Dr Nicki Cassel BSc BVSc MMedVet (Diagnostic Imaging) DipECVDI
VET IMAGING SPECIALISTS
Although ultrasound is less sensitive than computed tomography (CT) in assessing the adrenal glands it is the preferred primary method of assessment due to its accessibility in veterinary practice.
The indications to ultrasound the adrenal glands are largely to support a presumptive diagnosis of hyperadrenocorticism and to further differentiate between adrenal-dependant and pituitary-dependant hyperadrenocorticism. Further indications include investigating peritoneal or dorsal abdominal masses, hypertension and other clinical signs which may be related to phaeochromocytoma and to search for metastasis.
Normal Ultrasonographic Assessment
Patients can either be positioned in dorsal or lateral recumbency. It is essential to use a high frequency transducer to visualise the adrenal glands and a thorough knowledge of the regional vasculature is required to locate the adrenal glands. 1,2
With the patient in right lateral recumbency, the left adrenal gland can be visualised by positioning the transducer caudal to the last rib and ventral to the lumbar muscles at the level of the left kidney. Utilising this ultrasound window, the aorta should be seen running in a cranio-caudal direction and will be the closer of the two large vessels. The left renal artery can be seen exiting the aorta directed caudally where after it immediately deviates cranially to reach the renal hilus.
The adrenal gland is located between the aorta and the left kidney, cranial to the hook made by the renal artery (Figure 2 B).1
With the patient positioned in left lateral recumbency, and with the transducer in a similar position as on the contralateral side, the caudal vena cava and right kidney are located as landmarks to visualise the right adrenal gland. The right adrenal gland abuts the caudal vena cava. The right adrenal gland is more challenging to find than the left gland.1,2
A similar approach is used when locating the adrenal glands in feline patients, however the glands are consistently more cranially located along the main abdominal vessels, lying just cranial to the cranial pole of each respective (left and right) kidney. 2
The left adrenal gland is peanut shaped in smaller dogs and elongated and slender in medium to large breed dogs. (Figure 1 A and B) The right adrenal gland has been reported to have an “arrow” shape or L-shaped. The glands are typically uniformly hypoechoic however an outer hypoechoic rim and hyperechoic inner zone may be differentiated with progressive imaging techniques and equipment. This distinction represents the outer cortex and inner medulla.2 (Figure 1 C)
Feline adrenal glands are typically more oval bilaterally and homogeneously hypoechoic. It is rare to see the cortico-medullary distinction in a cat. Mineralisation of the adrenal glands is common in cats, occurring in up to 50% of the population.2 Such glands are hyperechoic with a distal acoustic shadow. This change does not however affect the size of the gland. Contrary, mineralisation in canine adrenal glands has a high probability of representing malignant change although rarely, it may be due to dystrophic change. 1,2,3 (Figure 3)
The cut-off for maximum adrenal gland size in the dog has been commonly referenced as 0.74mm1,2,4 for either the cranial or caudal pole in either a sagittal or transverse plane regardless of body weight of the patient. However a more recent study (n=45), it was found that the size of the adrenal gland in patients without clinical evidence of hyperadrenocorticism varied with three weight categories4. The guidelines from this study are as follows:
Maximum thickness of the caudal pole of the adrenal gland in sagittal plane:
- Dogs ≤10 kg: ≤0.54cm
- Dogs 10-30kg: ≤0.68cm
- Dogs ≥ 30kg: ≤0.80cm
However the authors do acknowledge limitations to the study, such as the low study numbers and the need to further investigate at risk populations of dogs in order to fine-tune these cut-off values.4 It has been found that the caudal pole thickness of either adrenal gland in a sagittal plane was the best dimension for evaluating adrenal gland size due to low variability, ease and reliability in measurement.4
Ultrasound is however, not a flawless technique and up to 25% of dogs with pituitary – dependant hyperadrenocorticism can have normal adrenal gland size on ultrasound and some healthy dogs will have adrenal glands larger than the recommended cut-off values.1-4 Therefore, the ultrasonographic findings should be interpreted in light of the clinical signs as well as the clinic-pathological test results.
In cats, the normal adrenal glands are 10-11mm in cranio-caudal length and up to 4.3 ±0.3mm in diameter. 2
Pathology Of The Adrenal Glands
Patients with PDH generally have bilaterally symmetrically adrenomegaly with a plump rounded appearance. This is attributable to the cortical hyperplasia secondary to pituitary disease. However in some patients with PDH there may be asymmetrical enlargement due to nodular hyperplasia. In these cases it may be difficult to distinguish the enlarged hyperplastic gland from an adrenocortical adenoma. 1,2 (Figure 2)
Another aetiopathogenesis for bilateral adrenomegaly is trilostane therapy.2,3 This is due to cortical hypertrophy secondary to reduced cortisol production and the diminished negative feedback mechanism. Following trilostane treatment, the glands can also become heterogenous in nature or have an enhanced cortico-medullary distinction. It is therefore imperative to perform ultrasonographic assessment of the adrenal glands prior to initiating trilostane treatment.
Primary adrenal tumours are generally unilateral but bilateral tumours have been reported. In patients with clinical signs of hyperadrenocorticism and the finding of an adrenal gland nodule on ultrasound can prove a conundrum. This finding may be due to an adenoma, an adenocarcinoma or a hyperplastic nodule and none of these changes have specific ultrasonographic changes.1-3 The following guidelines apply in such cases:
- Masses ≥ 2.0cm and/or showing mineralisation are considered less likely to represent hyperplastic change with a benign or malignant lesion more likely.
- Masses ≥ 4.0cm are more likely malignant than benign.
Besides a diagnoses of adrenocortical tumours, other tumours occurring in the adrenal glands include myelolipomas, phaeochromocytomas and metastatic tumours. Benign lesions such as cysts, granulomas and haematomas can also mimic neoplastic change in the adrenal glands.1,2
Myelolipomas are benign, endocrinologically inactive tumours. Their fatty component results in them been hyperechoic on ultrasound. 2
Phaeochromocytomas are rare catecholamine secreting tumours. Patients present with vague clinical signs either due to the secretion of catecholamines or due to the space occupying lesion in the retroperitoneal space. These tumours are incredibly rare in feline patients. 1,2,5
Several tumours metastasise to the adrenal glands; mammary, prostatic, gastric and pancreatic carcinomas, squamous cell carcinomas, transitional cell carcinomas, malignant histiocytosis, melanomas and haemangiosarcomas.1 Thus if a patient is suspected of having one of these tumours, a full metastasis search should be performed with careful interrogation of the adrenal glands.
Adrenalectomy is the treatment of choice for adrenal tumours. Regional vascular invasion or tumour thrombus is reported to be as high as 82% for phaeochromocytomas and between 11% to 41% for adrenocortical tumours.3 Vascular invasion has been reported to occur via the phrenicoabdominal vein with echogenic material reported in the phrenicoabdominal and renal veins and the caudal vena cava as a result of extension.1-3
Although vascular invasion is more common with tumours affecting the right adrenal gland, aggressive tumours of both glands can invade the caudal vena cava (Figure 3). Tumour thrombus has been associated with a shorter survival time. A negative finding for local vascular invasion on ultrasound is however not sufficient to exclude the possibility and it is advocated that if surgical treatment is intended, a computed tomography study is performed for optimal surgical planning.
Adrenal gland atrophy
If a patient is suspected of having hypoadrenocorticism, then a finding of small adrenal glands or the inability to find the adrenal glands supports this diagnosis. A cut-off value has been documented as ≤3.0mm thickness for the left adrenal gland and ≤3.4mm for the right adrenal gland. However, once again, ultrasound alone cannot be used to make a diagnosis of adrenal gland atrophy.1 Other causes for non-visualisation of the adrenal glands include incorrect transducer selection/ultrasound technique, poor image quality due to gas in the GIT or patient panting and exogenous steroid administration.2
Ultrasound is the preferred first choice modality for adrenal gland assessment in patients with suspected pathology. However, there is a certain degree of overlap in the ultrasonographic appearance of healthy and diseased glands as well as non-specific pathological changes making a definitive diagnosis bases on ultrasonographic findings alone impossible.
It is therefore imperative to correlate ultrasonographic findings with clinical signs and any clinic-pathological test results in order to make a definitive diagnosis. In many cases of adrenal tumours, a definitive diagnosis will only be made at necropsy.
If a nodule or mass is found, following the necessary function tests, it advised to do follow up ultrasound studies every 3 – 6 months for monitoring purposes.