Introduction
Background
Cysticercosis is the most common parasitic infestation affecting the central nervous system (CNS). CNS involvement is seen in approximately 90% of patients with cysticercosis; when cysticercosis involves the CNS, it is called neurocysticercosis (NCC). NCC is recognized as a common cause of neurologic disease in developing countries, and it is also seen in developed countries, including the United States. NCC is a chronic disease associated with substantial morbidity and high social and economic costs. A minimal estimate of annual treatment costs in the United States (a country in which NCC is not endemic) is $9 million; in Mexico and Brazil, costs are estimated to be nearly $90 million per year.
The pathogenesis and clinical presentation vary with the site of infection and the host immune response. NCC poses a complex diagnostic and treatment dilemma because of its varied presentation. Factors determining treatment include whether symptoms are present, the location of the cysts, and whether there is a host immune response.1,2,3,4
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Pathophysiology
Infectious organism
Cysticercosis of the CNS (neurocysticercosis, NCC) may be associated with poor sanitation; the disease is acquired after the consumption of infected food or food contaminated with infected feces (ie, fecal-oral transmission). More commonly, transmission occurs through the consumption of fruit and vegetables grown in soil fertilized with contaminated pig or human waste. Less commonly, it occurs after the ingestion of infected pork.
The larval form of the pork tapeworm, Taenia solium, causes NCC. T solium is the only tapeworm for which humans may be both the intermediate host (harboring the larval form) and the definitive host (harboring the adult form). The ingested larvae (ie, embryos or oncospheres) from contaminated food, drink, or soil are absorbed through the intestinal blood vessels into the venous circulation. They pass through the pulmonary circulation and systemically lodge in the skeletal muscle, eyes, and CNS.
In the CNS, the larvae may lodge in the subarachnoid space or in the brain parenchyma, commonly at the junction of gray and white matter. In tissues, oncospheres develop into cysticerci. The encapsulated larval forms contain clear fluid and a viable scolex. If humans ingest infected meat that contains cysticerci, the cysticerci may attach themselves to the intestinal mucosa and develop into mature tapeworms.
Adult worms are composed of hundreds of proglottids, each of which contains oncospheres that repeat the cycle when the proglottid is shed in feces. Cerebral lesions evolve from an active form to a transitional form and then to an inactive form.
The interval from infection to the onset of symptoms is probably lengthy. In a case series of British soldiers infected in Asia, the median time until the first appearance of symptoms was estimated to be 4 years. The proportion of infected persons who develop symptomatic disease is unknown. In various clinical trials, the spontaneous disappearance of all or some of the cysts has been described in a substantial number of patients. Antihelminths have been found to hasten the disappearance of active parenchymal lesions.
Types of NCC
CNS cysts are encountered in 4 types in NCC: (1) meningeal (racemose variety), (2) parenchymal (solitary or multiple cysts), (3) ventricular (usually solitary), and (4) mixed.
Meningeal cysts form mostly in the basal meninges, sometimes causing stroke and hydrocephalus. Parenchymal cysts are usually found in the cerebral cortex, including the cortical-subcortical junction. The white matter is rarely involved. Ventricular cysts are seen in 15% of patients with NCC; in 50% of cases, they are located in the fourth ventricle.5 They may cause intermittent hydrocephalus. In approximately 20% of cases, parenchymal cysts are found concomitantly with intraventricular cysts.
When cysticerci become inflamed, the granular ependymitis and accompanying fibrillary astrocytosis cause the cysticerci to adhere to the walls of the ventricles. The racemose form is characterized by proliferative lobulated cysts without a scolex; such cysts are usually found in the ventricular system and the subarachnoid space. Although infrequent, this is the most serious manifestation of NCC.
Spinal cord cysts are rare (1-3% of cases). Intramedullary NCC occurs from either hematogenous or ventriculoependymal spread. In two thirds of patients, the thoracic cord is affected.6,7
Ocular involvement is seen in approximately 5% of patients; it may be diagnosed by means of fundus examination or ultrasonography (US). Cysts may float freely in the anterior/vitreous chamber of the eye or adhere to retinal and subretinal tissues. Subretinal cysts produce vasculitis and retinal edema. If located in the vitreous, cysts result in chorioretinitis and vitreous detachment; they rarely occur in the eyelids or lacrimal glands.
Stages of NCC
The host may tolerate the worm as long as the embryo is alive. Viable cysticerci are associated with minimal inflammation (vesicular stage). The worm usually dies 2-6 years after infection, and the disintegration of the parasite triggers a vigorous tissue reaction. An inflammatory response to the degenerating cyst results in severe symptoms. As the cysticerci lose the ability to control the host's immune response, the cyst wall becomes infiltrated and is surrounded by predominantly mononuclear cells. Inflammatory cells enter the cyst fluid (colloid stage). As the host's immune response progresses, fibrosis encompasses the cysticercus, with concomitant collapse of the cyst cavity (granular-nodular stage). The dead parasite decays into eosinophilic desiccated material.
The final stage is a calcified nodule, which presumably forms as a result of dystrophic calcification of the necrotic larva (calcific stage). The various pathologic states that may be seen in NCC include the following: (1) meningoencephalitis, (2) granulomatous meningitis, (3) focal granuloma, (4) focal or diffuse multiple cysts, (5) hydrocephalus, (6) intraventricular cysts, (7) ependymitis, and (8) arteritis.
In India, NCC is characterized by small, multiple, diffuse parenchymatous involvement. In Latin America, NCC is characterized by solitary or few large parenchymal cysts, meningeal racemose, and the ventricular involvement.
Frequency
United States
Cysticercosis of the CNS (neurocysticercosis, NCC) is found mostly in immigrant populations; an estimated 1000 new cases are diagnosed each year.
International
Cysticercosis of the CNS (neurocysticercosis, NCC) has a worldwide distribution. NCC is endemic in most of Central America, South America, and Asia. In endemic areas, 25-50% of patients with adult-onset epilepsy have associated NCC. The average prevalence of seropositive findings in patients in endemic areas is 6-10%; 4-5 epilepsy cases per 1000 result from NCC. In Latin America, approximately 400,000 individuals have symptomatic disease. In Mexico, NCC is diagnosed in 13-33% of all patients with intracranial space-occupying lesions.8
Mortality/Morbidity
Results of postmortem studies in areas endemic for neurocysticercosis (NCC) suggest that approximately 80% of infections are asymptomatic.
- In Mexico, NCC is the cause of death in 2-3% of patients examined after death.
- In Latin America, where NCC is endemic, 400,000 people are estimated to have symptomatic disease. In endemic areas, 4-5 epilepsy cases per 1000 are presumed to be caused by NCC.
Race
- Multiple lesions are less common in India than in Latin American countries. Neurocysticercosis (NCC) with concomitant intracranial hypertension, as well as the meningeal form of NCC, is uncommon in India.
- Subcutaneous cysticercosis is more common in China than in Latin America and India.
- Differences in the human leukocyte antigen may determine the risk of intracranial infection or symptomatic parenchymal disease in a population infected with cysticercosis.9
Sex
- The male-to-female ratio of neurocysticercosis (NCC) is 1:1.
- Cysticercal encephalitis, a rare presentation of NCC, is more common in young girls than in others.
Age
- Rates of seropositive results tend to increase with age; seizures resulting from neurocysticercosis (NCC) typically begin in early adulthood (usually, in patients older than 15 y).
- The incidence of NCC is low in children because of the long incubation period; children probably account for 0.8-27.8% of patients with NCC.
Presentation
The clinical presentation of cysticercosis of the CNS (neurocysticercosis, NCC) is variable. NCC may manifest as any neurologic or psychiatric syndrome. Seizures are the most common clinical manifestation of intraparenchymal NCC at all stages. It is likely that NCC is the most common cause of symptomatic epilepsy worldwide; moreover, it is possible that NCC contributes to the relatively higher epilepsy rate in developing countries. Most patients with seizures have parenchymal cysticerci with associated edema or enhancement (colloid or granular-nodular stage).10,11,12
Headaches are common with the parenchymal, ventricular, and cisternal types of NCC. Headache and focal symptoms are more common during the active and transitional stages of the cysts. Meningeal and intraventricular cysts may appear as obstructive or communicating hydrocephalus. Symptoms usually result from a host inflammatory response to the parasite or the space occupation. The racemose form commonly seen in Latin America results in chronic meningitis, arachnoiditis, and hydrocephalus. Occasionally, patients with NCC present with progressive dementia, behavioral disturbances, or pseudohypertrophy of the calf muscles.
In general, the clinical presentation depends on the host immune response; on the number of lesions; and on the form (active or inactive), location, and duration of the disease. Clinical findings may be summarized as follows:
- Increased intracranial pressure
- Pseudotumor (diffuse parenchymal involvement)
- Obstructive hydrocephalus (intraventricular cysts, racemose meningeal cysts)
- Seizures (focal or multifocal, possibly intractable)
- Intracranial space occupation (parenchymal cysts)
- Meningoencephalitis
- Basal arachnoiditis
- Psychiatric disorders, including dementia
Hydrocephalus occurs less frequently in children with parenchymal disease than in adults with parenchymal disease. Cysticercus encephalitis occurs more often in children and young females than in others. Isolated non-neurologic manifestations, such as ocular or dermal cysts, occur in less than 5% of symptomatic cases.
Preferred Examination
The diagnosis of cysticercosis of the CNS (neurocysticercosis, NCC) is complex; no diagnostic test identifies all cases of cysticercosis. The diagnosis depends on a constellation of the clinical history, exposure history, laboratory results, and imaging findings.
CT scanning or MRI after the intravenous administration of contrast material is the imaging test of choice.13
Enzyme-linked immunotransfer blotting (EITB) is the most accurate serologic test and is the most practical screening tool. Its sensitivity for multiple intracranial cysticerci is 90-100%.
Limitations of Techniques
A negative serologic result does not exclude cysticercosis. When inflammation is absent, enzyme-linked immunotransfer blotting (EITB) results are negative in 60-80% of patients. Results are probably negative in more than 80% of cases of neurocysticercosis (NCC) involving only a single lesion. The sensitivity of serologic testing also considerably decreases late in the course of the disease and in patients with calcified lesions. Conversely, asymptomatic patients commonly have seropositive EITB results.
In most patients, neuroimaging findings are not pathognomonic for NCC. If an eccentric scolex is seen within the cyst, NCC may be diagnosed confidently. Neither CT scans nor MRI images are practical for screening a large population for NCC, particularly in developing countries.
Differential Diagnoses
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References
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Dumas JL, Visy JM, Belin C, et al. Parenchymal neurocysticercosis: follow-up and staging by MRI. Neuroradiology. Jan 1997;39(1):12-8. [Medline].
Gaur V, Gupta RK, Dev R, et al. MR imaging of intramedullary spinal cysticercosis: A report of two cases. Clin Radiol. Apr 2000;55(4):311-4. [Medline].
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Further Reading
Keywords
CNS cysticercosis, neurocysticercosis, NCC, pork tapeworm, Taenia solium, T solium, CNS parasite, tapeworm, tape worm, cysticercus disease, taeniasis, teniasis, cestode infection, helminthiasis, parasitic disease, meningeal cysts, parenchymal cysts, ventricular cysts
