Amoebic cerebral abscess
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"amoebic brain abscess" OR "amebic cerebral abscess"
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PMID: 35522800
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Amoebic Cerebral Abscess
Overview
Amoebic cerebral abscess is a rare but serious extraintestinal complication of infection with Entamoeba histolytica. Cerebral involvement is reported in fewer than 0.1% of patients in large clinical series, making it one of the rarest forms of extraintestinal amebiasis after liver and lung involvement. The prognosis depends heavily on the size and location of the lesion.
- Harrison's Principles of Internal Medicine 22E, p. 1800
- Goldman-Cecil Medicine, p. 3444
Aetiology and Pathogen
Causative organism: Entamoeba histolytica - a protozoan parasite that exists as cysts and trophozoites.
- Low infectious dose (<100 organisms)
- Resistant to chlorine; environmentally stable
- Named "histolytica" because of its tissue-destructive properties - mediated by:
- Gal/GalNAc lectin (enables adherence to host epithelium)
- Cysteine proteinases
- Contact-dependent cytotoxicity and apoptosis induction
- "Amebic trogocytosis" (nibbling of host cell membranes)
Pathogenesis and Route to the Brain
The sequence of invasion is:
- Ingestion of E. histolytica cysts via fecally contaminated food or water
- Excystation in the small intestine; trophozoites colonise the large bowel
- Invasion of the colonic mucosa with formation of flask-shaped ulcers
- Entry into the portal bloodstream
- Haematogenous dissemination - most commonly to the liver (amebic liver abscess, ~5% of intestinal cases)
- From the liver, further spread can occur to:
- Lung (pleuropulmonary - most common complication of liver abscess, 20-30%)
- Peritoneum (abscess rupture)
- Pericardium (left-lobe abscess rupture)
- Brain - by haematogenous spread from the intestine or contiguous spread from a liver abscess
- Skin, genitourinary tract (rare)
The trophozoites must overcome reactive oxygen/nitrogen species from the host. E. histolytica uses its thioredoxin-thioredoxin reductase system (not glutathione, which it lacks) as its key antioxidant defence. This is also the system targeted by metronidazole.
- Harrison's, p. 1797-1799
Clinical Features
- Symptoms depend on lesion size and location - headache, focal neurological deficits, seizures, altered consciousness, features of raised intracranial pressure
- History of intestinal symptoms (diarrhoea, dysentery) may or may not be present - fewer than one-third of patients with extraintestinal amebiasis have active diarrhoea
- Concurrent or antecedent liver abscess may be evident (fever, right upper quadrant pain)
- Should be suspected in any patient with amebiasis who develops neurological signs or symptoms
High-risk groups include the very young or old, malnourished persons, pregnant women, and patients on corticosteroids.
- Rosen's Emergency Medicine, p. 1552
Diagnosis
CSF examination: Microscopic identification of motile trophozoites (hematophagous trophozoites) in CSF - but this has low sensitivity.
Tissue biopsy: More specific; trophozoites must be differentiated from free-living amoebae (Naegleria, Acanthamoeba, Balamuthia) by:
- Trophozoite morphology
- Associated host response
- Clinical history
Imaging (CT/MRI): Round or oval hypodense/hypoechoic lesion(s); ring-enhancing on contrast. Non-specific - must be correlated clinically and serologically.
Serology (ELISA): Positive in >90% of invasive cases (colitis, ameboma, liver abscess). Serum antigen detection >95% sensitive for liver abscess. Less data specific to cerebral abscess, but positivity in appropriate clinical context is strongly supportive.
PCR: >95% sensitivity for invasive disease - the most sensitive method.
Stool examination: Cysts/trophozoites present in <50% of patients with extraintestinal disease at the time of diagnosis - not reliable alone.
Key diagnostic distinction: The microscopic diagnosis of E. histolytica (vs. the non-pathogenic E. dispar, E. moshkovskii, E. bangladeshi) can only be made by identifying trophozoites that have ingested erythrocytes on microscopy, or via antigen testing/PCR.
| Test | Sensitivity |
|---|---|
| Real-time PCR | >95% |
| Serum antigen | >95% |
| Stool antigen | ~40% (liver abscess) |
| Serology (IHA) - convalescent | >90% |
| Microscopy - abscess fluid | ≤20% |
- Goldman-Cecil Medicine; Henry's Clinical Diagnosis and Management
Treatment
Primary therapy - Invasive/disseminated disease:
| Drug | Dose | Notes |
|---|---|---|
| Metronidazole | 750 mg TID × 7-10 days (IV or oral) | Drug of choice; IV used for CNS disease |
| Tinidazole | 2 g/day × 5 days | Fewer side effects, better tolerated; preferred in some guidelines |
| Ornidazole | 500 mg TID × 5 days | Alternative nitroimidazole |
| Secnidazole | 2 g single dose | For uncomplicated disease |
CNS amebiasis: IV metronidazole is the primary treatment. Neurosurgical intervention may be required depending on lesion size, location, and clinical response.
Luminal agent (mandatory follow-up): Parasites persist in the intestinal lumen in up to 50% of patients treated with nitroimidazoles. All treatment should therefore be followed by a luminal agent:
- Paromomycin 500 mg TID × 7 days (preferred)
- Iodoquinol 650 mg TID × 20 days
- Diloxanide furoate (second-line)
Asymptomatic carriers: Iodoquinol, paromomycin, or diloxanide furoate alone.
- Harrison's, p. 1800; Goldman-Cecil, p. 3445; Rosen's Emergency Medicine, p. 1552; Henry's Clinical Diagnosis, p. 1157
Differential Diagnosis (Brain Abscess Context)
- Free-living amoebic meningoencephalitis - Naegleria fowleri (primary amoebic meningoencephalitis via olfactory route), Acanthamoeba, Balamuthia mandrillaris (granulomatous amoebic encephalitis) - treated with a 5-drug regimen including amphotericin B, miltefosine, fluconazole, rifampin, azithromycin
- Pyogenic brain abscess
- Toxoplasma encephalitis (especially in immunocompromised)
- Tuberculous brain abscess
- Primary or metastatic brain tumour
Prevention
- Adequate sanitation and safe water supply
- Avoid unpeeled fruits and vegetables in endemic areas
- Use bottled water; iodination (tetraglycine hydroperiodide) for water disinfection in the field - chlorine is not effective against amebic cysts
- No vaccine available
- Asymptomatic cyst carriers can excrete up to 15 million cysts/day, making carrier treatment a public health priority
Key Points Summary
- Cerebral amoebic abscess is the rarest form of extraintestinal amebiasis (<0.1% of cases), occurring by haematogenous spread from the gut or liver
- Suspect in any patient with amebiasis + neurological signs
- Diagnosis: CSF trophozoites (low sensitivity) or biopsy; serology, antigen detection, and PCR are supportive
- Treatment: IV metronidazole (drug of choice for CNS disease) ± neurosurgical drainage, followed mandatorily by a luminal agent
- Trophozoites in brain tissue must be distinguished from free-living amoebae, which require a different treatment regimen
A 2022 case report (PMID 35522800) documented an isolated amoebic brain abscess with an excellent response to therapy, confirming that early diagnosis and metronidazole treatment can lead to good outcomes even in this rare presentation.
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