Page 183 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 173

long as 5 days. Hypoparathyroidism resolves for most exhibit subnormal or marginal iMg concentrations. The
affected dogs in 8 to 12 weeks. Cats develop hypocalce- impact of magnesium supplementation in the treatment
mia less frequently than dogs after surgical correction of of hypoparathyroidism should be investigated. Although
primary hyperparathyroidism. 141,285 primary hypoparathyroidism is usually diagnosed in older
Hypoparathyroidism following spontaneous infarc- cats, it has been reported in a 6-month-old kitten initially
tion of a parathyroid gland tumor previously causing evaluated for lethargy, inappetence, muscle tremors, and
hypercalcemia is a rare condition that can result in acute seizures. 34
hypocalcemia in dogs. 485 The rapid correction of cancer- Most causes of primary hypoparathyroidism have been
associated hypercalcemia (e.g., with tumor excision and attributed to immune destruction of parathyroid tissue.
chemotherapy) can be associated with hypocalcemia Early reports of hypoparathyroidism in dogs and cats
and low PTH concentration, but hypocalcemia is usually did not consistently evaluate magnesium status and used
minor and transient. tMg when it was reported. Based on discordance of mag-
Both acute hypermagnesemia and severe magnesium nesium status using iMg versus tMg, hypomagnesemia
depletion may suppress PTH secretion. 63,465,572 As with based on tMg assessments may have underestimated a
hypocalcemia, mild acute hypomagnesemia stimulates role for hypomagnesemia in the genesis of hypoparathy-
PTH secretion, but severe magnesium depletion roidism in animals. Hypomagnesemia may decrease cell
decreases PTH secretion, increases end-organ resistance membrane receptor sensitivity to iCa and PTH, as well
to PTH, and may impair calcitriol synthesis. The end- as decrease PTH synthesis. 325 Serum iMg concentration
organ resistance to PTH that develops during magnesium should be measured when iCa and PTH concentrations
depletion may persist for days after magnesium repletion are determined.
and resumption of normal PTH concentrations in The potential role of magnesium depletion in the
humans. Until recently, hypomagnesemia has been development of post-thyroidectomy hypocalcemia in cats
reported rarely in dogs and cats with hypoparathyroidism. has not been explored. Magnesium depletion could play a
Normal serum tMg does not guarantee a normal iMg role in the development of postoperative hypocalcemia in
concentration because there is substantial discordance cats with hyperthyroidism, because hyperthyroidism can
between these two measurements. be associated with magnesium depletion. 179
Magnesium depletion can cause functional hypopara- Canine distemper virus (CDV)-induced parathyroid
thyroidism, and measurement of serum iMg concentra- hypofunction may contribute to development of hypocal-
tion is recommended to exclude or identify this form of cemia. Dogs infected with CDV had reduced serum tCa
hypoparathyroidism. Serum tMg concentrations when concentrations associated with ultrastructural evidence of
measured in dogs and cats with primary hypoparathyroid- parathyroid gland inactivity, degeneration, and viral
ism usually have been normal. 87,179 In 357 dogs with pri- inclusions. 608
mary hypoparathyroidism, mean iCa and mean PTH
concentrations were below the reference range. 521 The Miscellaneous Causes of Hypocalcemia
iMg concentration was below the reference range in Metabolites of ethylene glycol can chelate calcium and
39%, within the reference range in 55%, and above the ref- becomedepositedinsoft tissues,resulting inhypocalcemia.
erence range in 6% of dogs with hypoparathyroidism. Of Both dogs and cats exhibit hypocalcemia after ethylene gly-
the 55% of dogs with iMg within the reference range, 69% col ingestion. 569 Seizures have been observed in dogs
had an iMg concentration within the lower half of the ref- within hours of ingestion; renal function was normal at this
erence range, and only 31% had an iMg concentration time (Chew, personal observations). Hypocalcemia often
within the upper half of the reference range. develops later when renal function is severely reduced from
Despite the relative paucity of published reports from acute renal failure and when hyperphosphatemia is severe.
cats, hypoparathyroidism was diagnosed in 27 cats during Pansteatitis has been associated with severe hypocalce-
521 641
a 2-year period. Of cats with hypoparathyroidism, 59% mia in a cat. A 13-year-old cat had a history of
were domestic shorthairs, 22% were an unspecified breed, anorexia, lethargy, and abdominal enlargement;
and 15% were Siamese. Mean serum iCa concentration decreased serum iCa and tCa concentrations were
was below the reference range, and mean PTH concentra- observed with an elevation in PTH concentration (sec-
tion was in the lower half of the reference range. The iMg ondary hyperparathyroidism). No clinical signs of hypo-
concentration was below the reference range in 37%, calcemia were noted. Pansteatitis was confirmed on
within the reference range in 59%, and above the refer- histopathology. The exact mechanism of hypocalcemia
ence range in 4%. Of the 59% of cats with iMg within was not determined, although it was probably due to
the reference range, 88% had an iMg concentration the formation of calcium soaps.
within the lower half of the reference range, and only Acute decreases in iCa concentrations are most com-
12% had an iMg concentration within the upper half of monly caused by acute respiratory alkalosis in humans. 465
the reference range. These results suggest that a large It is likely that this phenomenon also occurs in dogs and
number of dogs and cats with hypoparathyroidism also cats subjected to the stresses of hypocalcemia and a visit to