Anesthesia for Thyroidectomy

Introduction

• A thyroidectomy is the surgical removal of all or a part of the thyroid gland. It is performed to treat thyroid cancer, a goiter, or to perform an excision biopsy of a nodule. A thyroidectomy can be total (the whole gland is removed) or partial (a single lobe is removed). The complexity of the surgical approach may vary from the excision of a simple nodule to performing a sternotomy to access the inferior pole of a large retrosternal goiter.
• Common indications for a thyroidectomy include:
  1. Hyperthyroidism (Graves’ disease, toxic solitary nodule, drug-induced hyperthyroidism (e.g., amiodarone, lithium)
  2. Thyroid cancer
  3. Benign large goiter with potential tracheal compression

Preoperative Assessment

• The two main goals of the preoperative assessment of patients presenting for thyroid surgery are to ensure euthyroid state and assess any potential for difficult airway management.

Assessment of Preoperative Euthyroid State

• Assessment of preoperative euthyroid state is critical as preoperative hyperthyroidism or hypothyroidism can induce severe intraoperative and postoperative complications.
• Most patients present for surgery in a relative euthyroid state.
• However, some patients may not be adequately stabilized, so the anesthesiologist needs to carefully look for clinical signs of thyroid imbalance and be prepared to deal with potential complications.

Hyperthyroidism

• Hyperthyroidism can manifest with anxiety, lethargy, weight loss, tremor, tachyarrhythmias, or angina. It can lead to cardiac complications (tachyarrhythmias, myocardial ischemia) or thyroid storm during the dissection of the gland (seen mostly in patients with Hashimoto’s disease).
• It is essential for patients to be euthyroid before surgery to avoid complications. This is achieved with antithyroid medications (carbimazole, methimazole, propylthiouracil, iodine) that decrease the synthesis of thyroid hormones and beta blockers to blunt the cardiac effects of thyroid hormones.
• Antithyroid medications take several weeks to induce an euthyroid state because they only inhibit the production of new thyroid hormones; they do not act on the existing hormones, which have a long half-life (the half-life of T4 is 7 days).
• A small group of patients won’t be properly euthyroid when presenting for surgery because of the serious adverse effects of the antithyroid drugs (agranulocytosis, hepatitis, aplastic anemia, lupus-like syndrome, etc.).
• Hyperthyroidism affects the pharmacology of propofol: propofol clearance is increased; therefore, infusion rates should be increased.¹

Hypothyroidism

• Hypothyroidism leads to decreased myocardial contractility and cardiac output, impaired baroreflex, anemia, hypoglycemia, hyponatremia, decreased hepatic drug metabolism resulting in delayed emergence from anesthesia, decreased respiratory response to hypoxia, and hypercapnia. Mean arterial blood pressure remains unaltered despite the decrease in cardiac output because of the increase in systemic vascular resistance.
• Some patients (20%) can be clinically hypothyroid despite receiving a proper dose of levothyroxine. This is mostly due to decreased absorption of levothyroxine (interaction with many foods and medications, anatomical or functional gastrointestinal malabsorption). Notably, pregnancy can increase the requirement of levothyroxine up to 50%.²
• In clinically hypothyroid patients, it is recommended to administer a stress dose of hydrocortisone before surgery because there is a high incidence of associated adrenocortical insufficiency.³

Assessment for Potential Difficult Airway Management

• Patients scheduled for thyroidectomies may have a higher incidence of difficult direct laryngoscopy, although this is controversial. Currently, with the widespread use of video laryngoscopy, difficult intubation is rare.
• In some rare cases, intubation can be extremely challenging. It is important to identify the risk factors for difficult intubation, which include
  • large goiter;
  • substernal involvement;
  • signs of tracheal deviation or compression
    • clinical (stridor, orthopnea);
    • imaging (chest x-ray, computed tomography, magnetic resonance imaging); or
    • preoperative fiberoptic laryngoscopy.
• A preoperative discussion with the surgeon is essential for anticipating airway challenges and planning their management.⁴
• With postural symptoms, it is important to try to find the position in which the symptoms are minimized (“rescue position”). This can be useful if unforeseen postintubation deterioration occurs due to aggravation of the tracheal compression distal to the ETT. Placing the patient in that rescue position may alleviate the compression symptoms.
• There is a risk of postoperative tracheomalacia and respiratory distress with long-standing goiters associated with tracheal compression. Tracheomalacia is an airway emergency requiring emergent postoperative reintubation. It presents as postextubation respiratory distress with expiratory stridor. This complication is rarely seen in the Western world, where goiters are usually removed before there is frank tracheal compression. This mostly occurs in underdeveloped countries where compressive goiters are sometimes detected late and present with respiratory symptoms at the time of surgery.

Assess for the Presence of Superior Vena Cava (SVC) Compression

• SVC compression by the goiter (plethora of head and neck associated with prominent neck veins) can increase the risk of intra- and postoperative bleeding. Intubation in a head-up position to reduce venous congestion in the pharynx should be considered.

Assess for Associated Pathology

• Thyroid cancer can be associated with multiple endocrine neoplasia (MEN) type 2 with concomitant presence of pheochromocytoma and/or hyperparathyroidism. Screening for the presence of hypercalcemia, hyperglycemia, clinical signs of autonomic instability, hypertensive, and tachyarrhythmia episodes should be completed.

Intraoperative Management

• Most thyroidectomies are done under general endotracheal anesthesia. In selected cases and in motivated patients, thyroidectomies can be performed successfully with bilateral superficial cervical blocks and mild or no sedation.⁵

Airway Management

• Difficult airway equipment should always be readily available in case of unforeseen difficult intubation. Although intubation is generally not a problem, it can be extremely difficult, especially in patients with large goiters.
• A short-acting muscle relaxant should be used to allow intraoperative monitoring of the recurrent laryngeal nerve. Usually, succinylcholine or a small dose of rocuronium is used to facilitate intubation.
• In cases of large substernal goiters with tracheal compression, intubation with spontaneous ventilation should be considered. Paralysis and controlled ventilation may worsen airway compression at the site of impingement and induce tracheal collapse distal to the ETT.⁴ In these situations, placing an ETT proximal to the obstruction cannot be regarded as definitively securing the airway. If there is significant difficulty in ventilating the patient after ETT placement, it is recommended to place the patient in the “rescue position” that was identified preoperatively to decrease tracheal obstruction distal to ETT (upright, lateral decubitus, or sometimes prone).

Recurrent Laryngeal Nerve Monitoring

• Intraoperative laryngeal electromyographic (EMG) monitoring of the vocal cords to monitor the integrity of the recurrent laryngeal nerve is becoming popular to protect the recurrent nerve during dissection. A neural integrity monitor (NIM) ETT with embedded bilateral surface recording electrodes above the cuff detects EMG signals from the vocal cords.
• For the NIM tube to function properly, the electrodes must have good contact with the vocal cords. A larger tube is recommended. The position of the tube in the trachea should be optimized. The color-coded contact band (blue) should be placed in between the vocal cords to allow optimal contact between the electrodes and the vocal cords. Positioning is often checked right after intubation using video laryngoscopy. It is also recommended that the position of the tube be rechecked after hyperextension of the neck for surgical exposure.

Intraoperative Hyperthyroidism

• In case of suspected baseline hyperthyroid clinical state, beta blockers should be available to rapidly treat potential tachyarrhythmias or thyroid storm that can be triggered by manipulation of the gland. This has been mostly described in patients with Graves’ disease.
• Thyroid storm is a life-threatening emergency that needs rapid identification and treatment. It is rarely encountered with the widespread use of antithyroid drugs and beta blockers before surgery.
• Thyroid storm is characterized by sudden onset of hypertension, sinus tachycardia, tachyarrhythmias, increased EtCO₂, and hyperthermia. Its presentation can mimic malignant hyperthermia. Supportive measures include high doses of beta blockers (to manage tachyarrhythmias and decrease conversion from T4 to T3), hydration, cooling, and high-dose steroids. They should be started as soon as the diagnosis is suspected.
• Antithyroid medications should be started quickly as well. They are not the first line of treatment because the improvement of thyroid function can take several weeks.
• Medications and conditions that induce adrenergic activation (ketamine, ephedrine, atropine, hypercapnia) should be avoided.

Other Intraoperative Considerations

• Monitoring the depth of anesthesia should be considered as hyperthyroidism increases the metabolism of anesthetics, and there is a risk of insufficient depth of anesthesia, especially when using propofol infusions.¹
• Postoperative nausea and vomiting (PONV) prophylaxis is essential. Thyroidectomies have a higher incidence of PONV retching that not only decreases patient satisfaction and delays postanesthesia care unit (PACU) stay, but it also increases venous pressure and the risk of postoperative bleeding.⁶ This is achieved by administering antiemetics and decreasing opioid analgesia. Total intravenous anesthesia should be considered because it leads to rapid recovery and has antiemetic properties.
• Massive hemorrhage is rare, and it is mostly seen after dissection of large retrosternal goiters where there is a potential for major hemorrhage from adjacent large vessels.
• Pneumothorax is rare. It is a potential complication of dissection of large retrosternal goiters.
• The eyes should be carefully protected from trauma, especially in cases of exophthalmos (Graves’ disease), as it makes the eyes especially prone to injury.
• Smooth emergence is important, as coughing and bucking may increase the risk of bleeding by increasing venous pressures. The surgeon will often request a Valsalva maneuver before closing to ensure proper hemostasis. Deep extubation or extubation should be considered with low doses of remifentanil infusion (0.025-0.05 mcg/kg/min).
• Superficial cervical plexus blocks (C2-C4) are becoming popular because they are easy to perform, have low rates of complications, and result in excellent postoperative pain relief, better patient satisfaction, less postoperative opioid administration (leading to decreased PONV), and help avoid the use of nonsteroidal anti-inflammatory drugs (minimizing the risk of postoperative bleeding). They are usually performed after intubation.
• Deep cervical blocks are not recommended. They do not improve postoperative analgesia when compared to superficial cervical blocks, and they have potential undesirable complications, such as recurrent and phrenic nerve transient paralysis, seizures due to intravascular injection, and epidural/subarachnoid spread.⁷
• Recovery from neuromuscular blockade should be ensured prior to monitoring, and paralytic agents should be avoided during the monitoring.
• Some surgeons request to examine vocal cord movement with direct laryngoscopy immediately after extubation to ensure proper bilateral movement of the vocal cords. This can be challenging; patients need to be extubated deep to tolerate the postoperative direct laryngoscopy. In those cases, consider replacing the ETT with a supraglottic airway (SGA) at the end of the surgery and examine the vocal cords with a fiberoptic bronchoscope through the SGA.

Postoperative Complications & Management

• Postoperative hypothyroidism requires lifelong thyroid hormone replacement.
• Postoperative hypoparathyroidism and symptomatic hypocalcemia result from damage to the parathyroid glands during surgery. It is mainly seen after total thyroidectomy.
• In the majority of the cases, hypocalcemia is transient, but it can be profound. Symptoms typically occur 24-72 hours after surgery (6-30% reported instances after total thyroidectomy).⁸ Patient may complain of paresthesia, muscle spasms, laryngospasm, tetany, altered mental status, seizures, laryngospasm, and may present with EKG changes (prolonged QT interval)
• A serum parathyroid hormone (PTH) level is usually drawn in the PACU to assess the potential for postoperative hypocalcemia. Because the PTH has a very short half-life (2-4 min), a measurement in the immediate postoperative period is a good predictor of postoperative hypoparathyroidism and symptomatic hypocalcemia. Early systematic calcium and vitamin D supplementation in at-risk patients effectively prevents postoperative symptomatic hypocalcemia.⁹
• Damage to the recurrent laryngeal nerve is a well-known complication of thyroid surgery. The incidence of postoperative recurrent laryngeal nerve paralysis (transient or permanent) ranges from 0.5-4% for benign goiters to 11% for thyroid cancer.¹⁰
• Typically, paralysis is transient and recovers within 3-6 months in 50% of the patients. It is usually unilateral and well tolerated (hoarseness or aphonia).
• When paralysis is unilateral, symptoms are mild and mainly include voice alteration (hoarseness and aphonia). Rarely, a sensory deficit of the larynx (associated with superior laryngeal nerve injury) may increase the risk of aspiration when drinking liquids. Before discharge, patients should be educated on swallowing precautions.
• Bilateral vocal cord paralysis is rare but can cause postoperative respiratory distress and stridor due to complete adduction of both vocal cords, requiring emergent reintubation.
• Acute postoperative neck hematoma (0.1-1% incidence). About 75% will occur in the first 6 hours, and the remaining will present in the first 24 hrs. This is a life-threatening complication; it usually presents as acute respiratory distress due to compression of the airway. Treatment is the emergent release of wound clips and sutures to decompress the trachea, emergent intubation, and exploration of the wound for hemostasis.¹⁰
• Postoperative tracheomalacia is associated with long-standing large compressive goiters with retrosternal extension. It presents as immediate post-extubation respiratory distress and expiratory stridor requiring emergent intubation. Consider seeking a cuff leak before extubation. This complication is extremely rare in the Western world. It is seen in large goiters compressing the trachea for many years.¹¹