CNS Imaging in Cysticercosis

Overview

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).
See the CNS images of cysticercosis below.
Nonenhanced CT scan of the brain demonstrates the multiple calcified lesions of inactive parenchymal neurocysticercosis. CT images of the brain in a patient with neurocysticercosis show numerous parenchymal lesions. MRIs of the brain show multiple, large, cysticercus cysts in the basal cisternal spaces in a patient with neurocysticercosis. 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]}

Types of NCC

CNS cysts are encountered in 4 types in NCC:

• Meningeal (racemose variety)
• Parenchymal (solitary or multiple cysts)
• Ventricular (usually solitary)
• 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:

• Meningoencephalitis
• Granulomatous meningitis
• Ocal granuloma
• Focal or diffuse multiple cysts
• Hydrocephalus
• Intraventricular cysts
• Ependymitis
• 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.

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.^[8] 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%.
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.

Radiography

Radiographs have been used as part of the evaluation during ventriculography for the diagnosis of intraventricular neurocysticercosis (NCC) and during myelography for the diagnosis of intraspinal NCC.
On radiographs, calcified cysticerci appear 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.
CT is more sensitive than plain radiography in detecting intracerebral calcification. The presence in the skeletal muscles of multifocal calcifications resembling grains of rice is suggestive of cysticercosis, particularly in patients from endemic areas. A solitary calcification is nonspecific and is of no diagnostic significance.

Computed Tomography

Spinal neurocysticercosis (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.
In most patients, the diagnosis of NCC may be reliably made on the basis of CT findings; this is particularly true for 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 with CT ventriculography or MRI.
In most cases, CT scans help in the detection 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 neurocysticercosis (NCC).
For cases involving small, ring-enhancing lesions in association with vasogenic edema, the differential diagnosis includes metastases, cerebral abscess, parasitic infection, primary neoplasm, and resolving subacute infarction and hematoma.
An inactive lesion represents a dead organism. Such lesions are densely calcified, with no enhancement. CT reveals 1 or more sites of calcification, typically 2-10 mm in diameter (see the image below).
Nonenhanced CT scan of the brain demonstrates the multiple calcified lesions of inactive parenchymal neurocysticercosis. 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 the image below). Symptomatic infection develops when the cysticercus loses the ability to control the host's inflammatory and immune responses.
Enhanced CT scan of the brain in a patient with neurocysticercosis demonstrates a live cyst with a minimally enhancing wall and an eccentric hyperattenuating scolex. 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 the image below).
Nonenhanced (left) and enhanced (right) CT scans of the brain in a patient with neurocysticercosis show multiple ring-enhancing lesions with perifocal edema. As the cysticercus becomes fibrotic or collapses, a focal area of enhancement suggestive of granuloma is seen (granular-nodular stage) (see the image below).
Nonenhanced (left) and enhanced (right) CT images of the brain in a patient with neurocysticercosis show an enhancing disk lesion with perifocal edema suggestive of the granulomatous form of disease (arrow). Multiple ring lesions are also apparent; these are suggestive of the colloid stage of neurocysticercosis. Finally, a focal area of calcification appears. At any stage, solitary or multiple lesions may exist. Multiple cysts are found in 80% of patients (see the image below). Cysticercal encephalitis results from infection with a large number of cysticerci that induce an intense inflammatory response in the brain parenchyma. Its appearance is that of diffuse cerebral edema with small, slitlike ventricles.
CT images of the brain in a patient with neurocysticercosis show numerous parenchymal lesions. Intraventricular cysts remain clinically silent until they degenerate. The symptomatic form results in obstructive hydrocephalus. Cysticerci may 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 the image below).
Left, CT scan of the brain shows marked dilatation of the right lateral ventricle in a patient with intraventricular neurocysticercosis. Right, Contrast-enhanced ventriculogram shows a fourth ventricular cyst as a filling defect in the contrast-enhanced spinal column. Ocular involvement may be intravitreal or subretinal. CT may demonstrate cysts in the extraocular muscles (see the image below).
Left, Enhanced CT scan of the orbit shows a calcified neurocysticercosis lesion in the subretinal space of the right globe. Right, Image shows a neurocysticercosis granuloma in the right lateral rectus muscle. There are 3 subtypes of subarachnoid NCC:

• In the first subtype, NCC is located in the gyri of the cerebral convexities; in this subtype, the appearance and presentation of cysticerci may resemble that of active parenchymal NCC.
• In the second subtype, cysticerci are found in the fissures (eg, sylvian fissure); they may measure several centimeters in diameter and are termed giant cysticerci (see the image below). Giant cysticerci may produce a mass effect or parenchymal inflammation. They are readily detected on CT scans, but small cisternal cysticerci may not be seen. Nonenhanced CT image of the brain shows large cerebrospinal fluid–attenuating cysts in the suprasellar cisterns in a patient with neurocysticercosis.
• In the third subtype, cysticercosis involves the basal cisterns. This subtype is characterized by arachnoiditis; it appears as focal or diffuse meningeal enhancement or as vasculitis with stroke. Patients often develop communicating hydrocephalus (see the image below). Nonenhanced (left) and enhanced (right) CT images of the brain in a patient with neurocysticercosis demonstrate peripheral meningeal enhancement around the cysts in the suprasellar and right sylvian cisterns.

Magnetic Resonance Imaging

MRI is superior to CT in imaging the lesions of neurocysticercosis (NCC), but it is less available than CT in regions of the world where NCC is endemic. MRI is better in detecting cystic lesions in the base of the brain, CSF spaces (eg, ventricular NCC, cisternal NCC), and intramedullary lesions. The scolex may be more readily apparent on MRI than on CT. MRI is superior in demonstrating inflammation around the cyst.
The differential diagnosis of spinal intramedullary NCC includes other infectious granulomas and neoplasms. 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 are indistinguishable from the CSF in the ventricle or cisterns; in such cases, a preoperative diagnosis may be difficult.
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.
Intraspinal NCC 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.
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).^{[9, 10]}
T1WIs clearly show an eccentric, hyperintense, 2- to 5-mm scolex with a pea-in-the-pod appearance (see the image below). The demonstration of a scolex is pathognomonic of neurocysticercosis.
T1-weighted (T1), T2-weighted (T2), and fluid-attenuated inversion recovery (FLAIR) MRIs show a typical cyst with a scolex (arrow) in a patient with neurocysticercosis (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. On T2WIs, the hypointense cyst wall stands out between the hyperintense cyst fluid and edema (see the image below). The cyst wall may be enhancing in the granular-nodular stage.
T1-weighted (T1) and T2-weighted (T2) MRIs show a degenerating cyst with a hypointense wall and hyperintense surrounding edema, which is best depicted on T2-weighted images. The patient has neurocysticercosis (NCC). Cisternal cysts (see first image below) and intraventricular cysts (see second image below) 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.
MRIs of the brain show multiple, large, cysticercus cysts in the basal cisternal spaces in a patient with neurocysticercosis. T1-weighted (T1) and T2-weighted (T2) MRIs show a dilated fourth ventricle containing a cyst in a patient with neurocysticercosis (NCC). The cyst wall (straight arrow) and the scolex (curved arrow) are seen clearly depicted. The cyst resulted in obstructive hydrocephalus. A shunt procedure was required.

Ultrasonography

Ultrasonography is useful in the diagnosis of ocular neurocysticercosis (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. 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. A cyst without a scolex cannot be differentiated from other cystic lesions of the eye.

Angiography

Cerebral angiography may be useful in the evaluation of vasculitis resulting from cisternal neurocysticercosis (NCC). Narrowing, occlusion, and the beading of vessels may be seen. Angiographic findings of vasculitis are nonspecific and may be seen in cases of tubercular meningitis, chronic meningitis, and vasculitis related to collagen vascular disease.