Ultrasonography of the Adrenal Glands

The improved resolution of newer ultrasound systems has allowed visualisation of the adrenal glands with more regularity.

Specific indications for ultrasonography of the adrenal glands include: 

• The presence of a mass or masses in the dorsal abdomen 
• Investigation for metastatic spread of neoplasia 
• Differentiating between adrenal and pituitary dependant hyperadrenocorticism 
• Investigation of hypertension and symptoms suggestive of phaeochromocytoma 

Normal Anatomy 

The adrenal glands are paired organs situated within the retroperitoneal space. Each adrenal gland is located craniomedially to their respective kidney. 

An illustration of the adrenal gland position and surrounding anatomy in the dog. 

The right adrenal gland is situated in close apposition to the vena cava, whilst the left adrenal gland is more commonly separated from the aorta by a pad of fatty tissue. The size and shape of each adrenal gland varies with body weight, age, and conformation. 

Generally, the left adrenal gland has a narrower central section with a wider cranial and caudal pole. This gives the left adrenal gland a “peanut” shape. In comparison, the right adrenal has a “comma” or “arrowhead” shape. 

The phrenicoabdominal artery passes dorsally to each adrenal gland and the phrenicoabdominal vein passes ventrally. 

In cats, the adrenal glands have a uniform oval shape. Incidental mineralisation is a normal finding seen in older cats1. 

Examination Technique 

Ultrasonographic visualisation of both adrenal glands can be difficult, especially in large, deep-chested, or obese patients.  

Due to the cranial and dorsal location of the adrenal glands, it is important to ensure that the patient is adequately clipped. Extend the clipped area to the edge of the lumbar musculature dorsally and, in deep-chested or large breed patients, include the caudal 2-3 intercostal spaces in the clipped area because it can be necessary to use an intercostal window to visualise the adrenal glands. 

Assessment of the adrenal glands can take time and may involve pressure on some areas of the dorsal abdomen. Therefore, sedation can be useful to improve patient comfort and facilitate the ultrasound examination. 

The highest frequency setting that gives adequate depth of penetration to view the area of the adrenals should be used. In most patients, a frequency of 5 – 7.5 MHz is adequate. However, higher frequencies (8 –15 MHz) will improve image resolution and can be utilised in smaller patients. Either a micro-convex or linear transducer can be used for the examination. Linear transducers are capable of higher frequency settings and are ideal for smaller patients. However, their larger footprint makes them harder to manoeuvre around the body. In comparison, micro-convex transducers produce a lower frequency range though their smaller, curved surface make them ideal in larger patients and for imaging using the intercostal spaces. 

Due to the small size of the normal adrenals, the use of anatomic landmarks to guide an examination of the adrenal glands is essential. 

The left adrenal gland is examined with the patient in right lateral recumbency; the patient is positioned in left lateral recumbency to examine the right adrenal gland. 

Left Adrenal Gland 

With the transducer marker orientated cranially, identify the left kidney. A dorsal imaging plane can be useful to avoid interference from neighbouring gas-filled intestinal structures. From the left kidney the transducer can be fanned towards the midline to identify the aorta. Small motions should then be used to achieve a longitudinal plane image of the aorta at this level. 

With small fanning motions dorsally or ventrally, the renal artery should be identifiable branching from the aorta and turning (“hooking”) cranially towards the hilar area of the left kidney. 

The left adrenal gland is situated cranial to the ‘hook’ of the renal artery between the cranial pole of the left kidney and aorta. Small fanning motions and/or slight rotation of the transducer at this level may be needed to achieve a longitudinal image of the left adrenal gland, revealing the typical “peanut” shape. 

Once identified, the transducer can be rotated through 90° to achieve a transverse plane image of the adrenal. 

Right Adrenal Gland 

With the transducer marker orientated cranially, identify the right kidney, and orientate the transducer to view the right kidney in a dorsal plane. 

By fanning the transducer towards the patient’s midline or sliding the transducer ventrally the caudal vena cava can be identified in a longitudinal orientation. It is important not to press too hard as this will compress the vena cava and make it harder to visualise. 

The right adrenal gland lies dorso-medial to the caudal vena cava. By fanning the transducer between the caudal vena cava and the cranial pole of the right kidney, the right adrenal is visible at the point the vena cava just disappears from view. In a longitudinal plane, the right adrenal has a more triangular/ “arrow-head” shape compared to the left. 

Once identified, the transducer can be rotated through 90° to achieve a transverse plane image of the right adrenal gland. In this plane the adrenal gland will appear oval in shape. 

Due to the more cranial position of the right adrenal gland, in larger patients it may be necessary to use an intercostal approach via the last 2 rib spaces to view the right adrenal gland. 

Normal appearance 

The adrenal glands usually appear uniformly hypoechoic. When using higher frequency ranges, a change in echogenicity between the outer cortex and inner medulla may be visualised in some patients.  

As the adrenals have a hypoechoic appearance, it can be difficult to identify the glands from adjacent vascular structures. Colour flow Doppler ultrasound modes can be used to help resolve the adrenal from the surrounding blood vessels. 

Examples of the appearance of the left adrenal gland in dogs. Image A) The left adrenal gland (red arrow, cranial pole; callipers, caudal pole) can be seen in a longitudinal plane adjacent to the cranial pole of the left kidney. Part of the aorta can be seen (green arrow). Image B) The typical “peanut” shape of the left adrenal gland can be seen (blue arrow, cranial pole; callipers, caudal pole). The adrenal gland is situated cranial to the ’hook’ of the left renal artery (yellow arrow). 

Feline Left Adrenal Gland. This image shows the normal feline left adrenal gland (red arrow). In comparison to dogs, the adrenal gland is smaller and more oval shaped. 

Right adrenal gland in the dog. This image shows a sagittal plane image of the right adrenal gland (red arrow and callipers). Note the triangular or “arrowhead” shape. 

There are published reference ranges for normal adrenal gland size (below). However, adrenal gland size varies among individual patients and there is an overlap in size range between normal dogs and those with adrenal or endocrine abnormalities. To measure the adrenal gland thickness, a sagittal or dorsal plane image of the adrenal is acquired, and the width of the caudal pole is measured. 

≤0.54cm (Dogs <10kg)2 

 ≤0.68cm (Dogs 10-30kg)2 

 ≤0.8cm (Dogs > 30kg)2 

In cats, the short axis diameter of the adrenal gland can be measured as the dorsoventral width of the adrenal in a sagittal or dorsal plane image. 

Cat: Short axis diameter of the adrenal = 1.5mm-5.9mm3 

Ultrasonographic appearance of adrenal pathology

Adrenal Gland Hyperplasia 

Bilateral enlargement of the adrenal glands can be seen in patients with pituitary dependant hyperadrenocorticism (PDH). This can appear as symmetrical enlargement and rounding of the adrenal glands. However, unilateral enlargement, bilateral unequal enlargement and adrenal masses or nodules are occasionally reported in cases of PDH4. Therefore, it is not always possible to differentiate PDH from adrenal neoplasia. Non-hormonal disease can also cause adrenal enlargement and ultrasound findings should always be interpreted alongside patient history, clinical exam, and laboratory test results. 

Enlarged Adrenal Gland. This sagittal plane image of the left adrenal in a dog shows enlargement of the adrenal gland. The digital callipers are placed to measure the width of the caudal pole. 

Adrenal gland hyperplasia is reported for patients undergoing therapy using trilostan. 

Adrenal Gland Hypoplasia 

Dogs with hypoadrenocorticism have a decreased adrenal gland size in comparison to normal dogs. In one study, a left adrenal thickness of less than 3.2mm was suggestive of hypoadrenocorticism. 

Adrenal Gland Neoplasia 

Primary adrenal gland tumours include adenomas, adenocarcinomas, myelolipomas and phaeochromocytomas. The adrenal glands can also be a site for the metastatic spread of other neoplastic lesions. 

Typically, unilateral enlargement of one adrenal gland will be seen. However, overlap in the appearance of adrenal conditions causing hyperplasia and neoplasia is possible. Ultrasonography does not show whether lesions are hormonally active or not and so ultrasound findings should always be interpreted alongside the patient history, clinical examination, and laboratory test results. 

Adrenal gland neoplasia is more likely to be present than hyperplasia where the diameter of the gland ≥ 2cm4,7. Another study suggests that malignancy should be suspected when adrenal gland lesions are ≥20mm in size8. 

Mineralisation can be seen in some cases of adrenal neoplasia. However, it is not specific for malignancy. 

A feature of malignant adrenal neoplasms is local invasion of blood vessels. This is more commonly seen in lesions of the right adrenal gland due to the close proximity of the caudal vena cava. Assessment of the local vasculature using ultrasound can identify lesions within blood vessels, and Doppler ultrasound can be used to document the lack of blood flow within vascular structures. 

Neoplasia of the Right Adrenal Gland. This sagittal plane image shows a hypoechoic mass in the region of the right adrenal gland (red arrow). There is invasion of the adjacent caudal vena cava (blue arrow) and interruption to the blood flow within the vena cava can be seen (red asterisk) by the lack of coloured pixels within the colour doppler sample gate (yellow box). 

Sampling Considerations 

Historically, ultrasound guided fine needle aspiration of adrenal gland lesions has not been commonly undertaken, due to the risk of a hypertensive crisis caused by the sampling of adrenal phaeochromocytomas9, as well as due to risks of haemorrhage from the procedure. 

However, some recent studies have shown the procedure to be a “minimally risky procedure”9 or “relatively safe”10. A 23- to 25-gauge 1.5-inch needle can be used for fine needle aspirates. 

References 

1. Benigni L. (2012) Adrenal glands. In: BSAVA manual of canine and feline ultrasonography. Eds. Barr F., Gaschen L. Gloucestershire: British Small Animal Veterinary Association, pp 147–154. 
2. Soulsby S.N., Holland M., Hudson J.A., Behrend E.N. (2014) Ultrasonographic evaluation of adrenal gland size compared to body weight in normal dogs. Veterinary Radiology and Ultrasound 56: 317-326. 
3. Griffin S. (2021) Feline Abdominal Ultrasonography: What’s Normal? What’s Abnormal? The Adrenal Glands. Journal of Feline Medicine and Surgery 23: 33-49. 
4. Nyland T.G., Neelis D.A., Mattoon J.S. (2015) Adrenal Glands, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 541-556. 
5. Mantis P., Lamb C.R., Witt A.L., Neiger R. (2003) Changes in ultrasonographic appearance of adrenal glands in dogs with pituitary-dependant hyperadrenocorticism treated with trilostane. Veterinary Radiology and Ultrasound 44: 682-685. 
6. Wenger M., Mueller C., Kook P.H., Reusch C.E. (2010) Ultrasonographic evaluation of adrenal glands in dogs with primary hypoadrenocorticism or mimicking diseases. Veterinary Record 167: 207-210. 
7. Besso J.G., Penninck D.G., Gliatto J.M. (1997) Retrospective ultrasonographic evaluation of adrenal lesions in 26 dogs. Vet Radiol Ultrasound 38: 448-455. 
8. Cook A.K., Spaulding K.A., Edwards J.F. (2014) Clinical findings in dogs with incidental adrenal gland lesions determined by ultrasonography: 151 cases (2007-2010). Journal of the American Veterinary Medicine Association 244: 1181-1185. 
9. Pey P., Diana A., Rossi F., Mortier J., Kafka U., Veraa S., Growth A., MacLellan M., Marin C., Fracassi F. (2020) Safety of percutaneous ultrasound-guided fine-needle aspiration of adrenal lesions in dogs: Perception of the procedure by radiologists and presentation of 50 cases. Journal of Veterinary Internal Medicine 34: 626– 635. 
10. Sumne J.A., Lacorcia L., Rose A.M., Woodwar A.P., Carter J.E. (2018) Clinical safety of percutaneous ultrasound-guided fine-needle aspiration of adrenal gland lesions in 19 dogs. Journal of Small Animal Practice 59: 357-363. 

Further Reading 

d’Anjou M.A., Penninck D. (2015) Adrenals, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds: Penninck D. and d’Anjou M.A., John Wiley & Sons, Chichester, pp 387-401. 

Griffin S. (2021) Feline Abdominal Ultrasonography: What’s Normal? What’s Abnormal? The Adrenal Glands. Journal of Feline Medicine and Surgery 23: 33-49. 

Nyland T.G., Neelis D.A., Mattoon J.S. (2015) Adrenal Glands, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 541-556. 

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