Page 174 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

164 ELECTROLYTE DISORDERS

434 sick cats, the prevalence was 27%. 204 On the basis of BOX 6-5 Clinical Signs Associated
serum tCa concentration, hypocalcemia is usually defined
as a concentration less than 8.0 mg/dL in dogs and less with Hypocalcemia
than 7.0 mg/dL in cats. When serum iCa concentration
is used, hypocalcemia is generally defined as a concentra- Common
tion less than 5.0 mg/dL (1.25 mmol/L) in dogs and less None
than 4.5 mg/dL (1.1 mmol/L) in cats. The most likely Muscle tremors or fasciculations
reason for submission of samples to measure calcium reg- Facial rubbing (paresthesia?)
ulatory hormones in animals with hypocalcemia is for Muscle cramping
those with persistent hypocalcemia that is moderate to Stiff gait
severe in magnitude and for which a known cause cannot Behavioral change
Restlessness or excitation
be identified; most will be submitted with suspicion for a
Aggression
diagnosis of primary hypoparathyroidism.
Hypersensitivity to stimuli
In human patients, large and unexplained differences
Disorientation
between ionized and tCa concentrations have been found
in hypocalcemic conditions. 320 This discordance is also Occasional
seen in dogs and cats and is not predictable. Based on Panting
serum tCa measurement in 1633 sick dogs, 27% were Pyrexia
Lethargy
classified hypocalcemic, but when iCa was measured, Anorexia
31% were hypocalcemic. 519 Using serum tCa measure-
Prolapse of the third eyelid (cats)
ment in 434 sick cats, 49% were classified hypocalcemic, Posterior lenticular cataracts
but when iCa was measured, only 27% were actually Tachycardia or electrocardiographic alterations
hypocalcemic. Thus, in dogs, tCa measurement (prolonged QT–interval)
underestimated ionized hypocalcemia, and in cats, hypo-
calcemia was overestimated when using serum tCa con- Uncommon
Polyuria or polydipsia
centration to predict iCa status.
Hypotension
CONSEQUENCES OF HYPOCALCEMIA Respiratory arrest or death
AND CLINICAL SIGNS

Clinical signs related to hypocalcemia are identical
regardless of the underlying cause (Box 6-5). Low serum hypocalcemia. 145,408 Patients with chronic hypocalcemia
iCa increases excitability of neuromuscular tissue, which often display intermittent clinical signs despite seemingly
accounts for many of the clinical signs of hypocalcemia. stable serum tCa concentrations. Although unpredict-
Animals with mild decreases in iCa concentration may able, clinical signs often follow periods of exercise or
display no obvious clinical signs. The duration and mag- excitement that may be associated with respiratory alkalo-
nitude of ionized hypocalcemia and the rate of decline in sis and subsequent decreases in iCa concentration. Rapid
iCa concentration interact to determine the severity of infusion of alkali to correct metabolic acidosis can cause
clinical signs. Clinical signs in dogs often are not obvious seizures in animals with marginal or previously
until serum tCa concentration is less than 6.5 mg/dL, compensated hypocalcemia through further reduction
and some dogs show surprisingly few signs despite severe in iCa concentration.
hypocalcemia (serum tCa concentration, <5.0 mg/dL), Clinical signs in dogs with chronic hypocalcemia (pri-
especially if the underlying disease has been chronic and mary hypoparathyroidism) include seizures, muscle
there has been sufficient time for physiologic adaptation. tremors or fasciculations, muscle cramping, stiff gait,
Acute development of hypocalcemia is usually associated and behavioral changes (e.g., restlessness, excitation,
with severe clinical signs. In its most severe forms, hypo- aggression, hypersensitivity to stimuli, and disorien-
calcemia can cause death as a result of circulatory effects tation). 87,115,145,530 Seizures often begin as focal muscle
(e.g., hypotension and decreased myocardial contractil- tremors that become more widespread. Most dogs in
ity) and respiratory arrest from paralysis of respiratory one series had a seizure during the initial 24 to 48 hours
muscles. Serum tCa concentration less than 4.0 mg/dL of hospitalization, a much higher frequency than that
can cause left-sided myocardial failure 154 and death, 179 encountered with idiopathic epilepsy. 179 Seizure activity
especially if the decline in serum calcium concentration associated with hypocalcemia may not be similar to that
was rapid. in idiopathic epilepsy because affected dogs may remain
Other electrolyte and acid-base abnormalities can partially conscious and retain urinary continence during
either magnify or diminish the signs of hypocalcemia. the seizure. 179,446 Seizures are often preceded by appre-
Correction of hypokalemia in cats with concurrent hypo- hension or nervousness. The seizures may be as short
calcemia may precipitate the onset of clinical signs of as 60 seconds or as long as 30 minutes in some dogs.