Amebic Liver Abscess

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Diagnosis and reasoning

This patient has presented with fever, jaundice, and abdominal pain - an ominous group of symptoms known as Charcot’s triad; this is highly suggestive of acute suppurative cholangitis, a surgical emergency. Considering the history of alcohol abuse, two other emergencies should also be kept in mind - acute pancreatitis and spontaneous bacterial peritonitis, both of which can be rapidly fatal if not diagnosed and treated promptly. However, the examination brings up a surprising finding: massive hepatomegaly. This is not compatible with the above diagnoses, and is instead suggestive of a hepatic abscess, hepatoma or severe malaria. The next step should be a liver profile and ultrasound scan of the abdomen; unfortunately, the former is only slightly deranged, and not of much diagnostic significance. However, the ultrasound scan is much more helpful; the sonographic findings of a cystic lesion with thickened walls are most compatible with a pyogenic or amebic abscess; a hepatoma would have shown up as a solid lesion. Ultrasound is also the imaging study of choice for detection of hydatid cysts; this diagnosis can be effectively ruled out, as their sonographic appearance is that of a multilocular cyst, with calcifications and daughter cysts often visible. Note also the presence of shaggy margins and a subhepatic collection; this hints that the cyst/abscess might have ruptured. This too argues against a hydatid cyst, as rupture often results in anaphylaxis. Further imaging with computerized tomography (CT) is unlikely to yield additional information; instead, the next step should be aspiration of the lesion under imaging guidance. In this patient, the aspirate has a typical ‘anchovy sauce’ appearance. This is particularly suggestive of an amebic abscess; note that a positive serology result is required for definitive confirmation. A few additional points should be mentioned here. First of all, he very likely acquired the infection from Kenya, where the disease is endemic. Furthermore, fever with chills is uncommon with an uncomplicated amebic abscess; in this patient, these symptoms might be due to secondary bacterial infection of the abscess itself; another rare possibility is peritonitis following rupture of the abscess. Metronidazole is the amebicide of choice in these patients; given the possibility of secondary infection and/or peritonitis, empirical antibacterial therapy (such as IV cefotaxime) should also be considered. Note that open surgical drainage carries a high mortality rate, and is rarely used. There is no justification for keeping him nil by mouth.


Discussion

Amebic liver abscesses are the most common extraintestinal complication of invasive amebiasis, occurring in around 1% of patients infected with E. histolytica, and disproportionately affecting men. The disease is acquired via ingestion of amebic cysts; following their digestion, the trophozoites thus released into the gastrointestinal tract reach the liver via the portal system; here, they cause cellular necrosis, resulting in development of a liver abscess. It is important to appreciate that the disease may occur in patients with no history of amebic colitis. Amebic liver abscesses are usually solitary and surrounded by thin walled granulation tissue; the right lobe of the liver is more frequently affected. The majority of patients present with right upper quadrant pain and fever; jaundice may also be present, with multiple or large liver abscesses potentially causing severe jaundice. On examination, tender hepatomegaly can often be elicited. Sonography is frequently the radiologic study first performed, and is also the imaging modality of choice, being able to diagnose almost all hepatic abscesses. Typical findings include a round or oval lesion with well defined margins; these are primarily hypoechoic, with a fine homogeneous low-density echogenicity; some distal enhancement may be present. While computerized tomography (CT) and magnetic resonance imaging (MRI) can be performed for follow up imaging, they usually do not yield much additional information; if used, CT may reveal a hypodense lesion with internal inhomogeneity, a smooth margin, and a contrast-enhancing peripheral rim. Often, imaging findings are unable to reliably distinguish between a parasitic cyst and a pyogenic abscess; thus, percutaneous drainage may be required for diagnosis; the contents of the amebic abscess are usually described as resembling ‘anchovy sauce’. Note that the most common causes of pyogenic abscesses include bacteroides spp, enterococcus, Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus and Streptococcus spp; Common parasitic causes include Entamoeba histolytica and Echinococcus granulosus. Serologic testing is the best way to confirm the diagnosis; indirect hemagglutination is the most commonly used test, with a sensitivity and specificity both which range from 85% to 95%. In about a quarter of patients, the cysts of E. histolytica may be detected in the stool. Amebic liver abscesses are usually sterile. Secondary infection is the most common complication, while rupture into adjacent cavities or structures is a rare but important complication. Abscesses located in the superior surface of the liver may rupture through the diaphragm and cause empyema, pleural effusion, or a bronchopleural fistula. On the other hand, those located in the inferior surface tend to rupture into the peritoneal cavity; abscesses may also rupture into the pericardium, skin, colon and other intra abdominal viscera. The first line treatment of an uncomplicated amebic liver abscess should be amebicides, with metronidazole being the drug of choice. The response to medical treatment is directly proportionate to the size of the abscess. Ultrasound or CT guided percutaneous drainage is another modality of treatment. While this is not routinely recommended, in many cases it is eventually required, especially if the abscess is large. Open surgical drainage carries a very high mortality rate, and is not recommended unless there has been repeated failure of conservative methods, or if severe complications are present. Relapses following surgical drainage are uncommon. Note that complete sonographic resolution may take 6 to 9 months; persistent sonographic abnormalities do not necessarily indicate a need for further treatment.


Take home messages

  1. Amebic liver abscesses are the most common extraintestinal complication of invasive amebiasis.
  2. Abdominal ultrasound can readily and reliably diagnose liver abscesses; confirmation of the amebic etiology requires serology or analysis of the aspirate.
  3. Amebicides such as metronidazole are effective in treatment of amebic liver disease.
  4. Complications of amebic liver abscesses include rupture into adjacent body cavities and structures, secondary infection, and less often, generalized sepsis.

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