Neurocysticercosis: Slideshow

Neurocysticercosis is an infection in the brain of cysticercus, the larval form of the pork tapeworm Taenia solium. It is the most common parasitic disease of the nervous system and the main cause of acquired epilepsy for people in developing countries.

T solium is a cyclophyllid cestode that infects humans and pigs. T solium exists in several forms throughout its life cycle. Adult organisms may grow up to 2 to 7 meters in length and have a scolex, or head, with 4 suckers and twin rows of hooks. Their bodies are lined with several hundred hermaphrodite proglottids, or segments, which are each capable of releasing thousands of eggs once mature.

The life cycle of T solium is complex and involves both humans and pigs. Pigs ingest eggs by eating fecally contaminated food. The eggs, or oncospheres, hatch in the intestines and develop into cysticerci in muscles. When humans eat contaminated undercooked meat, the cysticerci develop into adult organisms in the small intestines and then begin to shed thousands of eggs. If humans ingest eggs through autoinfection or via reverse peristalsis, then cysticerci can form in muscles, the liver, or the brain. This results in neurocysticercosis.

Neurocysticercosis is commonly considered a disease of the developing world because of the increased prevalence in areas with poor hygiene and locales where humans and pigs live in close proximity to one another. Although the prevalence is highest in developing countries, the increased rate of immigration to developed countries has made it a true global problem. Of note is the near absence of infection in Muslim countries, where the consumption of pork is forbidden by Islam.

The presentation of neurocysticercosis is highly variable based on the size, quantity, and location of lesions, as well as on the host's immune response. Patients with very few lesions may have major symptoms, and those with many lesions may be asymptomatic; however, the general progression is toward increasingly severe and varied symptoms as lesions grow and new lesions form.

The cysticercus is a liquid-filled vesicle with a 3-layer wall with or without a scolex. Three different presentations are found within the nervous system. The cystic form refers to single or multiple cysts anywhere within the brain but typically within the leptomeninges and cerebral cortex. The racemose form refers to the presence of multiple cysts in the basal cisterns without a scolex. Racemose types may form giant vesicles up to 10 cm in diameter. The mixed form is a combination of the two.

Cysticerci in the brain develop and involute through 4 distinct stages. In the vesicular stage, the cysts develop and there is downregulation of host cellular immunity leading to minimal surrounding edema (top right). In the colloidal stage, the worm dies, and as the cysts degenerate the host cellular immunity is no longer downregulated, which leads to intense inflammation as antigens leak out (bottom left and bottom right). In the granular-nodular stage, the host immune response leads to progressive fibrosis and collapse of the cysts. In the calcified stage, the dead cysts calcify, and the inflammatory reaction is minimized (top left).

Neuroimaging with computed tomography (CT) and/or magnetic resonance imaging (MRI) plus the administration of a contrast agent is the best means of detecting neurocysticercosis. CT can accurately detect cysts in all 4 stages. The vesicular stage appears as thin-walled, fluid-filled cysts without enhancement. The increased inflammation in the colloidal stage shows ring enhancement with focal enhancement as fibrosis occurs in the granular-nodular stage. Calcified lesions in the final stage are then readily identifiable. At any one time, lesions in any or all stages may be present.

Similar to CT, MRI can identify cysts in all stages. Cysts in the vesicular stage appear isointense to cerebrospinal fluid, but with the development of edema, the lesions become increasingly intense after the administration of gadolinium. Calcified cysts appear as areas of signal void. MRI is especially useful in identifying lesions that are intraventricular or too small for CT scan. MRI can also show changes in the meninges or the spinal cord.

Definitive diagnosis of neurocysticercosis requires identification of cysticerci in the brain; however, there are numerous adjuvant means of detection readily available. Taeniasis may be established by detecting T solium eggs and proglottids in a patient's stool. The immunoblot assay developed by the Centers for Disease Control using purified antigen is the immunodiagnostic test of choice for confirming a clinical and radiologic presumptive diagnosis of neurocysticercosis. The test is 95% sensitive and 100% specific, superior to both enzyme-linked immunosorbent assay and polymerase chain reaction.

The preferred treatment for all forms of taeniasis, including neurocysticercosis, is medical management, although for patients with calcified solitary lesions, treatment may be initially withheld. Antihelminthic drugs, such as albendazole or praziquantel, are used to kill the worms. Corticosteroids are administered to reduce the inflammatory cascade that develops after the death of the helminth and the subsequent antigen release. Anticonvulsant drugs help control the cyst-induced seizures. Of note, there is controversy regarding the appropriate use of antihelminthic medications in subsets of patients, as patients with severe edema cannot afford the additional inflammation caused by the antihelminthics. Image included with permission and copyrighted by First DataBank, Inc.

Surgery is reserved for patients who have failed medical management and for whom severe symptoms have developed that are localized to specific accessible lesions. Medical management is almost always the preferred method of treatment; however, surgery has been shown to be advantageous for patients with refractory intraventricular cysts. Shown is a CT cisternography in a patient with increased intracranial pressure, demonstrating a cysticercus in the fourth ventricle that was identified and removed during surgery. CT cisternography is reserved for rare refractory cases in which MRI is not diagnostic or is contraindicated.

T solium is considered potentially eradicable because pigs and humans are the only animal reservoirs. Public health strategies focus on breaking the cycle of infection in populations at risk. Improving sanitation and living conditions, increasing testing and surveillance, developing a cysticercosis pig vaccine, and treating populations with antihelminthic drugs are all methods currently being debated by international health organizations to eliminate the parasitic infection permanently.

Neurocysticercosis: Slideshow

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