Disclosure
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 and is called neurocysticercosis (NCC). NCC is recognized as a common cause of neurologic disease in developing countries as well as 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 (nonendemic country) is $9 million, and 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. Treatment depends on the presence of symptoms, the location of the cysts, and the presence of a host immune response. Pathophysiology: Infectious organism NCC can be associated with poor sanitation, and 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 can 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, they may attach 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 and usually located in the fourth ventricle (50%). They may cause intermittent hydrocephalus. Concomitant parenchymal cysts in the presence of intraventricular cysts are seen in approximately 20% of patients. When cysticerci become inflamed, the granular ependymitis and accompanying fibrillary astrocytosis cause the cysticerci to adhere to the walls of the ventricles. Proliferative lobulated cysts without a scolex characterize the racemose form, which is usually found in the ventricular system and subarachnoid space. Although infrequent, this is the most serious manifestation of NCC. Spinal cord cysts are rare (1-3%). Intramedullary NCC occurs from either hematogenous or ventriculoependymal spread. The thoracic cord is affected in two thirds of patients. Ocular involvement is seen in approximately 5% of patients and can 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, but they rarely occur in the eyelids or lacrimal glands. Stages of NCC The host can 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 is infiltrated and 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 collapse of the cyst cavity (granular-nodular stage). The dead parasite decays into an eosinophilic desiccated material. The final stage is a calcified nodule, presumably the 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, the variety of NCC is characterized by small, multiple, diffuse parenchymatous involvement. In the Latin America, the variety is characterized by solitary or few large parenchymal cysts, meningeal racemose, and the ventricular type of disease. Frequency:
Mortality/Morbidity: Results of postmortem studies in areas endemic for NCC suggest that approximately 80% of infections are asymptomatic.
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Clinical Details: The clinical presentation of 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. Likely, NCC is the most common cause of symptomatic epilepsy worldwide, and it possibly contributes to a relatively higher epilepsy rate in the developing countries. Most patients with seizures have parenchymal cysticerci with associated edema or enhancement (colloid or granular-nodular stage). Headaches are common in 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 can 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 can be summarized as follows:
Hydrocephalus occurs less frequently in children with parenchymal disease than in a similar subset of adults. 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 cases of symptomatic disease. Preferred Examination: The diagnosis of NCC is complex, and 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. Enzyme-linked immunotransfer blotting (EITB) is currently the most accurate serologic test, and it 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, EITB results are negative in 60-80% of patients. Results are probably negative in more than 80% of cases of 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 can be diagnosed confidently. Neither CT scans nor MRI images are practical for screening a large population for NCC, particularly in developing countries.
Brain, Abscess
Findings: Calcified cysticerci are seen as multiple elongated lesions shaped like cigars or grains of rice. These lesions are arranged in the direction of the muscle fibers in affected skeletal muscle. Calcified cysticerci are easily visualized on soft tissue images. Calcified intracranial cysts are occasionally seen on skull images. Radiographs have been used as part of the evaluation during ventriculography for the diagnosis of intraventricular NCC and during myelography for the diagnosis of intraspinal NCC. Degree of Confidence: CT is more sensitive than plain radiography in detecting intracerebral calcification. False Positives/Negatives: Multifocal rice-grain–like calcification in the skeletal muscles is suggestive of cysticercosis, particularly in patients form endemic areas. Solitary calcification is nonspecific and of no diagnostic significance. |
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Findings: An inactive lesion represents a dead organism. This lesion is densely calcified with no enhancement. CT reveals 1 or more sites of calcification typically 2-10 mm in diameter (see Image 1). Active parenchymal NCC is the most common form of disease. The viable cyst appears as a thin-walled fluid-filled cyst with a mural nodule (live scolex); the cyst causes no inflammatory reaction or edema, and it does not enhance (see Image 2). Symptomatic infection develops when the cysticercus loses the ability to control the host's inflammatory and immune responses. In the colloid stage, the cyst is encapsulated, it contains a high-protein fluid, and it demonstrates ring enhancement. Often, associated edema or enhancement is noted in the brain parenchyma (see Image 3). As the cysticercus becomes fibrotic or collapses, a focal area of enhancement suggestive of granuloma is seen (granular-nodular stage) (see Image 4). Finally, a focal area of calcification appears. At any stage, solitary or multiple lesions may exist. In 80% of patients, multiple cysts are found (see Image 5). Cysticercal encephalitis results from infection with large number of cysticerci that induce an intense inflammatory response in the brain parenchyma. This appears as diffuse cerebral edema with small, slitlike ventricles. Intraventricular cysts remain clinically silent until they degenerate. The symptomatic form results in obstructive hydrocephalus. Cysticerci can be seen in any of the ventricles. CT scans may reveal evidence of obstructive hydrocephalus or dilatation or distortion of the involved ventricle. CT scans obtained after the intraventricular administration of contrast material delineate the cyst and the site of the obstruction (see Image 6). Subarachnoid NCC is described in 3 subtypes. First, if it is located in the gyri of the cerebral convexities, the appearance and presentation of cysticerci may resemble that of active parenchymal NCC. Second, when cysticerci are found in the fissures (eg, sylvian fissure), they may enlarge to several centimeters and are termed giant cysticerci (see Image 7). They may produce mass effect or parenchymal inflammation. Giant cysticerci are readily detected on CT scans, but small cisternal cysticerci may not be seen. Thirds, cysticercosis of the basal cisterns is characterized by arachnoiditis and seen as focal or diffuse meningeal enhancement or vasculitis with stroke. Patients often develop communicating hydrocephalus (see Image 8). Spinal NCC may be localized to the subarachnoid space or spinal cord. Degenerating cysticerci become fixed at one level and induce inflammation. CT myelography may demonstrate an extramedullary block or filling defects in the intrathecal contrast column. Ocular involvement may be intravitreal or subretinal. CT can demonstrate cysts in the extraocular muscles (see Image 9). Degree of Confidence: In most patients, CT is reliable in the diagnosis of NCC, particularly in patients with multifocal parenchymal disease. In the vesicular stage, MRI is more accurate than CT in detecting an eccentric scolex. Intraventricular cysts are more clearly depicted by using CT ventriculography or MRI. False Positives/Negatives: CT scans help in detecting most cases of structural disease, but lesions in the brainstem and small cisternal and intraventricular lesions may be missed. If CT is performed without contrast enhancement, isoattenuating lesions may not be detected. In some asymptomatic individuals, CT findings may be suggestive of NCC. The differential diagnosis of small, ring-enhancing lesions with vasogenic edema includes metastases, cerebral abscess, parasitic infection, primary neoplasm, and resolving subacute infarction and hematoma.
Findings: On MRIs, the contents of live cysts (vesicular stage) are isointense relative to cerebrospinal fluid (CSF) on T1-weighted images (T1WI) and T2-weighted images (T2WI). T1WIs clearly show an eccentric, hyperintense, 2- to 5-mm scolex with a pea-in-the-pod appearance (see Image 10). The demonstration of a scolex is pathognomonic for NCC. When the larva begins to die (colloid vesicular stage), the fluid in the cyst becomes more turbid, and it is mildly hyperintense to CSF on both T1WIs and T2WIs. The surrounding edema is hypointense on T1WIs and hyperintense on T2WIs. The hypointense cyst wall stands out between the hyperintense cyst fluid and edema on T2WIs (see Image 11). The cyst wall may be enhancing in the granular-nodular stage. Calcified cysts are seen as areas of signal void, especially on gradient-echo images. All the stages may be seen simultaneously in the same patient. Cisternal cysts (see Image 12) and intraventricular cysts (see Image 13) are visualized better on MRIs than on CT scans because they stand out in comparison to CSF as a result of the relative T1 shortening. If the cysts induce meningitis, arachnoiditis, or ependymitis, contrast enhancement is clearly noted. Intraspinal NCC most commonly involves the subarachnoid space and, less often, the cord or epidural space. Contrast-enhanced MRI clearly defines intramedullary and extramedullary cysticerci. Intramedullary NCC usually occurs as a single lesion in the thoracic cord. Degree of Confidence: MRI is superior to CT in imaging lesions of NCC, but it is less available than CT in endemic areas. MRI is better in detecting cystic lesions in the base of the brain, CSF spaces (eg, ventricular NCC, cisternal NCC), and intramedullary lesions. MRI may more often reveal the scolex, which may not be visible on CT scans. MRI is superior in demonstrating inflammation around the cyst. The differential diagnosis of spinal intramedullary NCC includes other infectious granulomas and neoplasms. False Positives/Negatives: A parenchymal cysticercus cyst without a scolex may mimic other cystic lesions. A ring-enhancing lesion with surrounding edema may represent NCC, tuberculosis granuloma, fungal abscess, pyogenic abscess, neoplasm, or resolving hematoma and/or infarct. Rarely, cyst walls may not be distinguishable from the CSF in the ventricle or cisterns, and a preoperative diagnosis may be difficult.
Findings: US is useful in the diagnosis of ocular NCC. The cysts are well delineated when they occur in a subretinal location. The cyst wall is well depicted against the vitreous humor, and the cyst elevates the retina. Degree of Confidence: The degree of confidence in the diagnosis of ocular NCC depends on expertise of the ultrasonographer. An experienced ultrasonographer can diagnose most NCC lesions of the eyes. False Positives/Negatives: A cyst without a scolex cannot be differentiated from other cystic lesions of the eye.
Findings: Nuclear medicine studies are not helpful in the diagnosis of NCC.
Findings: Cerebral angiography may be useful in the evaluation of vasculitis resulting from cisternal NCC. Narrowing, occlusion, and the beading of vessels may be seen. Degree of Confidence: Findings of vasculitis are nonspecific for the diagnosis of NCC. False Positives/Negatives: Angiographic findings of vasculitis are nonspecific and may be seen in tubercular meningitis, chronic meningitis, and vasculitis related to collagen vascular disease.
Intervention: Currently, interventional imaging techniques have no definitive role in the diagnosis or treatment of NCC.
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