ENDOCRINOLOGY
shown to be sensitive for the detection of invasion of
blood vessels.
Although ultrasound was sensitive for the detection of
thrombi in the caudal vena cava, it was only 75% sensitive for detecting all forms of invasion. In one study,
invasion of the surrounding blood vessel was shown
to occur via the phrenico-abdominal vein as opposed
to the erosion of blood vessel walls. Pheochromocytomas seem to have predilection for invasion of blood
vessels. Between 36-70% of all adrenal masses (benign or malignant) are seen to invade the surrounding
blood vessels.
In humans, CT scanning is an additional factor that
is used to decide on adrenalectomy. Benign masses
tend to have a Hounsfield unit of <10% and a >50%
contrast washout. Inhomogeneous masses with an irregular border are less likely to be benign, especially
if the HU is > 20. In those cases that warrant surgical
removal, pre-operative investigations for metastasis
should be done.
A CT scan of the thorax and abdomen has a higher
sensitivity for detecting metastatic lesions than thoracic radiographs and abdominal ultrasound. As discussed previously, in human medicine, the CT density
of the adrenal mass has diagnostic significance.
Masses that are <3cm in size, those that are not producing hormones and those that show no invasion of
the surrounding blood vessels may be left for careful observation. The size of the adrenal mass is reassessed at 1, 2, 4 and 6 months after initial diagnosis.
Those masses increasing in size should be considered
for adrenalectomy. The non-enlarging, stable mass
should continue to be observed at 4-6 month intervals.
Pre-operative attempts to diagnose the etiological
cause for the adrenal mass are essential. Surgical excision of an undiagnosed pheochromocytoma carries
a significant risk for peri-anaesthetic / operative morbidly and mortality. These can be significantly lowered
by medically managing this endocrine disorder preoperatively. Functional adrenal testing is therefore essential prior to surgical removal – especially to detect
the presence of a pheochromocytoma.
Adrenal functional testing
Pheochromocytoma: Tachycardia and hypertension
are the hallmarks of catecholamine release, but hormone release may be episodic. Patients may, therefore, be completely normal during the physical examination – and be normotensive. A wide variety of
clinical signs may be noticed by the owner, including:
collapse/weakness; panting, tachycardia/arrhythmias,
restlessness, inappetence/anorexia, nonspecific lethargy, exercise intolerance, polyuria/polydipsia, and
weakness.
vet360
Issue 06 | DECEMBER 2015 | 10
Testing for the presence of a pheochromocytoma in
dogs has centred on the measurement of adrenalin
metabolites in the urine, either absolute amounts per
24hrs or as measured by a ratio to creatinine. Ideally,
a first morning urine sample is collected and a small
amount of acid is added to the prevent degradation of
the metabolites. Metanephrine and normetanephrine
to creatinine ratios are readily available to veterinarians in South Africa through use of the human clinical
pathology laboratories. Pheochromocytomas should
always be considered malignant in dogs, with metastasis occurring in up to 40% of affected dogs.
Hyperaldosteronism: (Conn’s syndrome). This condition is suspected in patients with hypokalaemia,
hypernatremia and hypertension. All findings are not
consistently present in all patients. So, while identifying these electrolyte disturbances in a patient with
an adrenal mass is supportive for this diagnosis, the
absence of these findings do not exclude a case of
Conn’s syndrome.
The condition is rare in dogs and cats. Low renin and
high aldosterone serum concentrations are useful in
diagnosing those suspected patients with unremarkable serum electrolyte concentrations. Aldosterone
levels are easily tested for, renin requires special collection practices.
Hyperadrenocorticalism: (Cushing’s disease) Both
the ACTH stimulation test and low dose dexamethasone may be used to diagnose Cushing’s disease.
Most patients should have clinical signs consistent
with Cushing’s disease. In some cases it may be more
appropriate to exclude hyperadrenocorticism as a
differential. The urine cortisol to creatinine ratio is a
highly sensitively test, but not very specific. Therefore
it has a high negative predictive value. Patients with a
negative result are unlikely to have Cushing’s; while
a dog that tests positive may or may not have hyperadrenocorticism.
References
Reference available from the
Author on request.