Page 603 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 26 Tumors of the Endocrine System 581

secretion by hyperplastic or neoplastic parathyroid tissue. Ultra- A routine ventral midline approach to the thyroid glands is
sound examination of the neck commonly is used in the diag- made. The area is explored bilaterally, even if the location of the
parathyroid nodule is known preoperatively, to evaluate for addi-
nosis of hyperparathyroidism in dogs and cats and is particularly
VetBooks.ir useful for localizing parathyroid mass(es) before surgery or other tional sites of disease. The nodule can be identified with a combi-
a
ablative procedures. The normal sonographic appearance of
nation of visualization and digital palpation. Once a nodule has
canine parathyroid glands has been described, 355 and parathy- been identified, it is bluntly dissected from the thyroid gland and
roid masses as small as 3 mm in greatest diameter have been hemostasis is achieved with bipolar electrocautery.
identified ultrasonographically. 337 Parathyroid scintigraphy and The postoperative care generally is more involved than the sur-
selective venous sampling to assess local PTH concentrations do gery itself. Patients must be monitored closely for hypocalcemia,
not appear to be helpful in localizing hyperplastic or neoplastic which occurs as a result of downregulation of normal parathyroid
parathyroid tissue. 353,356,357 tissue with prolonged hypercalcemia. Serum ionized calcium con-
The management of hypercalcemia is further addressed in centrations should be monitored at least twice daily for as long as
Chapter 5. Primary hyperparathyroidism in dogs and cats usu- 5 to 7 days after surgery or other ablative procedures. Hypocalce-
ally is associated with slowly progressing hypercalcemia, and the mia should be treated if the ionized calcium falls below 0.8 to 0.9
increased calcium itself rarely requires emergency treatment. mmol/L; the total calcium is less than 8 to 9 mg/dL; or the patient
Hypercalcemia is a risk factor for acute kidney injury (AKI); has signs of tetany. Intravenous (IV) calcium salts are used for
the mechanisms include altered glomerular capillary perme- acute therapy for hypocalcemia; subcutaneous (SQ) administra-
ability, reduced renal blood flow, and mineralization of the kid- tion should be avoided. Vitamin D and oral calcium are used for
neys. The risk of mineralization is increased when the calcium subacute and chronic therapy.
× phosphorus product exceeds 70. As noted previously, patients Several excellent references are available on the treatment
with hyperparathyroidism often have a decreased or low normal of hypoparathyroidism. 340,358 In summary, 1,25-dihydroxyvi-
phosphorus level, which reduces the risk of renal mineralization. tamin D (calcitriol) is recommended for vitamin D supple-
3
In fact, AKI appears to be rare in dogs with primary hyperpara- mentation because it has a rapid onset of action and a short
thyroidism. In a large canine case series, the mean blood urea half-life. This facilitates dose adjustments and reduces the risk
nitrogen (BUN) and serum creatinine both were significantly of hypercalcemia. Oral calcium supplementation alone is not
lower in 210 dogs with primary hyperparathyroidism compared sufficient to treat hypoparathyroidism, and in fact this therapy
with 200 control dogs. 337 In addition, 95% of the hyperparathy- can be withdrawn gradually once the calcium is stable because
roid dogs had BUN and serum creatinine values within or below most maintenance diets contain an adequate amount of calcium.
the reference range. This partly may be a result of the secondary The approach to these patients postoperatively remains some-
nephrogenic diabetes insipidus that causes polyuria/polydipsia what controversial; some clinicians treat with calcitriol and oral
in these patients. calcium immediately postoperatively, whereas others monitor
Definitive therapy for primary hyperparathyroidism requires the ionized calcium carefully for the development of hypocal-
removal of the hyperfunctioning gland(s). This is most com- cemia. In the author’s (SB) opinion, careful monitoring without
monly achieved by surgery in both dogs and cats; however, per- administering oral calcium or calcitriol is preferred because not
cutaneous ultrasound-guided ablation techniques also have been all patients will develop hypocalcemia, and it is more straight-
described in the dog. There are four parathyroid glands, and forward in those patients to allow them to regulate their own
two are closely associated with each thyroid lobe. The external calcium. Furthermore, in one study prophylactic calcitriol
parathyroid glands are outside the thyroid lobe but within the administration was not shown to have a protective effect for
capsule and generally associated with the cranial pole. 221 The preventing hypocalcemia in patients after parathyroidectomy. 359
internal parathyroid glands are within the thyroid capsule and Several studies have attempted to correlate the preoperative ion-
lobe and can vary in location, but they generally are located in ized calcium concentrations with the risk of hypocalcemia post-
the caudal portion of the lobe. 221 Normal parathyroid glands in operatively, with varying results. One study found a moderate
the dog are small (2–5 mm × 0.5–1 mm), disk shaped, and tan correlation with a high preoperative ionized calcium and post-
in color. They are distinct from thyroid tissue. 221 Parathyroid operative hypocalcemia, 360 whereas other studies have failed to
adenomas are larger than normal parathyroid glands, round, and show a correlation between preoperative ionized calcium 361,362
firm in texture. Once a diagnosis of primary hyperparathyroid- and PTH 362 and postoperative serum calcium concentrations.
ism has been made, a preoperative ultrasound examination of A small number of patients may be resistant to the postopera-
the neck may be useful to establish the side and site of the para- tive management of hypocalcemia, 363 and this may be the result
thyroid nodule. This can be an important tool to confirm the of “hungry bone syndrome,” marked by aggressive, unregulated
presence of a nodule and the surgical site of interest. However, uptake of calcium by the bones. 364 In human medicine this
it is important to note that false positive and negative results syndrome has been managed with preoperative bisphosphonate
are possible with ultrasound; for example, thyroid nodules may administration 365 or the use of recombinant PTH. 366 Neither
be incidentally found in hypercalcemic dogs undergoing cer- of these approaches has been used in veterinary medicine, and
vical ultrasound studies. 213 In addition, patients with primary most patients eventually respond to high doses of calcitriol and
hyperparathyroidism may have disease in more than one para- calcium supplementation.
thyroid gland and/or ectopic parathyroid tissue. Up to three of Ideally the calcium will decline into the normal range and then
the four parathyroid glands can be removed without risk of per- plateau. Once a plateau has been documented, the patient can be
manent hypoparathyroidism. Patients with involvement of all discharged from the hospital. Some patients become hypocalcemic
four glands present a dilemma, and it is important to ensure that and require administration of both calcitriol and calcium. Once
hyperplasia in these cases is not secondary. their serum calcium concentrations stabilize, they can be dis-
charged, but they must have careful and regular follow-up. When
a References 337, 338, 347, 353, and 354. adjusting the dose of calcitriol, the goal is to maintain calcium