21 Anesthesia for Strabismus Surgery

Rebecca Lee and Frederick M. Wang


Individualizing anesthesia choices for strabismus surgery based on patient considerations leads to a better and safer patient experience, improved surgical outcome, and a more efficient operating room. This chapter relates the essential techniques of anesthesia for strabismus patients. The basic preoperative evaluation to identify risks, allay anxiety and pinpoint the best anesthetic route is discussed. Targeted preoperative laboratory testing recommendations are outlined. The pros and cons of premedication are given. Anesthesia principles, airway considerations, and pharmacologic options, from induction through maintenance to emergence, are explained. Techniques to enhance a rapid fresh recovery and speedy discharge are covered. Specific risk factors, including thermoregulation, the oculocardiac reflex, malignant hyperthermia, the use of muscle relaxants, postoperative nausea and vomiting, and potential neurotoxicity in young children, are analyzed. Important postoperative considerations, including airway patency, pain control, and emergence delirium, are included. Special methods, including total intravenous (IV) anesthesia, local anesthesia, and anesthesia for adjustable sutures, are described. An understanding of this chapter’s parameters and techniques will help the strabismus surgeon interface with the anesthesia team and maximize the result.

21 Anesthesia for Strabismus Surgery

21.1 Introduction

Proper anesthesia allows for strabismus surgery to be accomplished safely on a patient who is comfortable during the procedure, who is as immobile as necessary, and who recovers easily and quickly. Attention to specific details leads to a better patient experience, improved surgical outcomes, and a more efficient operating room (OR). A host of potential complications are to be avoided. An understanding of particular individual patient parameters and anesthesia options will help the strabismus surgeon interface with the anesthesia team and maximize the result.

21.2 Preoperative Evaluation

Anesthesiologists meet the majority of their patients and their families for the first time on the day of their procedure; therefore, the surgeon is responsible for ensuring patient preparation for anesthesia. This includes screening for conditions that should be evaluated and cleared prior to the day of surgery, ordering appropriate preoperative laboratory testing, and giving proper Nil per os, nothing by mouth (NPO) instructions. In addition, conditions that would necessitate the cancelation of surgery, such as the development of an upper respiratory infection (URI), should be discussed.

The patient should undergo a history and physical examination by his primary physician in a timely fashion: close enough to the surgery date to assess for acute conditions, but allowing sufficient time for proper specialist evaluation when necessary. Two weeks is therefore recommended.

Preoperative assessment should focus attention on cardiorespiratory issues, diabetes, bleeding disorders, seizures, and renal or hepatic insufficiency. Prior anesthetic experience of the patient, such as prolonged emergence, airway difficulty, and postoperative nausea and vomiting (PONV), should be noted. A patient or family history of anesthetic complications should be questioned, specifically searching for anything that could raise a concern for malignant hyperthermia (MH). The patient’s medications should be recorded. Most nonelective medications may be taken with a small sip of water the morning of surgery without regard to other NPO orders. Any allergies (even food allergies, as there may be cross-reactivity issues with some anesthetics and latex) should be noted. Latex allergy should be communicated to the anesthesia team prior to the day of surgery to allow preparation. 1

Patients should call the surgeon/anesthesiologist prior to the surgical day if they do not feel that they are in their usual state of health. Even minor URIs are a significant risk factor for anesthesia, and surgery should be rescheduled. Multiple studies have shown that active URI causes adverse respiratory events such as breath-holding, bronchospasms, laryngospasm, desaturations, and significant coughing episodes. 2 Airway hyperreactivity can persist for a few weeks after resolution of URI symptoms. Particular caution after a URI should be used with with a history of reactive airway disease, prematurity (< 37 weeks), smoking in the home, presence of any secretions or nasal congestion, and prior airway surgery. If a child has a chronic allergic rhinitis or chronic asthma, examination on the day of surgery will determine if the patient is in his or her “optimal” condition. 3

21.2.1 Preoperative Laboratory Testing

Preoperative laboratory testing is accomplished to identify conditions that carry perioperative risk, especially those that would change surgical considerations, anesthetic techniques, and pre- and postoperative management. Historically a battery of preoperative tests was performed on a routine basis. Studies show that none of these tests are routinely necessary for the identification of anesthetic risk in healthy children and nonelderly adults. 4 Many institutions require an electrocardiogram (ECG) in asymptomatic patients over age 60. Even this recommendation has not been confirmed to be cost-effective.

Targeted testing is based on a patient interview and review of previous medical records and physical examinations. 5 Particular emphasis should be placed on cardiovascular, pulmonary, renal, hepatic, hematologic (anemia and bleeding), and diabetic histories. The patient’s medications will help target testing, e.g., coagulation studies on those patients taking anticoagulants or with a history of liver disease.

Pregnancy testing of women in the childbearing age group presents unique challenges. No currently used anesthetics have been shown either to be teratogenic or to cause neurodevelopmental effects on the fetus. There is evidence that anesthesia carries a risk of increased spontaneous miscarriage in the first or second trimester, and an increased incidence of premature delivery and intrauterine growth retardation, resulting in a higher rate of low-birth-weight infants. 6

Preoperative pregnancy testing should be offered to all women during their childbearing years. The American Society of Anesthesiologists feels this should not be mandatory, but strongly encouraged with informed consent. 7 Many institutions, however, do require testing for pregnancy in elective cases. When pregnancy testing is accomplished in adolescents, a mechanism to privately inform the patient if the test is positive should be in place. In many states, pregnancy emancipates a child from having to inform the parents of the pregnancy. Urine pregnancy tests may be negative in the first few weeks after conception, and testing by blood level of human chorionic gonadotropin may not trigger a positive result for 7 to 12 days. A menstrual history, therefore, should always be taken even in the face of a negative pregnancy test. When routine pregnancy testing is accomplished, 1 to 2% of patients are newly discovered to be pregnant, and we recommend that strabismus surgery should then be canceled. There are many medical, ethical, and legal considerations forming the basis of this opinion.

21.2.2 NPO Orders

Mechanisms that prevent aspiration when a conscious person vomits are lost under general anesthesia. The gastroesophageal junction, upper esophageal sphincter, and protective laryngeal reflexes provide the normal physiologic mechanisms to reduce the risk of aspiration, which are all attenuated by drugs used to induce and maintain general anesthesia.

Aspiration pneumonitis is one of the most serious anesthetic complications. In a recent British study aspiration accounted for more deaths than failure to intubate or ventilate. 8 The morbidity of aspiration pneumonitis is dependent on the pH and volume of the aspirated materials. Experimental evidence has shown that aspiration of over 25 mL of material with a pH of less than 2.5 is the average threshold to produce aspiration chemical pneumonitis. In addition, aspiration of solid material causes pulmonary obstruction with secondary effects. Minimizing aspiration is accomplished by withholding food and liquid for a certain period before the induction of anesthesia. Studies of gastric aspiration and ultrasound results have informed us of stomach emptying times. 9 NPO guidelines are given in Table 21‑1. 10

Table 21.1 NPO guidelines by the American Society of Anesthesiologists

Fasting recommendations

Ingested material

Minimal fasting period

Clear liquids

2 hours

Breast milk

4 hours

Infant formula

6 hours

Nonhuman milka

6 hours

Light mealb

6 hours

Fried foods, fatty foods, or meat

At least 8 hours

Note: Most nonelective medications may be taken with a small sip of water the morning of surgery without regard to other NPO orders.

aSince nonhuman milk is similar to solids in gastric emptying time, the amount ingested must be considered when determining an appropriate fasting period.

bA light meal typically consists of toast and clear liquids.

Patient factors that increase the risk of aspiration include those that delay stomach emptying such as diabetes mellitus, chronic kidney disease, opioid use, raised intracranial pressure, previous gastrointestinal surgery, and pregnancy. Incompetent sphincters, such as in hiatal hernia, pregnancy, and morbid obesity, increase regurgitation risk. Positive pressure ventilation through a mask or laryngeal mask airway (LMA) may insufflate the stomach, which adds to aspiration risk. Light anesthesia and long surgeries also predispose to vomiting.

Anesthetic techniques that minimize aspiration risk include head up positioning and the use of a clear mask for induction to be able to quickly see any initial regurgitation, as well as aspiration of the stomach contents. Rapid sequence induction, tracheal intubation or use of second-generation supraglottic airway devices, and cricoid pressure all help prevent aspiration complications. The Trendelenburg position and the nausea that particularly accompanies strabismus surgery increase aspiration risk. Compliance with NPO orders is therefore particularly important with strabismus surgery.

21.2.3 Preoperative Anxiety and Premedication

Preoperative anxiety is common in patients of all ages but more so in the pediatric age group. Over 65% of children experience preoperative anxiety. There are pharmacologic and nonpharmacologic methods to help decrease anxiety and stress both preoperatively and at induction in children. Minimizing anxiety in this patient population is important, as perioperative anxiety in children has been shown to lead to emergence delirium and maladaptive behaviors and other issues (bed-wetting, nightmares, separation anxiety, and eating problems) that can last up to a year after surgery. In addition, children with higher preoperative anxiety exhibit higher pain scores, requiring more analgesics postoperatively.

Factors that predispose children to preoperative anxiety include age over 7 years, excessive parental anxiety, an unfamiliar hospital environment, and a history of stranger/separation anxiety. At all ages, preoperative anxiety may be triggered by a previous unpleasant hospital experience, uncertainty about the outcome from the intervention (fear of the unknown), and fear of pain and reoperation.

Familiarizing the child with the OR through videos or pamphlets is useful. A visit to the surgical area prior to their procedure day, when they can practice breathing into an anesthesia mask, experience a blood pressure cuff, and put on a pulse oximeter, will make their upcoming experience a familiar one. Some hospitals also utilize child life professionals to help patients during preoperative assessment and induction. Children can bring games on a phone or tablet into the OR with them, which both lends an element of familiarity and creates a diversion.

Parental presence at induction is usually a helpful technique to decrease perioperative anxiety for the child and the parent. Parents who are present at induction are more satisfied with their perioperative experience and their overall hospital experience. The anesthesiologist/anesthesiology team and/or perioperative nursing staff should prepare the parent for the sequence of events of induction, especially for inhalational induction. The parents should be informed that an expected and normal excitation phase of inhalational induction will consist of eye rolling, tachypnea, obstructive breathing patterns, and unintentional limb or muscle movements. Parents should be asked if they feel they will be too anxious or queasy to perform the requested role of distracting and calming their child. The benefit of parental presence decreases if the parent is also anxious or emotional, as children pick up on these emotions. Also, the staff may need to be concerned with a light-headed parent. If this occurs, have the parent leave the room.

When indicated, the preoperative sedative/anxiolytic should be administered in the holding area while the patient is being monitored by perioperative nursing staff. The most common route is oral, but it may also be administered intranasally or intramuscularly. Rarely, rectal routes are utilized. For patients who require an intravenous (IV) induction, premedication may facilitate IV placement.

Adverse reaction to oral premedication is uncommon in healthy children. Patients with obstructive sleep apnea and tonsillar and adenoidal hypertrophy may develop sedation and apnea with even a small dose of premedication, leading to desaturations. This can also occur postoperatively in these children and delay discharge.

Common premedications include midazolam, ketamine, and dexmedetomidine. Unless the case is very short, these do not delay discharge from the facility postoperatively. Midazolam works by inducing somnolence, decreasing anxiety, and inhibiting memory formation. 11 ,​ 12

21.3 General Anesthesia

General anesthesia involves controlling the patient’s level of consciousness, pain, and movement while maintaining general homeostasis. 13 The three phases of general anesthesia are induction, maintenance, and emergence.

21.3.1 Induction

The patient should be placed supine on the operating table with the head all of the way toward the top of the table to allow the surgeon to look directly down on the field. The top of the head should be depressed (chin elevated) so that the brow ridge is at the level of the cornea. The surgeon then does not have to operate over the brow ridge. A “shoulder roll” under the nipples is helpful to facilitate this position, especially in children.

Anesthetic agents for induction may be delivered by either an inhalation or IV route. Most younger children and parents prefer inhalational induction, as it avoids the discomfort of IV placement. Older children and adults usually prefer IV induction, which may be facilitated by a whiff of nitrous oxide. Indications for IV induction include patients with aspiration risk, difficult airways, and medical conditions that carry significant anesthetic risks necessitating IV access, such as cardiac disease or a family history of MH. IV induction is most commonly accomplished with propofol. Inhalational induction requires the patient to breathe the volatile anesthesia gas via a mask. Anesthetic gases have an unpleasant smell, but sevoflurane is the least pungent of the inhaled anesthetics and is therefore commonly used. Induction may be performed gradually, starting with a nitrous oxide/oxygen mixture and slowly increasing the sevoflurane concentration, or induction may be done quickly with a mixture of a high concentration sevoflurane, with or without nitrous oxide, as a single-breath induction. The speed of induction can be titrated to the cooperation of the patient. The child’s awake time is lessened with rapid induction, but the high concentration of the noxious gases may disturb the child. If the gradual raising of the concentration allows for smooth induction, this is employed.

During inhalational induction, the patient will experience stage 2 or the excitement phase; this is rarely seen with IV induction. This is the most likely phase for laryngospasm to occur. During this excitement phase, the patient may exhibit tachycardia, obstructed and irregular breathing pattern with tachypnea, breath-holding, muscle rigidity, and unprovoked muscle movements. At this time external stimulation, such as repositioning the patient or starting an IV, is to be avoided, as these actions can precipitate laryngospasm. Once the patient has completed stage 2, a peripheral IV is inserted to facilitate administration of medications.

Ketamine may also be employed for premedication and induction and may be given orally. If a very short procedure (e.g., granuloma excision) is to be done, then ketamine may be all that is necessary. Ketamine reduces the incidence of the oculocardiac reflex (OCR) and lessens the incidence of PONV. Ketamine use may slightly increase the incidence of postoperative psychologic upset and nightmares. Concomitant midazolam not only helps prevent hallucinations but also may lead to pleasant dreams. 14

21.3.2 Choice of Airway Control

Airway control may be achieved with either an endotracheal tube (ETT) or an LMA. The most commonly utilized ETT would be the oral RAE (named after the inventors, Ring, Adair, and Elwyn), which has a natural curve that allows the ETT to be secured midline along the mandible, preventing the circuit or ETT from obstructing the surgical field.

Factors that favor the use of an ETT include airway concerns where a secured confirmed airway is critical, the need to move the head during the procedures (LMAs are much easier to dislodge), a long or complex procedure, and the need for neuromuscular blockade where ventilation will be particularly critical. An ETT should also be used if there will be significant irrigation that may enter the nasopharynx, as LMAs do not protect the airway. Fluids and secretions in the airway may precipitate laryngospasm.

An LMA is a plastic “mask” that seals the supraglottic pharynx and allows for sealed ventilation. It is less irritating than an ETT and allows for a deeper extubation with less stimulation, and a potentially smoother emergence.

21.3.3 Maintenance

Volatile or IV anesthetic agents can be used for maintenance of anesthesia. The choice of airway does not preclude either of these maintenance methods. Whether IV or volatile gases are used is dependent upon individual patient factors. For example, a patient history of previous PONV may preclude the use of volatile agents and instead indicate the use of total IV anesthesia.

For maintenance with volatile gases most practitioners initially continue the sevoflurane used for the induction and then transition to either isoflurane or desflurane. One benefit of isoflurane is its low cost. When using volatile agents, lipid solubility can affect how fast a patient will emerge from general anesthesia; the more soluble the agent, the longer it can take to emerge, especially in longer procedures. Desflurane is the least lipid soluble. When properly titrated, regardless of solubility, an efficient emergence can occur with any volatile agent. Some studies have shown that patients receiving sevoflurane or desflurane have an increased incidence of emergence delirium in the recovery room compared to isoflurane. Desflurane, as it is the least lipid soluble of these agents, leads to more rapid emergence in extreme obesity. Desflurane occasionally irritates the airway, triggering laryngospasm, coughing, breath-holding, and salivation. 14

For total IV anesthesia, propofol may be used throughout the case for maintenance. Its use results in less PONV. Research shows that only when propofol is slowly infused does it help decrease PONV. It has no effect on PONV if given as a bolus at induction. Utilizing propofol for maintenance can increase the incidence of the OCR.

21.3.4 Anesthetic Adjuncts Pain Control

Opioids, as part of a balanced anesthesia, are utilized for intraoperative and postoperative analgesia. Commonly used opioids, from longest acting to shortest acting, are morphine, fentanyl, and remifentanil. Onset of action can be as short as 1 minute with remifentanil to 15 minutes with morphine. Duration of action ranges from 5 to 10 minutes with remifentanil to 4 to 6 hours with morphine. Common side effects include nausea, vomiting, respiratory depression, sedation, and upper airway obstruction. Studies have shown that increased opioid use increases the incidence of PONV. Remifentanil is often combined with propofol and administered as an infusion for a total IV anesthesia (TIVA). Multiple studies comparing remifentanil to fentanyl or morphine have not shown an increase in PONV with remifentanil use but have shown an increased incidence of the OCR and suboptimal postoperative analgesia.

Nonopioid analgesics such as acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) are useful adjuncts to relieve pain and inflammation. Acetaminophen may be combined with oral premedication. The acetaminophen syrup has a more pleasant taste than the oral midazolam syrups. NSAIDs may increase the risk of bleeding. In addition, both acetaminophen and ketorolac (NSAID) have an IV form that can be administered in the OR. Muscle Relaxants

Muscle relaxants are either depolarizing (e.g., succinylcholine) or nondepolarizing (e.g., vecuronium, rocuronium). Depolarizing muscle relaxants can lead to prolonged contraction of the extraocular muscles, which may interfere with forced traction testing. Succinylcholine has unique concerns compared to the other paralytics, especially in the strabismus patient population. It is a known trigger for both MH and hyperkalemic cardiac arrest in populations with rare myopathies who also have a higher incidence of strabismus. 13

21.3.5 Temperature Regulation

During all anesthesias, the patient’s normal thermoregulatory response to a cold environment is impaired, especially in infants due to their larger relative surface area. For all patients under anesthesia, there is both decreased heat production and lessening of heat conservation. Heat production is decreased by the suppression of central nervous signaling for heat production. A lack of heat generated by spontaneous breathing also contributes to decreased heat production, as does the suppression of shivering. Heat loss is fostered by the vasodilatation produced by some anesthetic agents, a delay in vasoconstriction, or excess sweating. Perioperative hypothermia leads to an increased risk of cardiac events, blood loss, and surgical site infections. 15 In addition, hypothermia increases the length of both postanesthesia care unit (PACU) and hospital stays.

The four main mechanisms of heat loss in the OR are radiation, conduction, evaporation, and convection. The majority of heat loss during anesthesia occurs through radiation, the transfer of heat from the patient to the surrounding environment. This can be minimized by elevating the temperature of the OR to greater than 70°F or by using warming lamps. Conduction loss occurs by direct transfer of heat from the patient to items in contact with the patient such as the OR table. The advent of the forced air warmer, which circulates warm air through a special blanket, has helped minimize heat loss from radiation and conduction. These forced air warmers can be placed underneath or on top of the patient. When using forced air warming, core temperature monitoring to assess for hyperthermia is important. Convection heat loss occurs when the movement of air flowing past the patient draws heat away from the patient. Convection loss can be minimized by keeping the patient covered at all times. Evaporation heat loss occurs as the liquid used to prep the patient or irrigation solutions evaporate, and via sweating. It can also occur within the breathing circuit, as the anesthesia machine does not provide humidified air. Methods to limit evaporative heat loss include the use of warm irrigation fluids and placement of a humidification device within the breathing circuit.

21.3.6 Emergence

Emergence is the process of beginning to recover from general anesthesia, during which spontaneous ventilation recurs and consciousness is regained. Removal of any airway devices is an integral part of this phase of anesthesia, and this can be performed either “deep” (earlier in the emergence process) or “awake” (late in the emergence process). Prior to any extubation there must be a regular breathing pattern and a return of sufficient muscle strength, especially if any neuromuscular blockade was utilized. For an awake extubation the patient must be conscious enough to protect their airway and manage secretions, and maintain spontaneous ventilation with minimal risk of obstruction or apnea. The drawback of an awake extubation is that the stimulation from having an airway in the trachea or oropharynx can lead to coughing, which is not desirable, as the increased venous pressure produced leads to bleeding at the surgical site. Administering lidocaine IV can help decrease coughing. Avoiding coughing during emergence and having a “smoother” emergence is a benefit of a deep extubation. The drawbacks of a deep extubation are the need for airway support (i.e., jaw-thrust, oral airway) and the inability of the patient to protect against secretions entering the airway. A major risk of deep extubation is laryngospasm as the phases of anesthesia induction occur in reverse order during emergence, especially phase 2. The choice of an awake or deep extubation depends on patient and intraoperative factors. If the patient had a difficult airway, it is preferable to extubate awake so as to reduce the chance that reintubation will be needed.

A deep extubation is simpler with an LMA, as the lesser depth of anesthesia needed for this airway allows the patient to have a regular spontaneous breathing pattern earlier in emergence. In addition, there is less stimulation with removal of an LMA compared to an endotracheal tube removal. This allows for a slow smooth emergence, as the patient is comfortable and not coughing.

Airway obstruction can occur after the stimulation of an endotracheal tube or LMA is removed. This may result from lingering sedation and anesthesia, anatomical variants (large tongue, redundant oropharyngeal tissue, large neck), or a history of obstructive sleep apnea or craniofacial abnormalities.

Immediately after removal of any airway device, placement of an oropharyngeal airway and the anesthesia mask are employed. Airway support (e.g., jaw thrust) is given to ensure the patient continues to exchange air. This support should continue until the patient can ventilate without jaw thrust or until the patient has passed phase 2 of emergence. Depending on the institution, many recovery room nurses will have the skills and training to provide this type of airway management, which can allow for safe and efficient patient care and turnover.

It is not uncommon for children, especially infants, to breath-hold after extubation. It is also possible for the patient to experience laryngospasm, which can also lead to subsequent bradycardia and respiratory arrest.

Lidocaine gel 3.5% instilled into the eye at the end of a general anesthesia procedure lowers pain postoperatively and lessens nausea, allowing for earlier discharge from the facility.

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Feb 21, 2021 | Posted by in OPHTHALMOLOGY | Comments Off on 21 Anesthesia for Strabismus Surgery

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