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 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
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|
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)
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.
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)
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.
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