Ultrasonography of the Adrenal Glands
Welcome to our series of articles on small animal abdominal ultrasonography. The initial articles provided an overview of basic ultrasonography principles and a discussion about how to perform a systematic scan of the abdomen. The rest of the series discusses ultrasound evaluation of specific abdominal organs/systems.
Read the other small animal abdominal ultrasonography articles published in Today’s Veterinary Practice at tvpjournal.com.
LOCALIZATION AND SCANNING TECHNIQUE
Left Adrenal Gland
The following are anatomic landmarks that help identify the location of the left adrenal gland:
- The left kidney is the lateral landmark.
- The abdominal aorta is the medial landmark.
- The celiac and cranial mesenteric arteries are cranial to the left adrenal gland.
- The left renal artery is the caudal landmark.
- The left phrenicoabdominal artery is the dorsal landmark.
- The left phrenicoabdominal vein is the ventral landmark (Figure 1).1
The normal left adrenal gland is in a cranial and medial position relative to the left kidney (Figure 2).1–5 However, the position of the left adrenal gland relative to the left kidney may be altered when the left kidney is displaced by excessive transducer pressure. The significant mobility of that kidney is a result of its being suspended by the renal stalk (vessels and ureter) and bound ventrally by the mesothelial lining of the peritoneum.
The left adrenal gland is slightly lateral and ventral to the abdominal aorta and sits adjacent or caudal to the celiac and cranial mesenteric arteries.1,4,5
The caudal pole of the adrenal gland is localized cranial to the “hook” of the left renal artery and its origin from the left lateral margin of the abdominal aorta (Figure 3).
The left phrenicoabdominal artery is noted along the dorsal surface of the left adrenal gland. Without color Doppler ultrasound, it may not be visible.
The left phrenicoabdominal vein, seen in the near field or ventral aspect of the adrenal gland and transecting it into equal cranial and caudal poles,1 is an inconsistent finding using gray-scale 2D imaging but, when present, helps confirm the localization of the left adrenal gland.
In dogs, the transducer motion for finding the left adrenal gland starts lateral in the long axis of the left kidney. Angle medial (nondistance motion) to the level of the aorta and find the celiac, cranial mesenteric, and left renal arteries. Often you will find the caudal pole of the left adrenal gland and 2 of those arteries; all 3 arteries will not necessarily be in the same imaging plane.
Once you have the caudal pole of the left adrenal gland, rotate the transducer in a clockwise direction from 12 to 1 o’clock (about 10°) to bring the left adrenal gland into long axis (Figure 1). Identification of the left phrenicoabdominal vein along the ventral (near field) margin of the middle of the left adrenal gland is confirmation that you are evaluating the left adrenal gland.
In cats, the celiac and cranial mesenteric arteries are used as the target vessels for finding the left adrenal gland. Angle the transducer medial to the left kidney and move ventral to the long axis of the aorta, finding the celiac and cranial mesenteric arteries as two rounded anechoic structures adjacent to each other in the craniocaudal axis (the celiac artery is cranial to the cranial mesenteric artery). The left adrenal gland (hypoechoic) will be in the retroperitoneal fat (hyperechoic), adjacent to and in the near field relative to these vessels. Occasionally the left phrenicoabdominal vein can be seen.
It is uncommon to see a distinct cortex and medulla in cats unless using a high-resolution linear transducer (Figure 4).
Right Adrenal Gland
The right adrenal gland is located ventral and lateral to the caudal vena cava. In dogs, the adrenal gland is in contact with the caudal vena cava (Figure 5).1,4,5 In cats, it can be further away, separated from the caudal vena cava by retroperitoneal fat (Figure 6).
The right adrenal gland is located medial to the right kidney in the dog. In the cat, it is craniomedial to the right kidney, located along the caudal vena cava between the cranial pole of the right kidney and the caudate lobe of the liver.
The right phrenicoabdominal vein is ventral (near field) to the right adrenal gland when it is visualized. It will be seen less frequently than the left phrenicoabdominal vein adjacent to the left adrenal gland.
With the right kidney in long axis, angle the transducer medial to the kidney and identify the caudal vena cava. Angle the transducer lateral to the caudal vena cava slowly, and the adrenal gland will be parallel to or at a 30° angle relative to the long axis of the caudal vena cava. If at an angle, the caudal pole of the adrenal gland will be further away from the caudal vena cava (Figure 7).
NORMAL ULTRASONOGRAPHIC FEATURES
The left adrenal gland has variable shape in dogs (Figure 8) but is consistently oval or bean shaped in cats (Figure 4).6
The shapes seen in small-breed dogs include a dumbbell or biconcave disc shape (similar to a cross-section of a red blood cell). In medium- and large-breed dogs, the left adrenal gland may have a pancake or lawn-chair shape (Figure 8). All of these shapes are considered normal variants.
In dogs, the adrenal gland length and width are variable; however, a caudal pole width of 0.6 to 0.74 cm has been shown to be normal.2,3,7,8 Recent literature has suggested that the thickness of the caudal pole of the adrenal gland in the sagittal plane is correlated to the size of the dog: ≤0.54 cm for dogs <10 kg, ≤0.68 cm for dogs 10 to 30 kg, and ≤0.8 cm for dogs >30 kg.9 Adrenal gland length has not been correlated with the age or size of dogs; therefore, the caudal pole measurement is the key to determining enlargement of the adrenal glands in the dog.2,3,7,8
The normal width and/or height of the left adrenal gland in cats is 0.43 ± 0.03 cm.2,3,10
The left adrenal gland in dogs is hypoechoic relative to the surrounding retroperitoneal fat, and often a hyperechoic oval stripe can be visualized in the middle of the gland. This stripe is assumed to be the delineation between the adrenal medulla (hypoechoic) and outer cortex (hypoechoic).2,3
The right adrenal shape is typically evaluated in long axis and is oval in small dogs and more of a pancake in medium- and large-breed dogs. However, the true shape of the right adrenal gland is a V (Figure 9). In cats, the normal shape for the right adrenal gland is an oval or bean shape.
PATHOLOGY OF THE ADRENAL GLAND
Small adrenal glands can be seen in dogs and cats with iatrogenic or spontaneous hypoadrenocorticism (Figure 10).2,3,11,12
The adrenal glands of dogs with pituitary-dependent hyperadrenocorticism can be normal sized, be bilaterally enlarged (hypertrophic) with a normal shape and increased caudal pole width/height (Figure 11), or have nodular hyperplasia.2,3,7,13–15 Often there is an accompanying steroid-induced hepatopathy, such as a generalized increase in echogenicity, hypoechoic nodules that are ill-defined, hyperattenuation of the ultrasound beam, and biliary debris (Figure 12).2,3,16–19
Focal enlargement, such as hyperechoic or hypoechoic nodules, can be present in the adrenal glands and can represent benign or malignant lesions (Figure 13).7,13–15
Benign adenomas can be unilateral or bilateral and functional (glucocorticoid secreting) or nonfunctional.2,3,20,21
Marked enlargement >2 cm of the adrenal gland is usually associated with malignant etiologies, such as adenocarcinomas or pheochromocytomas (Figure 14). Adenomas and adenocarcinomas are more likely to be mineralized,22,23 and malignant tumors often invade the phrenicoabdominal vein and/or caudal vena cava (Figure 15).2,3,20–22,24
Mineralization of the adrenal glands in cats is an age-related change that has not been reported to have clinical significance (Figure 16).13,25
Adrenal tumors are much rarer in cats than in dogs; however, aldosterone-secreting tumors, resulting in Conn’s syndrome, have been reported (Figure 17).2,3,26–32
The vascular anatomy is key to adrenal gland localization. The ability to routinely identify the adrenal glands is a standard part of the complete abdominal ultrasound evaluation. The most common observed abnormality of the adrenal glands is enlargement, which can take the form of hyperplasia, nodules, and masses. In cats, mineralization is seen as an incidental age-related change.
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