Sedation and Anesthesia for the Cat with Hyperthyroidism

One major disadvantage of selecting surgical thyroidectomy as the treatment used for a cat with hyperthyroidism is that sedation and anesthesia must be given in order to complete the operation (1-3). Cats with hyperthyroidism are generally older and commonly have concurrent or multisystemic disease, which can increase anesthetic risk (4-6).

Although the mortality rate is very low, morbidity due to anesthesia, which is often overlooked, is not uncommon. Provided that the potential risk is recognized, suitable premedication, a smooth anesthetic induction, careful monitoring, appropriate attention to the oxygenation, ventilation and circulation will ensure a very low morbidity rate in cats.

In this post, I will review suggested protocols for sedating and anesthetizing cats with hyperthyroidism.

Considerations for Preanesthetic Management
Cats with hyperthyroidism are generally older and may suffer from a concurrent disease, in addition to the damage caused by the high circulating thyroid hormone concentrations.

The most significant risk pertaining to anesthesia results from cardiac changes in hyperthyroid patients. In addition to a rapid heart rate (tachycardia), cardiac murmur, and a gallop rhythm, many of these cats develop thickening of the left ventricle, predisposing them to myocardial hypoxia, arrhythmias, and cardiac arrest (3-5).

If possible, it is always recommended that we use antithyroid drugs (e.g., methimazole or carbimazole) to treat the hyperthyroid cat medically for a few weeks before surgery in order to render the cats euthyroid before anesthesia. For more information, see my previous post on Preoperative Preparation of the hyperthyroid cat.


Preanesthetic Preparation
An overnight fast is required to help ensure that the a cat will have an empty stomach. Water need not be deprived until premedication is given or until about 2 hours prior to 
anesthesia.

On the day in which anesthesia and surgery is planned, the veterinarians should conduct a thorough physical examination, including auscultation for cardiac arrhythmias and murmurs. If concurrent kidney or liver disease is suspected, repeat laboratory testing should be done and intravenous fluid therapy administered if needed.

An IV catheterization is placed, generally in either the cephalic and saphenous veins. Use of SC premedication is generally recommended prior to placement of the IV line in these cats.

Premedication (Preanesthetic Agents)
Even very friendly cats often object to physical restraint, sometimes making the anesthetic induction procedure extremely difficult to accomplish safely. Therefore, use of preanesthetic sedation is recommended in order to facilitate IV catheter placement and anesthetic induction in these hyperthyroid cats (3-5). This sedative also has an anesthetic-sparing effect during the maintenance anesthetic period.

In hyperthyroid cats, use of an opioid agent administered subcutaneously is suggested for its cardiovascular safety and analgesia (5-9). Such opioid drugs commonly used include hydromorphone (0.025-0.05 mg/kg), oxymorphone (0.025-0.05 mg/kg), or methadone (0.3-0.5 mg/kg). These opioid agents help create a sense of well-being or euphoria, but they do not typically sedate cats very well. Therefore, it is best to combine one of these opioid agents with another drug to produce the desired sedative effect.

Combining an opiate drug either with a mild tranquilizer or sedative is commonly done in veterinary practice. The benzadiazepine tranquilizers (diazepam or midazolam) cause only minimal cardiovascular depression and are therefore very useful preanesthetic agents for hyperthyroid cats (5,6,9,10). The dissociative agent ketamine, is a useful preanesthetic agent. However, this drug can produce an increased heart rate, cardiac output, and blood pressure and should never be used alone, especially if administered intravenously or at high doses (1,4).

A number of drug protocols are available for premedication prior to induction (3,4,5,7). Combinations of an opioid (e.g., hydromorphone, oxymorphone, or methadone) administered with a tranquilizer (e.g., midazolam [Versed]) is one commonly used protocol. Alternatively, use of a dissociative agent (e.g., ketamine [Ketaset]) given with a mild tranquilizer (midazolam) is also a popular choice among veterinarians.

As an alternative, a combination of 3 agents — a subcutaneous opioid (see doses above), midazolam (0.1-0.2 mg//kg, SC), and very low-dose ketamine (2 mg/kg, SC) — can be used to sedate fractious cats (5).

Such drug combinations also provide better restraint and analgesia than that achieved by single drug administration and have fewer side effects than the use of one class of preanesthetic alone. For example, when used alone, opioids can induce extreme excitement or “mania,” ketamine will produce increased severe muscle rigidity and excessive salivation, and midazolam may induce paradoxical excitement and dysphoria. When combined with other CNS depressants, these side effects will occur less frequency and are much milder.

Anesthesia Induction
Placement of electrocardiogram (ECG) and Doppler equipment prior to induction of anesthesia is ideal. The ECG detects rhythm abnormalities, and Doppler assesses blood pressure changes and provides an audible pulse signal during this critical time.

Again, a number of protocols can be used for induction of anesthesia in cats with hyperthyroidism. One protocol that is very safe for the cardiovascular system entails the intravenous administration of both etomidate (0.5–1.5 mg/kg) 
and a benzodiazepine (eg, diazepam or midazolam, 0.1–0.3 mg/kg). However, etomidate has the potential to cause adrenal suppression and lower serum cortisol levels, warranting supplementation with physiological doses of glucocorticoids in some cats.

Alternatively, slow IV administration of propofol
 (2-6 mg/kg) provides rapid induction but should be used judiciously, as this agent can produce vasodilation and
 hypotension. Respiratory arrest is not uncommon, particularly with rapid IV bolus. The drug is best given slowly and titrated to effect  in order to produce 
an anesthetic level just deep enough to allow endotracheal intubation.

Maintenance Anesthesia (Inhalation Agents)
Endotracheal intubation and use of an inhaled anesthetic agent is recommended. A number of inhaled anesthetic agents are available but isoflurane or sevoflurane, are most commonly employed (1,3).

The advantages of inhalation anesthesia in cats with hyperthyroidism include the following: a patent airway, rapid control of anesthetic depth, and quick and smooth 
recovery (11). Disadvantages of these anesthetic agents include a degree of cardiovascular depression (e.g., myocardial depression, hypotension, and slowing of the heart). An IV infusion of an opioid drug (e.g., fentanyl 5–10 μg/kg/hr) can be used concurrently to facilitate reduction in the inhaled agent dose and to improve cardiovascular performance (5).

Anesthetic Support and Monitoring
Anesthetic monitoring is important to maintain a proper plane of anesthesia and to prevent excessive insult to the cardiovascular, respiratory, and central nervous systems.

Heart rate, rhythm, and blood pressure should be monitored during surgery. Hyperthyroid cats are prone to a number of arrhythmias (e.g., premature ventricular contractions, supraventricular tachycardia), as well as hyper- and hypotension. If arrhythmias develop in hyperthyroid in cats during anesthesia, treatment with a short-acting beta-blocker, such as esmolol (50 μg/kg/min) is recommended (4-6). Alternatively, small doses of IV propranolol (a longer-acting beta-blocker) may be carefully administered (1).

A balanced electrolyte solution should be administered IV with goal of maintaining cardiac filling without overloading the heart. In hyperthyroid cats with concurrent kidney disease, the fluids should be administered before and during anesthesia, and continued during the recovery period.

Body temperature should be monitored in all cats undergoing general anesthesia. Because of the tendency for anesthetized cats to lose body heat, supplemental heat sources are often required to maintain adequate body temperature (100-103.5 ̊F).

Recovery
A quiet, stress-free environment is ideal. Fluids should be continued in patients with renal compromise and analgesics provided as needed. Oxygen should be provided at least until extubation of the cat but may be helpful until full recovery.

Body temperature must be maintained so as not to prolong the recovery, and lessen oxygen requirement by muscle tissues. Forced warm air blankets or circulating warm water blankets are very effective in maintaining the body temperature, but warmed fluid bags or infra-red lamps are also useful external heat sources.

Bottom Line

Many choices are available to tailor anesthetic protocols to meet the demand of the hyperthyroid cat. A smooth induction of anesthesia, careful monitoring in oxygenation, circulation and ventilation, and close attention to fluid balance and smooth recovery, all are important steps in ensuring a safe anesthetic protocol for the hyperthyroid cat.

References:

1. Peterson ME: Considerations and complications in anesthesia with pathophysiologic changes in the endocrine system. In: Short CE (ed), Principles and Practice of Veterinary Anesthesiology. Philadelphia, Williams and Wilkins Co. 1987;251-270. 
2. Peterson ME, Randolph JF, Mooney CT: Endocrine diseases, In: Sherding RG (ed): The Cat: Diagnosis and Clinical Management (2nd Ed) New York, Churchill Livingstone. 1994;1404-1506.
3. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Ed), Philadelphia, Saunders Elsevier, pp 327-342, 2006.   
4. Jacobson JD. Sedating and anesthetizing patients that have organ dysfunction. Vet Med 2005;100:518-526. 
5. Mama K. Anesthesia for thyroid gland disease. Clinician's Brief 2012;July:37-40. 
6. Keon TP, Templeton JJ. Diseases of the endocrine system.  In Katz J, Steward DJ (eds): Anesthesia and Uncommon Pediatric Diseases. WB Saunders, Philadelphia. 1987;311-344.
7. Bateman SW, Haldane S, Stephens JA. Comparison of the analgesic efficacy of hydromorphone and oxymorphone in dogs and cats: a randomized blinded study. Vet Anaesth Analg 2008;35:341–347. 
8. Pascoe PJ. Opioid analgesics. Vet Clin North Am Small Anim Pract. 2000;30:757-72. 
9. Schaafsma IA, Pollak YW, Barthez PY. Effect of four sedative and anesthetic protocols on quantitative thyroid scintigraphy in euthyroid cats. Am J Vet Res 2006;67:1362-1366. 
10. Nordt SP, Clark RF. Midazolam: a review of therapeutic uses and toxicity. J Emerg Med 1997;15:357-65. 
11. Clarke KW. Options for inhalation anaesthesia. In Practice 2008;30: 513–518. 

World Diabetes Day— 2012

World Diabetes Day is the primary global awareness campaign of the diabetes mellitus world and is held on November 14 of each year (1). It was introduced in 1991 by the International Diabetes Federation and the World Health Organization in response to the alarming rise of human diabetes around the world.  

World Diabetes Day is a campaign that features a new theme chosen by the International Diabetes Federation each year to address issues facing the global diabetes community. While the campaigns last the whole year, the day itself marks the birthday of Frederick Banting who, along with Charles Best, first conceived the idea which led to the discovery of insulin in 1922 (2).

Charles H. Best and Frederick G. Banting (on right) with one of the diabetic dogs used in their studies of insulin

This year's campaign theme is "Diabetes: Protect our Future" (1,3). In the past five years alone, diabetes rates among dogs in the U.S. have increased roughly 33% among dogs and 16% among the nation's cat population, per a national analysis of pet health (3).  This emphasizes the importance of this disorder in our cats and dogs as well as human patients with diabetes.

To successfully manage diabetes in animals, one must understand the disease and monitor and provide daily treatments to the cat or dog with diabetes. Treatment involves a combination of weight loss (if obese), diet, and insulin injections generally twice daily.

References 

1. World Diabetes Day – Official website.
2. Frederick Grant Banting (1891-1941), codiscoverer of insulin. Journal of the American Medical Association 1966;198:660-661.
3. World Diabetes Day 2012 - Expanding the circle of influence (Published in Diabetes Voice, Volume 57 Number 2 July 2012)
4. Washburn L. Human health risks on the rise in animals. The Record. April 20, 2011.

Thyroidectomy for Cats with Hyperthyroidism: Surgical Techniques

As discussed in my last post we have several different techniques that can be used when performing a thyroidectomy in cats with hyperthyroidism (1-8). These include the following:

• Extracapsular thyroidectomy technique
• Modified extracapsular technique
• Intracapsular thyroidectomy technique
• Modified intracapsular technique

The surgical technique chosen depends both on the surgeon’s preference and whether one or both thyroid lobes need to be removed. In other words, if we know that the cat has unilateral thyroid disease (a single thyroid tumor), only a unilateral thyroidectomy is needed to cure the hyperthyroidism. On the other hand, if the cat has bilateral thyroid disease (tumors in both lobes), a bilateral or total thyroidectomy is needed to effect a cure (1-7).

The aim of all of these thyroidectomy techniques is to remove all abnormal thyroid tissue and preserve at least 1 parathyroid gland. In addition, care are should be taken to avoid trauma to the adjacent vessels and nerves, as well as to the parathyroid glands.

The 4 Surgical Techniques for Thyroidectomy in Cats

Extracapsular technique
The “original” extracapsular technique (1) is most useful for cats with unilateral thyroid disease, in which only one thyroid lobe needs to be removed. With this technique, the affected thyroid tumor together with the associated external and internal parathyroid glands are removed.

This surgical procedure here is simple: once the thyroid tumor is identified, the cranial and caudal blood supply to the affected thyroid tumor is ligated, and the entire thyroid lobe is excised along with its capsule (Figure 1). Again, no attempt is made to preserve the external parathyroid gland with this method (1,2).

Figure 1: Performing a unilateral thyroidectomy in a cat with the extracapsular technique

Closure of the incision is by simple continuous suture pattern in the sternohyoideus muscle using absorbable suture, simple continuous pattern in the subcutaneous tissues with absorbable suture, and interrupted sutures in the skin with nonabsorbable sutures. As an alternative to skin sutures, use a continuous absorbable intradermal suture layer.

Because the entire thyroid tumor and its capsule are removed with this technique, the cure rate is high, with little chance of local recurrence. However, because the external parathyroid gland is also removed, this technique is not recommended for cats in which bilateral thyroidectomy is needed because of the very high incidence of hypoparathyroidism (4,5,9).

Modified extracapsular technique
The “modified” extracapsular technique was developed to decrease the risk of postoperative hypoparathyroidism and hypocalcemia that can develop when both thyroid lobes are removed in cats with bilateral thyroid adenomas (3,5,7,8).

Compared to the original extracapsular technique, this surgical procedure is more difficult. Once the affected thyroid tumors and external parathyroid glands are identified, the thyroid gland capsule is incised approximately 300 degrees around the external parathyroid gland (Figure 2), being careful to preserve the blood supply to the parathyroid gland.

Figure 2: Extracapsular dissection for removal of a thyroid tumor in a cat. 
Figure from reference 8 (with permission)

The surgeon then uses a moistened cotton-tipped applicator to dissect the external parathyroid gland and attached capsule away from the the main thyroid tumor. Ideally, bipolar cautery (rather than ligatures) is used to control hemostasis in order to reduce blunt dissection near the external parathyroid gland. After the caudal thyroid vein is ligated, the affected thyroid tumor and remaining capsule are removed. Skin closure is routine, as described above.

With the modified extracapsular technique, the thyroid tumor and approximately 90% of its thyroid capsule are removed, leaving a small rim of thyroid capsule around the external parathyroid gland (3,5,7,8). Use of this modified extracapsular technique helps ensure that the external parathyroid gland with its blood supply remain intact, greatly lessening the incidence of hypoparathyroidism. However, because a small amount of thyroid capsule is not removed, small remnants of adenomatous tissue that are left behind may regrow with time, leading to recurrence.

Intracapsular technique
With the intracapsular technique for thyroidectomy, a small nick incision is made in an avascular area of the thyroid capsule on the middle to caudal aspect of the gland (Figure 3A). This longitudinal incision is extended with a scalpel blade or fine iris scissors until the entire thyroid capsule is opened (Figure 3B).  The incised thyroid capsule is reflected off the gland with tissue forceps (Figure 3C). The thyroid tumor tissue is then gently teased away from the inside aspect of the thyroid capsule with a sterile cotton-tipped applicator, leaving the thyroid capsule and external parathyroid gland intact.

Figure 3: Intracapsular dissection for removal of a thyroid tumor in a cat. 
Figure from reference 8 (with permission)

Extreme care is required during manipulation of the cranial pole of the thyroid to avoid injury to the blood supply of the external parathyroid gland (located outside of thyroid capsule). As with the extracapsular technique, meticulous hemostasis is critical to maintain good visualization of the surgical field. The incision is closed as described under the extracapsular technique.

The advantage of the intracapsular technique over the extracapsular techniques, described above, are that this is a technically simple method to help ensure that the external parathyroid gland and delicate blood supply are preserved. Because of this, the risk of hypoparathyroidism is greatly reduced. However, because of retained remnants of abnormal thyroid tissue that remain attached to the thyroid capsule, the rate of recurrent hyperthyroidism is highest with this method (4,10,11).

Modified intracapsular technique
Because the intracapsular technique has the potential to leave a significant amount of thyroid tumor tissue behind, a “modified” intracapsular technique was subsequently developed for use in hyperthyroid cats.

The procedure is as described above for the intracapsular technique, but as a final step, the thyroid capsule (caudal to the parathyroid gland) is resected following removal of the thyroid tumor. This leaves only a small rim of thyroid capsule around the external parathyroid gland, thereby greatly lessening the chance of recurrence (3,7).

Staged Bilateral Thyroidectomy

Some surgeons recommend that bilateral thyroidectomy be performed in two stages or separate surgical procedures, in order to lessen the risk of hypoparathyroidism (4). A period of at least 3-4 weeks between procedures gives time for vascular or parathyroid damage to heal.

The necessity of two anesthetic episodes is the major drawback of the technique, considering the older age of the hyperthyroid cats often affected.

Parathyroid Gland Autotransplantation

Parathyroid autotransplantation has also been described as a treatment for accidental removal of the parathyroid or if complete devascularization occurs during thyroidectomy (7,9).

If the parathyroid glands are removed or damaged, the parathyroid gland can be minced into small 1-mm pieces and inserted into a small pocket made in the cervical musculature. With time, such transplanted parathyroid tissue can start to function again. This will decrease the severity and duration of postoperative hypocalcemia.

My Bottom Line

I greatly prefer the extracapsular technique for thyroidectomy in cats because it helps to ensure a permanent cure of the cat’s hyperthyroidism (i.e., the recurrence rate is extremely low). With the intracapsular techniques (especially the original technique), remnants of thyroid tissue are commonly left behind, which can regrow to cause recurrence of hyperthyroidism in some cats.

However, in cats in which the parathyroid glands can not be identified, we still rely on intracapsular dissection when an external parathyroid gland cannot be identified. With this technique, we open the thyroid capsule, which makes it easier to locate and preserve at least one of the parathyroid glands and prevent hypoparathyroidism.

Finally, in cats in which identifying or saving the parathyroid gland is difficult, use of the staged bilateral thyroidectomy approach, parathyroid gland transplantation, or both can help lessen the incidence of postoperative hypocalcemia. However, considering the increased cost and morbidity of a second operation, our preference is to perform bilateral thyroidectomy in one procedure if at all possible.

References:

1. Black AP, Peterson ME: Thyroid biopsy and thyroidectomy, In: Bojrab MJ (ed): Current Techniques in Small Animal Surgery. Philadelphia, Lea and Febiger. 1983; 388-396. 
2. Birchard SJ, Peterson ME, Jacobson A. Surgical treatment of feline hyperthyroidism: Results of 85 cases. Journal of the American Animal Hospital Association 1984;20:705-709. 
3. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21:29-33. 
4. Flanders JA, Harvey HJ, Erb HN. Feline thyroidectomy. A comparison of postoperative hypocalcemia associated with three different surgical techniques. Veterinary Surgery 1987;16:362–366. 
5. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
6. Flanders JA. Surgical options for the treatment of hyperthyroidism in the cat. Journal of Feline Medicine and Surgery 1999;1:127–134. 
7. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
8. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
9. Padgett SL, Tobias KM, Leathers CW, et al. Efficacy of parathyroid gland autotransplantation in maintaining serum calcium concentrations after bilateral thyroparathyroidectomy in cats. Journal of the American Animal Hospital Association 1998;34:219-224. 
10. Swalec KM , Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. Journal of the American Animal Hospital Association. 1990;26:433–437. 
11. Welches CD, Scavelli TD, Matthiesen DT, Peterson ME. Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Veterinary Surgery 1989;18:392-396. 

Thyroidectomy for Cats with Hyperthyroidism: Surgical Procedure

The primary indication for thyroidectomy in cats, as in other species, is thyroid neoplasia (1,2). In almost all of these cats, the thyroid tumors are benign and hyperfunctional (i.e., the cats are hyperthyroid).

Thyroidectomy can range from a straightforward procedure to one that is fairly complex. Benign, well-encapsulated tumors, such as those found in most cats, are easily resectable with minimal complications. Malignant, invasive tumors require extensive, careful dissection around many important and vital structures such as the trachea, esophagus, carotid arteries, jugular veins, and recurrent laryngeal nerves (3-6).

A good working knowledge of the regional anatomy of the thyroid and parathyroid glands, as well as proper pre- and postoperative care is necessary for successful patient management with thyroidectomy (3-7).

The purpose of this post is to provide an overview of the different surgical techniques for thyroidectomy in the cat.

Positioning of the Hyperthyroid Cat for Thyroidectomy

Thyroidectomy in the cat is performed by a ventral midline (neck) cervical approach (Figure 1).

Figure 1: Preparation and positioning of a cat for thyroidectomy

After induction of general anesthesia, the ventral cervical region is clipped from the angle of the mandible (jaw) to the upper part of the sternum and prepared for aseptic surgery. The cat is placed in dorsal recumbency with the forelegs pulled caudally and the head and neck hyperextended (3-6). The cat should be positioned over a rolled towel or small sandbag so that the neck is perfectly straight. The head is gently taped down to the table to keep the head and neck in a stable position (Figure 1).

Towel clamps should penetrate only through the skin to avoid trauma to the jugular veins. Bipolar cautery, fine scissors and thumb forceps, sterile cotton-tipped applicator swabs, and magnification are useful during dissection of the parathyroid glands and removal of a cat’s thyroid tumors.

Initial Surgical Exploration

After proper positioning of the cat (see Figure 1), the 2 thyroid lobes are exposed through a ventral midline cervical approach. To accomplish this, a skin incision is first made from the larynx to the upper part of the sternum (manubrium); the incision is then continued through the subcutaneous tissues and superficial oblique muscle fibers (3-6). The paired sternohyoideus muscles are then exposed and separated in the midline to expose the trachea. Care is taken to avoid cutting blood vessels that lie on the ventral surface of the trachea.

Identify both thyroid lobes
The two thyroid lobes are next located (Figure 2). Normally, both lobes are located just caudal (posterior) to the larynx (voice box) on the medial (inside) aspect of the sternothyroideus muscles. However, an adenomatous thyroid lobe can be located anywhere between the larynx and thoracic inlet, as gravity pulls the thyroid tumor down ventrally in the neck.

Figure 2: Locating both thyroid lobes in a hyperthyroid cat with bilateral thyroid disease

Both thyroid lobes are next closely inspected for size, shape, color, and the presence of any nodules. Thyroid adenomas (adenomatous lobes) are usually plump and dark-brown in color, while unaffected “normal” thyroid lobes are small, thin, and paler in color (3-7). If either of the thyroid lobes appears “normal” in size in a cat with hyperthyroidism, it is most likely affected with tumor, especially if any nodularity is also present.

Identify external parathyroid glands

Figure 3: Parathyroid gland
anatomy

Once the two thyroid lobes are found, the external parathyroid glands are next identified. The external parathyroid glands are normally 1-4 mm in diameter, paler than the adjacent thyroid tissue, and usually located at the cranial or top pole of each thyroid lobe (3-6). Rarely, however, the external parathyroid glands may be located on the caudal or bottom pole of the thyroid lobe.

The parathyroid glands can sometimes be difficult to differentiate from fat deposits on the surface of the thyroid capsule. Under magnification, each parathyroid gland will have a small vessel that splits and surrounds the gland.

Surgical Techniques for Thyroidectomy

Four different techniques have been described for performing a thyroidectomy in cats with hyperthyroidism (3-6). These include the following:

• Extracapsular thyroidectomy technique
• Modified extracapsular technique
• Intracapsular thyroidectomy technique
• Modified intracapsular technique

The surgical technique chosen depends both on the surgeon’s preference and whether one or both thyroid lobes need to be removed. In other words, if we know that the cat has unilateral thyroid disease (a single thyroid tumor), only a unilateral thyroidectomy is needed to cure the hyperthyroidism. On the other hand, if the cat has bilateral thyroid disease (tumors in both lobes), a bilateral or total thyroidectomy is needed to effect a cure.

The aim of all of these thyroidectomy techniques is to remove all abnormal thyroid tissue and preserve at least 1 parathyroid gland. In addition, care are should be taken to avoid trauma to the adjacent vessels and nerves, as well as to the parathyroid glands.

Figure 4: Performing a unilateral thyroidectomy in a cat with the extracapsular technique

In my next post, I'll go into greater detail to discuss the about the advantages and disadvantages of each of the four reported techniques for thyroidectomy in cats.

References:

1. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
2. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders. 2012; 571-592. 
3. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21:29-33.  
4. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
5. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
6. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
7. Waters DJ. Endocrine system. In: Hudson LC, Hamilton WP (eds): Atlas of Feline Anatomy for Veterinarians. Philadelphia: WB Saunders, 1993;127–134. 

Surgical Thyroidectomy for Cats with Hyperthyroidism: Intraoperative Considerations

Surgical thyroidectomy is a highly effective treatment for hyperthyroidism in cats. While thyroidectomy is most often successful, it can be associated with significant morbidity and mortality (1-4). Hyperthyroid cats are older to geriatric, can suffer from marked weight loss, and may have cardiac complications. Both before and during surgical thyroidectomy, a number of factors must be considered to ensure a successful outcome (4-7).

As discussed in a recent post on preoperative management of the hyperthyroid cat, all cats should, therefore, be prepared for surgery by administration of an antithyroid drug, a ß-adrenoceptor blocking drug, or iodide to decrease the metabolic and cardiac complications associated with hyperthyroidism. Establishing euthyroidism in these cats preoperatively will help make them much better candidates for anesthesia and surgery (5-7).

Just prior to anesthesia and surgery, it is important to rule out concurrent kidney problems that may have been masked by the untreated hyperthyroidism. The cardiovascular status should also be reevaluated to ensure that any cardiac issues or hypertension are controlled. Both renal and cardiac disease complicate anesthesia and may even mean that surgery is not a good option for a particular hyperthyroid cat.

Anesthetic Considerations
Anesthetic management of the hyperthyroid cat should include the judicious use of agents that have minimal cardiac arrhythmic effects (1-4,8). A variety of anesthetic agents and techniques can be used and none has advantages that exclude use of all others, especially if the hyperthyroid state has been controlled with methimazole prior to surgery. Due to their weight loss, poor body condition, older age, and overactive metabolic state, hyperthyroid cats tend to be very sensitive to many common drugs used to induce surgical anesthesia (8).

During the anesthetic period, continuous monitoring of the anesthetic level, blood pressure, and electrocardiogram is essential. Cardiac arrhythmias are common, especially in cats not rendered euthyroid prior to surgery. If arrhythmias develop, the anesthetic concentration should be lowered and the cat ventilated with a higher concentration of oxygen. If the arrhythmia persists, small doses of intravenous ß-adrenoceptor blocking drugs  (e.g., propranolol) usually helps to restore normal sinus rhythm.

Unilateral vs. Bilateral Thyroid Tumors?
About 30 percent of hyperthyroid cats have disease in only one thyroid lobe (unilateral tumor), whereas the remaining 70 percent have tumors in both thyroid lobes (i.e., bilateral tumors) (5-7).

As discussed in my last post on thyroid imaging for preoperative staging of hyperthyroid cats, nuclear scintigraphy is ideally done prior to surgery to determine with certainty which thyroid lobe should be removed or if bilateral thyroidectomy is needed. If this is not feasible, then the surgeon will have to make a decision to remove one or both thyroid lobes based on the visual appearance of the glands during surgery.

Unilateral disease
In cats with unilateral thyroid tumors, the adenomatous thyroid will be obviously large and abnormal, whereas the other “normal” thyroid lobe is of normal size or even small (Figure 1) (1-4). Removing the single abnormal thyroid lobe cures the hyperthyroid state in these cats. Relapse of hyperthyroidism will not occur in these cats unless they develop a “new” adenoma in the remaining thyroid lobe in the future. However, relapse is rare and, when it does occur, hyperthyroidism generally takes years to redevelop.

Figure 1: Unilateral thyroid adenoma (notice the large tumor on right).
The smaller thyroid lobe (on left) was normal.

After unilateral thyroidectomy, a low serum calcium will not develop, even if both of the parathyroid glands associated with the excised thyroid lobe have also been removed. Cats can easily maintain a normal serum calcium with the remaining 2 parathyroid glands that are still intact, associated with the other thyroid lobe that was not surgically removed.

Bilateral disease
With bilateral thyroid tumors, enlargement of both lobes can easily be identified at surgery in most cats (Figure 2) (1-4). However, about 15% of cats with bilateral lobe involvement have one lobe which is only slightly enlarged and may be easily mistaken as normal. If a cat has bilateral lobe involvement but the smaller thyroid lobe is mistaken as normal and not removed, many of these cats experience a temporary cure but relapse of hyperthyroidism will usually occur within 6 to 12 months of surgery (9).

Figure 2: Bilateral thyroid adenomas.
Notice that both thyroid lobes are large and nodular.

In cats with bilateral thyroid adenomas, removal of both thyroid lobes with preservation of at least a single parathyroid gland is necessary to cure hyperthyroidism and avoid postoperative hypocalcemia (low serum calcium level) secondary to parathyroid damage (hypoparathyroidism).

Uncertain if unilateral or bilateral disease
If it’s not clear if the cat has unilateral or bilateral thyroid tumors and preoperative thyroid imaging has not been done, I recommend removal of only the obviously enlarged thyroid tumor. However, the associated external parathyroid gland should be preserved. By saving a parathyroid gland during the first unilateral thyroidectomy, this helps minimizes the risk of hypoparathyroidism should removal of the second thyroid lobe be required in the future.

Figure 3: Hyperthyroid cat with one thyroid tumor (bottom of photo) that was clearly large,
whereas the other thyroid lobe is equivocally enlarged (top smaller lobe).
The final diagnosis was bilateral thyroid disease (bilateral adenomas).

In my next post, I’ll be discussing the specifics of the ins and outs of thyroid surgery for cats with hyperthyroidism.

References

1. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
2. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
3. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Ed), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Kintzer PP: Considerations in the treatment of feline hyperthyroidism. Veterinary Clinics of North America. Small Animal Practice 1994;24:577–585.
6. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
7. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders. 2012;571-592. 
8. Peterson ME: Considerations and complications in anesthesia with pathophysiologic changes in the endocrine system. In: Short CE (ed), Principles and Practice of Veterinary Anesthesiology. Philadelphia, Williams and Wilkins Co. 1987;251-270. 
9. Peterson ME, Randolph JF, Mooney CT:  Endocrine diseases, In: Sherding RG (ed): The Cat: Diagnosis and Clinical Management (2nd Ed) New York, Churchill Livingstone. 1994;1404-1506. 
10. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21,10-16.

Thyroid Imaging for Preoperative Staging of Hyperthyroid Cats

Thyroid scintigraphy (thyroid scanning or thyroid imaging) provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the staging and management of thyroid disease in cats with suspected hyperthyroidism.

Ideally, thyroid scintigraphy would be performed in all hyperthyroid cats prior to surgical thyroidectomy to identify the extent and location of the cat's hyperfunctioning thyroid tumor tissue (1-7).  In hyperthyroid cats, thyroid scintigraphy directly visualizes all functional thyroid tissue and will easily reveal whether the hyperthyroid cat has a thyroid tumor affecting only one lobe (unilateral disease) or both lobes (bilateral disease). See the scans in Figure 1 below.

Fig. 1: Thyroid scintigraphy in 3 hyperthyroid cats.
The cat on the left has a single thyroid tumor (Unilateral), whereas the cat on the right has 2 thyroid tumors that involve both thyroid lobes equally (Bilateral symmetric). The cat in the middle panel also has bilateral thyroid disease, but it is possible that the surgeon could mistake the smaller thyroid lobe as normal since the thyroid tumor enlargement is so asymmetric.

Some hyperthyroid cats develop very large thyroid tumors that descend into the chest cavity. making them difficult to palpate or identify at surgery. We often see these tumors in cats that have been hyperthyroid for many months to years (8,9). As the tumor grows progressively larger, gravity pulls progressively harder. This causes the tumor to descend into the chest cavity. Again, thyroid scintigraphy will detect these intrathoracic thyroid tumors that could easily be missed at surgery (Figure 2).

Fig. 2: Thyroid scintigraphy in a hyperthyroid cat with bilateral thyroid tumors.
In this cat, the larger thyroid tumor has descended into the chest cavity because of the effects of gravity. Without thyroid scintigraphy, this tumor could easily be missed at surgery, resulting in persistent hyperthyroidism.

Ectopic thyroid tissue is an embryological phenomenon that we see in about 3-5% of all hyperthyroid cats (8,9).  As the cells that develop into the thyroid gland traverse the embryo, they can leave traces of tissue anywhere from the base of the tongue to the base of the heart. These thyroid remnants can develop into functional thyroid tissue.  If such ectopic thyroid tissue develops a tumor, it will not be palpable or easily found at surgery, especially if the ectopic tumor is located on the tongue or in the chest cavity.  Thyroid scintigraphy can quickly and easily locate these tumors (see Figure 3).

Fig. 3: Thyroid scintigraphy in a hyperthyroid cat with a single ectopic thyroid nodule.
In this cat, no thyroid tumors could be palpated in the neck area where the thyroid gland is normally located. The large thyroid tumor has arisen from ectopic thyroid tissue that is located within the chest cavity. Without thyroid scintigraphy, this tumor could easily be missed at surgery, resulting in persistent hyperthyroidism.

Finally, thyroid scintigraphy also provides valuable information for diagnosing and evaluating cats with thyroid cancer (carcinomas) and metastasis.  With scintigraphy, a carcinoma will appear as a large, irregularly shaped tumor extending beyond the limits of the normal thyroid region (see Figure 4).  We can also see whether the tumor has metastasized or shows local invasion of the surrounding tissues. Thyroid carcinomas account for 2-3% of all thyroid tumors in cats (8,9).

Fig. 4: Thyroid scintigraphy in 6 hyperthyroid cats with a thyroid carcinoma (thyroid cancer).
In these cats with thyroid carcinoma, notice that the thyroid tumors are very large, multinodular, with extension and metastasis into the chest cavity. Surgery is unlikely to be successful in any of these cats, since these cancers are highly vascular and invasive, and it would be difficult to find all malignant thyroid tissue at surgery.

If thyroid scintigraphy is not available, the surgeon will have to make a decision upon what thyroid tissue to remove based on the visual appearance of the glands during surgery. In some cases, the surgeon may be fooled into leaving abnormal functioning thyroid tissue behind, especially if located in the chest cavity. Failure to identify and remove all hyperfunctional tissue will result in failure to cure the cat's hyperthyroidism.

Why Isn't Thyroid Scintigraphy Always Done Prior to Thyroidectomy?

Apart from expense and the licensing needed to handle and administer radioisotopes, few veterinarians have access to the nuclear medicine equipment needed to obtain thyroid images or perform thyroid uptake determinations. Only a few large referral hospitals in the country offer nuclear scintigraphy to do thyroid scanning. However, if available, I believe that thyroid scintigraphy can play an essential role in the preoperative staging and surgical management of cats with hyperthyroidism.

References:

1. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
2. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
3. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
6. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
7. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21,10-16. 
8. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012;26:754.
9. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Congress Proceedings, 22^nd ECVIM-CA Congress (The European College of Veterinary Internal Medicine – Companion Animals) 2012; 224

Surgical Thyroidectomy for Cats with Hyperthyroidism: Preoperative Preparation

Thyroidectomy is a highly curative treatment for cats with hyperthyroidism. However, these patients are generally senior cats, with 95% being older than 10 years of age (1-3).

Performing surgery and general anesthesia on these older to geriatric cats has inherent risks. Hyperthyroid cats frequently have disorders of other organ systems (e.g., heart, kidneys, liver) that should be recognized and appropriately treated prior to surgery (1-3). Anesthesia on hyperthyroid cats can be challenging as a result of their hypermetabolic state and underlying thyroid tumor, as well as their advanced age. Without adequate preoperative preparation, thyroidectomy can be associated with significant morbidity and mortality, especially in those cats with severe or long-standing hyperthyroidism (4-6).

A thorough diagnostic evaluation of the hyperthyroid cat is mandatory prior to the surgery. Using short-term antithyroid drugs to restore euthyroidism before surgery will make these cats much better candidates for anesthesia and surgery (1-3,7,8).

Preoperative Evaluation of the Hyperthyroid Cat

All hyperthyroid cats in which surgical thyroidectomy is planned should have a thorough physical examination and complete laboratory evaluation, including a complete blood count, serum chemistry profile, and complete urinalysis. Often this database simply lends support to the diagnosis of hyperthyroidism, but these screening tests are essential in determining if a hyperthyroid cat also has concurrent medical problems (e.g., kidney, heart, or liver disease) that will need to be addressed prior to surgery.

Many hyperthyroid cats will develop secondary cardiac problems, including a rapid heart rate, heart murmurs, heart enlargement, or abnormal heart rhythms (1-4). Occasionally, especially in cats with chronic or severe hyperthyroidism, congestive heart failure will develop. Therefore, if signs of underlying heart disease is found on physical examination, one or more heart procedures, such as a chest radiograph (x-ray), electrocardiogram (ECG), or echocardiogram, may also be indicated in the presurgical workup of a cat with hyperthyroidism.

Preoperative Medical Management

Antithyroid drug treatment

Ideally, the cat would be treated preoperatively with antithyroid drugs (i.e., methimazole or carbimazole) to lower the high serum T4 levels down into the normal reference range limits (4-9). Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily. This dose is adjusted according to T4 measurements done at 2-week intervals, as well as the cat's clinical response. (7-9)

After methimazole or carbimazole treatment has maintained euthyroidism for at least 1-3 weeks, anesthetic and surgical complications will be greatly minimized. The last dose of methimazole or carbimazole should be given on the morning of surgery.

Beta-adrenergic blocking agents (Beta blockers)

In cats that cannot tolerate antithyroid drug treatment, alternate preoperative stabilization with beta-adrenoceptor blocking agents (e.g., propranolol or atenolol) should be used. The basis for use of beta blockers in hyperthyroidism is that excessive thyroid hormone mimics many of the effects of excess adrenalin. Beta blockers act to block the action of endogenous catecholamines epinephrine (adrenaline) and norepinephrine (noradrenaline), which mediates the fight-or-flight response (7,8).

Propranolol and atenolol are used to symptomatically control the tachycardia (rapid heart rate), tachypnea (rapid respiratory rate), hypertension and hyperexcitability prior to surgery when the hyperthyroid cat cannot be made euthyroid with antithyroid drugs. In addition, some cats that have persistent tachycardia (heart rate >220 beats per minute) on antithyroid drugs will also be treated with beta blockers prior to surgery to decrease the cardiac risks associated with general anesthesia.

Alternative drugs used to prepare the hyperthyroid cat for surgery
In cats with advanced or chronic hyperthyroidism that do not tolerate antithyroid drugs, there are a number of alternative drugs, including stable iodine, iopanoic acid, or L-carnitine, that can be added to the beta blocker treatment (7,8,10).

None of these drug are as effective as methimazole or carbimazole for preoperative preparation of the hyperthyroid cat.

Monitoring of Thyroid and Renal Values before Surgery

As serum thyroid levels normalized, it is important to monitor for worsening of serum kidney values as renal disease can sometimes be unmasked by treatment. Clinically significant rises in renal values warrant medical management prior to surgery to avoid postoperative renal failure (1-4).

Hemorrhage from jugular venipuncture sites can discolor and obscure the parathyroid glands. Therefore, jugular venipuncture should be avoided the week prior to surgery. Venipuncture can always be done, however, from either the cat’s saphenous or cephalic veins, located on the inside of the rear leg or top aspect of the foreleg, respectively.

Bottom Line

In most cats, thyroidectomy is relatively simple surgical procedure for an experienced veterinary surgeon to perform. However, thyroidectomy can range from a straightforward operation to one that is fairly complex, and it can be associated with significant morbidity and mortality, especially in cats with severe or chronic hyperthyroidism.

Considering that the average hyperthyroid cat is a geriatric patient with potential for high blood pressure and heart disease, quite a bit of patient preparation is necessary to reduce anesthetic risk.

A thorough preoperative evaluation of the hyperthyroid cat is mandatory. Using short-term antithyroid drugs to restore euthyroidism before surgery will make these cats much better candidates for anesthesia and surgery (1-3,7,8).

References:

1. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
2. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
3. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
6. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
7. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
8. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
9. Animal Endocrine Blog. Treating Cats with Hyperthyroidism: Antithyroid drugs. October 23, 2011.  
10. Animal Endocrine Blog. Alternative Medical Treatments for Hyperthyroid Cats. September 13, 2012.