Page 164 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

154 ELECTROLYTE DISORDERS

lymphadenitis, 372 two cats with disseminated histoplas- underlying neoplasia following extensive diagnostic eval-
mosis 250 and dogs with coccidioidomycosis or schistoso- uation, survival for many months, and necropsy. 381 It
miasis. 178,581 In one dog with schistosomiasis, PTHrP appears that excessive PTH, 25-hydroxyvitamin D, or
levels were undetectable, 476 but in two other dogs with calcitriol concentration is not the cause of IHC in most
schistosomiasis, PTHrP levels were increased with no cats. However, normal concentrations of calcitriol could
malignancy found at necropsy. 197 In cats, elevated result in hypercalcemia if there are mutations of the VDR
calcitriol concentrations were documented in cases of or an increase in the number of calcitriol receptors. Nor-
Nocardia and atypical mycobacteria infection. 368 Cats mal concentrations of iMg indicate that PTH secretion is
with blastomycosis, cryptococcosis, actinomyces, and not inhibited by decreased or excess iMg. 520 Renal func-
injection site granulomas (Chew and Peterson, unpub- tion, based on BUN and serum creatinine concentration,
lished observations on injection site granu- is usually normal initially, but some cats develop CRF sec-
loma) 368,511,547 have been noted with hypercalcemia ondary to long-standing IHC. 381 Results of serology
possibly because of enhanced synthesis of calcitriol. 150 testing for feline leukemia virus and feline immunodefi-
Severe hypercalcemia was observed in association with ciency virus have been negative, and serum thyroxine
noninfectious granulomatous dermatitis in two dogs in concentrations have been normal. Chronic acidosis could
which excess synthesis of calcitriol was suspected explain chronic elevations of iCa, 121 but venous blood gas
(Kwochka and Chew, unpublished observations). PTH, analysis has not revealed significant acid-base
PTHrP, and 25-hydroxyvitamin D concentrations were disturbances. Exploration of the cervical region has not
not increased. Hypercalcemia resolved as the inflamma- identified primary hyperparathyroidism, and subtotal
tion subsided. In a dog with granulomatous lymphadeni- parathyroidectomy has not resolved hypercalcemia in cats
381
tis, serum PTH concentration was low but serum in which this procedure was performed.
calcitriol concentration was elevated. 372 Nodular As many as 35% of cats with calcium oxalate urinary
panniculitis with hypercalcemia has been reported in stones have hypercalcemia. Even though the specifics of
dogs, and calcitriol concentrations were two to three the underlying diagnoses were not detailed, 424 it is likely
times the normal in one instance. 166,448 that most had IHC. The occurrence of ureterolithiasis in
cats was very uncommon before 1993. Eleven cases of
Idiopathic Hypercalcemia of Cats calcium oxalate ureterolithiasis were recently described
Hypercalcemia may be less common in cats than in dogs, in cats, and four had mild to moderate hypercalcemia. 319
although the incidence of hypercalcemia from primary It appears that the frequency of hypercalcemia in calcium
care practices is not reported. Within the past 10 years, oxalate stone-forming cats has decreased substantially
IHC has been recognized in cats 365,381 and is now the (Lulich, personal communication, 2003).
most common cause of ionized hypercalcemia in cats in Specific treatment for IHC is impossible because the
the United States. Even though some suggest that IHC pathogenesis remains unknown. Increased bone resorp-
is a local geographic phenomenon, 178 it is widespread tion, increased intestinal absorption, or decreased renal
across the United States and is being recognized in other excretion of calcium or combinations of these
parts of the world. mechanisms could be responsible for hypercalcemia.
In IHC, serum calcium concentration may be The feeding of increased dietary fiber decreased serum
increased for months to more than 1 year. In 427 cases calcium in some cats 365 but not in others. 381 The benefi-
of feline IHC, 46% had no clinical signs, 18% had mild cial effect of a higher fiber diet may be because of
weight loss with no other clinical signs, 6% had inflamma- decreased intestinal absorption of dietary calcium. The
tory bowel disease, 5% had chronic constipation, 4% were effects of fiber on intestinal absorption are complex and
vomiting, and 1% were anorectic. 520 Uroliths or renoliths depend on the types and amounts of fiber in the diet
were observed in 15%, and calcium oxalate stones were and other nutrients present.
noted in 10% of cases. Cats ranged in age from 0.5 to The feeding of veterinary renal diets may result in
20 years, and long-haired cats accounted for 27% of the normocalcemia in some cats with IHC. These diets are
cases (compared with an overall submission rate of 14% generally low in calcium and phosphorus and are consid-
from long-haired cats). There was no sex predilection. ered alkalinizing or at least less acidifying than mainte-
Serum iCa concentration was increased; PTH concentra- nance diets. Some cats that show an initial decrease in
tion was in the lower half of the reference range; and serum calcium concentration following any type of die-
PTHrP was negative in all samples. Concentration of tary change will have a return to hypercalcemia over time.
iMg was normal, and mean concentration of 25- In those cats that do not respond to a change in diet,
hydroxyvitamin D was within the reference range. prednisone therapy may result in a long-term decrease in
Calcitriol was measured in a small number of these cats iCa. The effects of glucocorticosteroid treatment may last
and was suppressed. In another study, 1 of 7 cats for months to years in some cats with doses of 5 to 20 mg
exhibited an increased concentration of calcitriol, and prednisone/cat/day. There is an escape from the effects
2 of 11 cats had increased PTHrP in the absence of of glucocorticosteroid treatment in some cats and a