Thyroid Pathology and Indication for Surgery
Thyroid Pathology and Indication for Surgery
Hyperthyroidism results from excess circulating T3 and T4. The vast majority of cases are caused by intrinsic thyroid disease.
Indications for surgery include:
- Grave's disease: An autoimmune (diseases arise from an inappropriate immune response of the body against substances and tissues normally present in the body) condition associated with diffuse enlargement and increased vascularity of the gland caused by antibodies mimicking Thyroid Stimulating Hormone (TSH). It is the only cause of hyperthyroidism associated with eye signs and pretibial myxoedema.
- Thyroid secreting adenomas (a benign tumor originating from the gland) often presenting as a solitary nodule.
- Toxic Multinodular Goiter: Is a form of hyperthyroidism where there is excess production of thyroid hormones. More common in women.
The Major Manifestations of hyperthyroidism are weight loss; diarrhea; skeletal muscle weakness and stiffness; warm, moist skin; heat intolerance; and nervousness. Hypercalcemia, thrombocytopenia, and a mild anemia may be present. A fine tremor, exophthalmos (is a bulging of the eye anteriorly out of the orbit.), or goiter may be noted, particularly when the cause is Graves' disease. Cardiac signs range from sinus tachycardia to atrial fibrillation and congestive heart failure. The diagnosis of hyperthyroidism is confirmed by abnormal thyroid function tests, which may include an elevation in total (bound and unbound) serum T4, serum T3, and free (unbound) T4.
Medical Treatment of Hyperthyroidism (Table 6.2) relies on drugs that inhibit hormone synthesis (e.g, propylthiouracil, methimazole), prevent hormone release (e.g, potassium, sodium iodide), or mask the signs of adrenergic over activity (e.g, propranolol). Although beta -adrenergic antagonists do not affect thyroid gland function, they do decrease the peripheral conversion of T4 to T3 (an active form of thyroid hormone). Radioactive iodine destroys thyroid cell function but is not recommended for pregnant patients and may result in hypothyroidism. Subtotal thyroidectomy is an alternative to medical therapy.
Table 6.2 Anti-Thyroid Drugs
|Drugs||Dose||Mechanism of Action|
Maintenance: 5- 15mg daily
Takes 6-8 weeks to work
|Pro-drug rapidly converted to methimazole.
Prevents synthesis of T3 and T4
|Propylthiouracil||Initial:200- 400mg daily
Maintenance: 50- 150mg daily Takes 6-8 weeks
|Inhibits conversion of T4 to T3|
|Iodine/Iodide||Lugol’s solution: 5g Iodine solution in 10g
Potassium iodide: 0.1-0.3ml TDS
|Large doses of Iodide inhibit hormone production.
Marked reduction in thyroid vascularity over 10-14days
|Propranolol||Oral: 40-80mg TID (May need higher dose as metabolism increased) IV: 0.5mg titrated to effect||Controls sympathetic effects of thyrotoxicosis crisis.
Blocks peripheral conversion of T4 to T3
Table 6.2.1 Anti-Thyroid Drugs
|Carbamizole||Rashes, arthralgia, pruritis, myopathy.
Bone marrow suppression, fetal hypothyroidism
|Propylthiouracil||Thrombocytopenia, anemia, hepatitis, nephritis, Crosses placenta: fetal hypothyroidism|
|Iodine/Iodide||Antithyroid effects diminish with time.
Hypersensitivity reactions. Crosses placenta: foetal hypothyroidism
|Propranolol||Negative inotropy & chronotropy. Bronchospasm Poor peripheral circulation.
It is a life-threatening exacerbation of hyperthyroidism that most commonly develops in the undiagnosed or untreated hyperthyroid patient because of the stress of surgery or non thyroid illness. Operating on an acutely hyperthyroid gland may provoke thyroid storm, although this is probably not due to mechanical release of hormone. Its manifestations include hyperthermia, tachycardia, dysrhythmias, myocardial ischemia, congestive heart failure, agitation, and confusion. Although free T4 levels are often markedly elevated, no laboratory test is diagnostic. Treatment involves large doses of propylthiouracil and supportive measures to control fever and restore intravascular volume. It is essential to remove or treat the precipitating event.
Management of thyroid storm
- Administer cold IV fluids.
- Administer sodium iodide, 250 mg PO or IV q6h.
- Administer propylthiouracil, 200-400 mg PO or via NGT q6h.
- Administer hydrocortisone, 50-100 mg IV q6h.
- Administer propranolol, 10-40 mg PO q4-6h, or esmolol infusion to treat hyper-adrenergic signs.
- Cooling blankets and acetaminophen and meperidine (25-50 mg) IV q4-6h may be used to prevent shivering.
- Use digoxin for heart failure especially in the presence of atrial fibrillation with rapid ventricular response.
Anesthetic Considerations of Hyperthyroidism
Preoperative Anesthetic Management
- All elective surgical procedures, including subtotal thyroidectomy, should be postponed until the patient is rendered clinically and chemically euthyroid, the state of having normal thyroid gland function, with medical treatment (Table 6.1).
- Preoperative assessment should include normal thyroid function tests, and a resting heart rate less than 85 beats/min has been recommended. Benzodiazepines are a good choice for preoperative sedation. Antithyroid medications and beta-adrenergic antagonists are continued through the morning of surgery.
- A large goiter that has been present for some time may be associated with tracheomalacia (characterized by flaccidity of the supporting tracheal cartilage, widening of the posterior membranous wall, and reduced anterior-posterior airway caliber. These factors cause tracheal collapse, especially during times of increased airflow) postoperatively. Symptoms of dysphagia, positional breathlessness with a difficulty lying flat, change in voice or stridor may alert the anesthetist to possible difficulties with airway compromise on induction. Evidence of other systemic disease, cardio- respiratory compromise and associated endocrine or autoimmune disorders should also be sought.
- An examination of the goiter or nodule should be performed to assess size and extent of the lesion. A fixed hard nodule suggests malignancy with possible tethering to surrounding structures and limited movement. An inability to feel the bottom of the goiter may indicate retrosternal spread. The trachea should be examined to check for any deviation or compression. Retrosternal or large goiters can compress surrounding structures and may elicit signs of superior vena cava (SVC) obstruction, Horner's Syndrome, pericardial or pleural effusions. A mandatory detailed airway examination would also include assessment of atlantoaxial flexion and extension, thyro-mental distance, Mallampatti, mandibular protrusion and incisor distance.
- Routine blood tests include Full Blood Count, electrolytes, thyroid function and corrected calcium levels.
- Administration of propylthiouracil and methimazole is particularly important because of their relatively short half-lives.
- If emergency surgery must proceed, the hyper dynamic circulation can be controlled by titration of an esmolol infusion.
Intraoperative Anesthetic Management
- Cardiovascular function and body temperature should be closely monitored in patients with a history of hyperthyroidism. Temperature, EKG, B/P, pulse oximetry, urine output, if possible neuro muscular function should be checked.
- Thiopental may be the induction agent of choice as it possesses some antithyroid activity at high doses.
- Ketamine, pancuronium, indirect-acting adrenergic agonists, and other drugs that stimulate the sympathetic nervous system are best avoided because of the possibility of exaggerated elevations in blood pressure and heart rate.
- In most cases, the patient can be given an intravenous induction and intubated with a reinforced tube (Figure 6.3 and 6.4). It is advisable to demonstrate manual ventilation prior to giving a non-depolarizing muscle relaxant.
Reinforced (armored) endotracheal tube contain spiral of metal in the tube wall which is useful to prevent kinking during thyroid surgery, neurosurgery, maxillo-facial surgery and in patients in the prone position. Stylet can be used to align the direction of the tube
- For optimal surgical access the head is fully extended and rested on a padded ring with a sandbag between the scapulae. Access to the airway will be limited during the procedure so the endotracheal tube should be taped securely. Neck ties should be avoided.
- A head up tilt is preferable to allow venous drainage although care must be taken to ensure arterial pressure is not compromised. As the arms are extended by the patient’s side, long extension leads on the drips are useful.
- Patients' eyes should be well protected, as the exophthalmos of Graves' disease increases the risk of corneal abrasion or ulceration. The head of the operating table can be raised 15-20° to aid venous drainage and decrease blood loss, although doing so increases the risk of venous air embolism.
- Hyperthyroid patients can be chronically hypovolemic and vasodilated and are prone to an exaggerated hypotensive response during induction of anesthesia. Adequate anesthetic depth must be obtained, however, before laryngoscopy or surgical stimulation to avoid tachycardia, hypertension, and ventricular arrhythmias.
- Neuromuscular blocking agents (NMBAs) should be administered cautiously (initial dose should be reduced), because thyrotoxicosis is associated with an increased incidence of neuromuscular disease. Hyperthyroidism does not increase anesthetic requirements i.e., there is no change in minimum alveolar concentration.
- If there is concern regarding distorted anatomy or that the airway neck X-ray both PA and lateral has to be done and comented and discused with the seurgion. an awake intubation may be used when fisible. This technique should be avoided in those patients with marked symptoms of airway obstruction as complete obstruction may be provoked. A tracheostomy under local anaesthetic by the surgeons may be an alternative way of controlling air way.
- All elective surgeries with airway compromise has to be managed in better centers where there is specialist anesthesiologist
- Emergence: At the end of the procedure, if there have been any concerns regarding the integrity of the recurrent laryngeal nerve, then the vocal cords are visualized for movement with a laryngoscope. Neuromuscular blockade should be fully reversed, the patient sat up and endotracheal tube cuff deflated to ensure a leak prior to extubation. Extubation shall be done intravenous lidocaine 1.5mg/kg helps reduce coughing against the endotracheal tube. Steroids (e.g. dexamethasone 8mg) may help to reduce airway edema if the procedure has been long or difficult.
- Prepare your self for reintubation/ trachiostomy after extubation, make sure the surgeon is around for possible airway obstruction mangment
Hemorrhage: Postoperative bleeding can cause compression and rapid airway obstruction. Signs of swelling or hematoma formation that is compromising the patient's airway should be immediately communicated to the surgeon for decompression, reintubation should be performed early.
Laryngeal edema: This is an uncommon cause of postoperative respiratory obstruction. It can occur as a result of traumatic tracheal intubation or in those who develop a hematoma that can cause obstruction to venous drainage. It can usually be managed with steroids and humidified oxygen
Recurrent Laryngeal Nerve (RLN) Palsy: Trauma to the recurrent laryngeal nerve can be caused by ischemia, traction, entrapment or transection of the nerve during surgery and may be unilateral or bilateral. Unilateral vocal cord palsy will present with respiratory difficulty, hoarse voice or difficulty in phonation whilst bilateral palsy will result in complete adduction (to draw inward toward the median axis of the body) of the cords and stridor. Bilateral RLN palsy requires immediate reintubation and the patient may subsequently need a tracheostomy.
Hypocalcaemia: Unintended trauma to the parathyroid glands may result in temporary hypocalcaemia. Permanent hypocalcaemia is rare. Signs of hypocalcaemia may include confusion, twitching and tetany (a muscle spasm or sharp flexion of the wrist and ankle joints). This can be elicited in Trousseau’s (carpopedal spasm precipitated by cuff inflation) or Chvostek’s sign (spasm of the facial muscles elicited by tapping the facial nerve in the region of the parotid gland; anterior to the ear and just below the zygomatic bone) Calcium replacement should be instituted immediately as hypocalcaemia can precipitate layngospasm, cardiac irritability, QT prolongation and subsequent arrhythmias
Tracheomalacia: The possibility of tracheomalacia should be considered in those patients who have had sustained tracheal compression by large goiters or tumors. A cuff leak test just prior to extubation is reassuring but equipment should be available for immediate reintubation and consecutive tracheotomy.