Strabismus Surgery Case File

Strabismus Surgery Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 37

3-year-old otherwise healthy girl is brought to her pediatrician because her parents notice that her eyes do not line up in parallel. She is referred to an ophthalmologist, who confirms that the visual axes of her eyes are indeed not parallel, and that her vision is impaired in the right eye (amblyopia). She is diagnosed with strabismus, which requires surgical repair. The patient’s medical history is otherwise unremarkable. Her childhood vaccines are up-to-date, and she is not allergic to any medications. Her parents are young, and have never had surgery with the exception of the mother’s receiving an epidural for delivery. She is scheduled for outpatient surgery, and arrives to the surgical suite after appropriately observing preoperative fasting guidelines.

➤ What are the anesthetic considerations in a pediatric patient undergoing surgical correction of strabismus?

➤ How can an appropriate anesthetic plan either prevent, or treat each of the above?

ANSWERS TO CASE 37:

Strabismus Surgery

Summary: A 3-year-old girl presents for outpatient strabismus surgery.

➤ Anesthetic considerations: For a pediatric patient undergoing strabismus repair, the anesthetic considerations include stimulation of the oculocardiac reflex, the likelihood of postoperative nausea and vomiting, and the coexistence of malignant hyperthermia with muscle dystrophies, some of which may cause strabismus.

➤ Appropriate anesthetic plan: The treatment for oculocardiac reflex is to cease the exciting stimulus, and to use atropine if severe bradycardia persists. Postoperative nausea and vomiting is prevented by the prophylactic administration of antiemetics in combination. In addition, this patient’s anesthetic must include a careful family history, as well as efforts to reduce the risk of malignant hyperthermia (MH) by avoiding triggering agents insofar as possible, and the ready availability of dantrolene sodium in preparation for its possible occurrence.

ANALYSIS

Objectives

1. Become familiar with the anesthetic issues that may be encountered during strabismus surgery.

2. Understand the oculocardiac reflex, the use of prophylactic medication, and its treatment should it occur.

3. Understand the etiology, triggering agents, and treatment for malignant hyperthermia.

4. Develop a plan of prevention and treatment of postoperative nausea and vomiting in the pediatric patient.

Considerations

This young patient has strabismus sufficiently severe as to require a surgical correction. She has no other comorbidities. However, it is possible that her strabismus may be caused by an underlying myopathy, which may place her at increased risk to MH. Strabismus surgery itself can present challenges to the anesthesiologist. Traction on the extraocular muscles can excite the oculocardiac reflex, which is associated with potentially serious cardiac dysrhythmias. Atropine or glycopyrrolate should be prepared, and the dose for this patient (given her size) determined before the case begins. Strabismus surgery also predisposes a patient to postoperative nausea and vomiting. Prevention by prophylactic medications and preparation for treatment are important aspects of the anesthetic plan.

APPROACH TO

Strabismus Surgery

DEFINITIONS

STRABISMUS: A condition in which the visual axes of the eyes are not parallel and the eyes appear to be looking in different directions. Strabismus can lead to amblyopia in one eye, a form of blindness.

MALIGNANT HYPERTHERMIA: A hypermetabolic state that includes severe prolonged muscle contractions and a rapid rise in body temperature (up to or above 105°F). Susceptible patients carry an autosomal dominant gene, which has variable penetrance. The disease is caused by exposure to triggering agents (see Table 37–1).

OCULOCARDIAC REFLEX: A reflex reaction to pressure on the globe of the eye or to traction on the extraocular eye muscles that results in cardiac dysrhythmias, most commonly sinus bradycardia.

Table 37–1 ANESTHETIC MEDICATIONS AND MALIGNANT HYPERTHERMIA
TRIGGERING AGENTS	SAFE AGENTS
Succinylcholine All volatile anesthetic agents: Isoflurane Desflurane Sevoflurane	N2O
	Benzodiazepines
	Propofol, etomidate, pentothal
	Ketamine
	Narcotics
	Local anesthetics
	Nondepolarizing muscle relaxants

CLINICAL APPROACH

A child arriving to the operating room for correction of strabismus presents a set of challenges for the anesthesiologist. Two of these challenges are related to the surgery itself, and the third, to the patient population in which the surgery is required.

Traction on the extraocular muscles and/or pressure on the eye can elicit the oculocardiac reflex (OCR), which can result in severe bradycardia as well as other cardiac dysrhythmias. The OCR is initiated by an afferent signal traveling along the ophthalmologic division of the trigeminal nerve to the trigeminal’s sensory nucleus. Then an efferent signal travels via the vagus nerve to the heart, initiating dysrhythmias. The most common dysrhythmia is sinus bradycardia which can be severe, although a wide variety of dysrhythmias have been reported. The treatment for the OCR is to stop the surgical stimulus. This treatment is usually sufficient, and the heart rhythm typically returns to normal. However, if bradycardia (or other dysrhythmia) persists, atropine or another antiarrhythmic drug can be given. Since children have greater vagal tone than adults, some surgery centers routinely administer atropine prophylactically to prevent this complication. 

Patients undergoing strabismus repair are also likely to experience postoperative nausea and vomiting. Traction on the extraocular muscles and or pressure on the eye is hypothesized to cause this unpleasant complication. Because the incidence of vomiting after strabismus surgery is so high (>67%), it is appropriate to treat patients undergoing strabismus repair prophylactically to prevent this response. Dexamethasone and the HT3 antagonists such as ondansetron are the most effective prophylactic medications and should be given to all patients undergoing strabismus repair, as well as patients who have experienced postoperative nausea and vomiting following previous anesthetics. The combined use of these drugs has been shown to be more effective than either given alone, and can reduce the incidence of PONV to less than 20%.

Finally, some experts believe that children undergoing strabismus repair are at an increased risk for MH, a potentially fatal complication of exposure to some anesthetics (see Table 37–1). The incidence of MH is 1 in 15,000 anesthetics in children, and 1 in 50,000 anesthetics in adults. Every anesthesiologist needs to watch vigilantly for signs of MH in every patient, and be prepared to treat it at any time with every anesthetic given (see Table 37–2). ECG, end tidal CO2 levels, and patient temperature are always monitored continuously.

Patients susceptible to MH carry an autosomal dominant genetic mutation leading to faulty calcium reuptake in muscle cells. This failed mechanism results in prolonged muscle contraction, rhabdomyolysis, and a dramatically increased metabolic rate. The mutation is most often silent until the patient is exposed to a triggering agent. Since patients diagnosed with a myopathy may be at an increased risk of carrying this mutation, the safest anesthetic plan is to avoid the use of all triggering agents in this setting. However it

Table 37–2 DIAGNOSIS AND TREATMENT OF MALIGNANT HYPERTHERMIA
SIGNS OF MALIGNANT HYPERTHERMIA	TREATMENT OF MALIGNANT HYPERTHERMIA
Increasing carbon dioxide production	Discontinue use of the triggering agent and stop surgery as soon as possible.
Hyperthermia (up to 2°C [35.6°F] per hour)	Apply a cooling blanket to the patient.
Rhabdomyolysis	Give intravenous fluids to maintain kidney function.
Muscle rigidity	Give dantrolene, a medication that decreases the release of calcium from the sarcoplasmic reticulum.
Increased oxygen consumption

should be stated that the connection between strabismus and an underlying myopathy (and hence a propensity for MH) is controversial and many practitioners will not avoid triggering agents unless a family history suggests a genetic problem with anesthesia.

Succinylcholine is a depolarizing neuromuscular blocking agent that is usually avoided in young strabismus patients. First, succinylcholine is, in general, avoided in children. It has been associated with unpredictable and intractable cardiac arrest in apparently healthy children. Many of these children were ultimately shown to have an undiagnosed myopathy, as many of these diseases are not clinically apparent until past the first decade of life. Succinylcholine could cause a rapid and dangerous rise in serum potassium, causing cardiac dysrhythmias and even cardiac arrest. Second, in patients undergoing strabismus repair, succinylcholine can interfere with the surgeon’s forced duction test (to evaluate muscle tone) for up to 15 minutes. Third, given the possibility that the strabismus might be associated with a myopathy, some would avoid succinylcholine in the setting of strabismus repair out of concern that it is a triggering agent for MH.

Most children scheduled for strabismus surgery are apparently otherwise healthy. Nevertheless, they are at increased risk of two serious complications: the oculocardiac reflex and malignant hyperthermia. Moreover, the likelihood that these patients will vomit warrants prophylaxis with antiemetic drugs. These factors necessitate that the anesthesiologist plan the anesthetic carefully and have a heightened awareness of potential known adverse events associated with this operation.

Comprehension Questions

37.1. A 2-year-old boy is having correction of ptosis of the right eyelid. The patient is stable during induction of anesthesia, but his heart rate drops to 50 bpm soon after the surgical incision. Which of the following is your first step in treating this patient?

A. Do nothing; his heart rate is in the normal range.

B. Give intravenous atropine.

C. Tell the surgeon to stop what he is doing.

D. Give intravenous fluids.

37.2. A 10-year-old boy requires repair of an inguinal hernia. He has no other health-related issues and has no surgical history. When asked about the surgical history of family members, his mother states she has never had surgery, but her brother (his uncle) died unexpectedly during surgery for a broken arm at age 12. Which of the following should

be included in your anesthetic plan?

A. Not include the use of succinylcholine or volatile anesthetics

B. Not be altered by this information

C. Include prophylactic dosing of dantrolene

D. Include prophylactic dosing of acetaminophen

37.3. A 5-year-old girl has a history of repetitive vomiting after strabismus surgery when she was 3 years old. She is now scheduled for treatment of multiple dental caries under general anesthesia. This type of surgery is not associated with a high incidence of postoperative nausea and vomiting. Anesthesia for this young girl should include which of the following?

A. Intraoperative ondansetron for nausea and vomiting prophylaxis.

B. No prophylaxis for postoperative nausea and vomiting; this surgery

doesn’t cause nausea and vomiting.

C. Intraoperative dexamethasone and ondansetron for prophylaxis.

D. No general anesthesia, the patient should have her dental caries repaired under local anesthesia.

ANSWERS

37.1. C. The patient is having a vagal response to something the surgeon is doing on or around his eye. The first step in treating his bradycardia is to have the surgeon stop until the patient’s heart rate returns to normal (110-115 bpm). If the pause in surgery does not rapidly return the heart rate to normal, intravenous atropine should be given.

37.2. A. The patient’s uncle may have had malignant hyperthermia during his arm surgery, although such a story is not definitive proof of a positive family history. Since malignant hyperthermia has a 5% mortality rate, the safest approach to this patient is to avoid all triggering agents during his anesthetic. Prophylactic dosing of dantrolene does not prevent malignant hyperthermia, but may attenuate early signs of the disease and slow the initiation of proper treatment. However, dantrolene is not indicated in this case.

37.3. C. Although the little girl’s previous strabismus surgery may have been the only cause of her postoperative vomiting, there are many other possible etiologies, and no way to be certain exactly what caused the problem. General anesthesia alone can cause some patients to have severe postoperative nausea and vomiting. The safest anesthetic plan will include the best prophylactic measures available to try to prevent vomiting. These include a multidrug regimen such as dexamethasone plus ondansetron, avoiding the use of N2O, and being certain that the patient is well hydrated at the conclusion of surgery.

Clinical Pearls

➤ The oculocardiac reflex, caused by pressure on the eye or surgical traction on extraocular muscles, results in a vagal response, most often sinus bradycardia. Removal of the stimulus is the first step in treatment.

➤ Postoperative nausea and vomiting occurs with high incidence after strabismus surgery, indicating prophylactic treatment with a combination of antiemetic medications.

➤ Patients who have strabismus may have an increased risk for malignant hyperthermia. A thorough family history is important, especially in this setting.

References

Bingham R (ed). Hatch and Sumner’s Textbook of Paediatric Anaesthesia. 3rd ed. London, UK: Hadder Arnold;2008: 524-525. 

Miller RD (ed). Miller’s Anesthesia. 6th ed. Philadelphia, PA: Elsevier, Churchill Livingstone; 2007: 2530; 2535-2536. 

Motoyama EK, Davis PJ (ed). Smith’s Anesthesia for Infants and Children. 6th ed. Philadelphia, PA: Mosley;1996: 637-638; 641. 

www.medterms.com/script/main/art.asp?articlekey=12157.

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