Page 161 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 151

present in 29% and urolithiasis was present in 31% of dogs Ultrasonographyoftheneckishelpfulinthediagnosisof
in this study. Other studies have shown that calcium- primary hyperparathyroidism in dogs and cats, but it
containing uroliths (calcium phosphate, calcium oxalate, requires an ultrasound unit with a high-frequency (7.5-
or mixtures) occurred in approximately 30% of dogs and to10-MHz)transducer toachievethenecessarylevelofres-
in a cat with primary hyperparathyroidism. 178,300,353 olutionratherthanthewidelyavailable5-or7.5-MHzunits
Urolithiasis is attributed to hypercalcemia and used for abdominal studies. 178,618 With a 10-MHz linear
subsequent hypercalciuria. Interestingly, hypercalcemia transducer, the parathyroid glands of normal dogs can rou-
arising from other causes has not been associated with tinely be identified especially in larger dogs. 469 Parathyroid
urolithiasis except in cats with idiopathic hypercalcemia gland masses greater than 5 mm can usually be identified,
(IHC). 520 More prominent clinical signs and serious and some masses as small as 2 mm may be detected.
consequences can occur when hyperparathyroidism and Enlarged parathyroid glands are expected to be hypoechoic
severe hypercalcemia are long standing and associated or anechoic, well marginated, and easily contrasted with
with renal failure. 115,116 thyroid tissue. False-positive results are rare, but false-neg-
The diagnostic workup to confirm primary hyperpara- ative findings may occur. Ultrasonography correctly
thyroidism often begins with the fortuitous finding of identifiedthepresenceandlocationofasolitaryparathyroid
increased serum calcium concentration on routine clinical gland mass in 10 of 11 dogs in a prospective study in which
chemistry testing. 178 The diagnosis of primary hyper- themasswasconfirmedatsurgery. 180 Sonographyidentifies
parathyroidism is easy in dogs and cats that have increased the location of the parathyroid gland tumor and allows
serum tCa concentration, normal renal function, and presurgical planning.
increased concentration of immunoreactive PTH. The Double-phase scintigraphy of the parathyroid glands
99m
appropriateness of the PTH concentration must be using Tc sestamibi was useful in the diagnosis of para-
interpreted in relation to the serum iCa concentration. thyroid gland adenoma in initial reports from two
Additional support for the diagnosis of primary hyper- dogs. 362,626 In a study of 15 dogs with hypercalcemia,
parathyroidism is provided by the finding of increased scintigraphy correctly identified 3 of 3 dogs with hyper-
serum iCa concentration, increased serum ALP, low calcemia of malignancy as negative for hyperfunctioning
serum phosphorus concentration, increased or normal parathyroid glands. 361 Scintigraphy identified only one of
calcitriol concentration, undetectable PTHrP, and cal- six dogs with a parathyroid gland adenoma and only one
cium-containing uroliths. The most consistent laboratory of six dogs with parathyroid hyperplasia. Based on these
abnormality in dogs with primary hyperparathyroidism is results, parathyroid gland scintigraphy is not
increased serum calcium concentration. 178 recommended to identify abnormal parathyroid glands
Hypercalcemia results from a combination of effects because of very poor sensitivity and specificity.
following PTH binding to receptors in kidneys and bone. Surgical exploration of the cervical region in patients
PTH also acts indirectly to increase serum iCa concentra- with parathyroid gland adenoma or carcinoma usually
tion by enhancing renal conversion of 25-hydroxyvitamin reveals enlargement of one parathyroid gland, and the
D to calcitriol. Hypophosphatemia secondary to PTH- remaining three are small or impossible to identify
enhanced urinary excretion of phosphorus was observed because hypercalcemia results in atrophy of normal
in 5 of 21 dogs. 40 Serum phosphorus concentration is parathyroid tissue. Primary parathyroid gland hyperplasia
typically low, 178 and calcitriol concentrations were mildly may affect more than one gland, and clinical signs can
increased or in the high-normal range in three of four recur if only the largest gland is removed surgically. Para-
dogs with primary hyperparathyroidism. 489 thyroid gland tumors may be difficult to identify if the
The diagnosis of primary hyperparathyroidism is more tumor is embedded in fat or if it arises from the internal
challenging when PTH is within the reference range. A parathyroid gland. Failure to visualize a parathyroid gland
PTH concentration in the upper part of the reference tumor is rarely attributed to the occurrence of a tumor in
range in association with hypercalcemia is inappropriate. ectopic parathyroid tissue. Methylene blue infusion to
Confirmed primary hyperparathyroidism has been noted enhance visualization of parathyroid glands should be
in dogs and cats with hypercalcemia and reference range reserved for patients in whom a tumor is strongly
PTH concentrations. 178,285 In a cat with persistent suspected but not readily identified during surgery
hypercalcemia related to primary hyperparathyroidism, because clinically relevant side effects of methylene blue
PTH concentration was increased on two occasions but administration include hemolytic anemia and acute renal
within the reference range on five other occasions. 141 failure. 185 In 47 dogs with primary hyperparathyroidism
PTH concentrations measured in blood collected from treated with parathyroidectomy, 94% of surgeries resulted
either the left or right jugular vein did not differ, and sam- in control of hypercalcemia. 464 Hypercalcemia resolved
pling from a specific side was not valuable for localizing within 1 to 6 days, and remained within the reference
the site of an enlarged parathyroid gland. 180 Circulating range for a median of 561 days. The use of a rapid
PTHrP concentrations were undetectable in six dogs with PTH assay for intraoperative measurement of PTH may
primary hyperparathyroidism. 489 be helpful to demonstrate a decrease in PTH