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Abdominal ultrasonography is a readily available, non-invasive, and commonly used imaging modality that is often utilized to evaluate the liver, gallbladder, and spleen in dogs. However, like any diagnostic test, it has its own limitations, and it is often only an initial step in achieving a diagnosis. It is vital to understand these limitations in order to prepare clients appropriately when referring for an ultrasound to investigate suspected hepatobiliary or splenic disease.
Elevated liver enzymes are a common reason for patients to be referred for an abdominal ultrasound, but ultrasound has frequently been shown to be a poor predictor of underlying hepatic disease, particularly in cases of diffuse disease.1 While non-specific changes such as alterations in echogenicity or echotexture are common, many dogs with significant hepatic disease have a completely normal appearance to their liver on ultrasound. In one study of dogs with suspected liver disease, 63% of dogs with an unremarkable liver on ultrasound had histopathologic abnormalities noted on biopsy. Importantly, most patients with chronic hepatitis were noted to have a normal ultrasonographic appearance to the liver.2 This is crucial to understand when pursuing ultrasound as the next step for dogs with elevated liver enzymes, as it indicates that a large percentage of dogs with liver disease will have no significant ultrasonographic changes, and this does not effectively rule out liver disease. While ultrasound is still recommended as a first step to look for other abnormalities such as a hepatic mass or biliary disease that could explain liver enzyme elevations, it is important that owners understand that liver biopsy is almost always required for definitive diagnosis of hepatic disease in dogs.
Although ultrasound’s utility for diagnosis of diffuse liver disease is limited, it is useful for determining if a hepatic mass is present. In one study, 76% of dogs with liver masses noted on ultrasound had neoplasia diagnosed histopathologically. However, it is worth noting that some dogs had benign liver masses, and only 64% of dogs with hepatic neoplasia had a mass noted on ultrasound, indicating that the absence of a mass does not rule out cancer.2 In another study, a larger mass size (>4.1 cm) and a heterogenous appearance were more likely to indicate that focal hepatic lesions were cancerous.3 The finding of a hepatic mass on ultrasound should prompt discussion about the likelihood of cancer and should inform next steps, such as CT angiogram, to better evaluate the resectability of a given mass — though a CT cannot definitively diagnose cancer without histopathology.
It is also important to consider the very high prevalence of benign lesions noted in the liver of older patients. Up to 70% of older dogs have benign liver lesions,4 indicating that there is a high risk of over-interpreting changes seen in older dogs. A recent study of dogs with hemoabdomen secondary to a ruptured splenic mass found that dogs with liver lesions noted on preoperative ultrasound did not have an increased risk of either splenic malignancy or hepatic metastasis when compared to dogs without visible hepatic lesions on ultrasound. In fact, only 19% of the hepatic lesions noted on ultrasound were confirmed to represent metastatic disease on histopathology.4 Given the high prevalence of benign hepatic changes such as nodular hyperplasia in older dogs, the finding of liver nodules on ultrasound is not specific for cancer and should be interpreted with caution.
Ultrasound has been used for diagnosis of portosystemic shunts, but has been shown to have variable accuracy for identification of extrahepatic congenital shunts, indicating that failure to identify a shunt on ultrasound does not rule one out. CT angiogram is the preferred method for shunt identification in most cases,5 but if ultrasound is pursued as a first step, it should be a specific “shunt hunt” ultrasound with a radiologist in which the patient is sedated to allow adequate visualization of the vasculature.
Ultrasound of the liver is a reasonable first step in investigating elevated liver enzymes in dogs, but it is important to note the significant limitations that ultrasound has when evaluating dogs with diffuse liver diseases, such as chronic hepatitis or copper hepatopathy. The primary role of ultrasound in these patients is to rule out a focal lesion such as a mass, or to rule out an issue outside of the liver that could explain the enzyme elevations. If no significant findings are found on ultrasound, there is still significant potential for hepatic disease, and biopsy is required for definitive diagnosis.
Ultrasound of the gallbladder can be used to evaluate for gallbladder wall thickening, masses, cholelithiasis, and gallbladder mucocele in dogs. Of these, gallbladder mucocele is the most important pathology diagnosed ultrasonographically, as gallbladder masses and cholelithiasis are uncommon in dogs.
Gallbladder mucocele is an increasingly common and potentially life-threatening problem in dogs that relies on ultrasound for diagnosis. Although the “kiwi” is the classic appearance of a gallbladder mucocele on ultrasound, this is not the only or even the most common ultrasonographic appearance for a mucocele. More commonly, mucoceles appear as organized debris with stellate strands attaching to the gallbladder wall, although there is significant variability in the appearance of mucoceles, and the kiwi pattern does occur in some patients.6 Although ultrasound is good at finding gallbladder mucoceles, it is less accurate in diagnosing gallbladder rupture. One study showed that only a 56.1% sensitivity of ultrasound to determine gallbladder rupture from a mucocele, indicating many cases that are identified as having been ruptured at the time of surgery were missed with pre-operative ultrasound.7 Ultrasound is a useful tool for evaluating the gallbladder and for the diagnosis of gallbladder mucocele in dogs, but the limitations for diagnosis of gallbladder rupture should be understood and conveyed to owners. Importantly, gallbladder rupture should never be ruled out on the basis of ultrasound alone in a patient with consistent clinical signs.
Ultrasound of the spleen can reveal diffuse changes such as splenomegaly or a change in echotexture or echogenicity, or can find focal lesions such as masses or nodules. Like in the liver, benign changes in the spleen are common in older dogs and may include lymphoid hyperplasia, extramedullary hematopoiesis, myelolipoma, or hematoma, among others. This high prevalence of benign change in the spleen means that just like with the liver, the spleen’s ultrasonographic appearance cannot be used to make a definitive diagnosis.
Diagnosis of a splenic mass is an indication that cancer is possible, and splenectomy should be considered but does not necessarily indicate that cancer is present. In fact, more recent studies have shown a higher incidence of benign lesions than what has been reported historically. One study showed that 70.5% of incidentally identified splenic nodules or masses in patients without hemoabdomen were benign. Of the 29.5% that had a malignant splenic mass, 58% of these had hemangiosarcoma.8 The incidence of cancer is significantly higher in patients with hemoabdomen due to a ruptured splenic mass; in this cohort, only 35.7% of these masses were benign in a recent study. Of the 64.3% of patients with cancerous masses in this study, 87.4% were hemangiosarcoma.9 Although the prevalence of benign lesions is much lower in patients with hemoabdomen than in those with incidentally identified splenic masses, over a third of dogs with hemoabdomen have a benign splenic mass, and thus would be expected to have a good prognosis with surgery alone. This is crucial to emphasize in communication with owners when they are making decisions about surgery for a dog with a hemoabdomen. It is also important to note that it is not possible to determine whether a splenic mass is cancerous based on its appearance alone, as a cavitated appearance has not been found to be sensitive or specific for predicting malignancy.10 However, having multiple splenic masses is associated with a higher risk of cancer.11 As with the liver, ultrasound of the spleen may detect non-specific, diffuse changes, or may find masses that would indicate the possibility of cancer, although histopathology is required for definitive diagnosis.
Ultrasound of the liver, gallbladder, and spleen in dogs is a useful first step in evaluating these organs for signs of disease, but it is important to set realistic client expectations that further diagnostics will almost always be indicated. Ultrasound is an important diagnostic test that can help guide next steps, such as a CT angiogram to determine if a liver mass is surgically resectable, or a splenectomy with histopathology to definitively diagnose a splenic mass. Ultrasound is almost never the last step in the path to a diagnosis, and an informed discussion of its limitations and benefits will help ensure that client expectations of ultrasound are appropriate.
