trunk block of the facial nerve. The block is done at the mandible neck in front of the tragus of the ear and near the condyloid process. The needle is placed at this level, and around 4 mL of local anesthetic is injected while removing the needle.
Atkinson’s block: The facial nerve’s superior branch is blocked by administering anesthetic solution at the lower zygomatic bone margin.
Nadbath block: In this block, the facial nerve is blocked at the stylomastoid foramen. It can cause vocal cord paralysis, laryngospasm, dysphagia, and respiratory distress.
Retrobulbar block (intraconal block)
Herman Knapp first described this method in 1884. A local anesthetic is injected behind the eye into the cone formed by extraocular muscles (Fig. 29.2). A blunt-tipped 25-gauge needle penetrates the lower lid at the junction of the middle and lateral one-third of the orbit (usually 0.5 cm medial to the lateral canthus). Awake patients are told to gaze supranasally when the needle is advanced 3.5 cm to the muscle cone apex. Patients receiving this eye block would usually receive brief sedation during the block. After aspiration to avoid intravascular injection, 2 to 5 mL of local anesthetic is injected. Local anesthetic choices vary, although lidocaine 2% or bupivacaine 0.75% is more commonly used. The addition of epinephrine (1:200,000) can minimize bleeding and prolong anesthesia. Hyaluronidase (3–7 U/mL) is also applied to improve local anesthetic retrobulbar distribution. A retrobulbar block is followed by anesthesia, akinesia, and oculocephalic reflex abolition. Typically, the superior oblique muscle outside the muscle cone is not paralyzed. The retrobulbar block has a quicker onset than a peribulbar block, which is associated with less chemosis (i.e., conjunctiva swelling).
The retrobulbar block should not be practiced in selected patients like:
The retrobulbar block complications include:
Peribulbar anesthesia (extraconal block)
This technique was first applied by Davis. Unlike the retrobulbar blockade, the needle in the peribulbar blockade technique does not penetrate the cone formed by extraocular muscles. In peribulbar block, 6 to 12 mL of local anesthetic is administered into the orbit’s peripheral spaces. The anesthetic agent diffuses through the muscle cone and eyelids, inducing global and orbicular akinesia and anesthesia. After topical conjunctiva anesthesia (Fig. 29.2), an inferotemporal injection is given halfway between the lateral canthus and the lateral limbus. The needle is progressed under the globe, parallel to the orbital floor; after reaching the eye equator, it is guided slightly medial (20°) and cephalad (10°), and 5 mL of local anesthetic is administered. A second 5-mL injection can be delivered through the conjunctiva on the nasal side, medial to the caruncle, and directed straight back parallel to the medial orbital wall, pointing slightly cephalad (20°). After injection, orbital compression is implemented for around 15 minutes.
The advantages of peribulbar block involve fewer injury chances to the globe, optic nerve, and artery, and less discomfort on injection. However, it has slower onset and a higher risk of ecchymosis.
SubTenon (episcleral) block: Tenon’s fascia covers the globe and muscles. Under it, local anesthetic extends circularly across the sclera to the extraocular muscle sheaths (Fig. 29.2). For a subTenon block, a blunt 25-mm or 19-gauge curved cannula is used. After topical anesthesia, the conjunctiva is lifted along with Tenon’s fascia in the inferonasal quadrant. A tiny nick is then produced with blunt-tipped scissors and slid underneath to create a track in Tenon’s fascia which follows the globe’s contour and extends beyond the equator. When the eye is still fixed, the cannula is inserted, and 3 to 4 mL of local anesthetic is applied. Complications of subTenon blocks are much less than those of retrobulbar and peribulbar techniques.
