BrainTF7-Neurocysticercosis

Pathogenesis

Stages

On MRI

Suggested Reading

Cysticercosis is a parasitic infection that afflicts immunocompromised and immunocompetent individuals. Neurocysticercosis is commonly found in India, China, Central and South America and Mexico.The causative agent is the pork tapeworm, Taenia Solium. Humans may be the definitive host (tapeworm) or the intermediate host (cysticercus).
On eating insufficiently cooked pork, man becomes the definitive host. This tapeworm releases eggs that pass into the stool.
On ingesting ova contaminated food or water humans become the intermediate host. In the stomach the outer shell dissolves and oncospheres are released. They penetrate the stomach and intestinal lining to enter the blood stream. These may deposit in any soft tissue but have a predilection for the brain.
These oncospheres may burrow into the brain parenchyma, meninges, ependyma and choroid plexus (four patterns-parenchymal, subarachnoid, intraventricular and mixed).
Spinal cord involvement is rare.

• Initial infection is asymptomatic and there is a small edematous lesion.
• Cysticerci (secondary larvae) develop into cysts, two to three months after the ingestion of ova. Usually asymptomatic. Occasional seizures. Cyst measures 3 to 20 mm in diameter. It has a scolex. It is alive and stays viable for 2 to 6 years.
• When the cyst dies it releases antigens and metabolic products. These produce an inflammatory reaction with edema and resultant mass effect. Patient may have seizures or focal neurological deficits. The clear cyst fluid becomes turbid.
• Eventually the cyst collapses, degenerates and calcifies.
• If the oncospheres lodge in the meninges or choroid plexus, subarachnoid cysts may form. These cysts may be multiloculated (bunch of grapes - racemose form - 5mm to 9 cm). Scolex is absent. Chronic meningitis and ventriculitis may result. There may be obstructive hydrocephalus. This type does not usually calcify.
• Repeated ingestion of ova leads to intracranial cysts in different stages of evolution. Also the lesions resolve at differing speeds.
• In an untreated host, resolution takes 2 to 10 years.
  

• Initially an edematous lesion that is hypointense on the T1W images and hyperintense on the T2W images is seen. Occasional peripheral or punctate enhancement is seen. Enhancing nodules (protoscoleces - within the cysticerci but without cyst fluid) may be seen later.

• The mature cysts - vesicular stage - measure 3 to 20 mm. They are usually seen at the grey-white junction. They may also be found in the basal ganglia, brainstem and cerebellum. Cyst wall is thin and smooth. The cyst fluid is isointense to CSF. Scolex is seen as a focal nodule within the cyst. Occasional enhancement of the cyst wall is seen.

• As the cyst dies a fibrous capsule which enhances forms. Cysts are hyperintense to CSF (proteins + debris). Perilesional edema (hypointense on the T1W images and hyperintense on the T2W images) is seen. This is the colloidal vesicular stage.

• Subsequently the cyst retracts and forms a granulomatous nodule that may show ring or solid enhancement. On the T2W images they are usually hypointense  This is the granular nodular stage.

• Finally the lesion becomes gliotic and calcifies. They are better seen on CT. On the T2W images they are hypointense (better appreciated on the gradient rephased images). This is the calcified nodular stage.

• Intraventricular lesions are usually located in the fourth ventricle with obstructive hydrocephalus. These may enhance.

• In the subarachnoid space, the suprasellar and cerebellopontine angle cisterns are common sites. These usually do not enhance. There is focal widening of the CSF space or an inflammatory reaction within the adjacent brain parenchyma. Infection of the space may lead to chronic meningitis and a communicating hydrocephalus with enhancement of the basal cisterns.

Suggested Reading:

1. Chang K-H, Lee JH, Han MH, Han MC: The role of contrast-enhanced MR imaging in the diagnosis of neurocysticercosis: AJNR 1991: 128:509-512.
2. Whiteman MLH, Bowen BC, Post MJD, Bell MD: Intracranial Infection. In: Atlas SW: Magnetic Resonance Imaging of the Brain and Spine, Lippincot-Raven, pp:757-763, Second Edition.