Amoebic hepatic abscess

Amoebic hepatic abscesses are a form of hepatic abscess resulting from Entamoeba histolytica infection.

Epidemiology

The parasite causes up to 40 million infections annually, up to 100,000 deaths per annum ^7,8. However, clinical disease only presents in a minority of patients ⁸. 

Although the causative pathogen is found worldwide, it is endemic to the Middle East and Eastern Asia, and travel history should always be sought in suspected cases.

Hepatic abscess is the most common extraintestinal form of E. histolytica infection ⁶.

It has a strong male predominance, about 10:1 ⁶. However, pregnant women are more prone to develop the infection ⁷.

Clinical presentation

Patients present with fever, right upper quadrant pain and general malaise.

The etiology of the abscess can be inferred based on ⁶:

• identification of E. histolytica specific antigen or DNA in stool samples

• antiamoebic antibodies in blood serum 

Pathology

E. histolytica infection occurs in humans due to the consumption of contaminated food or water. Cysts in the small intestine undergo excystation, allowing trophozoites to penetrate the bowel mucosa ⁸. Via this port of entry, the parasite enters the portal venous system and may then spread to the liver, resulting in abscess formation ⁸.

Radiographic features

Amoebic hepatic abscesses can sometimes be difficult to differentiate from other liver abscesses. They tend to be round or oval and variable in size, although most are around 2-6 cm in diameter. An enhancing wall is present in most cases.

Other described features include:

• an incomplete rim of edema

• the margin of the abscess tends to be smooth in around 60% of cases and nodular in around 40%

• internal septations: present in around 30% of cases

• focal intrahepatic biliary dilatation peripheral to an abscess is an uncommon manifestation

Extrahepatic pathology can be present in a considerable amount of patients, including:

• pleural effusion

• perihepatic fluid collection

• gastric or colonic involvement

• retroperitoneal extension

Ultrasound

It may appear as a hypoechoic lesion with low-level internal echoes and absent significant wall echoes.

CT

Usually, they appear as rounded, well-defined lesions with attenuation values that indicate the presence of complex fluid (e.g. 10–20 HU). An enhancing wall and a peripheral zone of edema may be seen with wall thickness around 3-15 mm. The central abscess cavity can show septations and/or fluid-debris levels.

Gas can be present within an abscess if there is a complicating hepatobronchial fistula or a hepatocolic fistula.

MRI

Described signal characteristics include:

• T1: generally homogeneous low signal intensity (signal homogeneity within the abscess can be present more often on T1- than on T2-weighted images ⁵)

• T2: generally homogeneous high signal intensity; perilesional edema may be seen in half of the cases

Treatment and prognosis

The cornerstone of management is initiating anti-parasitic agents, most commonly metronidazole, which also treat amoebic dysentery ⁶. Traditionally, amoebic abscesses are treated with medical therapy as the first line, as opposed to percutaneous or surgical drainage, which is the first line of treatment in pyogenic liver abscesses.

In practice, percutaneous drainage may frequently be required, particularly if:

• uncertain diagnosis

• larger abscesses which are at risk of spontaneous rupture into the peritoneal, pleural, or pericardial spaces

• failed response to metronidazole therapy - proposed as persistent symptoms after four days ⁶

Practical points

• as the abscess liquefies, abscess size may increase on imaging despite clinical improvement. Therefore. The finding alone should not cause panic to alter the line treatment.

• residual amoebic abscesses may take up to 2 years to completely resolve on imaging; consequently, persistent imaging findings alone should not guide further management ⁶