Page 88 - Clinical Small Animal Internal Medicine

56 Section 2 Endocrine Disease

The likelihood of falsely low ACTH values in dogs with recommended to differentiate PDH from ADH. Since
VetBooks.ir PDH is increased by: suppression in response to dexamethasone supports a
diagnosis of PDH, a dog with dexamethasone resistance
intraassay and interassay variability (increased at lower
●
can have either AT or PDH.
cACTH concentrations)
pulsatile ACTH secretion
● Dexamethasone Suppression with UCCR
inappropriate sample handling allowing ACTH
● Decreased blood cortisol concentration after dexameth­
degradation.
asone administration is reflected in decreased UCCR.
Dexamethasone Suppression Test Results After the patient’s owner/handler collects a
Dexamethasone administration in:
morning urine sample on two consecutive days, three
normal dogs: causes rapid and prolonged suppression doses of dexamethasone (0.1 mg/kg PO) are adminis­
●
of cortisol secretion tered at 6–8‐hour intervals, with a third urine sample
patients with an AT: at any dosage does not suppress collected the next morning.
●
cortisol secretion Decrease in the third UCCR to <50% of the mean cor­
dogs with PDH: tisol basal values is consistent with PDH. Lack of sup­
– does not appropriately suppress ACTH secretion pression does not confirm AT. In 160 dogs with HAC (49
(therefore does not suppress cortisol) when a low with ATs, 111 with PDH), the UCCR in 72% of dogs with
dose (0.01 mg/kg) is administered PDH suppressed to <50% of the basal UCCR, while the
– in 75% of dogs with PDH, ACTH and cortisol con­ other 28% of those with PDH were dexamethasone
centrations decrease when a high dose (0.1 mg/kg) is resistant. In dogs with ATs, maximum suppression was
administered 44% of the baseline sample.
– in 25% of dogs with PDH, suppression of ACTH and
cortisol does not occur even after administration of Differentiating PDH from ADH: Imaging
higher dosages; in these patients, a large pituitary While imaging can be very helpful in differentiating PDH
tumor or tumor developing from the pars interme­ from ADH, it cannot be used to establish a diagnosis of
dia is more likely. HAC. Moreover, finding normal adrenal glands on imag­
ing studies does not rule out HAC.
Test Results The largest study evaluating both suppres­
sion tests (LDDS and high‐dose dexamethasone sup­ Radiography
pression [HDDS]) included dogs with PDH (n = 181) and Imaging results may include:
ATs (n = 35). With LDDS, criteria for identifying dogs
with PDH included: ● abdominal distension
four‐hour post‐LDDS cortisol concentrations below ● good contrast due to abdominal fat deposition
● ● hepatomegaly
laboratory cut‐off or <50% of basal cortisol bladder distension
concentration ● mineralization of bronchi and pulmonary interstitium,
eight‐hour post‐LDDS cortisol concentrations <50% ●
● and of dermal and subcutaneous tissues in areas pre­
of the basal cortisol concentration and greater than the disposed to calcinosis cutis.
laboratory cut‐off.
A small liver makes HAC unlikely. An AT may be visual­
With HDDS, criteria for cortisol suppression were a four‐ ized due to either mass effect or tumor calcification.
and/or eight‐hour cortisol concentration below the labo­
ratory cut‐off or <50% of the basal cortisol concentration. Adrenal Gland Imaging
Approximately 75% of dogs with PDH met at least one
criterion for suppression on either LDDS or HDDS. Of Adrenal gland width is the most informative parameter
identified on ultrasonography. However, the following
those with PDH, 88% suppressed with the LDDS and may affect correct measurement.
12% demonstrated suppression with HDDS.
Dexamethasone resistance (i.e., no criteria were met) ● The long axis of adrenal gland often is misaligned with
occurred in all dogs with AT and the remainder (25%) of either the medial or dorsal plane of the body.
the dogs with PDH. In another study of 41 dogs with AT, ● Cross‐sectional images may lead to oblique views and
28 LDDS and 30 HDDS tests were performed, with no miscalculation of glandular dimensions.
suppression seen on any test. ● Breed and body size differences.
In dogs demonstrating lack of suppression with ● Macronodular hyperplasia (a rare form of PDH) and
LDDS, use of endogenous ACTH rather than HDDS is some ATs can be difficult to differentiate.