Amoebic liver abscess: MRI findings

Middle-aged male with unremarkable past medical history, recently (1 year earlier) immigrated to Western Europe from his native country (Bangladesh). Suffering since a month from recurrent fever, abdominal discomfort and cough, unresponsive to empiric antibiotics.
Laboratory evidence of infection including leukocytosis (18.000 cells/mmc), C-reactive protein >300 mg/L, plus elevated gamma-glutamyl-transpeptidase.

Initial ultrasound (Fig. 1) revealed a large (8x5.5 cm), well-demarcated hypoechoic and avascular mass located in the dorsal aspect of right liver lobe, which caused compression on the inferior vena cava and mild ventral dislocation of the portal vein.
Blood and stool cultures tested negative. Serology revealed positive Entamoeba histolytica IgG antibodies, and fecal parasitic trophozoites were found.
Liver MRI (Fig. 2) confirmed a sizeable ovoid lesion with internal fluid-like appearance, thin regular peripheral rim with low T2- and intermediate T1-weighted signal intensity, and mild oedema of the adjacent parenchyma. On diffusion-weighted images, the lesion showed visually hyperintense inhomogeneous appearance, more pronounced at the periphery. Corresponding apparent diffusion coefficient (ADC) maps showed strong peripheral hypointensity and moderately restricted diffusion centrally. After intravenous gadolinium contrast, the lesion showed abscess-like peripheral enhancement.
With laboratory and imaging features consistent with amoebic liver abscess, the patient started appropriate antibiotic treatment including metronidazole and improved clinically.

Transmitted via the fecal-oral route, the intestinal protozoan Entamoeba histolytica is endemic in India, Southeast Asia, parts of Africa, and vast regions of Central and South America including Mexico and Brazil. Favoured by malnutrition, colonic trophozoites may enter the portal system. Although occurring in ~1% of infected people, amoebic liver abscess (ALA) represents the commonest extraintestinal disease site [1-3].
In Western countries, ALA is increasingly encountered in immigrants and occasionally in short-term travellers, after variable time intervals (weeks to years) after leaving high-prevalence regions. ALA typically affects young adults (mean age 35 years) with a striking (10:1) male predominance. Manifestations include high fever, upper abdominal pain following gastroenteritis; sometimes presentation is insidious (diarrhoea, malaise, weight loss). Liver enzymes, function and bilirubin are generally normal. Colonic ulcerations are found endoscopically in half of the patients [1-4].
Diagnosis relies on symptoms and relevant epidemiology, coupled with consistent imaging features, serology and detection of trophozoites, antigens or DNA in stools. The typical solitary ALA (75-80% of cases) is unilocular with variable size (mean 8 cm), located in the right lobe near the capsule. Sonographically, ALAs appear as round or oval masses with regular contour, homogeneous low-level internal echoes and distal through-transmission. CT shows well-defined cavities containing complex fluid (10-20 Hounsfield Units attenuation), with moderately thick (3-15 mm) peripheral enhancement [1, 5-7].
As in this patient, MRI optimally depicts the ALA content with more or less homogeneous, low T1- and high T2-weighted fluid-like signal intensity. Reflecting the fibrin lining and scant perilesional inflammatory reaction, MRI also shows the thin, regular ALA margin with variable signal features, mild oedema of the adjacent liver parenchyma (in 50% of cases), and peripheral “rim” enhancement [5-8].
The key differential diagnosis is pyogenic liver abscess, which is more prevalent in industrialised countries, occurs at a higher mean age without sex bias, often in association with diabetes or underlying hepatobiliary disorders. Features suggesting bacterial aetiology include marked leukocytosis, multifocality, bilobar involvement and complex septated appearance. However, solitary lesions are unreliably differentiated between the two entities on the basis of size and sonographic appearance [1, 4, 6-7].
Possible complications include abscess rupture in the pleuropulmonary system (40%), peritoneum (7%) or pericardium, obstructive jaundice and inferior vena cava obstruction. Antiprotozoans (particularly metronidazole) are curative in 90% of patients. Therapeutic aspiration is reserved for huge lesion at risk of impending rupture, those in which bacterial infection is considered or not responding to antibiotics [1, 2].