- Email: [email protected]
Feline thyroid surgery
Vet Clin Small Anim 32 (2002) 851–859
Feline thyroid surgery Sheldon Padgett, DVM, MS Metropolitan Veterinary Hospital, 1053 South Cleveland-Massillon Road, Akron, OH 44321, USA
Thyroid disease in the cat presents a unique challenge to the surgeon, because the patients commonly have concurrent secondary metabolic abnormalities that can predispose them to significant and sometimes life-threatening complications. It is necessary not only to consider the best method of treatment but the effects that thyroid disease can have on other organ systems.
Surgical indications Thyroid surgery in the cat is generally limited to removal of the thyroid (thyroidectomy), although a thyroid biopsy may occasionally be indicated. Thyroidectomy is most commonly performed as therapy for hyperthyroidism. Additionally, because of the intimate anatomy of thyroid and parathyroid glands, the thyroid is usually removed as part of a parathyroidectomy performed as therapy for functional parathyroid disease.
Hyperthyroidism Since the first report of the disease in 1979 by Peterson et al [1], hyperthyroidism has become the most commonly diagnosed endocrine disease in cats. The disease is caused by functional benign adenomatous hyperplasia in approximately 98% of cases. Functional thyroid tumors (carcinomas) account for the remaining 2% of hyperthyroid cats [2]. Feline hyperthyroidism is usually treated by one of the following methods: surgical removal of abnormal tissue (thyroidectomy), daily oral medication (methimazole [Tapazole]), or radioactive iodine therapy. Each treatment has pros and cons depending on the patient’s medical condition, availability of the therapy, and client’s tolerance for complications or ability to medicate.
E-mail address: [email protected] (S. Padgett). 0195-5616/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 1 9 5 - 5 6 1 6 ( 0 2 ) 0 0 0 2 3 - 2
852
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
Medical therapy options for hyperthyroidism Long-term oral antithyroid drug administration is widely available to all owners and requires no anesthesia or specialized equipment. The initial cost of therapy is lower, and no hospitalization is necessary. Methimazole is not a curative therapy but controls hyperthyroidism by blocking thyroid hormone synthesis. This medication may not be appropriate for animals that are difficult to medicate or experience side effects associated with the drug. Approximately 18% of cats have been reported to have mild to severe reactions to the drug (self-excoriation, hematologic abnormalities, vomiting, anorexia, and hepatopathy) [3,4]. Long-term thyroid hormone level monitoring and methimazole dosage adjustment are necessary, which adds to the expense of medical therapy. Radioactive iodine therapy is a safe and effective method of curing hyperthyroidism. Only one treatment is necessary, no systemic side effects are seen, and no anesthesia risks are necessary. Although it is an excellent procedure, it is still not widely available. The cost is significant, and there is a chance that up to 8% of cats may need a second treatment to achieve euthyroidism [3]. Depending on regulatory mandates in the area, posttreatment isolation is necessary, often for a number of weeks.
Surgical considerations Surgical anatomy The thyroid glands are paired and located on either side of the trachea in the paratracheal fascia, normally just distal to the caudal larynx (thyroid cartilage). The normal feline thyroid gland is approximately 1 cm long, 3 to 5 mm wide, and 1 mm thick [5]. When a thyroid gland is pathologically enlarged, the thyroid can be found further caudal than normal because of gravitational migration; therefore, the entire cervical area should be available for exploration (Fig. 1). Each thyroid gland has two associated parathyroid glands. The external parathyroid gland is found lying just beneath the thyroid capsule, usually at the cranial pole of the thyroid gland, although there is tremendous variation in this location. The internal parathyroid gland is found embedded in the thyroid tissue at the middle to caudal aspect of the thyroid gland. The carotid sheath is also found in the paratracheal fascia near the thyroid glands. Each thyroid gland is supplied by a cranial thyroid artery, which is a branch of the carotid artery. This enters the thyroid at the cranial pole and gives rise to a small vascular branch supplying the external parathyroid gland. The caudal thyroid artery seen in the dog is usually absent in the cat. The recurrent laryngeal nerves are fine in the cat and run in the paratracheal fascia near the thyroid glands. The left recurrent laryngeal nerve is
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
853
Fig. 1. Intraoperative view of a unilateral thyroid tumor in a mature cat. The histologic diagnosis was multinodular adenomatous hyperplasia. The external parathyroid gland (PT) is located cranially (to the left).
located dorsolateral to the trachea, and ventral to the esophagus. The right recurrent laryngeal nerve is located lateral to the trachea and dorsomedial to the sternothyroideus muscle [5]. Preoperative evaluation Elevated circulating thyroid hormone levels should be confirmed by laboratory analysis. Another method of documenting hyperthyroidism is technetium 99m radionuclide imaging [6]. This test documents hyperthyroidism, the distribution of the abnormal tissue (one gland or both), the presence of functional ectopic thyroid tissue, and metastatic thyroid carcinoma. Unfortunately, the equipment necessary for this imaging modality limits its availability. Assessment of the systemic health of the patient is important. Preoperative blood work and urinalysis should be performed to reveal any concurrent metabolic abnormalities. More than 90% of hyperthyroid cats have an increase in liver-associated enzymes, and more than 20% have a high blood urea nitrogen or creatinine level [7]. Because the typical feline hyperthyroid patient is geriatric, concurrent metabolic or neoplastic abnormalities are common. Electrocardiograms and thoracic radiographs are recommended before anesthesia. Hyperthyroidism has been found to lead to tachycardia, gallop rhythm, cardiomegaly, and heart murmurs in many cats [7,8]. This has been attributed to increased sympathetic tone, hypertrophic cardiomyopathy in response to increased thyroid hormones, and cardiac changes that compensate for altered peripheral tissue function [7,8]. Ventricular arrhythmias and
854
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
heart failure secondary to cardiac disease are not common but are seen more frequently in older hyperthyroid cats. Thoracic radiographs are also useful to demonstrate obvious metastatic disease from thyroid carcinoma. Both tachycardia and ventricular arrhythmias should be controlled before anesthesia. This can usually be achieved by controlling the hyperthyroidism with methimazole (10–15 mg/kg of body weight administered orally) [3] for 2 to 4 weeks before surgery. This is not appropriate for cats that have demonstrated adverse effects to this medication. Propranolol therapy (0.5 mg/kg administered orally 2–3 times a day) for 3 to 5 days before surgery has also been suggested to decrease anesthetic risk associated with cardiac changes [5]. Propranolol not only decreases heart rate but has also been shown to lower serum T3 levels [9]. This drug should be used with extreme caution in cats with congestive heart failure because of the negative inotropic effects of propranolol. Anesthetic considerations Premedications containing acepromazine (0.1 mg/kg administered intramuscularly) are commonly recommended to decrease autonomic tone in the hyperthyroid patient, thereby decreasing the potential for arrhythmias [10]. Drugs that potentiate arrhythmias should be avoided because of the frequency of hypertrophic cardiomyopathy, tachycardia, and arrhythmias. Atropine should be avoided in the hyperthyroid patient because it may induce arrhythmias. Ketamine should not be used because it sensitizes the heart to catecholamine-induced arrhythmias [11]. Isoflurane via mask and propofol are suggested for use as induction agents. An electrocardiogram should be used during anesthesia to monitor for arrhythmias. Support of circulating volume and renal function via careful administration of a balanced electrolyte solution is prudent. Surgical technique Surgical preparation of the entire ventral cervical area to the level of the thoracic inlet should be performed, because the enlarged thyroid can travel caudally as a result of gravity. The patient is positioned in dorsal recumbency with the forelimbs pulled caudally. The neck should be slightly extended. The approach is via a ventral midline incision caudal to the larynx. Blunt dissection in the midline between the strap muscles of the neck (sternohyoideus and sternothyroideus) is performed. The trachea is visualized, and blunt dissection is undertaken in the paratracheal fascia lateral to the proximal trachea. Care is taken not to damage the tracheal vascularity, which occurs as a pedicle on the lateral aspects close to the trachea. The recurrent laryngeal nerve and the carotid sheath should be identified and preserved. If the thyroid glands are not found immediately caudal to the larynx, the paratracheal area should be explored to the level of the thoracic inlet.
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
855
If no preoperative radionuclide scan has been performed, careful inspection of both thyroid glands is necessary to determine whether the disease is unilateral or bilateral. Bilateral disease is manifested as two enlarged glands. Unilateral disease should lead to atrophy of the normal gland as a result of negative feedback induced by the autonomous hormone production from the abnormal gland. If the thyroid gland opposite an enlarged gland seems to be normal, it is almost certainly hyperplastic as well. In this case, the normal-appearing gland should at least be biopsied if not removed. Approximately 70% of hyperthyroid cats have bilateral disease [7,8]. Different methods of removing the thyroid glands are reviewed below. Resected thyroid tissue should always be submitted for histopathologic examination. Routine closure of the neck muscles and skin is then performed. Surgical thyroidectomy techniques differ primarily according to the method by which parathyroid function is preserved. No matter which of the following techniques is used, the underlying principles of meticulous dissection, careful attention to preserving vascularity, and avoiding other vital structures in the area are crucial. Extracapsular thyroidectomy technique This technique involves no dissection of the thyroid parenchyma. The vascularity to the thyroid gland is ligated, the parathyroid gland is sharply dissected from the capsule of the thyroid, and the entire thyroid gland (with capsule) is removed [12]. This technique is not recommended because of the high incidence of postoperative complications. Modified extracapsular thyroidectomy technique This technique is similar to the extracapsular technique except that the external parathyroid gland vascularity is preserved, thereby decreasing postoperative morbidity. Using a fine-tipped electrode, the thyroid capsule is cauterized around the external parathyroid gland and its vascularity, with at least 2-mm margins. A number 11 scalpel blade or fine scissors are used to cut through the cauterized area. The rim of thyroid capsule containing the external parathyroid gland and vessel is dissected free from the thyroid parenchyma, which is still encompassed by the rest of the thyroid capsule. Intracapsular thyroidectomy technique This technique minimizes damage to the blood supply of the external parathyroid gland by leaving the thyroid capsule in place. A small incision is made in an avascular area of the thyroid capsule. Using a cotton-tipped applicator or hemostat, meticulous blunt dissection is performed to remove the thyroid gland from its capsule. It is particularly important not to damage the area of the capsule associated with the external parathyroid gland and its associated vascularity. Any pieces of the thyroid remaining in the capsule should be carefully removed, because adenomatous tissue left within the capsule has been associated with hyperthyroid recurrence [12–14].
856
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
Modified intracapsular thyroidectomy technique This technique differs from the routine intracapsular method by removing more of the thyroid capsule, leaving only the capsule associated with the external parathyroid gland and its blood supply. After the thyroid gland is carefully removed from the capsule, a number 15 scalpel blade or fine scissors are used to create a peninsula of capsular tissue containing only the gland and its blood vessel. The remainder of the capsule is excised, decreasing the likelihood of adenomatous tissue remaining in situ. Staged bilateral removal technique To avoid damage to the two remaining external parathyroid glands during a bilateral thyroidectomy, some authors suggest a staged unilateral removal [5]. The time between surgeries allows resolution of transient damage to the parathyroid gland or its vascularity. The surgeries are staged by at least 3 weeks [5]. Although this technique does decrease the incidence of postoperative hypocalcemia, there is added risk as a result of two anesthetic events. Parathyroid autotransplantation In the event there is inadvertent removal or complete devascularization of the external parathyroid gland during surgery, autotransplantation can save the function of the parathyroid gland. This is achieved by mincing the parathyroid gland into approximately 1-mm cubes and inserting them into a stab incision in one of the local neck muscles [15]. This allows revascularization and minimizes the duration and severity of postoperative hypocalcemia [15,16]. This has been recommended as routine procedure [17], which may not be prudent, because there is a chance that diseased thyroid may be transplanted with the parathyroid gland [16]. Postoperative complications and management Hypocalcemia The most serious complication of bilateral thyroidectomy is postoperative hypocalcemia as a result of acute hypoparathyroidism, which is reported in 11% to 82% of cats depending on the method of thyroidectomy (Table 1) [12,14,18]. Hypoparathyroidism usually develops because of inadvertent removal of all parathyroid glands or disruption of the vascular supply to the remaining parathyroid glands. Hypocalcemia usually occurs 24 to 72 hours after surgery. Not all hypocalcemic patients warrant therapy, and the decision to treat is based on the severity of clinical signs. Clinical signs of hypocalcemia can be mild (irritability and decrease in appetite) or severe (ear and facial twitching, seizure-like muscular tetany). Clinical signs do not usually occur until total serum calcium is less than 6.5 mg/dL [5]. It is recommended that serum calcium concentrations be measured at least once 24 hours after bilateral thyroidectomy. Mild signs of hypocalcemia can be treated with oral supplementation alone. Cats showing moderate to severe signs of hypocalcemia should have
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
857
Table 1 Incidence of hypocalcemia after bilateral thyroidectomy Bilateral Thyroidectomy Technique Intracapsular
Modified intracapsular Extracapsular Modified extracapsular Staged intracapsular
Incidence of Hypocalcemia
Reference
15% 22% 36% 33% 82% 23% 11%
18 14 12 14 12 14 12
(8/53) (11/50) (7/19) (10/30) (9/11) (6/26) (1/11)
their calcium levels confirmed by laboratory analysis if possible. If immediate results are not possible, clinical signs alone warrant treatment, because severe hypocalcemia can be fatal. Moderate to severe hypocalcemia is treated by slow intravenous infusion of calcium gluconate (1 mL of a 10% calcium gluconate solution containing 9.3 mg of ionized calcium administered over 10 minutes). This should be followed by intravenous infusion of a calcium gluconate solution (10 mL of a 10% calcium gluconate solution in 250 mL of 0.9% NaCl at 2.5 mL/kg/h for 8 to 12 hours [5,11,15]. If serum calcium concentrations decrease after discontinuing the slow intravenous calcium infusion, another 8 to 12 hours of therapy is indicated. Oral supplementation of calcium (500–700 mg/kg of calcium gluconate administered orally and divided into 3 doses per day) [5] should be started as well as a vitamin D analogue (dihydrotachysterol solution at a rate of 0.03 mg/kg/d for 3 days and then decreased to 0.01–0.02 mg/kg/d) [10]. Dihydrotachysterol often does not show an effect until 48 to 72 hours after starting therapy. As the serum calcium concentrations increase over a period of days to weeks, the oral medications can be tapered. Although some patients require lifelong oral calcium therapy, most do not need oral supplementation for more than 3 to 6 weeks. Azotemia Many patients with hyperthyroidism have concurrent renal insufficiency. It is possible that treatment of hyperthyroidism may lead to clinical evidence of a previously masked chronic renal insufficiency, because treatment has been shown to decrease mean glomerular filtration rate and to increase serum creatinine and blood urea nitrogen concentrations [19,20]. It may be prudent to achieve temporary euthyroidism via medical management and assess the degree of azotemia before achieving permanent euthyroidism with surgical therapy. Nerve damage The recurrent laryngeal nerve can be damaged on exploration of the paratracheal fascia, leading to voice change or laryngeal paralysis. For this reason, it is important to know the normal anatomy of the area and strive to
858
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
avoid undue tension or trauma to the recurrent laryngeal nerve. Laryngeal paralysis can be temporary if the nerve is mildly damaged. If clinical signs in this area are caused by permanent laryngeal paralysis, surgical correction is required. Horner’s syndrome can also occur if the sympathetic trunk is damaged. Careful retraction of the cervical musculature should minimize this risk. Recurrent hyperthyroidism Patients undergoing bilateral thyroidectomy can experience recurrence of disease months to years after surgical treatment [13,14]. This has been reported to occur more commonly with the intracapsular technique [14]. Recurrence is attributed to leaving nests of abnormal thyroid tissue at the surgical sight, which become functional after hypertrophy [13,14], and to adenomatous ectopic thyroid tissue [13]. Alternatively, metastatic thyroid carcinoma could cause recurrence of disease. The best method of diagnosing the location of the abnormal tissue is a technetium 99m thyroid scan. Hypothyroidism Although it has been suggested that cats undergoing bilateral thyroidectomy should receive thyroid hormone replacement, most patients show no clinical signs referable to hypothyroidism. Hormone replacement therapy is not necessary in most patients.
References [1] Peterson ME, Johnson GF, Andrews LK. Spontaneous hyperthyroidism in the cat. In: Scientific Proceedings of the American College of Veterinary Internal Medicine [abstract]. Guelph (ON): Aqim College; 1979, p. 108. [2] Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981–1986). JAVMA 1988;193:359–64. [3] Kintzer PP. Considerations in the treatment of feline hyperthyroidism. Vet Clin North Am Small Anim Pract 1994;24:577–85. [4] Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med 1988;2:150–7. [5] Flanders JA. Surgical therapy of the thyroid. Vet Clin North Am Small Anim Pract 1994;24:607–21. [6] Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Vet Radiol Ultrasound 1984;25:23–7. [7] Liu S, Peterson ME, Fox PR. Hypertrophic cardiomyopathy and hyperthyroidism in the cat. JAVMA 1984;185:52–7. [8] Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. JAVMA 1983;183:103–10. [9] Foster DJ, Thoday KL. Use of propranolol and potassium iodate in the presurgical management of hyperthyroid cats. J Small Anim Pract 1999;40:307–15. [10] Feldman EC. Feline hyperthyroidism (thyrotoxicosis). In: Feldman EC, Nelson RW, editors. Canine and Feline Endocrinology and Reproduction. 2nd edition. Philadelphia: WB Saunders; 1996. p. 118–66.
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
859
[11] Muir W, Hubbell JAE. Handbook of veterinary anesthesia. St. Louis: CV Mosby; 1989. p. 74–86. [12] Flanders JA, Harvey HJ, Erb HN. Feline thyroidectomy: a comparison of postoperative hypocalcemia associated with three different surgical techniques. Vet Surg 1987;16:362–6. [13] Swalec KM, Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. J Am Anim Hosp Assoc 1990;26:433–7. [14] Welches CD, Scavelli TD, Matthiesen DT, et al. Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Vet Surg 1989;18:392–6. [15] Flanders JA. Surgical treatment of hyperthyroid cats. Mod Vet Pract 1986;67:711–5. [16] Padgett SL, Tobias KM, Leathers CW, et al. Efficacy of parathyroid gland autotransplantation in maintaining serum calcium concentrations after bilateral thyroparathyroidectomy in cats. J Am Anim Hosp Assoc 1998;34:219–24. [17] Norsworthy GD. Feline thyroidectomy: a simplified technique that preserves parathyroid function. Vet Med 1995;90:1055–63. [18] Birchard SJ, Peterson ME, Jacobson A. Surgical treatment of feline hyperthyroidism: results of 85 cases. J Am Anim Hosp Assoc 1984;20:705–9. [19] Dibartola SP, Broome MR, Stein BS, et al. Effect of treatment of hyperthyroidism on renal function in cats. JAVMA 1996;208:875–8. [20] Graves TK, Olivier NB, Nachreiner RF, et al. Changes in renal function associated with treatment of hyperthyroidism in cats. Am J Vet Res 1994;55:1745–9.
Feline thyroid surgery Sheldon Padgett, DVM, MS Metropolitan Veterinary Hospital, 1053 South Cleveland-Massillon Road, Akron, OH 44321, USA
Thyroid disease in the cat presents a unique challenge to the surgeon, because the patients commonly have concurrent secondary metabolic abnormalities that can predispose them to significant and sometimes life-threatening complications. It is necessary not only to consider the best method of treatment but the effects that thyroid disease can have on other organ systems.
Surgical indications Thyroid surgery in the cat is generally limited to removal of the thyroid (thyroidectomy), although a thyroid biopsy may occasionally be indicated. Thyroidectomy is most commonly performed as therapy for hyperthyroidism. Additionally, because of the intimate anatomy of thyroid and parathyroid glands, the thyroid is usually removed as part of a parathyroidectomy performed as therapy for functional parathyroid disease.
Hyperthyroidism Since the first report of the disease in 1979 by Peterson et al [1], hyperthyroidism has become the most commonly diagnosed endocrine disease in cats. The disease is caused by functional benign adenomatous hyperplasia in approximately 98% of cases. Functional thyroid tumors (carcinomas) account for the remaining 2% of hyperthyroid cats [2]. Feline hyperthyroidism is usually treated by one of the following methods: surgical removal of abnormal tissue (thyroidectomy), daily oral medication (methimazole [Tapazole]), or radioactive iodine therapy. Each treatment has pros and cons depending on the patient’s medical condition, availability of the therapy, and client’s tolerance for complications or ability to medicate.
E-mail address: [email protected] (S. Padgett). 0195-5616/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 1 9 5 - 5 6 1 6 ( 0 2 ) 0 0 0 2 3 - 2
852
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
Medical therapy options for hyperthyroidism Long-term oral antithyroid drug administration is widely available to all owners and requires no anesthesia or specialized equipment. The initial cost of therapy is lower, and no hospitalization is necessary. Methimazole is not a curative therapy but controls hyperthyroidism by blocking thyroid hormone synthesis. This medication may not be appropriate for animals that are difficult to medicate or experience side effects associated with the drug. Approximately 18% of cats have been reported to have mild to severe reactions to the drug (self-excoriation, hematologic abnormalities, vomiting, anorexia, and hepatopathy) [3,4]. Long-term thyroid hormone level monitoring and methimazole dosage adjustment are necessary, which adds to the expense of medical therapy. Radioactive iodine therapy is a safe and effective method of curing hyperthyroidism. Only one treatment is necessary, no systemic side effects are seen, and no anesthesia risks are necessary. Although it is an excellent procedure, it is still not widely available. The cost is significant, and there is a chance that up to 8% of cats may need a second treatment to achieve euthyroidism [3]. Depending on regulatory mandates in the area, posttreatment isolation is necessary, often for a number of weeks.
Surgical considerations Surgical anatomy The thyroid glands are paired and located on either side of the trachea in the paratracheal fascia, normally just distal to the caudal larynx (thyroid cartilage). The normal feline thyroid gland is approximately 1 cm long, 3 to 5 mm wide, and 1 mm thick [5]. When a thyroid gland is pathologically enlarged, the thyroid can be found further caudal than normal because of gravitational migration; therefore, the entire cervical area should be available for exploration (Fig. 1). Each thyroid gland has two associated parathyroid glands. The external parathyroid gland is found lying just beneath the thyroid capsule, usually at the cranial pole of the thyroid gland, although there is tremendous variation in this location. The internal parathyroid gland is found embedded in the thyroid tissue at the middle to caudal aspect of the thyroid gland. The carotid sheath is also found in the paratracheal fascia near the thyroid glands. Each thyroid gland is supplied by a cranial thyroid artery, which is a branch of the carotid artery. This enters the thyroid at the cranial pole and gives rise to a small vascular branch supplying the external parathyroid gland. The caudal thyroid artery seen in the dog is usually absent in the cat. The recurrent laryngeal nerves are fine in the cat and run in the paratracheal fascia near the thyroid glands. The left recurrent laryngeal nerve is
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
853
Fig. 1. Intraoperative view of a unilateral thyroid tumor in a mature cat. The histologic diagnosis was multinodular adenomatous hyperplasia. The external parathyroid gland (PT) is located cranially (to the left).
located dorsolateral to the trachea, and ventral to the esophagus. The right recurrent laryngeal nerve is located lateral to the trachea and dorsomedial to the sternothyroideus muscle [5]. Preoperative evaluation Elevated circulating thyroid hormone levels should be confirmed by laboratory analysis. Another method of documenting hyperthyroidism is technetium 99m radionuclide imaging [6]. This test documents hyperthyroidism, the distribution of the abnormal tissue (one gland or both), the presence of functional ectopic thyroid tissue, and metastatic thyroid carcinoma. Unfortunately, the equipment necessary for this imaging modality limits its availability. Assessment of the systemic health of the patient is important. Preoperative blood work and urinalysis should be performed to reveal any concurrent metabolic abnormalities. More than 90% of hyperthyroid cats have an increase in liver-associated enzymes, and more than 20% have a high blood urea nitrogen or creatinine level [7]. Because the typical feline hyperthyroid patient is geriatric, concurrent metabolic or neoplastic abnormalities are common. Electrocardiograms and thoracic radiographs are recommended before anesthesia. Hyperthyroidism has been found to lead to tachycardia, gallop rhythm, cardiomegaly, and heart murmurs in many cats [7,8]. This has been attributed to increased sympathetic tone, hypertrophic cardiomyopathy in response to increased thyroid hormones, and cardiac changes that compensate for altered peripheral tissue function [7,8]. Ventricular arrhythmias and
854
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
heart failure secondary to cardiac disease are not common but are seen more frequently in older hyperthyroid cats. Thoracic radiographs are also useful to demonstrate obvious metastatic disease from thyroid carcinoma. Both tachycardia and ventricular arrhythmias should be controlled before anesthesia. This can usually be achieved by controlling the hyperthyroidism with methimazole (10–15 mg/kg of body weight administered orally) [3] for 2 to 4 weeks before surgery. This is not appropriate for cats that have demonstrated adverse effects to this medication. Propranolol therapy (0.5 mg/kg administered orally 2–3 times a day) for 3 to 5 days before surgery has also been suggested to decrease anesthetic risk associated with cardiac changes [5]. Propranolol not only decreases heart rate but has also been shown to lower serum T3 levels [9]. This drug should be used with extreme caution in cats with congestive heart failure because of the negative inotropic effects of propranolol. Anesthetic considerations Premedications containing acepromazine (0.1 mg/kg administered intramuscularly) are commonly recommended to decrease autonomic tone in the hyperthyroid patient, thereby decreasing the potential for arrhythmias [10]. Drugs that potentiate arrhythmias should be avoided because of the frequency of hypertrophic cardiomyopathy, tachycardia, and arrhythmias. Atropine should be avoided in the hyperthyroid patient because it may induce arrhythmias. Ketamine should not be used because it sensitizes the heart to catecholamine-induced arrhythmias [11]. Isoflurane via mask and propofol are suggested for use as induction agents. An electrocardiogram should be used during anesthesia to monitor for arrhythmias. Support of circulating volume and renal function via careful administration of a balanced electrolyte solution is prudent. Surgical technique Surgical preparation of the entire ventral cervical area to the level of the thoracic inlet should be performed, because the enlarged thyroid can travel caudally as a result of gravity. The patient is positioned in dorsal recumbency with the forelimbs pulled caudally. The neck should be slightly extended. The approach is via a ventral midline incision caudal to the larynx. Blunt dissection in the midline between the strap muscles of the neck (sternohyoideus and sternothyroideus) is performed. The trachea is visualized, and blunt dissection is undertaken in the paratracheal fascia lateral to the proximal trachea. Care is taken not to damage the tracheal vascularity, which occurs as a pedicle on the lateral aspects close to the trachea. The recurrent laryngeal nerve and the carotid sheath should be identified and preserved. If the thyroid glands are not found immediately caudal to the larynx, the paratracheal area should be explored to the level of the thoracic inlet.
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
855
If no preoperative radionuclide scan has been performed, careful inspection of both thyroid glands is necessary to determine whether the disease is unilateral or bilateral. Bilateral disease is manifested as two enlarged glands. Unilateral disease should lead to atrophy of the normal gland as a result of negative feedback induced by the autonomous hormone production from the abnormal gland. If the thyroid gland opposite an enlarged gland seems to be normal, it is almost certainly hyperplastic as well. In this case, the normal-appearing gland should at least be biopsied if not removed. Approximately 70% of hyperthyroid cats have bilateral disease [7,8]. Different methods of removing the thyroid glands are reviewed below. Resected thyroid tissue should always be submitted for histopathologic examination. Routine closure of the neck muscles and skin is then performed. Surgical thyroidectomy techniques differ primarily according to the method by which parathyroid function is preserved. No matter which of the following techniques is used, the underlying principles of meticulous dissection, careful attention to preserving vascularity, and avoiding other vital structures in the area are crucial. Extracapsular thyroidectomy technique This technique involves no dissection of the thyroid parenchyma. The vascularity to the thyroid gland is ligated, the parathyroid gland is sharply dissected from the capsule of the thyroid, and the entire thyroid gland (with capsule) is removed [12]. This technique is not recommended because of the high incidence of postoperative complications. Modified extracapsular thyroidectomy technique This technique is similar to the extracapsular technique except that the external parathyroid gland vascularity is preserved, thereby decreasing postoperative morbidity. Using a fine-tipped electrode, the thyroid capsule is cauterized around the external parathyroid gland and its vascularity, with at least 2-mm margins. A number 11 scalpel blade or fine scissors are used to cut through the cauterized area. The rim of thyroid capsule containing the external parathyroid gland and vessel is dissected free from the thyroid parenchyma, which is still encompassed by the rest of the thyroid capsule. Intracapsular thyroidectomy technique This technique minimizes damage to the blood supply of the external parathyroid gland by leaving the thyroid capsule in place. A small incision is made in an avascular area of the thyroid capsule. Using a cotton-tipped applicator or hemostat, meticulous blunt dissection is performed to remove the thyroid gland from its capsule. It is particularly important not to damage the area of the capsule associated with the external parathyroid gland and its associated vascularity. Any pieces of the thyroid remaining in the capsule should be carefully removed, because adenomatous tissue left within the capsule has been associated with hyperthyroid recurrence [12–14].
856
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
Modified intracapsular thyroidectomy technique This technique differs from the routine intracapsular method by removing more of the thyroid capsule, leaving only the capsule associated with the external parathyroid gland and its blood supply. After the thyroid gland is carefully removed from the capsule, a number 15 scalpel blade or fine scissors are used to create a peninsula of capsular tissue containing only the gland and its blood vessel. The remainder of the capsule is excised, decreasing the likelihood of adenomatous tissue remaining in situ. Staged bilateral removal technique To avoid damage to the two remaining external parathyroid glands during a bilateral thyroidectomy, some authors suggest a staged unilateral removal [5]. The time between surgeries allows resolution of transient damage to the parathyroid gland or its vascularity. The surgeries are staged by at least 3 weeks [5]. Although this technique does decrease the incidence of postoperative hypocalcemia, there is added risk as a result of two anesthetic events. Parathyroid autotransplantation In the event there is inadvertent removal or complete devascularization of the external parathyroid gland during surgery, autotransplantation can save the function of the parathyroid gland. This is achieved by mincing the parathyroid gland into approximately 1-mm cubes and inserting them into a stab incision in one of the local neck muscles [15]. This allows revascularization and minimizes the duration and severity of postoperative hypocalcemia [15,16]. This has been recommended as routine procedure [17], which may not be prudent, because there is a chance that diseased thyroid may be transplanted with the parathyroid gland [16]. Postoperative complications and management Hypocalcemia The most serious complication of bilateral thyroidectomy is postoperative hypocalcemia as a result of acute hypoparathyroidism, which is reported in 11% to 82% of cats depending on the method of thyroidectomy (Table 1) [12,14,18]. Hypoparathyroidism usually develops because of inadvertent removal of all parathyroid glands or disruption of the vascular supply to the remaining parathyroid glands. Hypocalcemia usually occurs 24 to 72 hours after surgery. Not all hypocalcemic patients warrant therapy, and the decision to treat is based on the severity of clinical signs. Clinical signs of hypocalcemia can be mild (irritability and decrease in appetite) or severe (ear and facial twitching, seizure-like muscular tetany). Clinical signs do not usually occur until total serum calcium is less than 6.5 mg/dL [5]. It is recommended that serum calcium concentrations be measured at least once 24 hours after bilateral thyroidectomy. Mild signs of hypocalcemia can be treated with oral supplementation alone. Cats showing moderate to severe signs of hypocalcemia should have
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
857
Table 1 Incidence of hypocalcemia after bilateral thyroidectomy Bilateral Thyroidectomy Technique Intracapsular
Modified intracapsular Extracapsular Modified extracapsular Staged intracapsular
Incidence of Hypocalcemia
Reference
15% 22% 36% 33% 82% 23% 11%
18 14 12 14 12 14 12
(8/53) (11/50) (7/19) (10/30) (9/11) (6/26) (1/11)
their calcium levels confirmed by laboratory analysis if possible. If immediate results are not possible, clinical signs alone warrant treatment, because severe hypocalcemia can be fatal. Moderate to severe hypocalcemia is treated by slow intravenous infusion of calcium gluconate (1 mL of a 10% calcium gluconate solution containing 9.3 mg of ionized calcium administered over 10 minutes). This should be followed by intravenous infusion of a calcium gluconate solution (10 mL of a 10% calcium gluconate solution in 250 mL of 0.9% NaCl at 2.5 mL/kg/h for 8 to 12 hours [5,11,15]. If serum calcium concentrations decrease after discontinuing the slow intravenous calcium infusion, another 8 to 12 hours of therapy is indicated. Oral supplementation of calcium (500–700 mg/kg of calcium gluconate administered orally and divided into 3 doses per day) [5] should be started as well as a vitamin D analogue (dihydrotachysterol solution at a rate of 0.03 mg/kg/d for 3 days and then decreased to 0.01–0.02 mg/kg/d) [10]. Dihydrotachysterol often does not show an effect until 48 to 72 hours after starting therapy. As the serum calcium concentrations increase over a period of days to weeks, the oral medications can be tapered. Although some patients require lifelong oral calcium therapy, most do not need oral supplementation for more than 3 to 6 weeks. Azotemia Many patients with hyperthyroidism have concurrent renal insufficiency. It is possible that treatment of hyperthyroidism may lead to clinical evidence of a previously masked chronic renal insufficiency, because treatment has been shown to decrease mean glomerular filtration rate and to increase serum creatinine and blood urea nitrogen concentrations [19,20]. It may be prudent to achieve temporary euthyroidism via medical management and assess the degree of azotemia before achieving permanent euthyroidism with surgical therapy. Nerve damage The recurrent laryngeal nerve can be damaged on exploration of the paratracheal fascia, leading to voice change or laryngeal paralysis. For this reason, it is important to know the normal anatomy of the area and strive to
858
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
avoid undue tension or trauma to the recurrent laryngeal nerve. Laryngeal paralysis can be temporary if the nerve is mildly damaged. If clinical signs in this area are caused by permanent laryngeal paralysis, surgical correction is required. Horner’s syndrome can also occur if the sympathetic trunk is damaged. Careful retraction of the cervical musculature should minimize this risk. Recurrent hyperthyroidism Patients undergoing bilateral thyroidectomy can experience recurrence of disease months to years after surgical treatment [13,14]. This has been reported to occur more commonly with the intracapsular technique [14]. Recurrence is attributed to leaving nests of abnormal thyroid tissue at the surgical sight, which become functional after hypertrophy [13,14], and to adenomatous ectopic thyroid tissue [13]. Alternatively, metastatic thyroid carcinoma could cause recurrence of disease. The best method of diagnosing the location of the abnormal tissue is a technetium 99m thyroid scan. Hypothyroidism Although it has been suggested that cats undergoing bilateral thyroidectomy should receive thyroid hormone replacement, most patients show no clinical signs referable to hypothyroidism. Hormone replacement therapy is not necessary in most patients.
References [1] Peterson ME, Johnson GF, Andrews LK. Spontaneous hyperthyroidism in the cat. In: Scientific Proceedings of the American College of Veterinary Internal Medicine [abstract]. Guelph (ON): Aqim College; 1979, p. 108. [2] Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981–1986). JAVMA 1988;193:359–64. [3] Kintzer PP. Considerations in the treatment of feline hyperthyroidism. Vet Clin North Am Small Anim Pract 1994;24:577–85. [4] Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med 1988;2:150–7. [5] Flanders JA. Surgical therapy of the thyroid. Vet Clin North Am Small Anim Pract 1994;24:607–21. [6] Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Vet Radiol Ultrasound 1984;25:23–7. [7] Liu S, Peterson ME, Fox PR. Hypertrophic cardiomyopathy and hyperthyroidism in the cat. JAVMA 1984;185:52–7. [8] Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. JAVMA 1983;183:103–10. [9] Foster DJ, Thoday KL. Use of propranolol and potassium iodate in the presurgical management of hyperthyroid cats. J Small Anim Pract 1999;40:307–15. [10] Feldman EC. Feline hyperthyroidism (thyrotoxicosis). In: Feldman EC, Nelson RW, editors. Canine and Feline Endocrinology and Reproduction. 2nd edition. Philadelphia: WB Saunders; 1996. p. 118–66.
S. Padgett / Vet Clin Small Anim 32 (2002) 851–859
859
[11] Muir W, Hubbell JAE. Handbook of veterinary anesthesia. St. Louis: CV Mosby; 1989. p. 74–86. [12] Flanders JA, Harvey HJ, Erb HN. Feline thyroidectomy: a comparison of postoperative hypocalcemia associated with three different surgical techniques. Vet Surg 1987;16:362–6. [13] Swalec KM, Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. J Am Anim Hosp Assoc 1990;26:433–7. [14] Welches CD, Scavelli TD, Matthiesen DT, et al. Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Vet Surg 1989;18:392–6. [15] Flanders JA. Surgical treatment of hyperthyroid cats. Mod Vet Pract 1986;67:711–5. [16] Padgett SL, Tobias KM, Leathers CW, et al. Efficacy of parathyroid gland autotransplantation in maintaining serum calcium concentrations after bilateral thyroparathyroidectomy in cats. J Am Anim Hosp Assoc 1998;34:219–24. [17] Norsworthy GD. Feline thyroidectomy: a simplified technique that preserves parathyroid function. Vet Med 1995;90:1055–63. [18] Birchard SJ, Peterson ME, Jacobson A. Surgical treatment of feline hyperthyroidism: results of 85 cases. J Am Anim Hosp Assoc 1984;20:705–9. [19] Dibartola SP, Broome MR, Stein BS, et al. Effect of treatment of hyperthyroidism on renal function in cats. JAVMA 1996;208:875–8. [20] Graves TK, Olivier NB, Nachreiner RF, et al. Changes in renal function associated with treatment of hyperthyroidism in cats. Am J Vet Res 1994;55:1745–9.