- Most common parasitic CNS infection, Taenia solium in uncooked pork (intermediate host)
- Cyst with a dot, usually parenchymal, causing seizures.
Description
Neurocysticercosis is a neurological infection caused by the cystic larvae of the tapeworm Taenia solium. It is the most common helminthic infection of the nervous system globally and is associated with significant morbidity, particularly in developing countries.
Pathogenesis
Ingestion of undercooked pork containing cysticerci, or more commonly, ingestion of food or water contaminated with T. solium eggs from a human carrier leads to the oncospheres hatching in the stomach, then migrating through the intestinal wall, entering the bloodstream and lodging in tissues including the brain, muscle, and eyes. In the brain, the larvae form cysts, causing a vigorous inflammatory response as the cysts degenerate. 75% of infected patients have CNS involvement.
Definitive Host: Humans serve as the definitive host for Taenia solium when they ingest undercooked or raw pork containing the cysticerci (larvae). In the human intestine, these larvae develop into adult tapeworms, which can produce thousands of eggs during their lifespan.
Intermediate Host: Pigs typically act as the intermediate host. They become infected when they ingest materials (such as human feces) contaminated with Taenia solium eggs. These eggs hatch in the pig’s intestine, releasing oncospheres that penetrate the intestinal wall, enter the bloodstream, and develop into cysticerci in the pig’s muscles and other tissues.
Stages
- Vesicular stage: This is the initial stage after oncospheres have reached the nervous system and developed into cysticerci. The cysticerci are viable, with a clear vesicular wall, an uninflamed host tissue interface and a visible scolex. The cyst fluid is isosmolar with cerebrospinal fluid, and the cyst appears as a fluid-filled structure on neuroimaging.
- Colloidal vesicular stage: In this stage, the cysticercus degenerates, leading to an inflammatory response. The cyst becomes turbid and the scolex begins to degenerate. The vesicular wall thickens and becomes irregular, leading to perilesional oedema and possibly contrast enhancement on imaging. Clinically, this stage often correlates with symptoms due to inflammation, such as seizures.
- Granular nodular stage: Continued degeneration of the cysticercus leads to further inflammation, with gliosis and infiltration by inflammatory cells. The cyst shrinks, and the scolex disappears. On imaging, the lesion appears as a nodular enhancing structure with surrounding oedema.
- Nodular calcified stage: This is the final stage of involution, characterised by complete involution of the cysticercus into a calcified nodule, and resolution of the inflammation and oedema. Patients may be asymptomatic at this stage or may have symptoms related to the location of the calcification or to seizures triggered by the calcification. On imaging, calcifications appear as hyperdense on CT and hypointense on T2-weighted MRI sequences.
Subtypes
There are four anatomical forms of neurocysticercosis, defined by the location of the cysts:
- Parenchymal: Most common form (~70% of cases), usually presenting with seizures
- Subarachnoid: Causes chronic meningitis and can lead to hydrocephalus
- Intraventricular: Can cause obstructive hydrocephalus. Usually associated with poorer prognosis.
- Spinal: Rare, but can cause radiculopathy or myelopathy
Epidemiology, Risk Factors & Associations
- Highly endemic in developing countries (Asia, Africa, and South America).
- Most common in areas where pig farming is common and sanitation is poor
- Immigrants or travellers to endemic areas are at risk
- Poor personal hygiene and autoinfection are significant risk factors
- Typically seen in adults, but can occur in all age groups
Clinical Features
Presentation is variable and depends on the number and location of the cysts. Common features include:
- Seizures (most common)
- Headaches
- Focal neurological deficits
- Meningismus
Complications
- Chronic meningitis
- Hydrocephalus
- Long-term neurological deficits due to parenchymal damage
- Vision loss (ocular cysticercosis)
- Death
Pathological Features
Histopathology
- Macroscopic: Cysts are often visible to the naked eye
- Microscopic: Cysts contain larvae of T. solium, often surrounded by a granulomatous inflammatory response
Radiological Features
The imaging features depend on the stage of the disease (vesicular, colloidal vesicular, granular nodular and nodular calcified).
General Features
- Small enhancing rim-lesion with a peripherally enhancing nodule (scolex). Can be large.
- Can cause seeding to entire neuraxis.
- Characteristically located in the parenchymal (most common), subarachnoid, intraventricular or spinal regions with various stages of involution.
- Intraventricular lesions may cause obstructive hydrocephalus. 4th ventricle is the most common location.
- Basal cistern cysts may have a racemose appearance (grape-like)
- Parenchymal lesions are often hemispheric, at the grey-white matter junction and may cause significant oedema
- Non-enhancing cyst suggest live larvae. Ring-enhancing cyst suggest dying larvae causing inflammatory reaction. Calcification suggest an old lesion.
- Other findings include: hydrocephalus, chronic meningitis, calcification of the skeletal muscle
- MRI most sensitive modality:
- FLAIR and CISS/FIESTA most sensitive for detecting intraventricular lesions
- GRE/SWI helpful in young adults presenting with seizures
- DWI and FLAIR are helpful to look for a scolex
- Vesicular stage: Cyst fluid similar to CSF. Subtle rim enhancement may be present. No perilesional oedema.
- Colloidal vesicular stage: Cyst with rim enhancement and perilesional oedema.
- Granular nodular stage: Continued degeneration of the cysticercus leads to further inflammation, with gliosis and infiltration by inflammatory cells. The cyst shrinks, and the scolex disappears. On imaging, the lesion appears as a nodular enhancing structure with surrounding oedema.
- Nodular calcified stage: Scattered calcifications.
CT
- Vesicular stage (viable larva): Smooth, thin-walled cyst with no or minimal enhancement, isodense to CSF, no oedema. Eccentric intracystic hyperdense dot representing protoscolex (parasite head)
- Colloidal vesicular stage (degenerating larva): Hyperdense cyst fluid with surrounding oedema. Thicker, ring-enhancing fibrous capsule
- Granular nodular stage (healing): Mild oedema. Involuting, enhancing nodule
- Nodular calcified stage (healed): Interval shrinkage of cyst, calcified nodule
MRI
- T1: Cysts are hypointense, the scolex can be isointense or slightly hyperintense.
- T2: Cysts are hyperintense with a hypointense wall. The scolex is of variable signal intensity. There is extensive surrounding vasogenic oedema.
- FLAIR: Hypointense intralesional eccentric nodule representing scolex
- Gd+: Ring enhancement – indicates inflammatory response caused by dying larvae
Diagnosis
Diagnosis confirmed by ELISA of serum or CSF testing for anticysticercal antibodies.
Differential Diagnosis
The differential diagnosis depends on the imaging appearance and includes:
- Tuberculomas: Typically show rim enhancement
- Brain abscess: Encapsulated lesion which also shows smooth complete rim enhancement with central T2 hyperintensity but demonstrates central diffusion restriction. Dual-rim sign (low-signal intensity outer ring (paramagnetic free radical producing macrophages) with high-signal enhancing inner edge (abscess capsule) on T1 weighted scans).
- Gliomas: Usually solid and show enhancement, can have cystic components. GBMs demonstrate low signal (haemorrhagic contents), enhancing irregular incomplete and rim, hetergenous central signal (necrotic core).
- Metastases: Typically multiple lesions with solid and cystic components, strong enhancement of the solid parts
Management
Management involves antiparasitic therapy (albendazole or praziquantel), corticosteroids to control the inflammatory response, and symptomatic treatment for seizures. Surgery may be needed in cases with large cysts, hydrocephalus, or spinal involvement.