Page 175 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 165

Most seizures resolve without treatment but often recur Patients with low iCa and low PTH concentrations have
despite treatment with anticonvulsants. Growling attrib- absolute hypoparathyroidism (parathyroid dependent).
utable to pain or behavior change occurred in approxi- A normal reference range PTH when iCa is low is inappro-
mately 40% of dogs, and intense rubbing of the face priate, because normal parathyroid glands should respond
with the paws or on the ground was observed in more than with increased PTH. Hypocalcemic patients with
50% of dogs. These signs were attributed to either increased PTH are classified as having parathyroid-inde-
paresthesia or pain from facial muscle spasms. 87,179 pendent hypocalcemia. Normograms to determine the
Pyrexia may be caused by increased muscular activity adequacyoftheincreasedresponseofPTHtolow iCahave
with or without seizures. Lethargy and weakness are seen not been established for dogs or cats. In cases of parathy-
inapproximately33%,andpolyuriaandpolydipsiaoccurin roid-independent hypocalcemia, hypocalcemia exists
about 25% of cases as a result of psychogenic mechanisms from redistribution of calcium into other body spaces,
or renal injury (nephrocalcinosis) from hypercalciuria excess phosphorus effects, or from deficiencies of vitamin
associated with PTH deficiency in animals with hypopara- D or dietary calcium. Patients with persistent moderate to
thyroidism. 504,530 Anterior and posterior lenticular severe hypocalcemia based on serum tCa should be
cataracts occurred in more than 33% of affected dogs 87,308 evaluated for iCa and PTH concentrations; measurement
and also in cats. 179,444 Tachycardia and electrocar- of 25-hydroxyvitamin D and serum phosphorus is also
diographic abnormalities (increased QT–interval) may helpful, and in rare circumstances, measurement of
also be encountered. Both hypertension and hypotension calcitriol may help provide a definitive diagnosis. The
have been reported during hypocalcemia in humans. 97,154 conditions associated with hypocalcemia in dogs and cats
Neuromuscular signs in cats with chronic hypocalce- are listed in Box 6-6 according to their relative frequency,
mia associated with primary hypoparathyroidism are sim- regardless of clinical signs or severity of decreased serum
ilar to those in dogs (e.g., muscle tremors, weakness, and calcium concentration. The anticipated changes in
generalized seizures). 444 Anorexia and lethargy appear to calcium hormones and serum biochemistry in disorders
be more common in cats than in dogs with primary hypo- causing hypocalcemia are noted in Table 6-4.
parathyroidism, but seizures have not been reported to be
induced by excitement, as occurs in dogs. Prolapse of the DIFFERENTIAL DIAGNOSIS AND
third eyelid is occasionally observed in cats with acute MECHANISMS OF HYPOCALCEMIA
hypocalcemia but is not a prominent finding during
chronic hypocalcemia. Hypoalbuminemia
Clinical signs associated with acute postoperative Hypoalbuminemia is the most common associated condi-
hypocalcemia are similar in dogs and cats and are related tion but perhaps the least important for clinical
to neuromuscular excitability. Focal twitching of facial consequences, and it occurs in nearly one half of the dogs
muscles and vibrissae may be noticed before more with hypocalcemia. 115 Hypocalcemia associated with
generalized muscle tremors or seizures develop. Tetany hypoalbuminemia is usually mild (serum tCa concentra-
or facial twitching has not been observed in cats after thy- tion, 7.5 to 9.0 mg/dL in dogs), and no signs referable
roidectomy until serum tCa concentration is less than to the functional effects of low serum calcium concentra-
6.9 mg/dL. 179,444,446 Severe hypocalcemia (<6.5 mg/ tion are observed. Application of calcium correction
dL) is often associated with muscular twitching, tetany, formulas to serum tCa concentrations in dogs or cats with
or seizures. Anorexia and lethargy are not often consid- hypoproteinemia or hypoalbuminemia has been
ered primary signs of hypocalcemia, but both signs advocated in the past. However, these correction
decrease in cats during calcium infusion after thyroidec- formulas do not improve the prediction of actual iCa con-
tomy, suggesting a relationship between hypocalcemia centration and in many cases increase the level of diagnos-
519
and these signs. tic discordance. Use of correction formulas to adjust
serum tCa concentration to serum total protein or albu-
APPROACH TO HYPOCALCEMIA min concentration is not recommended.
Hypocalcemia develops when bone mobilization of cal-
cium is reduced, skeletal calcium accretion is enhanced, Renal Failure
urinary losses of calcium are increased, gastrointestinal Renal failure is the second most common disorder
absorption of calcium is reduced, calcium is translocated associated with hypocalcemia in dogs. 115 Decreased
intracellularly, or as a result of a combination of these calcitriol synthesis by diseased kidneys and, to a lesser
mechanisms. Much like the initial approach to hypercalce- extent, mass law interactions of calcium with markedly
mia, it is helpful to make the initial distinction as to increased serum phosphorus concentration are probable
whether hypocalcemia is parathyroid dependent or para- causes of the hypocalcemia observed in dogs and cats with
thyroidindependent.Ionizedcalcium concentrationmust CRF. To decrease iCa concentration by 0.1 mg/dL,
be evaluated in conjunction with PTH concentration to serum phosphorus concentration must increase by
determine whether PTH production is appropriate. 3.7 mg/dL. 7 Calcitriol deficits are more important