Page 87 - Clinical Small Animal Internal Medicine

7 Pituitary-Dependent Hyperadrenocorticism in Dogs and Cats 55

especially when combined with abdominal ultrasound Evaluation of Pituitary–Adrenal Axis
VetBooks.ir evaluation and interpreted in light of the history and the following circumstances.
Evaluation of the pituitary–adrenal axis is indicated in
physical examination.

● Patients with clinical signs and laboratory findings
Synthetic vs Compounded ACTH Synthetic polypeptides, consistent with PDH and in whom nonadrenal illness
such as Cortrosyn® (cosyntropin) or Synacthen® (tetra­ has been ruled out or is well controlled.
cosactrin), contain the biologically active first 24 amino ● Patients in whom an adrenal/pituitary mass or bilat­
acids of ACTH; however, their potencies have not been eral adrenal hyperplasia has been discovered in con­
compared. junction with compatible clinical signs.
In several studies, after administration of cosyntropin ● Patients with an incidentally discovered adrenal mass,
(5 μg/kg or 250 μg/dog IV or IM), peak cortisol concen­ with adrenalectomy being considered.
trations occurred at 60–90 minutes. After administra­ ● Diabetic dogs with insulin resistance.
tion of 5 μg/kg IV, no difference was detected between
60‐ and 90‐minute cortisol concentrations. Cosyntropin
can be reconstituted, divided into aliquots in plastic Differentiating PDH from ADH: Laboratory Analysis
syringes, and frozen at –20 °C for six months. Whether Given that PDH and adrenal‐dependent hyperadreno­
tetracosactrin can be frozen has not been investigated; corticism (ADH) are the most common forms of HAC,
according to the manufacturer, store it at temperatures and that their treatment options and prognoses differ, it
from 2 to 8 °C. is important to recommend additional testing to deter­
Compounded ACTH products have been evaluated mine the exact etiology of HAC. Several diagnostic
in healthy dogs. Sixty minutes after administration, modalities are commonly used to differentiate between
cortisol concentrations were similar among four PDH and ADH.
compounded products (2.2 U/kg IM) and cosyntropin ● Endogenous ACTH
(5 μg/kg IV). However, at later times, cortisol concen­ ● Dexamethasone suppression
trations varied considerably with the compounded ● Dexamethasone suppression with UCCR
products.
Endogenous ACTH
Urine Cortisol to Creatinine Ratio Canine ACTH is secreted from the pituitary gland
The UCCR provides an integrated reflection of cortisol in an episodic, pulsatile fashion in both healthy
production, adjusting for fluctuations in blood concen­ dogs and those with PDH. A circadian rhythm has
trations. Determination of basal UCCRs can be per­ not been convincingly demonstrated, although one
formed alone or in tandem with other endocrine testing. study reported higher plasma concentrations of canine
ACTH (cACTH) in the late afternoon than in the
Indications The UCCR can be used as a screening test morning.
for hypercortisolemia, although a single positive result
should not be overinterpreted. Adding oral dexametha­ Indications Concentrations of cACTH do not differ
sone suppression to UCCR testing (see later) has the between healthy dogs and those with PDH, and its
advantage of potentially demonstrating both increased measurement is not useful to screen for HAC. However,
cortisol production and decreased sensitivity to gluco­ measurement of cACTH is the most accurate stand‐
corticoid feedback. alone biochemical test for differentiating PDH from
adrenocortical tumor (AT), but the sensitivity of the
Sensitivity/Specificity When a single, random urine assay differs with methodology. Normal or elevated
sample is collected in veterinary hospitals, reported sen­ concentrations of cACTH are consistent with PDH
sitivity and specificity of UCCR for diagnosis of HAC while suppressed values are consistent with AT.
range from 75% to 100% and 20% to 25%, respectively. In
some dogs, there is considerable day‐to‐day variation in Sensitivity/Specificity The most common problem with
UCCR results. the cACTH assay is poor sensitivity. The largest study of
However, in dogs with physical and biochemical cACTH in dogs with HAC used a two‐site solid‐phase
changes consistent with HAC, when two basal UCCRs chemiluminescent immunometric assay (Immulite 2000
were above the cut‐off level:
Immunoassay System for ACTH; healthcare.siemens.
sensitivity was 99%; 95% confidence interval 94–100% com) and showed excellent discrimination between
●
specificity was 77%; 95% confidence interval 64–87%. PDH and AT.
●